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constant flow variable speed pump for heat exchanger

constant flow variable speed pump for heat exchanger

constant flow variable speed pump for heat exchanger

(OP)
Hello guys, I have searched high and low for clues to my problem and can't seem to get help. I have a centrifugal pump controlled by a VFD. The pump transfers milk from one tank to another through a milk-to-water and milk-to-glycol heat exchanger. The liquid levels in the two tanks are constantly changing. The source tank fills and empties and the destination tank fills, then is changed to an empty tank. I can monitor the levels of both tanks and use them in my PLC networks. I would like to arrive at an equation for my PID loop that varies the speed of the pump to keep the flow rate constant while the head pressure is changing. The number I want to arrive at is, how much do I need to change the speed of the pump for each change in total head to keep the flow constant. I have tried substituting the affinity equations in each other but I find myself running in circles. If anyone has done anything like this or knows how to approach it, I would appreciate hearing from you.

RE: constant flow variable speed pump for heat exchanger

It will be impossible to enable a constant flow in this system without an automatically set control valve some where in this line that will absorb the variable pressure drop in this system, which is the sum of the static head differential plus the friction pressure drop at the required flow. An inline FE and flow transmitter will be also most likely be required.

RE: constant flow variable speed pump for heat exchanger

Shouldn't be that hard, but you will need the pump curves at different speeds from the vendor.

An approximate guess though is this.

Total head loss across the pump = Hp + Hd -Hs
where Hp is head loss in the piping (static for the same flowrate), Hd is height of delivery tank, Hs is height of source tank

Now depending on the ratio of Hp to (Hd-Hs) the system will work or not. If Hp is quite big compared to the max head difference ( i.e. >70%) the you should be ok.

I would just start with pump speed is proportional to head^2. It won't get you the same flow but then you can add a bit.

SO using some basic numbers, if your head loss is 100 and you're at 100% speed

then at head loss of 70, you're at about 83% speed, but you're not at the same flow, so add a bit to say 86%. A bit of trial and error as nothing is exact and you won't be more than 10% out once you establish a set of points or a simplified line. As said much easier with a set of pump curves for different speeds.

Of course it would be a lot easier to just add a flow meter (mag flow or UT) and control on that using your PLC??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: constant flow variable speed pump for heat exchanger

An indication of flow rates and heads involved wouldn't hurt, people could then see if they are wasting time on answers completely unsuited for the application.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: constant flow variable speed pump for heat exchanger

Not sure if it is an option, but a positive displacement pump is better suited for constant flow, no programming required.

I used to count sand. Now I don't count at all.

RE: constant flow variable speed pump for heat exchanger

as per the pump laws ( or fan laws) the volumetric flow varies with the rpm while the develop head varies with the RPM^2. If the required head varies then it would be necessary to run thepump at the RPM needed for the max developed head and then throttle the discharge pressure as the need for a lower flow is met.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

RE: constant flow variable speed pump for heat exchanger

I disagree. what happens is that you will move further along the curve for the same flow but lower head. You need to remember here that the OPs system curve keeps changing as the tanks fill and empty.

Hence the speed reduction is proportional to SQR of flow, but a bit more speed is need to match up the flow.

Speed variation on a VFD is similar to changing impellor size.

Hence in the picture below, which is a little extreme, if you want to maintain say 200 GPM, then you can do that if your system curve changes ( which it does for the OP) so that at say 200 GPM his head across the pump could be 18 ft to 90 ft corresponding to 900rpm to 1800 rpm.

I think that range is a bit large myself so I would be more inclined to go for 300 gpm at min diff head of 40ft vs max diff head of 90 ft

VFDs run on flow control all the time to maintain a set flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: constant flow variable speed pump for heat exchanger

There are also purely mechanical flow control fittings you could use on the discharge of the pump. Have a Tee after the discharge, one side with the flow control, the other as a re-circ line.

I used to count sand. Now I don't count at all.

