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VSD energy savings
18

VSD energy savings

VSD energy savings

(OP)
Hi,

I would like to perform some detailed calculations to determine the energy saving in the appliance of a VSD (variable speed drive) in electric motors mainly for pumps.
I searched in many references and there are some explanations, but i didn't find yet one that suits for my intentios. Complete enough...
So, my base point is that i can only measure in site the power consumption of the motor and with this i would like to perform some calculations to estimate the energy saving after apply a VSD.

Can anyone help me?

RE: VSD energy savings

I have been studying this as well.  I found these quotes on this forum and others.

"Saving energy means reducing the energy cost per gallon of water pumped, not reducing the cost per hour to run the motor turning the pump."

"If a 10 HP, 100 GPM pump could be slowed by 15% and still deliver the head required, it would be producing 10 GPM, and drawing 6.1 horse power.  This would only be 1.66 GPM per horse power, compared to 10 GPM per horse power at BEP, which means a VFD system is using 6 times or 600% more energy per gallon produced."

"Also when running the pump at full speed, maximum flow, or BEP, the energy used by the VFD itself, along with motor losses from the VFD's sub-standard voltage wave form, causes extra energy to be used per the gallons produced."

"High flow or low, a VFD always causes more energy to be used per gallon produced."

RE: VSD energy savings

(OP)
jonr12

I don't know for shure if i agreed with that...sorry.
I can't consider that all people around the world that applies this equipments and measures clearly the energy cost savings are mistaken...
I didn't understand well those affirmations and i challange those people to prove it by means of calculations, as i am trying to prove the energy savings...

Thanks anyway for the answer!

RE: VSD energy savings

It took some time before it started making sense to me as well.  Even when reducing the speed of a pump with a VSD, the reduction in power used is not linear to the drop in gallons produced.  The reduction in energy when using a VSD is figured by the cube of the speed, and is no where close to a linear reduction in flow.  I have found that most of the claims of energy savings are being made by people who sell VSD's, and I have not found it to be true in real life.  A VSD cannot make a pump more efficient, and therefore cannot save any energy.

RE: VSD energy savings

Q2=Q1 x n2/n1
Q2=100 x 0.85 x n1/n1
Q2=85gpm


control valve + pump replaced by VFD controlled pump:
installation cost:
power supply cable section =1.5 nominal (reduced section through skin effect),VFD
added losses:
-heat from switching electronics
-increased heat loss in electric motor (skin effect)
savings:
pump always operates on BEP:
kW ofset BEP (currently)-kW BEP =savings
soft start/stops inncreases relyability

 

RE: VSD energy savings

6
Saving energy with a VSD requires that you must operate a lot of the time within 50% and 85% of the pump's BEP flowrate and that the head delivered when at reduced speed is sufficient to circulate the reduced flow.

Below 50% speed, motor and VFD efficiencies at reduced load starts to hurt the economics considerably (I have the typical tables), which is why I don't recommend that, not to mention that you are only producing a head of 0.5^2 * BEP_H = 0.25 At 33% speed, that's 10% of BEP_H.  Those two factors combined will kill VSD applications right there.

Its true that control valves waste energy, but at reduced flows, they don't waste a lot of energy and what they waste is usually less than the energy used with a VFD, due to the above inefficiencies, unless you are in a range of 50% to 85% of BEP_Q.

Above the range of 85%, the energy saved by a VFD OVER that of a properly sized pump and control valve is simply not worth (IMO) the extra maintenance headaches and sometimes the power quality problems that VFDs often bring along with them.

Once the energy use at various flowrates is established, to know if you will save energy or not requires that you know what flowrates you will have to run in the system over a given time period.  Then you can determine the energy you will save over that time period.  As I say, typically between 50% and 85% BEP_Q is where there is a potential to save energy, so you have to be operating inside that range to do it.  Look at your system history and see if and how much time you operate there.  Calculate the energy savings over those times.  

Since variations in flowrate are needed in order for a VFD to produce savings over a properly sized pump, there are a few other possibilities you can consider.  Look at how you can eliminate flowrate variations.  Can you provide a tank and let the level in the tank fluctuate to smooth down those peaks and valleys in the flowrate?  Can you adjust your process so that there is less variations in the batch sizes?  Can you operate the system for longer time periods and pump at a constant average flowrate over, say 24 hours rather than just pumping at a big flowrate for only 8 hours per day and a small flowrate during the night?  Fill a tank at the average flowrate then have the process draw from the tank at high demand periods.  There may be many possibilities you could draw upon to eliminate any need for a VSD at all.  I simply recommend that you consider those too.

VSDs do serve very well in certain situations.  I find them most useful for,

Adjusting flowrates in a process where the output required today is typically less than the original design parameters of the system.

Adjusting the head in a system that has an unusual fluid, such as a heavy nonNewtonian hot crude oil that requires very high pressures to get it moving, but as the pipeline fills with hot oil and forces out the cold heavy oil, less and less pressure is needed to hold the design flowrate.  Having said that, it is often just as economical to use two pumps arranged in series, operating 2, then 1.  Which one is better?  That depends on the specific fluid properties, the pipeline thermodynamics and the flowrates you need to run during startup and shutdown.

VSDs may also serve a purpose where you have many pipeline branches that are operated individually and each branch has a widely varying head requirement, such as in an irrigation distribution system, but then you must be very carefull to get the head you need at each flowrate.  Too much head needed and a VSD won't deliver it at reduced flow.  For that reason, in irrigation systems I usually prefer to open each branch with a valve, flow a certain time to deliver the needed water and close, then move to another branch.  That way a pump can be sized for a constant flowrate.  If head varies considerably a VSD may help, but again subject to its ability to deliver that head at a different flowrate.  Because when a VSD changes speed to adjust head, it also changes flowrate, so be carefull if it can deliver the water needed during the operating time available for any given branch.

In general, I think I can agree with jonr, that VSDs are not the energy savings panicea that many manufacturers claim, but its not only their fault.  Some guy (hopefully not an engineer) still has to install them in a system where they do not belong.  If flow varies between 50-85% of BEP_Q, and you have to operate there, and at the same time head varies from 0.25-0.72 of BEP_H, you stand a chance to save some bucks.

  

 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Jonr12 - how do you get that a 100 GPM pump with a speed reduction of 15% would only deliver 10 GPM?

The laws affinity are well proven and states that

F1/F2=n1/n2, and H1/H2=(n1/n2)^2 and HP1/HP2=(n1/n2)^3

So assuming that the system you had initially would throttle about 30% of the head - then the 15% reduction in speed would mean a 15% reduction in flow - or 85 GPM.

And your power would drop by 39% (as you also wrote) - but not with 10 GPM but with 85 GPM. IMO you would expect a lower efficiency and the actual HP would be higer e.g. 7 HP. So not you get 12GPM/HP - better than originally.

Best regards

Morten
 

RE: VSD energy savings

I let someone convince me that a 10 HP pump on a VSD  would drop to a 1.25 HP load, when my demand dropped from 100 GPM to 10 GPM.  The pump could only be reduced to 85% of speed, because I could not reduce the head requirement, only the flow.  Figuring a reduction in load by the cube of the speed, this leaves it drawing 6.1 HP, even when the demand is only 10 GPM.  I was told the BEP would follow the speed but, if it does, then the best efficiency is not as good at reduced speed as it is at full speed.  Also the RPM can't be reduced as much as I first thought.  The owner wants to go back to filling a big tank at full speed to save energy.  This got me in trouble with the owner and I have been studying several things that claim to save energy.  What I am finding out is the only way to reduce energy consumption is to increase the efficiency of the pump.  A VSD did not make the pump more efficient.

RE: VSD energy savings

You won't save energy if the head cannot be reduced.  

The operating point occurs where the VSD adjusted curve intersects the system curve, then look down and get the flow.  

The pump efficiency tracks the flow adjusted for rpm.  Example:  Say a flow of 80 gpm on the pump's 100% speed curve has an efficiency of 66%.  When the pump's speed is reduced to some percentage N of rated rpm, say N=60%, the Pump flow will tend to be 60% * 80 = 48 gpm and the efficiency at that flow of 48 gpm will be (more or less) the same 66%.

But, if the system curve and the pump curve intersect at some other flow, not the 48 gpm, the efficiency will be thrown off that track.  Say the pump curve and the system curve intersected above a flow of 55 gpm.  Then you have to regress the 55 gpm efficency from that 60% speed back up to 100% speed and then get the efficiency off the 100% speed curve at that new flowrate.  55 gpm/60 * 100 = 92 gpm.  Maybe that efficency is around 69%.  A slight improvement over the 66%, so you'd gain a little pump efficiency with that one.

If the flow at the curve intersection point was less than the 48 gpm, Say instead of the 55 gpm we had before, this time its 43 gpm, you regress that back up to 100% speed, 43/60 * 100 = 72 gpm, so look at the efficiency on the 100% curve for 72 gpm and maybe that's 62%, so its less than 66 and you lose some pump efficiency with that flowrate.

If you can only reduce speed to 85% because of head, then that fits you into my general rules for NOT using a VFD.  I agree you will not save money with a VSD on this system, or if you do, it will be such a very small amount it won't be worth the headache of installing the thing.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Another thing I've noticed lately, after having done quite a lot of hard calcs on the subject, is that it usually isn't cost effective to design a new system with a VSD.  If you take the usual rules and method for defining a flowrate and making an economic decision on pipe diameter verses energy consumed and then sizing the pump for the head required by that diameter, you have to operate very close to that flowrate in order for the pipe diameter solution to be valid.  Just a small drop in average flowrate is often enough to affect the economic selection of the pipe diameter to such an extent that the pipe diameter needs to be made smaller.  Since the need for a VSD is usually based on the necessity to run smaller flowrates, a VSD can hardly ever be an economic decision for a new design, since reducing the pipe diameter also tends to decrease the flowrate.  The net result is that you wind up pegging the absolute minimum flow required for your process with the pipe diameter and pump at BEP, without the need for a VSD at all.  That reinforces my opinion that you must have a defined and very necessary flow variation in order to ever consider using a VSD to begin with.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

(OP)
Dear BigInch,

First of all, that you very much for your extended and careful answer!

