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Duct static pressure simulation
6

Duct static pressure simulation

Duct static pressure simulation

(OP)
I am currently working on a problem with a 20,000cfm VAV AHU.  This has system powered terminal units (Carrier Moduline) which require 1" of static at the terminal unit in order to modulate.  The problem is that there is not enough static pressure at most of the units for them to modulate.  This is a roughly 30 year old setup, which I don't believe has ever had the ability to modulate.

I believe the fan was undersized to begin with.  It was originally specified with a 20,000 @ 1.5" esp fan, which is currently in place.  I am trying to specify a new fan which will provide 1" of static pressure at the furthest terminal.  Can anyone assist me with a general approach to modeling this?  If i am lacking .5" at the furthest terminal, I do not think it is as easy as adding .5" to the new fan specs, as i highly doubt this thing responds linearly.

I have tried using Revit MEP, however this does not allow you to simulate different static pressure conditions (it basically adds up all of the pressure drops to calculate ESP).

Please help!

RE: Duct static pressure simulation

2
In the mentioned circumstances, yes, it is as simple as adding needed pressure boost to obtain new desired working point.

RE: Duct static pressure simulation

The best design for VAV ahu applications is to have a static pressure sensor approx 2/3 the way down the most strenuous run of duct, and modulate the supply fan VFD to maintain a static pressure setpoint of say, 1.5" at the sensor.  This allows energy to be saved when many of the boxes are closed down to minimum flow.   

RE: Duct static pressure simulation

If you can get a VFD on the supply fan do it.  It makes balancing a breeze as well.  Or you can re-sheave the fan you have.  Check its curve with the manufacturer, if .5" is all you need, a sheave package will probably work and run you under $1000.   

RE: Duct static pressure simulation

(OP)
The blower motor has an adjustable pulley.  We have adjusted it all the way, and gained .2" of static.  The unit already has a VFD on it.  

I have done this:  Varied the drive from 0-100% and measured downstream SP and ESP.  I then extrapolated this (exponential looking) in order to come up with the new ESP at the desired downstream SP.  The original AHU was specified with 1.5" ESP and from this method I show it would need to be 2.8" ESP.  How does this sound?

RE: Duct static pressure simulation

Well, I think you could get manufacturer's fan curve easily enough, and just check the static at the rpm you're operating at.  There are also static pressure "ductloss" worksheets you can get to do a proper static pressure calculation using the ductwork drawings and cfm's.  ASHRAE fundamentals has a great ductloss section for this calculation.  

If you are at 100% on the VFD and your sheave is already expanded, then I would just replace the sheave to provide 3" esp.  Talk with the sales rep for the unit - they will spec the appropriate sheave for you.

RE: Duct static pressure simulation

As NCPE mentioned, pressure sensing that governs VFD is practically a norm, whatever final solution would be.

Your estimate of 2.8" comes very close to 2.5" acquired by simple addition, but question of nominal flow  remains.

When you change fan rotation, without changing anything on ducting, you are actually "walking" over duct system curve, which means higher pressure will also mean higher volume flow.

Duct system curve constant is k = delta p/second power of volume flow

Your system, however, changes, as it can be assumed additional 0.5" of local pressure drop is imposed, so working point should go up the vertical line of constant volume flow to 0.5" higher pressure.

That is why I stated that fan with +0.5" and with the same volume flow will do the job.

If change of volume flow does not concern you, what you could do is to impose additional 0.5" of pressure drop by some local damper, and than try to play with fan regimes to see what flow you can get.

Did you estimate diversity? Total flow needed could be lower than in constant-flow system, so maybe you can even use your old fan if you can properly estimate total flow needed.

This depends very much on whether you have original project documentation, as many constant-flow systems are not designed for peak flow, but for system average flow (not simple average, but average plus some sort of modification, which is still lower than total peak).

0.5" is, however, not small difference in range of systems I normally design, which poses question of your current fan abilities.

Anyhow, if you can add 0.5" to your ducting, you can see in reality what your fan can do. It works only if you can apply it to main supply fan, to "suffocate" the whole volume flow by 0.5". If you have filter with differential pressure gauge downstream the fan on main duct, maybe you can play with that.