RE: constant flow variable speed pump for heat exchanger

(OP)
Thanks to everyone, especially LittleInch, for answering. I think I see what to do, but want to post a response to you all to see if I understand what you have been saying. First, a little more clarification as to my installation. I left out some details earlier because I did not think they would be relevant to your answers, but I see that they may be interesting nonetheless. We are controlling the speed of the pump via a VFD where the process variable is the temperature of the milk leaving the heat exchanger. The PID loop response needs to be fairly sluggish to compensate for the reaction time of the temperature sensor and the 6 compressors that are chilling the glycol. If we respond too quickly, we speed up our pump and flowrate long before the chillers can start and take a couple degrees out of the glycol. Because of that, the perturbations from the rapid head changes make the system inherently unstable. For instance, the dairy will run 40 cows into the parlor, connect the milking units to them in about a minute, and they will start producing an average flow of about 16 lbs/min of milk. That flow, along with the residual flow from the other side of the parlor where the cows are finishing up, hits our tank at instantaneous rates reaching 80 gpm. Our supply tank is a circular, 275 gallon tank that stands about 7 feet tall with its bottom 1.5 feet off the floor. The 2 hp milk pump is connected right to the bottom of the tank, pumps the milk through about 20 feet of 2" pipe and a heat exchanger, then into a 7000 gallon truck whose bottom is about 4 feet high and whose top is about 10 feet high. You can see that our suction head (Hs) v aries between 0 and 5.5 feet and our discharge head (Hd) varies between 4 and 10 feet. Chiller capacity limits our flow rate to about 18 gpm, which can vary as they lose charge, blow fuses, overheat, etc. We want to correct our PID output to include the information about the changing static head to enable the PID loop to respond more rapidly and yet remain stable.

What I take from your answers is that I need to use the Affinity Law regarding the relationship between head and pump speed to arrive at my answer. This law states: dp1 / dp2 = (n1 / n2)2 where dp = head and n = rpm. Manipulating this equation, I get that n2 = n1 / sqrt(dp1/dp2). For example, if my head increases by 10% during my 'scan time' of 10 seconds since I last checked, then dp2 = 1.1 * dp1, so dp1/dp2 = 0.91 and sqrt(dp1/dp2)=0.95 and n2 = n1/0.95. Following this, I should expect that if my temperature sensor has not changed its reading, I should expect my new speed for my pump should be 105% of what it was before the head changed. If my head increases by 20%, I expect my speed should increase to 109.5% of what it was. If 100%, 141% and so on.

After talking about it with my boss, because our Hp changes very slowly, due to a filter plugging and a large tank filling, we may need to monitor only the head changes in the supply tank for the above formula. The supply head changes at least 5 times faster than our system's ability to respond, but the discharge head changes 50 times slower and is not an issue in our installations.

Thanks again. If your head is spinning, don't feel bad, so is mine.

RE: constant flow variable speed pump for heat exchanger

You could run the discharge to the top of the last tank, pump at constant speed & constant flow against constant elevation head and throw the VFD away.

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RE: constant flow variable speed pump for heat exchanger

The factor that is missing from your equations is that as the speed decreases so does the flow when following the same system curve.

Therefore to increase head, but not increase flow you need an increase in speed, but maybe not quite as much as you calculate above as flow ratio = speed ratio.

BI - his issue as described is that it is his inlet static head which changes a lot, not his discharge head.

however if you really want to do it (strict flow control) this way as another poster said, swap out your centrifugal pump for some sort of PD pump ( screw maybe) and then speed of motor = flow virtually regardless of inlet and outlet head. Or fit a flow meter and control on that and not level.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: constant flow variable speed pump for heat exchanger

Seems all too complicated and expensive for a 2 hp pump, think BigInch had given you the solution to your problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: constant flow variable speed pump for heat exchanger

I think so. He's trying to fix something that isn't broke.