1)    I told that you have "typical tables". Can you tell me where I can found them?
2)    You mentioned that, for example, we can apply a buffer tank, that in my opinion is very correct from the point of view of the energy losses. But from the financial point of view, it might not be so advantageous  in comparison with the VSD...
3)    When we determine that a pump is functioning at a lower point that it ment to, isn't it better to consider the motor substitution? And what about PWM  controllers?...
4)    It was a good example the one that you gave about crude oil, because I have many situations of installations with fuel oil that have the same specifications. In this kind of applications, usually there are positive displacement pumps. Is there any special problem in VSD appliances?

Thank you very much for your kindly answer. It was very clear for me!
 

RE: VSD energy savings

Thank you Big Inch. That is one of the best ways I have ever heard it explained.  I wish I had understood this earlier.  I am going to figure out how to give you a star.   So in my case with the pump at 10 Gpm and speed of 85%, 10/85 * 100 = 11.7 GPM at 100% speed.  The VSD makes this pump only about 15% efficient.  But I guess you can still say that is the Best Efficiency Point at this speed and flow.  And you can still say BEP follows the speed, it just changed the Best Efficiency from 66% to 15%.  Now I understand why the VSD actually increased the electric bill.  Even with a system that can live with a 75% reduction in head, so the efficiency stays up at low RPM, it is at best just a linear reduction in flow rate and energy use, not energy savings.  Researching energy saving devices the VSD or VFD comes up a lot.  These calculations and my own experience prove this is not true.  So are all the people around the world that applies this equipments and measures clearly the energy cost savings mistaken?

RE: VSD energy savings

(OP)
MortenA

Correct. That was what made me a bit confused :)

Dear Bing Inch

Thanks for that last explanation. In fact it is very complex to determine effectively what will be the benefits.
My only experience (and that is why i posted this topic) is that the only way that we can make this is to install one VSD in a pump that fits those conditions that you mentioned and see how it works for a month and determine the potencial annual savings.

Thanks! I think i understood correctly.
1) the system must fit in the 50%-85% flowrate and have a variable behaviour;
2) consider first other options: analyse the whole system, consider some changes in the process or even to install a buffer tank
3) make the calculations to detect the potencial savings with the installatio  of a VSD

In fact, i have another question that i would like to have a opinion. I will give my point of view and you can tell me if is a correct approach.
To make those calculations in the specific case that i may detect that a VSD is possible to apply, i must make some measurements in loco. So, my problems relates with this. The only thing that i can measure is the power consumption of the pump, and maybe the differential pressure across the pump if there is installed some instrumentation. With the power consumption i can calculate the flowrate through the affinity laws, but my dilema is:

1) In systems with control valves near each consumer but with no bypass line witch can balace the circuit, the power consumption oscilates, even if this oscilation is little, and i can measure it during time and so on...

2)The power consumption should be the same in a system where are control valves in each consumer and with a bypass line also with a control valve that balances the circuit. This is a BIG problem, because in this kind of systems i can't calculate the aproximate flowrate during time.

What should i do in this cases?
Thanks  

RE: VSD energy savings

You have little hope of making a decision for a complex system and supporting it by calculations unless you can model the system operation quite accurately and you know the flow you must have going through the pump at all times.  Your only alternative may be to make that decision by direct testing.  

The only alternative to having proper head and flow measurements is to assume some typical operating scenarios, calculating the flows and heads and any resulting energy loss or savings for each scenario.  Then you will have to guess how much time you spend in each scenario.  It may be able to get an idea about if you should proceed with conducting actual tests, or if there appears to be such little chance to save energy that you should abandon the tests entirely.  It should be possible to at least estimate some basic scenarios based on the power you are consuming now.  The disadvantage in that method is you will know the power consumed, but not the actual combination of head and flow that resulted in that power consumption.  Since flow varies directly with speed and head with the square, its slightly more important to know head required and guess the flow.  Can you install suction and discharge pressure gages?  

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

A good break point for the decision could be my 85% flow rule.  If you can't reduce speed below that, and assuming that you do in fact still spend the most time nearer to 100% flow than 85%, the money you save will be very small indeed.  If less than half of your operating time is below 92% speed, don't do it.  If more than half your operating time is between 85% and 92%, go ahead and test.  Don't have very high expectations about big percentage changes in the bill.

If you do the testing, please let us know how it works out.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

(OP)
Dear BigInch

thanks once more.
I suspected that...So what you say (and its logic) is that in most cases, does not worth it to perform so many calculations do obtain few savings. Ok.
About the pressure gauges i have mentioned that before. That is one thing that i migh be able to obtain if that is already installed, and i think so... What you are saying is that i can use those values and with the affinity laws obtain the flowrates? This means that i have to register all values for some hour or twon in site, correct?

Another thing, previously, you mentioned the buffer tanks. How can i estimate the energy savings in this case? May i consider that those points that the power consumed should be the same as in normal conditions and the the excess  is eliminated by the buffer tank?
I ask this because i am studying the same thing for compressed air: the installation of buffer tanks near big consumers. Here i also am needing to calculate those savings, witch is quite dificult...

In relation to testing VSD, as soon as i have that situation, i will make some kind of report and post it here, because i am very interested in this subject.

thanks!

RE: VSD energy savings

Use the discharge press - suction press gages to estimate the head that the pump is adding.  With the power reading and the head from the pump, you may be able to estimate flow in the pump by looking at the pump curve.

Tanks can save energy by moderating the maximum flow in certain cases.  Consider that in most systems you have a range of flowrates that a process can run.  Let's consider a process that flowrates vary from a low of 50 gpm for 30 minutes, with a time adjusted daily average of 60 gpm (for 24 hours) and a maximum flowrate, needed for only emergency conditions of 100 gpm only for about 10 minutes.  When designing this system with a pump only, no tank, you must be able to reach all flowrates over the range of 50 to 100 gpm and the pump and piping would have to be sized to deliver the full 100 gpm.  But there may be a more optimum solution if you consider providing a tank.  With a tank of the proper size, you could design the pump and pipe for any flowrate you choose between the average flowrate and the maximum flowrate.  As system flow capacity increases from the average flowrate, the cost of pipe and pump increase, but cost of tank volume goes down.  At 100 gpm, tank cost is zero.  The optimum tank size is the tank volume that balances the cost of providing the tank against the cost of increasing the flowrate of the system and operating the system at the higher flowrate too.  As you can see, providing a tank reduces flowrate and reduces operating cost, so very large savings are sometimes possible if a tank is provided and the system can be designed to run at a lower flowrate.  

The amount you can save on the pipe size varies with the location of the tank in the system, because the pipe coming from the tank still has to be able to reach the maximum flowrates.  If its a pipeline feeding a delivery tank, you can feed the tank at average rates and draw from the tank at maximum rates, so the long pipeline could be designed for average flow with only the final delivery piping from the tank sized for the maximum flow.  If its a supply tank, the saving from reducing pipe size will not be possible.   

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

(OP)
Allrigth. I understood. Its logic that this kind of tanks should be installed near the consumers.
I will spend some hours to perform some calculations, because my intention is to obtain energy savings to include in energy audits. So i need to make that.
As soon as i do that i will try to share it.

Thank you!!!
I think its all for now in this topic.
Thank you all, specially to BigInch!

Next time i will post some results regarding my VSD experiences!

RE: VSD energy savings

These are the figures I have for VFD Efficiencies at partial load.  There is virtually no public information, but after a lot of searching, I managed to find these in a report done for the US DOE.  The original table had some missing values in it that I had to estimate.  If anyone has more or better info on this, please post it in this forum and/or e-mail me a copy and I'll do it.
 
               Percent of full load
        25    33    50    66    75    100
             
VFD HP    Eff_25  Eff_33  Eff_50  Eff_66  Eff_75  Eff_100
1    0.09    0.21    0.44    0.63    0.71    0.83
2.5    0.20    0.34    0.62    0.78    0.82    0.90
5    0.30    0.44    0.75    0.86    0.88    0.92
10    0.35    0.50    0.79    0.88    0.90    0.94
25    0.36    0.51    0.79    0.89    0.91    0.94
50    0.43    0.58    0.84    0.91    0.92    0.94
100    0.55    0.66    0.89    0.94    0.95    0.97
250    0.61    0.72    0.91    0.95    0.96    0.97

I have attached a typical chart for motor efficiencies at partial loads.  I would also appreciate any better info someone might have about these values too.
See attachment.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

4
Something to be taken into account is that the VFD takes up room in the switchroom. The VFD loses 3-5% in heat. This heat has to be disposed of by airconditioning , at least in Australia. So there is added energy in the extra heat load.

The increased size of switch room needs to be taken into the Cpaex and Opex calculations.

Motor efficiency data is predicatd on a sine wave form AC supply. By having a chopped square wave or PWM the motor efficiency is affected. teh motor manufacturers do not publish this data even if they have tested the motor.

If you intend overspeeding your pump to get more out of it you will not doubt need a special motro that may not work with a DOL supply. this increases maintentance costs. If the motor fails and there is no stock to hand you have the spectre of loss of availability of plant. FMECA is required .

Biggest challenge is the inability of engineers to select pump correctly. They add factor to factor and drive the efficiency backewaards. Then they add a VFD to be sure, to be sure. This all spurred on by the VFD salesfolks.

How many engineers allow for the future without doing a NPV evaluation of replacing the pump "when the future comes" rather than the cost of the VFD and its ineffiiencies.As we cannot see the future it may never come or it may sneak up on your and bite you on the @rse.