RE: Duct static pressure simulation

(OP)
Thank you all for your insight into this, as it is helping me to understand the concept of the system curve.

I realize now that by sheaving the motor, I was just walking the fan along the system curve of the ductwork.

I need to adjust my system curve (by physically modifying ductwork), so that I keep volume flow constant and raise static pressure.  Am I on the right track?

Drazen you suggest imposing a pressure drop in my duct in order raise my system curve vertically on the SP vs CFM plane.

Now i am struggling with this:  Imagine a Static Pressure vs Length chart...If I impose a great resistance early on in the duct, won't it drop my static back down to where I was initially?  And if I don't impose enough friction, then I won't effectively be "rasing my system curve vertically."  So to me it seems almost impossible to find my happy medium.  

Ho

RE: Duct static pressure simulation

You should find someone who understands air systems to help you where you work or hire a consultant.

This is an area of Engineering that requires more than a web forum.

RE: Duct static pressure simulation

I agree willard.  I do believe he could contact the manufacturer and come up with a sheave package (maybe even new motor if more hp is required) in order to provide the static he needs.  But your right, he should contact a Mechanical HVAC consulting PE (like myself) haha, especially because if what he imposes does not work, he would be up the creek.

RE: Duct static pressure simulation

(OP)
gee thanks.  what is the point of this forum then?  it allows people to network and discuss ideas who wouldn't typically get the chance...in a non-binding way (so much so that we'll never meet).  if you don't have anything to contribute to the topic, then don't.  don't suggest that the individual who sought the advice of other experts should seek the advice of other experts.

All input will be taken at face value!  

RE: Duct static pressure simulation

You're welcome.

RE: Duct static pressure simulation

2
Probably not the best path to take on a forum where you're asking for help scottlsu.

Willard was making the, very reasonable, statement that you don't appear to know what you are doing and sound like a very junior engineer. We can't solve your problems, but only try and point you in a direction.

The direction Willard is suggesting is hiring someone with experience to solve your problem. Not a bad solution.

RE: Duct static pressure simulation

Yes, as colleagues stated, forum cannot be place for through examination of technical problem up to final solution, but tip can be valuable to point at direction or concept.

To briefly explain previous words - improsing additional pressure can just give you idea how much flow you can get at current setting. Initial curve of the fan obviously suggests that you will not have the same flow, but than you can try to work with VFD, to see how close you can get to initial flow with imposed pressure drop.

All this does not relieve of other duties like proper setup of flow measurement apparatus, informed decision about needed design flow for VAV refurbishment, which is engineer's decisiion based on previous designe analysis, and probably need for additional calculations.

"Simplest" way would be to buy new fan as previously stated, while again you need to be certain which design flow you actually need, exisiting one or modified one, decision that carries financial responsibility.

 

RE: Duct static pressure simulation

Well said Drazen, the vast amount of knowledge required to design complete systems, and the hours put into it just goes to show that this is only a place for "tips" and not meant to provide someone without experience a design for their application.

RE: Duct static pressure simulation

It seems to me that the original premise of this discussion and the comments that followed were based on the assumption that the system was perfictly balanced and that the SP sensor was throughly checked for accuricy  and this premice was never discussed although Drazen tuched on it It seems you were all focased on a solution without making sure all the boxes were calabrated and maybe with proper diversity this system could actually work

RE: Duct static pressure simulation

"The problem is that there is not enough static pressure at most of the units for them to modulate."

This lends me to believe that there actually was a TAB done on the system, but if there wasn't, that should be the first step.

