Duct static pressure simulation 6

[scottlsu]

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!

 

[Drazen]

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

 

[MechEngNCPE]

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.

 

[MechEngNCPE]

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.

 

[scottlsu]

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?

 

[MechEngNCPE]

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.

 

[Drazen]

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.

 

[scottlsu]

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

 

[willard3]

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.

 

[MechEngNCPE]

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.

 

[scottlsu]

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!

 

[MechEngNCPE]

You're welcome.

 

[ChrisConley]

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.

 

[Drazen]

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.

 

[MechEngNCPE]

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.

 

[imok2]

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

 

[MechEngNCPE]

"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.

 

[Allen1973]

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

 

[willard3]

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.

 

[scottlsu]

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.