Have you considered that many authorities require the string testing of pump, motor and the duty VFD. What a nightmare getting VFDs from Sweden to a pump manufactuer in Brazil with a motor from Taiwan. The who is to bame if it does not work when you get it to site, unpack it install it and try to make it work??? Without a VFD this becomes a whole lot simpler.

Go to a refinery where the competence of engineers is recognised as the highest. You dont see VFDs. Why? They can size a pump and do the costings and risk assessments. the have a better take on the future and they are not bnervous nellies. They do not rely onthe supply to select the pump/control valve/VFD as do the "catalogue" engineers commonly found in the water and mechaanical services industries.

RE: VSD energy savings

3
"Go to a refinery...You dont see VFDs. ...they do not rely on the supply to select the pump/control valve/VFD as do the "catalogue" engineers commonly found in the water and mechaanical services industries."

Total bull. They are applied all over refineries, but in applications where they can make a useful difference. Same applies in power plants, where the engineers are of at least the same calibre, if not higher, than the refineries.

In the right application they can work, in the wrong application they are a nightmare. Anyway, who are the real guilty ones - the over-zealous salesmen pushing product where it isn't suited, or the lazy engineers who don't bother to do the calcs before buying?
 
 
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

I don't think the engineers are lazy.  I think they have been convinced by the manufacturers of VSD's, as well as many utilities and government entities, that a VSD will make any pump match any condition, and still save energy.  Why bother doing the calculations when we can just match the pump to the maximum demand, then use a VSD to "save energy", while making the pump match any demand from 100% flow to 0% flow?  

I think a lot of facts like the loss of efficiency at partial load, and that head is reduced by the square of the speed, are being purposely omitted or ignorantly neglected.  Purposely omitted by the manufacturers, government entities, and utilities, that want us to believe that a VSD always saves energy.  The manufacturers benefit from tremendous sales, by being able to claim they produce a product that will help the world save energy.  Government entities and utilities benefit from being able to claim, that they are helping the world by promoting or even mandating the use of products that "save energy".  Facts are being ignorantly neglected by engineers, plant operators, and the general public, because they believe that manufacturers, government entities, and utilities would not purposely deceive, to justify their existence or simple greed.

The fact that input power is reduced as the speed is reduced, makes it easy to dupe us into believing that a VSD saves energy.  The average person sees the amps or watts being reduced as the speed is reduced, and thinks energy is being saved.  Sadly, the energy use or electric bill actually increases because a VSD always increases the energy use per gallon produced.  The owner in my case is still so convinced that a VSD will save energy, that I as the engineer am taking the blame for applying it incorrectly.  There is so much propaganda on this subject that I am having a hard time convincing the owner, that a VSD will always increase the energy use per gallon produced, no matter how it is applied.  My suggestion to go back to DOL controls and use a big tank which will save energy, is being met with resistence.  Now a couple of other engineers have been brought in to reveal the error of my ways.

These other engineers are also of the mind that a VSD always saves energy, and are convincing the owner that I do not know what I am doing.  I am sure that their attempt to "save energy" by "correctly" applying a VSD will also fail.  By the time the owner is convinced that a VSD will not save energy, he will be too embarrassed to admit that I was correct, and I will have lost one of my best customers forever.  

I believe that there are applications where VSD's can "make a useful difference".  I do not believe that a VSD can save energy. What I have learned from this is,  "all the people around the world that applies VSD equipment and measures clearly the energy cost savings are either mistaken" or outright lying.

RE: VSD energy savings

Simple.  In fact I've been looking for a client like that.  Tell him you're so sure that he won't save energy with this VFD that you will make a special contract to pay him equally for any savings he makes over the entire year, if he agrees to pay you equally the amount he loses.  Settle up each month.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

BigInch,

Always enjoy the VFD/VSD conversations. Don't overlook the fact that is easier to put a VFD on a pump than it is to select the proper pump curve. If the customer is sold on VSD's, I have a site where the customer is convinced his inlet guide vanes are controlling air flow to 17% turndown. I have a special hat made out of tin foil just for meeting with that customer.

Jonr12,

How is this part of gub'mint blame? Being a gub'mint energy engineer, I assure you I find it all very funny excpet that tax money is wasted. Gub'mint has enough ignorance to dance around without having private sector nonsense added. VSD's and VFD's clearly can save money, being stupid can clearly waste money. Gub'mint clearly does not have a monopoly on stupid, or pet rocks would never have been sold.

RE: VSD energy savings

Right Maurice.  Its a band aid for tax credits.  Just like wind turbines, IMO.

The Fed. government has made it too easy to get tax credits
for any system that installs energy savings equipment.  I would imagine the proof requirements are nonexistant, or at best quite lax.  From what I understand there are also state aid programs that fund utility rebate plans.  They would pay a good portion of cost and installation and I'm told that many VFDs went on irrigation or wate supply systems with water wells with high static heads, etc.  The latest news is that a lot of the utilities wised up last year and the practice has slowed down somewhat.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

VSD wizards, what about VSDs on boiler feed pumps? I suspect this is not an application in which energy will be saved. Specifically, putting VSDs on boiler feed pumps in a small power plant with multiple pumps serving multiple boilers, each boiler having a modulating drum level control valve.
In this application, as I understand it, the speed of the pumps is modulated to meet a pressure setpoint in the pump outlet header. The flow rate will vary, since the power demand of the plant varies.
I appreciate any insights...

RE: VSD energy savings

Maurice, I meant no disrespect.  I believe the government is being duped by the manufacturers, like everyone else.  A VSD/VFD will show to save energy when it is compared to an extremely inefficient system.  But compared to a pump running DOL at it's Best Efficiency Point, a VSD/VFD is wasting energy.  

My simple math tells me that the most efficient system would be to run the pump DOL and always be filling a tank at BEP.  Then I can draw from the tank at any flow rate, and every gallon I draw from the tank was produced at the lowest cost per gallon possible.

I can get fairly close to the same efficiency with a two pump system, using a 3 HP and a 7.5 HP pump, instead of a single 10 HP.

At any flow rate, 10%, 50% to 85%, or even 100%, a single pump controlled by a VSD/VFD will use more energy per gallon than either of the above systems.  IMO a VSD/VFD causes more energy use per gallon.  So I don't see how they can be promoted as energy saving devices.

I am beginning to believe that the only reason to use a VSD/VFD  is that, "it is easier to put a VFD on a pump than it is to select the proper pump curve".  That would be the mark of a lazy engineer, government or not.   I have a special hat made out of tin foil just for anyone who thinks a VSD/VFD can efficiently turn down a pump to 17%, or really any flow.   

RE: VSD energy savings

Boilers usually have a narrow range of high operating pressures which have to be maintained for most all flowrates, therefore VSDs may lose too much head when rpm is reduced to match lower flowrates.  At 80% of rated speed, you produce 0.80^2, or 64% of rated head, at 50% speed only 25% head.  Check your operating pressure variation with flowrates.  If your required head doesn't follow those same head variations with flow, its probably not a VSD project.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

There was a major VFD manufacture promoting the use of VFDs to avoid waterhammer. When I explained to them that VFD s do not function when there is a loss of power and thus adds nothing to this form of mitigation they were not the slightest embarassed.

If a pump is being stop/started and needs a VFD for this purpose then a larger suction tank, flywheel on the pump, three smaller pumps instead of two larger pumps may be the way to go. Or perhaps the pump is too large.

If a large pump is needed for flood conditions then have a small pump for routine duties and the large pump for the flood conditions.

My work with engineering students and trying to recruit them to the Institution of Mechanical Engineers has left me dismayed as to what is being taught at university. Students seem to be right up to speed with management topics, the environment, risk analysis, industrial relations, computer graphics etc etc. However they cannot size a pump or control valve. When they get into the market place they aim straight for management as there lies the money.

RE: VSD energy savings

BFW pumps are usually high speed, high power machines. Once you cross the line from LV to MV drives the costs jump significantly, and the payback is much longer if ever achieved. MV drives appear to have a higher failure rate and repairs take longer and cost more. For large utility-class power plants VFDs are unlikely to be a good retrofit. If the BFW pump was selected from the outset for variable speed operation and the system designed around a VFD-controlled pump then they're likely to be more successful, but VFD control of BFW pumps is a bad example for the reasons BigInch has clearly noted.


Stanier,

The IMechE is not alone, the IEE-accredited courses are no different in their bias toward the soft skils and in the neglect of principles and the 'old' subjects. And I know it is now 'IET' but I opposed the creation of the IET when it happened and, having seen what a disaster the idiots at Savoy Place have masterminded, I oppose it even more strongly now.
 
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

I found this in one of my searches.

"Even if a VSD/VFD is programmed to operate very fast, it is still a programmed amount of time, instead of a direct and instant reaction to a change in pressure.  Because the reaction time of a VSD/VFD is just a bit off, it is like landing on a trampoline a split second after someone else.  It causes a catastrophic collision and shoots the person (pressure) extremely high.  Under normal operating conditions, VSD/VFD controls can accentuate or even perpetuate transient pressure waves, which can actually cause water hammer, not prevent it."

On top of all that as Stainer said, "a VSD/VFD can do nothing to mitigate water hammer problems when the power goes off".  Surge tanks, and/or surge anticipator/pressure relief valves are still needed to help with water hammer during a power outage, and apparently during normal operation of the VSD/VFD system as well.  

As BigInch said, "Boilers usually have a narrow range of high operating pressures which have to be maintained for most all flow rates, therefore VSDs may lose too much head when rpm is reduced to match lower flow rates."  Also hot water has no lubricating or cooling value for the pump.  The minimum flow required for a pump moving hot water is fairly high.  The pump will still probably require a re-circulation or by-pass line, which limits the minimum flow possible, and further limits the usefulness of a VSD/VFD.

As ScottyUK said, "The IMechE is not alone, the IEE-accredited courses are no different in their bias toward the soft skills and in the neglect of principles and the 'old' subjects".  The manufactures have convinced most people that a VSD/VFD will make any pump match any job, so we no longer have to waste our time selecting a proper pump curve.  They also claim a VSD/VFD makes any pump, doing any job, save energy, and eliminate water hammer.  I think these are just more instances where, "all the people around the world that applies VSD equipment, measuring clearly the energy savings and water hammer reduction, are either mistaken" or outright lying.