Kind of rediculous to run a pump at constant flow (basically constant speed) using a VFD. You might get really fine flow control, but I seriously doubt you need it that fine and you're having a hard time working out how you're going to control it. I also refuse to believe it won't work with a slightly variable inlet head. NPSH is critical with milk?

Technology is stealing American jobs. Stop H1-Bs for robots.

RE: constant flow variable speed pump for heat exchanger

I had a look at a 2hp (1.5kW pump) and at 18 gpm you could have a huge differential head ( 75m+) There must be some large pressure drop in the HX or the pump is operating so slowly it's struggling to work properly and maybe that's more of the problem here.

Gaalfred - can you please give us some real number to work with here for head or discharge pressure of the pump please and also confirm your steady flow is 18 GPM / 4100 litres/hour.

what frequency / percent of speed is it operating at?

At 18 GPM it's going to take you 6 1/2 hours to fill your 7000 gallon truck. That sounds like an awfully long tome to me...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: constant flow variable speed pump for heat exchanger

(OP)
I need to respond to a couple of your comments, guys:

So...Artisi, would you say that accurate control of a morphine pump in a hospital is "too complicated and expensive" because it is a 1/400th horsepower pump? What does the size of the pump have to do with the accuracy or value of the process?

BigInch...How far are you from Pipestone, MN? You could come out and see it for yourself. You could visit the National Monument while you are at it. I need accurate control of the milk exiting the heat exchanger, not accurate control of the flow. The milk needs to be 38 degrees, not 35, not 42, and not wildly varying between 38 and 42. When the suction head goes up by 4 feet, the temperature of the milk goes up by 4 degrees until the VFD can reduce the speed of the pump to get the temperature back down. The response time needs to be slow because the system response time is slow. Like I said in my long post yesterday, the suction head changes at least 5 times faster than the system can respond and causes a properly quick response time to overshoot and the milk temperature to oscillate. Believe me, I am not trying to fix something that is not broken. I asked for help with the formula, not criticism of my project. Using a VFD to run this pump is not ridiculous. The pump needs to run at full speed or greater for the wash process, and it needs to run at just the right speed for cooling and transferring the milk. If a chiller compressor stops for some reason, I can't be standing there to slow down the pump, it needs to do that itself based on the milk temperature. No one wants to dump $10,000 worth of milk down the drain one morning because a compressor failed and someone thought a VFD was ridiculous.

LittleInch...You are exactly right, I could do better with a PD pump or a meter. A 3A sanitary pump costs about $5000, 5x a centrigugal one and a 3A meter costs about $5000. I don't know if you are at all familiar with daires, but they are extremely price-conscious and operate on the brink of bankruptcy most of the time. Just when they think they are getting ahead, some government decision screws them, like Canada's recent decision not to import milk from a certain supplier in WI. Now all the farmers that sell to that supplier suddenly have NO MARKET for their product since all the other creameries are running at capacity. I either provide them with an affordable solution or they don't do the project. Efficiently cooling milk to the optimum temperature saves them a LOT of money and is well worth the effort we put into these designs.
Also, why is it that you think my equations will not be accurate using flow ratio = speed ratio? Do those affinity laws not apply to all centrifugal pumps within their operating range? Or are they rules of thumb and not laws? Or can I not combine two of them like I did? It seems like I should be able to combine them since n1/n2 occurs in both. Thanks so much for your careful analysis and understanding of what I am trying to do.

RE: constant flow variable speed pump for heat exchanger

(OP)
Also...to LittleInch, you are correct that it takes 6 1/2 hours to fill the truck. The large dairies have dispensed with the bulk milk tank with cooling capability and have gone to direct-load into trucks. The trucking company backs the tanker up to a hole with a foam seal around it in the milk room. The pipe is connected to the tanker and the valves are opened and the milk begins to flow. The tractor part of the truck unhooks from the tanker, moves over to the next bay, connects to the full tanker that is waiting there, and heads down the road with it. There are usually at least 3 bays, one for an empty tanker, one for a full tanker, and one for the tanker that is connected to the pipe and being filled. The tankers are insulated so the milk stays cold. This is why we need to get the milk accurately to its desired temperature as it is going into the tanker. There is no provision for cooling it later.