RE: Duct static pressure simulation

Hi scottlsu,
Why are you focused on the fan as the problem?
Do you know that none of the VAV manufacturers make system powered VAV boxes anymore? They don't because the system powered boxes don't work. The fan cycles off because of Time of Day (TOD) and restarts the next day, there is not enough static in the primary duct, the VAV boxes won't work.
You will save an incredible amount of fan energy (money) if you replaced the VAV boxes with boxes that have pressure independent controls. Not to mention the comfort complaints that will go away because of a properly operating VAV system.
Yes, if you throw enough money at the problem, adjusting the fan speed to build the static then maybe you get enough static to the system powered VAV boxes. But after that you are going to be paying higher energy cost for the increased fan operation (BHP).
Good luck,
Allen
 

RE: Duct static pressure simulation

Scottlsu should hire an Engineer familiar with air systems as he plainly new to air systems and can use help. It's good Engineering to know what you don't know and ask for help.

I have seen way too many junior engineers come to grief on air systems. They are complex and yet they seem so simple. There is no simple solution to the question originally posed with the given information.

RE: Duct static pressure simulation

(OP)
Let me re-iterate my problem.  

To address whether a T&B was done:  This is a system which has operated for 30 years.  Yes, a t&b was done at some time in the past.  However, I do not think that it was successful.  There is simply no means of calibrating the VAV terminal unless there is 1" SP available at the terminal.  (The way they work is there is a bellows which inflates or deflates .  It needs to inflate to seal off the diffuser air passage.  This can be calibrated, as there is a small regulator at each VAV terminal.  However, calibration is useless because 1" is required to inflate!  This is basically a constant volume system at the present time.  Only a few terminals have enough SP to do their thing.

I have a full mechanical layout of the system.  I know designed cfm at every terminal.  I know the duct size and length and shape everywhere.  I have recreated the entire system in autocad revit, which does a ESP calculation.  Everything that I've done confirms the original specification of 20000cfm @ 1.5" ESP (Including physically taking measurements with manometer). I believe the ductwork was sized well, because I have taken flow measurements from many of the diffusers and we are close to the design cfm for each.

As far as the VFD: A controls contractor has sold the owner VFD's for all AHU's to "save energy".  There is not even a 2/3 length SP sensor to trim the drive!  So forget it is even there!  This will be valuable if I can get enough SP for some of the terminals to start closing off, thus giving room for the VFD to back off.  If I can successfully get the terminals to modulate then I explore getting the VFD set up to maintain downstream SP.

Also, the reason this has sparked is that one of the areas in this system (~6000cfm) has been re-modeled and they are trying to recommission it.  We want at least this zone to work!

I hope this has cleared things up a little.

RE: Duct static pressure simulation

Why not just re-sheave the dang fan!

RE: Duct static pressure simulation

(OP)
The fan is already at max RPM after adjusting the existing pulleys...

RE: Duct static pressure simulation

Got it.  Now find a larger fan "pulley" or sheave, rather, that provides the fan with a faster RPM...specifically one that will give you the correct static.  Did you contact the fan vendor yet?   

RE: Duct static pressure simulation

(OP)
No i mean the fan is at its mechanical limit.  Its got a tag on it that says max RPM 900.  

RE: Duct static pressure simulation

Seriously, hire someone who knows what they are doing.

RE: Duct static pressure simulation

Is it centrifugal?   

RE: Duct static pressure simulation

If it really is at its mechanical limit, I would just spec a fan that can provide the correct static & cfm.  Looks like a fan replacement.

RE: Duct static pressure simulation

I'd verify that that's the real "max rpm" with the manufacturer.  Did you get the fan curve yet?   

RE: Duct static pressure simulation

Boxes typically are available at 0.5-0.6 IWG and are pressure independent. Being able to cut out 0.5 IWG would be the same as having about 0.5 HP available, and would proably save $300-$400 a year on your electrical bill. I would consider replacing the boxes before the fan, or both.

RE: Duct static pressure simulation

(OP)
mauricestoker, I totally agree.  someone else mentioned early that the vav terminals should be replaced with pressure independent terminals to completely solve the problem and also make it more energy efficient.  Unfortunately the owner just had all of the terminal units replaced and relocated to suit the new floorplan!  I will definately present them with this option but i'm 99% sure they will just want to get the proper SP and move on with their lives.

MechEngNCPE, Yes its centrifugal and yes i have the curve.  Its a Lau OEM blower for another equipment manufacturer's air handler (Carrier).  I am working with the equip manufacturer on a replacement fan section.  They want ME to spec the static pressure and cfm.