I fell for these things just like everybody else.  One of the largest manufacturers of VSD/VFD controls had very little problem convincing me these things were true.  Now I am having to educate myself about VSD/VFD controls, and work my way through all the problems these lies created.  Isn't there an old saying about "the bigger the lie, the easier it is to get people to believe it".  Aren't there laws against such mis-representation?

RE: VSD energy savings

The more reputable manufacturer's say this in the fine print, so its all legal... kind of.  


Now.  I'd like to talk to you about this bridge right in the middle of NYC.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Another possible conclusion could be that energy is too cheap (at least in the US).

A 15% saving on power can buy you some hardware here in Denmark where energy taxes are higher.

There was a link to some very interesting articles on the subject:

http://canterburyengineeringassociates.com/index.php?option=com_docman&task=cat_view&gid=13&;Itemid=33

They tell why you often dont see the saving that the affinity equations promise on first glance. Read them!

Best regards

Morten

RE: VSD energy savings

Or on the second and third glances either.

Yes, nobody said that VSD don't have their place and, if certain conditions are meet, they will save more than 15% too.  Its just that those systems where VSDs will save money I think are more common in industrial fabrication or food processing plants where static heads are not high, fluids may be thixotrophic and flows vary anywhere from 0 to 100%, with all flowrates of more or less equal probability.  I just think that those conditions are not nearly so prevalent in continuous fluid transfer processes at industrial sites, product pipelines, irrigation systems and water supply.  And when they are its often cheaper to stop during the day and pump all night when power rates are even cheaper.  Even in product pipelines where specific gravities vary a lot and pressures change, delivery rates vary with viscosity, etc. you could control a lot of things with a VSD, but over the long term, its still better to just load the product as fast as you can and run at 100% with all products, because that maximizes volume delivered  ... and sold ... and, after all, that is what pays the bills.  VSDs need to fit the system and the objective too.  I've been doing product handling for quite a while now and I can count the times I needed  a VSD on one hand in a new system.  

Take away lesson.  You just can't ever beat the energy efficiency of running a properly sized pump at BEP 100% rated flow, or the volume delivered.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Thanks MortenA for that link.  Mr. Vaillencourt does an excellent job describing how the second Affinity law prevents the VSD/VFD from being able to reduce the pump's RPM by as much as the salespeople will describe.  However, even Mr. Vaillencourt says that the .9 reduction in horse power in his first example, and the 1.7 HP reduction in his second example are savings in energy, when this is absolutely not true.

In his Example 1, the second Affinity law shows that when the flow is reduced by 50%, the speed of the pump can only be reduced by 10% and still produce the head required.  This reduces the horse power from 3.3 to only 2.4 HP.   Although Mr. Vaillencourt says this is "not sufficient to economically justify the use of a VFD", he still claims the .9 reduction in horse power is a savings.  In reality the BHP equation and even pump curves I checked show that this flow and pressure can be accomplished with only 1.65 HP.  So a VSD/VFD reducing a 3.3 HP load to 2.4 HP is still wasting .75 of a horse power.  This means the VSD/VFD system is wasting 32% more energy than a smaller pump running at BEP, or the larger pump filling a tank at BEP.

In his Example 2, Mr. Vaillencourt shows the speed can be reduced by 21.5%, and that reduces the horse power from 3.3 to 1.6 HP.   He says that "clearly the 0.413 bhp calculation would result in a gross overstatement of the savings".  But he also says that the actual drop from 3.3 to 1.6 HP "is a significant amount of savings from the application of a VFD in this example".  Again this is not true. The BHP equation shows that 1.6 HP is the correct amount of power for this flow and pressure so, the VSD gives only a linear reduction in HP to flow, not energy savings.  To top it off, neither of these examples takes into account the loss of motor efficiency at partial load as described by BigInch, which would limit the reduction of speed, horse power, and energy even more.

MortenA says, "A 15% saving on power can buy you some hardware here in Denmark where energy taxes are higher".  If this is true then there should be tax credits or incentives paid to anyone who removes a VFD from a system.  There are considerable savings to be had for correctly sizing a pump to run at BEP, or for using a big tank where the pump always runs at BEP regardless of the flow rate, compared to using a VSD/VFD.  

BigInch says, "The more reputable manufacturer's say this in the fine print, so its all legal... kind of".  You have to study the fine print to see that VSD/VFD's actually waste energy.  Saying that a VSD/VFD saves energy is absolutely not true, so I don't see how it could be legal.  The best thing I got from Mr. Vaillencourt's paper was his use of the word "Myth-Applied".  I now believe almost every VSD/VFD is "Myth-Applied".

RE: VSD energy savings

"I now believe almost every VSD/VFD is "Myth-Applied". "

Dunno about "almost every", but then I deal with people whose systems cry out for VFDs and other technologies.  Your post is true and correct when the design point is just that - a single operational condition (or, as in BigInch's examples, the maximum possible flow, e.g. pipelines delivering commodities).  But, if the design condition is a range of flow rates, or if rates vary unpredictably over time throughout a system (variable heating loads in chilled water piping systems, e.g.), and TDH is not a concern, then a VFD can save significant power.

RE: VSD energy savings

I feel your pain.  Even with all I have learned recently, I have heard it so many times that I still want to believe a VFD can save energy.  I now understand that a VSD/VFD cannot reduce the energy used per gallon under any condition.  It is like a Tar Baby.  Once people see the amps drop and have it in their head that a VFD/VSD will save energy, it is hard to shake it off.  You are the third person just in this discussion that still holds to this myth, even after several reputable people have carefully explained why a VSD/VFD cannot save energy.

TDH is always a concern, otherwise there would be no need for a pump.   A VFD cannot magically cause the pump to produce more flow with less horse power.  I have been talking about a system with flow rates that vary unpredictably over time, and the installation of a VFD increased the electric bill by 300%.   

My calculations show that there are several ways to maintain the best efficiency as flow rates vary but, VFD control is not one of them.  I can see where with things such as a chiller, it would be "easier" to just add a VFD to vary the flow, but it would also be less efficient than a multiple pump set up.  A VFD may even be a good way to control a multiple pump system and round the edges between the pumps but, it is still the multiple pumps that would save energy, not the VFD.

I have been reading many articles where a VSD/VFD was praised for "saving energy".  In every case I find the VSD/VFD is not the reason for the energy savings.  Usually the system was altered to reduce the head required, or a smaller pump was installed.  Yet just like this discussion, somebody always tries to end the article with, "the VFD is saving $90,000 a year", which is total BS.

Once I finally understood that a VFD waste energy, and I got the VFD idea out of my head, I came across several other good ideas and ways of controlling pump systems.  I wonder how many real energy saving ideas are being neglected because everybody already mistakenly thinks the VFD is the end all, be all, of energy savings.  I even wonder how much energy is being wasted using motors that were designed to withstand the abuse of VFD controls, instead of   designing motors and pumps to be more efficient.

As BigInch said, there is nothing more efficient than a properly sized pump running at BEP.  The soft skill of using a VFD is no substitute for the "old school skill" of choosing the right pump curve(s).  

My take away lesson from all this is; Any VSD/VFD that was specified to supposedly "save energy" has been "myth-applied".

RE: VSD energy savings

Now wait a minute.  Even I still believe in the "myth".

While kW/gal is more with a VFD, that statistic alone does not tell the whole story.  

Say you have to pump less than BEP for some reason (whatever it might be), say 80% BEP, for a specific time interval.  With a VFD the energy bill will be less, because WITHOUT a VFD and a control valve you are effectively pumping 100% BEP at 100% head INSIDE THE PUMP, as 20% of the BEP flow is being pumped up to the top of the pump curve head is actually recirculating within the pump.  That is an energy loss too, which is bigger than the energy used by the VFD.

You have to compare kW/gallon in the discharge pipe, not just kW/gallon, as a lot of that might just stay inside the pump.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

"You have to compare kW/gallon in the discharge pipe"

Right.  Or better, $/gallon.  Or euros/liter.

"Any VSD/VFD that was specified to supposedly "save energy" has been "myth-applied""

Um, change "Any" to "Many", and "save energy" to "pay for itself" and I think we all will mostly agree.   

RE: VSD energy savings

Yes, its the cost of USEFUL work that we need to minimize.  That's delivered flow, not flow circulating around in the pump.  Of course the cost of mostly useLESS work per hour would, and should, be low.   

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

How many myths have morphed into legends to be considered facts by the unitiated?

If anyone is in Sydney Australia on Tursday 18th february I shall be presenting a paper to a joint meeting of ASME/ Engineers Australia/IMechE on this topic. Address is engineers Australia 8 Thomas St Chatswood NSW 2067. No charge refreshments served from 1730 hours. Dinner afterwards.

RE: VSD energy savings

I would suggest using kWh/gal rather than kW/gal.

RE: VSD energy savings

I think I may have found another myth.  This discussion is getting long but is quite good and is helping me a lot.  I don't know if I should start another discussion with this but it is all related so I will just keep going here.  Please let me know if I need to start another discussion.  

Using the example of needing 80% flow as described by BigInch, I went back to my pump curve. At 100% flow this pump is using 9.2 HP.  Because of the second affinity law I would only be able to slow this pump down by 4% and still produce the head required.  4% reduction in speed gives an 8% head reduction and 12% reduction in horse power.   12% reduction in horse power brings this pump down from 9.2 HP to 8.1 HP.

I can't figure out how to paste the curves.  However, the full speed curve shows this pump will reduce to the same 8.1 HP load without decreasing the speed.  I could simply let the system head or pump discharge pressure increase by 8%, which is only 12 PSI, or I could use a valve to maintain the same discharge pressure.  Simply reducing the flow by 20% either way, seems to require the same horse power as if slowing the RPM with a VSD/VFD.