RE: constant flow variable speed pump for heat exchanger

The milk needs to be 38 degrees, not 35, not 42,
Fine. Now we're getting somewhere. Then you need temperature control, NOT flow control. Let the output temperature control pump speed. Too cold, slow the flow down. Too hot, speed the flow up.

Technology is stealing American jobs. Stop H1-Bs for robots.

RE: constant flow variable speed pump for heat exchanger

What Big said, but also - you should control the pump speed based on delta-T (diff. between entering warm milk and chilled milk temperatures) also, as flow rate and delta-T are directly proportional. Presumably, the milk tank temperature varies as the milking process runs, from whatever temp. the cows are at to some lower temperature as it sits and cools.

RE: constant flow variable speed pump for heat exchanger

Gaalfred,
Understand your issues but if you can provide the information that would be good. I think one cheap way out of your problem may be to increase the fixed head loss so that the inlet head becomes less important. I think your pump is running very low speed and maybe something like 20m of 1" pipe after the pump but before the cooler might work. Or you might need 40m of pipe. I can't judge unless I can see what your pump discharge pressure range is.

It's difficult to get the same flow but different head by varying speed when also having a different system curve so an exact ratio doesn't work.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: constant flow variable speed pump for heat exchanger

A large source of dead time error ( which cause these dreaded oscillations in the final controlled temp) in your current process control loop setup would be in the final discharge milk temperature sensor. An RTD sitting in a thick thermowell is the root cause of this dead time. Solutions to reducing dead time errors are (a) use specially engineered thin wall thermowell to reduce the thermal inertia in the thermowell (b) use fins on the thermowell if the fluid is nonfouling to enable faster heat conduction into / out of the thermowell. A thermocouple also gives faster response in comparison to an RTD.

Your later posts give a more complete picture, and agree with others that in principal, a VFD may be part of the solution here. So your new proposal is to (a) monitor fast moving suction tank level and directly vary pump speed to get you constant flow (b) slower moving transients such as discharge tank head and suction tank temp can be handled by final milk temp TIC modulating coolant glycol flow into the milk to glycol HX.

A better estimate of the Q vs speed mathematical relationship is to draw a constant flow line on the flow map for the actual pump, derive the required polynomial expression for this and substract the constant friction drop on this at the desired flow ( friction piping press drop from suction tank exit to discharge tank inlet ) to get to the required differential level. Generalised affinity laws only work for small changes in flow / head / power / speed around any reference point, and are no use for larger deviations.

RE: constant flow variable speed pump for heat exchanger

A minor refinement to the new control scheme proposed in (b) would be to split range the final milk TIC output, such that the earlier output response modulates glycol flow at the final glycol-milk HX and the later response acts as an override on the XIC output(that derives pump speed) which goes to the pump VFD. This later response would address cases where there are temporary shortfalls in chilling capacity at the refrigerant compressors.

RE: constant flow variable speed pump for heat exchanger

Don't know what a 3A meter is, but why not look at a UT clamp on meter like this (example only)

https://www.coleparmer.co.uk/i/dynasonics-dttslp-0... or introduce a smaller section of pipe and get one with a better flow range like this https://www.coleparmer.co.uk/i/dynasonics-dttsdp-0...

I still think though a major part of your issue is that the pump, even at 2Hp, is just too big for your duty and hence to make the system insensitive to inlet tank height, make the fixed head element (at your rather low flow rate) much bigger.

coupled with Georges idea of a faster temperature pick up you have two ways to do thinks better without a lot of expense.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: constant flow variable speed pump for heat exchanger

Gaalfred, If you want cheap, get rid of the VFD. Open mind. I'll talk you through it. OK we change rediculous to unfortunate??