I am going to pull the trigger and just go with the 20k cfm @ 3.0 esp.  Once the terminal units can close off, I think bernoulli's principal will come into effect and i will see some increase in SP rather than just CFM.  Thanks for all of your suggestions i will post results in a few weeks when it is done.

RE: Duct static pressure simulation

I think 900 RPM is the slowest max rpm on a centrifugal supply fan I have ever seen....  are you sure?  cause a new sheave would be a heckuva lot cheaper than a new fan section.

RE: Duct static pressure simulation

30 years.... Is the duct clean and pressure tested? Does the leading edge of the fan blades look sand blasted?  People usually put in manual vfd's because there is too much static and they want to trim it off a bit, or make a seasonal adjustment.  Was anything else done along these lines?  

Blank off your system in sections to commission the terminal units.

Revit is probably the worst duct calc out there, 10 points for obtaining a result.

 

RE: Duct static pressure simulation

(OP)
I should say, that the AHU was replaced 5 years ago.  No specs changed from the original prints.  My point is that it is in very good, clean condition.  And all of the ductwork is very well done and well sealed.  Also I have confirmed the wheel's max RPM.  It is a Lau H Series 26"

RE: Duct static pressure simulation

scottlsu
Could you please give me an idea about Revit program.
Can you use this program to recalculate ESP or required fan pressure in case you changed the terminals duct connections size from 6" to 8" for eaxample

 

RE: Duct static pressure simulation

Yes Revit can redo the calc if you change the inlet sizes.

RE: Duct static pressure simulation

I am of the opinion that Revit's ability to estimate all of the parameters that it can is a curse.

I would have never thought to draw an entire system in Revit just to figure out what an ESP was.  

www.ellisconsultingengineers.com

RE: Duct static pressure simulation

Who would do that?  I'm just saying it can if you do it from the getgo.  A much easier way would be a calculation spreadsheet obviously.

RE: Duct static pressure simulation

Twice above by scottlsu:

1- "I have tried using Revit MEP, however this does not allow you to simulate different static pressure conditions (it basically adds up all of the pressure drops to calculate ESP)."

2- "I have recreated the entire system in autocad revit, which does a ESP calculation. "

A parametric modeling program with some design/analysis capabilities can be dangerous in the hands of a user not adequately familiar with the underlying principles.

You know the saying: "A little bit of knowledge is dangerous."

www.ellisconsultingengineers.com

RE: Duct static pressure simulation

(OP)
Is this forum for criticism or for support?  What I mean by Revit MEP's inability to "simulate different static pressure conditions" is that you cannot adjust your fan parameters to see its affect on ESP.   

RE: Duct static pressure simulation

thanks scottisu for sharing your problem with us.  The reason I like this forum so much is because people bring their problems in their various shapes and sizes and often poorly defined (it is a forum after all!!!).  The 'brain storming' I have witnessed on some problems here is brilliant on occasion.  I dont think we regularly hit the mark or really solve someones problems directly via the forum (I agree that life is to complicated for that!!).

I can honestly say that some of you guys sitting at home with a beer and chewing on one of my badly defined, ill informed problems has really helped at times, so thanks.  I hope to return the favour!

Also, I agree that it is very important to know when to call it quits and get a more senior engineer
involved, even if it blows your fee.  It will likley be much cheaper than having your ass sued for the modifications that didnt work!!  You need to weigh up risk versus reward.  Your client will probably be impressed that you knew when to call in a higher power to solve the issue.  No one thinks that a single engineer knows what the solution to every problem is!!!

cheers

RE: Duct static pressure simulation

I agree with Warmanga. I own my own firm, and usually have a pretty good idea of what's going on and what to do about it. But I'm not a 40+ year veteran and haven't seen everything.

For that reason I maintain relationships with other engineers and peers, esp. those I worked with in the past or went to school with. It's nice to have somebody to call and bounce ideas off of/ ask if they have seen the latest craziness, etc.

Sometimes "I don't know" is the right answer.

www.ellisconsultingengineers.com

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