Because the second affinity law says I can only slow the RPM of this pump by 4% and still produce the head required, the third affinity law says that 8.1 HP should be the lowest power possible at any flow rate.  However, I plugged this minimum speed into the curve and can see that the actual power required will drop even lower than the third affinity law states, as flow continues to decrease.  So there is something wrong there?  I have read several places where the affinity law assumes two points of a curve that have the same efficiency.  How can this be if the efficiency is different at different flow rates?  Does that mean that the affinity law only works when reducing the head, not the flow?  Otherwise why does the affinity law calculations not match the horse power from the curve?

Anyway using the curve at the minimum speed required, I can see that at 50% flow the power required drops from 9.2 to 6.1 HP.  However, the full speed curve shows the horse power drop from 9.2 to 6.5 HP by simply decreasing the flow, not the RPM.  At 50% flow, that is only 6% less power being used by varying the RPM, than by just riding the curve or restricting with a valve.  If you can say that using a VSD to reduce the horse power from 9.2 to 6.1 is saving energy, then using a valve to reduce the horse power from 9.2 to 6.5 is also saving energy.  Then if you take into account that at full speed the losses from a VSD adds 5% to the energy consumption, and a fully open valve does not, it seems there is little if any difference between using a valve and a VSD throughout the entire flow range.  So I guess it is also a myth that valves burn energy?

I was under the impression, as BigInch stated, that using a control valve would cause the pump to run at 100% BEP and 100% head inside the pump.  However calculations and the curves show that using a control valve reduces the power required almost exactly the same as using a VSD to vary the RPM.  The only thing I can't see from the curve is the loss of motor efficiency at partial load.  Is this efficiency the same for a motor that is slowed down to partial load, as compared to a motor that is still running at full RPM and just drawing a partial load?  If not, then maybe this is where my problem lies.  Otherwise I would  say that replacing a valve with a VSD to save energy is another "myth-application".


Still nothing produces as many gallons per horse power as letting the pump run at maximum flow and filling a big tank.  Second to this a two pump system is still more efficient than using a VSD on a single pump.  Now I also believe that using a control valve on one pump, or to round the edges between a two pump system, may be just as good as using a VSD.

I wonder how many more "myths have morphed into legends to be considered facts by the un-initiated"?  I also wonder how much energy the world is wasting by falling for these myths?   I wish I could go to Sydney to hear you Stanier.  Is there anyway you could put your paper here after you have presented it?   

RE: VSD energy savings

Weekend.  I'll get back to you on Monday.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Jonr,

lets make a new thread.  The data you have is getting hard to follow through this thread.  Let's start with a fresh slate.  Be sure to give flow, head, rpm, power and pump efficiency values for any point you want to talk about.  I'll assume its fresh water, unless you say differently.
-------------------------------

For what its worth department.

I had a look at the Vaillencourt paper and its pretty good.  I only have one disagreement.  The pump is being considered alone without any pipe flow dH-Q relations.  The system's static head is considered, but the speed change relationship he uses is based on the piping carrying whatever flowrate the pump makes using the total discharge head provided by the pump at any speed from minimum speed to 100% speed.  While his analysis is valid at the minimum speed and at the 100% speed Q-H points, the operating point will not necessarily follow his relationship between those points.  The operating point will follow the system curve between those points, which could differ significantly from his relationship.  However, if the pipe and fluid parameters are not well known, its the best approximation you can make under the circumstances.

If you need to evaluate the economics of operating at intermediate points between the minimum and maximum speeds, it could be very important to use the correct system curve head values.  You may do so using his method by considering each intermediate head a "design head", but you must use the system head at the correct intermediate speed, not at the 100% speed where the design head would normally be located.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Because the second affinity law says I can only slow the RPM of this pump by 4% and still produce the head required, the third affinity law says that 8.1 HP should be the lowest power possible at any flow rate.  However, I plugged this minimum speed into the curve and can see that the actual power required will drop even lower than the third affinity law states, as flow continues to decrease.  


W/o checking the numbers, I'll try to answer as much as I can.
8.1 HP I think is at your min speed to still deliver flow, so anything under that is wasted energy in any case.  You should turn the pump/VFD off.


I have read several places where the affinity law assumes two points of a curve that have the same efficiency.  How can this be if the efficiency is different at different flow rates?  Does that mean that the affinity law only works when reducing the head, not the flow?  Otherwise why does the affinity law calculations not match the horse power from the curve?

Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it.  Thus the two points referred to are any two points along that curve.

If you can say that using a VSD to reduce the horse power from 9.2 to 6.1 is saving energy, then using a valve to reduce the horse power from 9.2 to 6.5 is also saving energy.  Then if you take into account that at full speed the losses from a VSD adds 5% to the energy consumption, and a fully open valve does not, it seems there is little if any difference between using a valve and a VSD throughout the entire flow range.  So I guess it is also a myth that valves burn energy?

Yes.  But the supposition is that in that situation the VSD would (maybe) still save a TINY little bit more than using a valve.  There is no doubt that "Valves burn energy", it isn't a "myth".  VFDs also use 5% or so.

I was under the impression, as BigInch stated, that using a control valve would cause the pump to run at 100% BEP and 100% head inside the pump.  However calculations and the curves show that using a control valve reduces the power required almost exactly the same as using a VSD to vary the RPM.  

Yes, the pump brings up the flowrate carried in the impeller to full internal discharge pressure, but since flow cannot exit due to a valve restriction, it is recirculated and the efficiency drops according to what is shown on the pump chart.

In many situations the amount of energy saved by using valve or VFD is very small and essentially immaterial.  In those cases I believe a valve is far superior due to the lesser complexity of the system required to do so.  In other cases it is significant and a proper choice should be made.


The only thing I can't see from the curve is the loss of motor efficiency at partial load.  Is this efficiency the same for a motor that is slowed down to partial load, as compared to a motor that is still running at full RPM and just drawing a partial load?  If not, then maybe this is where my problem lies.  Otherwise I would  say that replacing a valve with a VSD to save energy is another "myth-application".


The loss of efficiency of the motor will never appear in the pump curve.  That curve is reserved for hydraulic efficiency and shaft power to the pump only.  You must adjust the power rating requirement for all additional electrical equipment yourself using the tables I provided for the motor and VFD above, whether that is at full or partial loads too.


Still nothing produces as many gallons per horse power as letting the pump run at maximum flow and filling a big tank.  Second to this a two pump system is still more efficient than using a VSD on a single pump.  Now I also believe that using a control valve on one pump, or to round the edges between a two pump system, may be just as good as using a VSD.

With two pumps operating within recommended ranges it may not be possible to reach all flowrates between operating scenarios of running one pump and two pumps, if your flow range is high.  For example, lets say your normal allowable range is 50% to 110% for each pump.  With a BEP for each pump of 100 gpm your ranges would be 50-110 and 100-220, but operating at 130 gpm would be a poor choice as that would involve running two pumps at 65 gpm each, both with poor efficiencies.  A VFD and 1x200gpm pump might be a far superior choice as long as you could still make the required head at lower flows.  It might be tough going down to 50 gpm.  Not a MYTH.  IT DEPENDS.  

I wonder how many more "myths have morphed into legends to be considered facts by the un-initiated"?  

As you have found out, making a proper evaluation of these types of problems is often not a simple task for experienced engineers.   
Only the uneducated perpetuate and believe in myths.  

I also wonder how much energy the world is wasting by falling for these myths?

Conservation and proper application is the best method to save energy.  
   I wish I could go to Sydney to hear you Stanier.  Is there anyway you could put your paper here after you have presented it?

I'll let Stanier handle that one.


Knowledge is power^3    

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

jnor12,

The presentation will be made available on the Engineers Australia' website after the event. I will make sure I make a posting here to give a link. We post our presentations there as many members are spread across this wide land of ours and cannot get to meetings.

It is up to the older engineers to get out there and spread the word on this subject. With 35% of the world's energy involved in pumping fluids it is an important issue.

Forget the greenies and their climate change dogma there is a need to use enrgy more efficiently. There is not an environmental issue in the world that will be solved by an environmentalists. It is engineers who will be solving the challenge. Let us start now.

RE: VSD energy savings

Engineers can be environmentalists.
 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

There is a lot of good info here.

I (my company) sells VFDs. We do not push VFDs as a pump energy savings device because the "cookie cutter" energy savings arguements typically are not true and in many cases the complete system can use more energy once the VFD is installed.

I also agree that many, if not all, of the glossy articles I've seen claiming a VFD was a wonderful energy saver on a pump also discuss how other parts of the system were changed at the same time totally obscuring the real saving or lack of savings due to the VFD. These articles are also typically written by VFD manufacturers.

The post about certain people seeing a drop in input power (kW) believing that energy savings are occuring is true and happens more than I care to admit. There is also no point aruing with these people because they have "seen the light" - I like BigInch's approach of getting them to agree to a 2-way money contract because that would be the only way to make a point to them.

I would say that a VFD can be an effective way to achieve process control. You can maintain a certain flow rate or maintain a certain pressure.

I remember writing this quote;

"Saving energy means reducing the energy cost per gallon of water pumped, not reducing the cost per hour to run the motor turning the pump."

It was in a discussion where someone had commented that the kW of their motor was reduced with the application of a VFD. I have no idea if they had done a kWh/gallon pumped or $$/gallon pumped study but from their post I have my doubts.
The discussion was also related to an application with a fairly constant head requirement which is not a good place to try and save energy with a VFD.

A big thanks goes out to BigInch for trying to spread the truth and cut through the BS.
 

RE: VSD energy savings

BigInch,

Engineers are the true environmentalists as they can actually achieve energy reductions, reduce pollution and cut out waste.

The "greenies" are like castrated tom cats on the yard post. They wail on all night about what they would like to do, but are ill equipped for the task to hand. Hence they will never solve an environmental issue but can just wail about them.