Pump head output (affinity law) varies with the square of pump speed. That's only 1/2 of the problem. The other half of the problem is the system curve; ie what's going on with flow and head in the pipe and tanks. The system flows vary more or less with the square root of pump head. Additionally they can have an initial H0 representing static head of the discharge tank that the pump must begin to pump against when starting. NPSH can also adversely affect this. So head obviously changes as the pumping continues and the discharge tank fills. How that changes is affected more by the dimensions of your tanks; ie how much head has increased in the discharge tank, and maybe the supply tank too, for any given quantity of fluid pumped. That is related more directly to tank fluid elevations than it is to flow rate. As you can see the resulting complex relationship is nowhere near direct to pump speed. If you could make a good mathematical relationship to describe it, it would not match how your PID would take temperature input and then output an instantaneous pump speed.

You've got two problems going on. Flow control and heat control. You apparently need flow control, I imagine to be as constant as possible, so you can know tank fill and process stop times and all those scheduling things that are good to know, and so that your heat exchanger can operate at a nearly constant power output. Wild variations in exchanger output power output doesn't do anything any good. Radical flow changes would cause wild temperature and power fluxes at the exchanger even if the milk remained the same temperature during the process.

When faced with multiple tasks, I like to break them down. Flow control is one, temperature control the other. Pump control works well for flow control. Exchanger power control works well for temperature. Those two working independently takes away the ability of each to fight the other, which would happen if you tried to control flow and temperature with only one feedback loop. So, use some kind of flow control for the pump and a temperature control to adjust the exchanger power input for discharge temperature.

Once you separate the two control loops, the pump is free to operate at constant speed for the process, if the flowrate is right at that constant speed, however you also have a wash cycle, WHICH IS COMPLETELY ANOTHER FUNCTION. So you need another button to turn these loops off and switch to the wash cycle. But forget that for now. If the flowrate of the pump is not right at it's syncronous speed, you could change the speed with a VFD, or you might could run at synced speed and use a control valve to control flow. Control valves can be far easier AND CHEAPER means of flow control than buying and installing VFDs due to cost of the VFD over the small control valve the previous mismatches I mentioned in pump head output vs flow in the system with tanks. Control valves don't suffer those head vs flow problems as much as pumps can. So I think that with proper tank dimensions, you could run the pump at sync speed with a contrl valve for flow and use a temperature sensor feedback to control the exchanger's function directly with the output temperature.


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RE: constant flow variable speed pump for heat exchanger

Back to the wash cycle.
The "control valve" might be just a valve with two positions, one for process flow, the other full open for wash cycle. That could work very well with a constant speed pump running at synced speed. When on wash cycle, just turn off the heat exchanger.

Technology is stealing American jobs. Stop H1-Bs for robots.

RE: constant flow variable speed pump for heat exchanger

(OP)
Thanks guys,
I found a performance curve for my pump. As far as I can tell, Thomsen doesn't publish full-range charts and I have never seen a chart for the Boumatic milk pump that is often used in daries. This one has a set of head vs flow curves for different sized impellers. I can convert the impeller diameter to speed and get a chart of head vs flow at 6 different speeds, none of which are the speeds we usually run for milk flow. I may have to set up a test bench with a paddle-wheel meter and a valve and pressure gauge to build my own chart of head vs flow at several different speeds in the range that is useful to me. It is a Thomsen #4 pump with 1 1/2" inlet and outlet running at 3450 rpm max.

I picked 20 gpm flow and followed the vertical line on the chart to each curve, then read across to get the total head at that flow and speed. From the two sets of 6 numbers, flow vs speed, I let Excel run a regression analysis on it and Excel came up with the formula y=0.74x+74.6. Using this formula with y=head and x=speed, I am able to arrive at a new speed to run my pump when a change in head occurs to keep the flow rate constant. I believe this answers my original question. Thanks, george for the tip on doing this.