What an industrial hero Al Gore is in that he has the "greenies" chanting for nuclear power stations and uranium mines, all while he flies around in his Lear jet.

VFD salespeople love "greenies" as they are so inept that they can sell heaps of VFDs without engineering.

RE: VSD energy savings

Just thought I'd chime in on post # 50 here... be very cautious about VFD's on boiler feed applications, or any pumps operating in parallel...  BFW pumps typically have flat head rise curves to shut off. The drives must be synchronized to all run at the same speed as drum levels increase or decrease. If the VFD decreases the speed on one pump, but remains the same on the other, that one pump will likely go to shut off while the other runs out past BEP.

As for the other comments about cost savings and operation, good discussions. I know pipeline companies and municipal water districts really like using VFD's in applications where the changes in system curves are severe
 

Did you know that 76.4% of all statistics are made up...

RE: VSD energy savings

"Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it." BI

I don't think this is true. I have been playing with curves and even the slightest reduction in RPM also gives a reduction in efficiency. The efficiency curve moves slightly to the left as the RPM is reduced but, is no where close to the reduction of flow. The only way the efficiency stays the same is when the flow rate stays the same. So if it is true that "the affinity law assumes two points of a curve that have the same efficiency", then the affinity law only works for a reduction in head, not flow.

"You must adjust the power rating requirement for all additional electrical equipment yourself using the tables I provided for the motor and VFD above, whether that is at full or partial loads too." BI

I understand this but, it doesn't answer my question about efficiency at partial load being the same for VSD and DOL? If it is, then it doesn't make much difference for a comparison.

"Yes. But the supposition is that in that situation the VSD would (maybe) still save a TINY little bit more than using a valve. There is no doubt that "Valves burn energy", it isn't a "myth"." BI

OK, we know now that the real myth is that VFD's save energy. But "valves burning energy" is a big part of that myth. Every VFD salesman I talked to started the conversation with "valves burn energy". This led me to believe that valves were out of the question, and that VFD was my only choice. After seeing VFD's in their real light, my calculations now correctly show that "VFD's burn energy". Because of the second affinity law and the fact that my head condition does not vary, comparisons between Valve and VFD shows very little if any difference. Add back in the motor efficiency at partial load and the power the VFD itself uses, and the VFD may actually use more energy than a valve. So we shouldn't say "valves burn energy", we should say "VFD's and Valves both burn energy".

"The presentation will be made available on the Engineers Australia' website after the event. I will make sure I make a posting here to give a link." Stainer

Thanks Stainer, I will be looking forward to seeing it.

"I (my company) sells VFDs. We do not push VFDs as a pump energy savings device because the "cookie cutter" energy savings arguments typically are not true and in many cases the complete system can use more energy once the VFD is installed." LionelHutz

I appreciate such honesty from someone who sells VFD's. I have not found this to be the case with most other manufacturers and salespeople. I also believe a VFD can be an effective way to achieve process control. There are many good uses for a VFD but, claiming they save energy when they do not, has me wondering what else some manufacturers are lying about?

"Engineers are the true environmentalists as they can actually achieve energy reductions, reduce pollution and cut out waste." Stanier

I agree but, engineers who are "myth-applying" VFD's are increasing energy consumption, increasing pollution, and increasing waste. Engineers who have "seen the light", think every time they spec a VFD they are helping save the world, by saving energy and reducing green house gasses. Most get really angry when being told, this is not true, and that they do not understand what they are doing. This discussion has been an unbelievable amount of help. I just hope those who would like to keep the myth alive, do not have the connections here to get this discussion deleted. I have copied everything just in case. When I figure out how to paste a curve, I will start a new discussion. I would like opinions from others on the best way to "really" save energy in my application.

RE: VSD energy savings

jonr12,

Quote:


"Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it." BI

I don't think this is true. I have been playing with curves and even the slightest reduction in RPM also gives a reduction in efficiency.
...
So if it is true that "the affinity law assumes two points of a curve that have the same efficiency", then the affinity law only works for a reduction in head, not flow.

Jon, The efficiency curve theory has been developed from real tests.  Let's don't make new ones.  I don't want to turn this thread into an exercise in debunking your theories, but I guess we have to start with that one.

Efficiency tracks with speed reduction and flow quite well, also with head.  It only gets a little difficult around the reverse points, but that bit can be easily ignored without excessive loss in accuracy.  See this chart here by Goulds.  I've circled two points to show exactly which two points the theory refers to.  I'm sure you can follow how the others track too.



Also see pump specific speed charts.  Specific speeds can tell you a lot about efficiencies of pumps and characteristic shapes of their cross-sections.  They can also tell you about how efficiencies vary with changes in speed, rpm and head.  Look at the formula, which is dependent on all three.  

SpSpeed = rpm * flow_gpm^0.5 / Head_ft^0.75
Does it track with affinity laws?  Yes it does.
Using a unit curve at 100% speed = 1, varying flow and head by affinity laws and then calculating SpSpeed as rpm is reduced shows that the SpSpeed won't change and therefore one wouldn't expect the efficiency to change very much either.
rpm    gpm    ft    Specific Speed
1    1    1    1.000
0.9    0.9    0.81    1.000
0.8    0.8    0.64    1.000
0.7    0.7    0.49    1.000
0.6    0.6    0.36    1.000
0.5    0.5    0.25    1.000
0.4    0.4    0.16    1.000
0.3    0.3    0.09    1.000
0.2    0.2    0.04    1.000
0.1    0.1    0.01    1.000


Quote:


I understand this but, it doesn't answer my question about efficiency at partial load being the same for VSD and DOL? If it is, then it doesn't make much difference for a comparison.

Please rephrase that question, if you want.  What's DOL?
But then again, if it doesn't make a difference, maybe we should just forget it.


Quote:


But "valves burning energy" is a big part of that myth. Every VFD salesman I talked to started the conversation with "valves burn energy". This led me to believe that valves were out of the question, and that VFD was my only choice.

Caveat emptor.


Quote:


After seeing VFD's in their real light, my calculations now correctly show that "VFD's burn energy". Because of the second affinity law and the fact that my head condition does not vary, comparisons between Valve and VFD shows very little if any difference. Add back in the motor efficiency at partial load and the power the VFD itself uses, and the VFD may actually use more energy than a valve. So we shouldn't say "valves burn energy", we should say "VFD's and Valves both burn energy".

  Yes, that conclusion has been drawn way, way up above.  I've also warned specifically about boilers and other applications with high static heads.  Most good pump mfgrs do too.  But what we really should say is actually a question, "Which option burns LESS energy?


Why are you worried about this thread getting deleted?  If we discuss this rationally with a view towards seeking real optimum solutions, there shouldn't be any problem.  When one goes wild discussing thoughts that make no engineering sense and conflict with well proven methods and test data, that's when threads lose credibility, cross into rant territory... and rightfully get deleted.

Upload a curve or other document by clicking right below this where it says,

"Step 3 attachmenbt ...or upload your file
       to ENGINEERING.com"

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

BigInch,

DOL = Direct On Line, i.e. the motor is connected straight to the mains power via a contactor or circuit breaker without any fancy tricks like star-delta, VFD, soft-start, auto-transformers etc. The motor gets full voltage as soon as the contactor closes, so it accelerates quickly, draws lots of current while accelerating, and runs at one fixed speed. It is the most common type of motor starter because it is simpler and cheaper than any of the alternatives.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

You mean like one of those mysterious little black boxes called a switch, don't you.

Whether that makes a difference is probably dependent on the number of starts.  In most cases, with anything near a properly sized storage reservoir and a properly sized pump, there shouldn't be so many to make a difference anyway.  Its another band-aid to fix the symptoms of a screwed up system design, or improper operation, not cure the disease.  But where it works, it works.  I'd rather not muddy the water discussing those in the context of VSDs vs Control Valves, so let's make another thread out of it, if its a problem.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

P.S.

Scotty, I hope you're working the night shift. smile

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Up to now no one has mentioned the effect of the cable supplying the VFD. For a start the cable has to be shielded to prevent RFI/EMI effects. So when doing your costing allow to replace the cable for an existing motor. Given that old cables tend to harden, removing them may be a challenge without excavation, if buried, or from common cable ladders in building/plants/corridors/conduits etc.

Toshiba , a manufacturer of VFDs has some fine information for the design for VSDs. www.toshont.com/vfdapp.htm . The attached file is just one of many. these are engineering guides not sales guides. They even make sense to a mechancial engineer like me so get your electrical engineer to have a look as well.

Look at the Application Notes. nparticulalr pay attention to the voltage stress on motor windings caused by long cable lengths. Modern plants have centralised control and switchrooms. Cable lengths for motors over 60m are a problem.

RE: VSD energy savings

There are well-documented solutions to the voltage magnification problem, as there are to most of the other 'problems' identified. A good electrical engineer would be able to identify those which are a problem with the specific installation and incorporate appropriate mitigation where necessary.

On a more general note, it is unfair and unreasonable to blame poor performance of a badly designed system on the drive manufacturers who are only responsible for part of the system. What some of you are doing you're doing is akin to blaming a pump manufacturer for poor system performance caused by a bad piping design.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

Just because VFD's have been sold like snake oil and frequently misapplied does not mean they cannot save money.

I generally work on the air side, so that would be my example. But first, I'd suggest looking at ASHRAE 90.1-2004, section 6.5.4.2.1 then considering how you would control a VAV fan over 15 HP.

The motor and VFD effiency generally decrease with reduction in percentage full load rating for a non-linear curve, meaning empirical data would work best, the fan curves. If you look at a typcial fan curve at two different speeds, note the static pressure in I.W.G., then apply the equation

FLOW * IWG/6,356 = W

you will get a different answer than using the affinity laws. The fan curves not only address change in efficiency, but also the reduced work from operating at lower static.

For a constant flow application without variable loading resistance, first thing I would look at would be reducing pressure for required delivery, just like setting minimum operating pressure for an air system. Re-sheaving or impeller change would make more sense in this case than a VFD.

The ASHRAE HVAC Systems and Equipment Handbook might be helpful on the matter.

 

RE: VSD energy savings

One application that can be well suited to benefit from the use of a VSD/VFD/ASD (everyone can pick the terminology that they happen to prefer) is powering the induced draft and forced draft fans of a large steam generator that must operate a significant portion of its time at much less than full load.  This is an application that is nearly pure circulation so when the fans are well sized, they function at near BEP throughout their operating range, and the unit's auxiliary power savings are dramatic compared to other arrangements--such as inlet guide vanes.  Not all such applications are suitable.  The amount of time spent how far below full load is very important in determining the potential benefits, and it is very important that the fans be properly sized.  If fan sizing is off, then the VSD assures that the fan will spend its entire service life well away from BEP performance with energy being wasted needlessly under all conditions.

Based on my observations, nearly all VSD's applied to pumps represent a thorough waste of money and energy.  They seem to be applied mainly as a substitute for sound engineering in the sizing of the pump and in the design of the associated piping and equipment systems.  The associated attitude seems to amount to, "Why bother with all of those time consuming calculations and when you can just plug in any old pump and VSD and it should work just fine since we won't be wasting any energy through a control valve."  I consider finding a VSD attached to a pump to amount to an alarm signal to be wary that this is likely to be a poorly chosen pump connected to a poorly designed system.  Shortcuts are seldom taken in only one part of a system or plant.

In general, a VSD driven pump is likely to spend all of its operating life well away from its BEP, and the cumulative waste of energy is almost certain to be completely ignored because of the simplistic (and politically encouraged) presumption of energy savings associated with the mere presence of a VSD.

I agree with the earlier comments about not operating VSD pumps in parallel.  Maintaining stable operation for such a system is sure to be found to be in the range from very difficult to impossible.

RE: VSD energy savings

"Why are you worried about this thread getting deleted? If we discuss this rationally with a view towards seeking real optimum solutions, there shouldn't be any problem. When one goes wild discussing thoughts that make no engineering sense and conflict with well proven methods and test data, that's when threads lose credibility, cross into rant territory... and rightfully get deleted." BI

I am just worried that saying "VFD's burn energy", even though it is true, will be seen as "making no engineering sense and conflicting with well proven methods and test data".   I have seen many places where the facts and figures have been deleted, obscured, or altered to keep the VFD from being shown in it's true light.  I am seeking real optimum solutions and have been trying to discuss things rationally but, the engineers for the VFD manufacturers I have been talking with, will have none of it.  Some even get red face mad, and refuse to discuss it further.  They are committed to saying a VFD always saves energy, and are quoting test data which gives the VFD credit for energy savings, even though one can clearly see the VFD is not the real reason for the documented energy savings.

"Caveat emptor." BI

It is extremely hard for the buyer to beware when nearly all the data available is "myth-information", and nearly all the salespeople and engineers refuse to discuss this issue rationally.  After the third engineer got red face mad and told me I was stupid for even considering that a  VFD would increase energy consumption, they convinced me that the myth was true.  This was the beginning of my problems and is the reason I want to get the facts straight here.  

Stanier, that link you gave is excellent.  I would love to discuss some of the other problems with VFD controls but, I would first like to put this energy savings and energy efficiency thing to rest.  Partial load efficiency, cable losses, VFD losses, reactor or filter losses, and many other things will only add to the problem.  I would like to discuss these things after I get my head wrapped around the curve issue.

Mauricestoker and ccfowler; I don't work on the air side.   I am sure there are applications such as air handlers and conveyor systems where a VSD can save energy.  However, now I also wonder if there are better ways than VSD to save energy in these applications.  Could it be that since we are so convinced that VSD will save energy, that we stopped looking for other means of control.  Could something like multiple motors and fans or reducing the pressure required, save more energy than a VSD, as it does in pumping applications?  If 20% of the worlds energy is used in pumping applications, we really need to get the facts straight.

This has gotten long so I will start a new discussion and see if I can paste the curves I have been working on.  The new discussion is at this link;

http://www.eng-tips.com/viewthread.cfm?qid=261532&page=1

RE: VSD energy savings

Sometimes they do, sometimes they don't.
Here we keep it rational with support calculations.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

jonr12,

There are some other forms of VSD other than VFD that in the right applications give savings. A motor was developed in the UK with multiple windings. Teh number of windings used determined the speed of the motor. Efficiency of the motor did not change.

Then there are eddy current clutches, DC motors, slip ring motors, scoop controlled fluid couplings, Vee belts and pulleys, cone and disc variators, gearboxes etc etc. All have their place in the engineering world.

The point is selection of a pump in the first place for current conditions is the starting point. Stuff this up and yes, you may consider changing the pump/impeller , adding a control valve or perhaps a VFD. The VFD may save energy compared to the ill applied "over sized pump" however the right sized pump will always be more efficient as it does not have the burden of the VFD's energy waste, cable replacement, larger switch room, special motor etc etc.

The acid test for the VFD sales people is that:- if the pump is "correctly" selected for the current duty point with an adequately sized suction tank, then how does the VFD save energy? Look at your curves from that point of view.

RE: VSD energy savings

Just saved this post to a Word document in case it gets pulled. Too much good stuff to waste here.

RE: VSD energy savings

OK - just a little input: I just finished an expansion to my house. Theres two new rooms with wooden floors (massive Douglas pine) and here i got floor heating.

The system is from Roth, and this uses a Grundfoss Alpha pump. A cute little thing with - lo and behold - VFD. Its not even hooked up to acontrol loop - you just choose your rpm on a dial. The rooms can be individually controlled so there are valves on each forward flow line. Very neat.

Anywhy - why did they put a pump in with a VFD when its a fixed speed pump for all practival matters?

Its cheaper i guess (for them - but maybe not for me in the long run?). The system can be installed in buildings of many different sizes, and be conncted to up to 6 individually controlled curcuits. So if they wanted to pick and choose the optimum pump they would have to engineer each system seperately. Now they can just put in the little Grundfoss - fits all.

Best regards

RE: VSD energy savings

No static head, so a valid VFD solution to widely varying flow requirements.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Here's their animation justifying the use of a VFD.

I think this is one of the applications where it might be justifiable because there is virtually no static head, just a variable system resistance to overcome (assuming zone valves or TRVs). There is presumably a discharge pressure sensor which causes the pump to speed up or down, although it doesn't explcitly say so. In my opinion even if the application is reasonably sound there are basic problems with mounting consumer-grade electronics on a pump casing which is likely to reach 80C or 90C in service. It just seems to be asking for trouble.

The manual is a disgrace - dumbed down to the utmost.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

ah - i didnt think of the discharge sensor. I will go home and check the manual smile

Oh - and the system is very clever. The forward temperature can be adjusted via an inlet valve with a thermostat that only let sufficent fresh water into the secondary loop to maintain the required temp.

This is quite important especially during the initial heat-up where too hot water could cause the wood to dry out, get cracks and perhaps start to "squeack" (dont know the xact english term).

Best regards

Morten

RE: VSD energy savings

Hopefully your floor slats are placed so the outside of the tree, largest tree rings are up, and the heart of the tree, the smallest tree rings, are on the bottom.  That will minimize tripping hazards.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Thanks Morten. The link to the English version is in my previous post - that's the manual I don't like. smile
  

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If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

That comparison is certainly designed to make the VFD look good.  If the max power needed is only 22 watts as shown on the e-pump, then a standard 22 watt pump should be used.  Then if the pump is a good design, a valve closing should drop the 22 watts to about 11 watts, without a  VFD.  

"In my opinion even if the application is reasonably sound there are basic problems with mounting consumer-grade electronics on a pump casing which is likely to reach 80C or 90C in service. It just seems to be asking for trouble." ScottyUK

Maybe that is why they do it.  The high temp on the electronics is like a timed fuse.  They can accurately predict and plan the life of the pump.  Planning the life of the pump is important because, the pump company only makes money when they sell a pump.  If you believe it is saving a lot of energy, you shouldn't mind having to replace the pump every year or two, right?  I believe that graphic may lead to a myth-understanding?

RE: VSD energy savings

jonr12,

If you look at how a TRV-equipped heating system behaves as it approaches nominal temperature you'll find that the radiators slowly gag in as they near setpoint. Eventually a fixed-speed circulator will be developing a high head against the few remaining radiators in service, wasting energy in doing so because the radiator doesn't really benefit from the increased flow. So there is merit in what they are saying. Whether it would yield the same energy savings as teaching my wife that light switches are capable of turning lights off as well as on is a different question. smile

I'd like to think that a market leader like Grundfos wouldn't risk their market position with a product with a latent weakness. Maybe there is enough thermal isolation between the pump casing and the drive components. Personally I'd rather see the drive electronics in a separate enclosure mounted to a nearby structure but that wouldn't suit the the average plumber. Frankly the current brown / blue / green-yellow wiring to the fixed-speed pumps seems to be beyond the majority of the domestic and light commercial plumbers in the UK.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

"That comparison is certainly designed to make the VFD look good.  If the max power needed is only 22 watts as shown on the e-pump, then a standard 22 watt pump should be used.  Then if the pump is a good design, a valve closing should drop the 22 watts to about 11 watts, without a  VFD. "

There is trouble in that approach, on mild days when the heating is nearly shut down.  A typical (commerical) zone heating valve, globe pattern, has trouble throttling below about 10% flow, and will tend to over-flow (or bang shut) as the pump head backs up.  Also, continous operation of the pump at full head with hot water ain't much good for the impellers.  Again, speaking of commercial grade stuff.

RE: VSD energy savings

OK we are getting a little off with this last one.  I know there are lots of good applications for VFD controls, and I am not bashing all VFD systems.  I think this may be part of the problem.  VFD's work so well for closed loops, hot water, blood, cement, and a lot of other process controls, that we think they should be just as advantageous for every application.  