I will also investigate can be done to improve the response of the temperature sensor per george's suggestion.

The term "3A" refers to the 3-A Sanitary Standards that specify the criteria for the design and fabrication of equipment that comes into contact with food. All equipment in Grade A dairies must meet these standard. I am sure that any 'fins' on a thermowell would be frowned on by the federal inspectors and harbors of bacteria, but I will check.

One feature of our systems is that the VFD controlling the milk flow causes the milk flow to exactly match the capacity of the chiller with its current operating conditions. These conditions vary quite a bit from winter to summer as the ambient temperature of the air across the condenser varies from -30F to 110F. By setting the set point of the glycol to just below the milk temperature set point, the chillers are used to their full capacity. It is very important to do this as there is not much excess cooling capacity, especially on a hot day when the high-producing group of cows is being milked and the milk must be instantaneously cooled to its marketable temperature.

RE: constant flow variable speed pump for heat exchanger

That sounds better now - The y in this formula is really y1 + y2, where y1 is the differential head between the 2 tanks, and y2 is a constant and is the frictional head loss, which would be constant at a constant flow of 20gpm.

The use of a thermocouple instead of an RTD would be a big improvement on TT response time also.

Yes, would suppose that in this plant, you would be operating the refrig. compressors to adjust speed and discharge pressure to produce a constant glycol delivery temp(to the milk-glycol heat exchnager) at the refrigerant-glycol heat exchanger. And that you would be running the compressors at high discharge pressure and high speed in summer to make up for the reduced refrigerant condensor cooling capacity.

RE: constant flow variable speed pump for heat exchanger

I'll add a bit more tomorrow but from looking at the pump curve, you will find making this system better difficult because for the main duty (18gpm)your pump is simply far too big.

This is like driving your car in top gear at idle. Very lumpy and any small hill and it can't maintain a steady speed.

You can do things about it but it would be really good to know what the normal discharge pressure is and what speed you find yourself at at 20 gpm.

Remember - More details = better answers
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RE: constant flow variable speed pump for heat exchanger

I really don't understand your equation given that maximum differential head is only 72 feet for the largest impellor ( you don't say which size impellor you have)?? Unless your speed is a fraction of max speed, but even then doesn't add up to me.

However as below, your 20GPM is just too far left on the curve. I don't know what your head loss is, but if 5 or 6 feet is enough to change flow then total can't be much more than 30? or lower. Hence your motor, if max frequency is 3450, will be very low, with low torque and low levels of adjustment.



So what can be done?

Basically I think you need to possibly increase your resistance to flow to reduce the impact of change of level plus increase the flow through the pump.

1) can be done by adding some lengths of tubing
2) can be done by having a long run of piping teeing off the discharge which you can calculate using the tables in the thomsen catalogue to equal your head loss in the cooler section and return it to the tank. I don't know if milk wouldn't like to be sheared across a control valve so this is the easy way, but a valve return can be done this is a lot simpler.
3) buy a smaller pump

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RE: constant flow variable speed pump for heat exchanger

LI where did that curve come from. What are the red lines? It seems that he doesn't have more than a max of 10 ft of system head and at startup it might be -1.5 You can see that our suction head (Hs) varies between 0 and 5.5 feet and our discharge head (Hd) varies between 4 and 10 feet.

I think he needs the VFD so he an run it, but at a nearly off speed.



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RE: constant flow variable speed pump for heat exchanger

Curve came from the vendors website. The red lines seem to be power requirement.

He's referring to static head only. We still have no idea what the friction losses are through his equipment at 18 gpm

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RE: constant flow variable speed pump for heat exchanger

looks like the inverse of power.

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RE: constant flow variable speed pump for heat exchanger

gaalfred - this post seems to have petered out which was unfortunate as there still seemed some unanswered questions / issues. Any further data / response?

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