I have been specifically talking about pumps with centrifugal impellers, moving relatively cool water, with high static head.  In these type pumping applications I believe a VFD actually increases the energy consumption, and I know it decreases reliability.  Like the one in the curve I am attaching because I still can't figure out how to paste it on the page.

RE: VSD energy savings

We agree then, jonr.

 

RE: VSD energy savings

Back a few posts to the subject of VFD's in power plants, BFW pumps, ID fan drives, and the like.  That industry has been using VFD's for decades now.  They were called steam turbine drivers and they did what a VFD would do if it were to be an electric drive.  If the variable speed function that the ST's achieved was a viable process control, then the question of whether to apply ST's or VFD electric drive is only an economic one.  (Assuming of course, similar rangeability.)

rmw

 

RE: VSD energy savings

Isn't that more of a drive type evaluation problem, rather than a VFD question, as in you wouldn't need a VFD with a diesel either.   

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: VSD energy savings

Jonr12,

I'm working on converting a constant primary/constant secondary central chiller plant to variable primary configuration. All load calaculations and life cycle costing has been done, and variable primary was analyzed as best life cycle cost and best life cycle energy usage. This will include relatively high pressure and relatively cold water temperature. If you are looking for an application of VFD to pumps, then variable primary chiller operation is a good one.

Nothing beats right-sizing for energy savings, so VFD's only make energy sense when there is no single right size, and the variation is enough to justify VFD loss (I typically use 0.96 for initial calculations, and variable guide vanes are more accurate down to 90%); the number I use for screening is 85%, same as mentioned above.

VFD's have good aplications outside of energy which should not be overlooked. Differential pressure/volume requirements, such as with labs, hospital, and most HEPA applications, are much easier using a VFD. With preset failover frequencies, they open up doors for easier, safer design (and TAB) than were available 15 years ago. A lot of the VFD's I've had installed had nothing to do with energy.

RE: VSD energy savings

BigInch,

No, my point was along the line with what you have presented repeatedly in this thread.  The driven equipment (pumps, fans, etc) would have to be just as carefully chosen with any variable speed driver (Steam turbine, liquid coupling, etc) with respect to its ability to meet the head and flow requirements for the process at speeds less than full speed.

So there is nothing new here under the sun.  Just a new device on the scene.

rmw

RE: VSD energy savings

RE: VSD energy savings

Thanks Stanier, that little lot will take some reading!
  

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If we learn from our mistakes I'm getting a great education!
 

RE: VSD energy savings

To marcosdias :
The following article could be of help in answering the questions you initially posed.

http://www.mty.itesm.mx/etie/deptos/ie/profesores/jabaez/clases/E886/Agosto_2003/Ahorro_Sistemas_bombeo/Energy_savings_ASD(IEEE).pdf

The author is a senior IEEE member with ample practical experience on this subject.
The article starts by covering the correct use of Affinity Laws. In this thread the comment has been made that "Affinity Laws overestimate energy savings with VSDs". A more accurate statement would be that the incorrect use of Affinity Laws often leads to overestimation of energy savings. The same could be said of any law or equation when it is misapplied, be it Affinity, Bernoulli or the First Law of Thermodynamics.
Affinity laws are often used without adequate understanding of their origin, the assumptions behind them and their limitations (Mr. Stepanoff's postulates from back in 1948, I think). This can result in over-estimations and, therefore, in bad engineering and economic decisions. When properly used however, these are helpful engineering tools.
The article also presents sample calculations for energy savings (the ones you asked about) and a case study.

I have some additional material on the subject in connection with different applications that may also be of help and which I would share when possible. Contact information may be found in my web page www.manuelsuarez.com
For those in the Houston area in May (5th to 7th), there will be a seminar where the subject of VSDs will be covered to some extent within the general title of "Energy Efficiency in Refineries". Information may be found in www.petroleumrefining.com (Refining Process Services).

I would also like to add some comments on the subject based on my own experience
(chemical engineer who arrived at  AC  VSDs through Process Control & Automation and Energy Use Optimization).

VSD: Final Element of Control or Energy Savings device ??
Probably a final element with good energy savings potential in some cases.
Proper analysis of its applicability, configuration and implementation requires significant joint work of ME / EE / ChE to produce optimized - improved - better processes and equipment.

Not always the only justification for its use is energy savings, although that is perhaps the easiest to quantify and therefore the one that gets more attention. To quantify the others requires more inter-disciplinary work and analysis.
There are cases where process control reasons will outweigh energy savings. Decisions on this may require, for instance, the calculation of Speed (rpm) Control loops Response Time (e.g. response time of flow rate for the control of pressure, temp. etc). The VSD's Frequency switching (micro-seconds) is only part of the story. Enter Moment of Inertia of the Load (rotational equipment) and the need for proper VSD and Motor power ratings specs to obtain the required acceleration / deceleration for the specified loop response time.

For Engineers, using a VSD should be very much like using a computer. Ignorance about the internal details of the computer's CPU architecture does not preclude its use. As long as the user has  sufficient understanding of its functionality, limitations and constrains, it is possible to make use of its applications. The same goes for the VSD and its 'internal details' vs the use of its applications.   

It has been said that nothing beats the energy efficiency of properly sized pump running at the BEP at 100 % of rated flow all the time. That is true but it probably is not always possible to operate in that mode. If it were, control valves would not exist.  
There is always some amount of DP across the control valve (even when 100 % open) which requires some amount of Work (e.g. Energy) to overcome. If an alternative exists to eliminate that resistance and still comply with Flow and Head constrains, then an objective comparison should be made based on rigorous Energy Balances. However, that would only account for the Energy Savings issue. For the comparison to be meaningful (e.g. choice between two final elements), other aspects of their characteristics and performance should be included in the evaluation.

To say that engineers using VSDs are simply too lazy to work out the math to develop proper equipment specs and sizing is simply not true. I assume the engineering personnel in Aker Kvaerner, Shell, Ebara Elliot, Statoil-Hydro and many others could take exception to that and to the notion that they get carried away with 'myths'. In the Ormen Lange project they spec 40 MW  AC VSDs to drive 3 compression units and some 7 MW re-compression units for this world largest Gas pipeline between Norway and Great Britain running at approx. 70 MM  Sm3/day. And in the water cooling system (sea water) at Ras Laffan industrial city it was 250 MW of water pumps total capacity (27 units at 7 MW each). Both cases, on the high-end of power rating, are good examples of quadratic, variable loads very good for VSD application (aside from other issues like motor start, compressor  or pump enhanced operation / control etc.).
Whenever a system involves a variable load, it may be worthwhile to examine the use of speed variation compared to fixed speed and restricted flow.

By comparison, Burckhardt Compressors is delivering 23 MW units for Ethylene service where they only contemplate 'soft starters', not full VSD capability.
The reason: process requirements do not merit a VSD. No special process control advantage and the energy savings are not significant in the intended operation regime.
Axens on the other hand has a process license that includes Hydrogen recirculation compressors. The spec clearly indicates "Speed Variation". This was accomplished via Steam Turbines years ago. Today, the same spec is fulfilled with Electric Motors + AC  VSDs (more efficient and avoids implementing a high pressure steam system for the turbine).

One comment specifically about Steam Boilers (presently working on an application for this type of unit). Without a doubt, the Damper in the Forced Draft Fan is an excellent candidate for replacement with a VSD, specially in swing boilers. More accurate combustion control, increased turn-down ratio, reduced fuel, electricity and emissions.  In the case of the Feed Water Pump yes, the speed control range is narrow. However, we are looking at the design power rating of the pump and motor. They must be rated for peak conditions but will probably operate at approx. 80 % of top loads most of the time. A potential 20 to 40 % margin of loss that could be avoided with the VSD. We are looking at VSD providing full torque at -0- rpm / soft start of motor / no Current peak at the start / constant power factor over the speed range, maybe help with water hammer reduction (although this is taken care of by proper hydraulic design), avoid resonances (VSD programmed to "jump' over critical frequencies) etc etc. (It's a simple one pump / one boiler case. Even so, there may be some departure from the BEP so a low L3/D4 spec is considered).

One comment that came up is that with the VSD we can run the motor at over 100 % (nominal) rpm for limited time (or even on permanent regime if properly sized) but could not, for instance, open the control valve 110 % for any amount of time for instance.....

In closing (and kindly excuse the length ..):

Nothing beats good design based on solid engineering principles and good economic analysis and evaluation.

There is good and bad sales engineering just like there is good and bad process or mech. or electrical engineering. They each have roles and responsibilities, complementary and not interchangeable, in producing good, solid and safe processes and systems.

Some times it will  be VSD and others it will be Control Valve.

It is possible to miscalculate a VSD system just like it is possible to miscalculate a pump and valve combination.

If possible, resort to installed base references. In most cases and applications there is plenty of solid historical data and information to use as basis for judgment and decisions.  
The "myth" seems to have convinced a lot of very capable professionals all over the world... and now they have the operational data and information to back up their decisions.

VSDs use technology (power electronics) that is rapidly evolving, it takes time to stay abreast of developments to be able to apply it correctly and in its full potential. Advanced Vector Control (not PWM), regenerative equipment and 4-quadrant operation, IGCTs, Sine Filters, adaptive motor control models etc., result in a wider spectrum of applications that require ME / EE / ChE combined work to put to practical uses.

Thanks to all, picked some very good comments and advice in this thread.

MS
www.3rps.com

RE: VSD energy savings

Correction / clarification on my previous post:
Ras Laffan cooling sea water units: 27 units is for stages 1 and 2 of the project. Additional stages will be added for the total capacity requirement of 256 MW. Single unit capacity is 7 MW.
Thanks

MS
www.3rps.com

RE: VSD energy savings

Not a bad article, but I'm not sure I believe the 66% efficiency number in Figure 8.

And I have this feeling that I could have saved Saudi Aramco even more money by replacing the pumps and looping the line after 10 years.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/

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