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!

 

[MechEngNCPE]

Why not just re-sheave the dang fan!

 

[scottlsu]

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

 

[MechEngNCPE]

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?

 

[scottlsu]

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

 

[MechEngNCPE]

Seriously, hire someone who knows what they are doing.

 

[MechEngNCPE]

Is it centrifugal?

 

[MechEngNCPE]

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.

 

[MechEngNCPE]

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

 

[mauricestoker]

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.

 

[scottlsu]

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.

 

[MechEngNCPE]

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.

 

[KiwiMace]

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.

 

[scottlsu]

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"

 

[317069]

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

 

[MechEngNCPE]

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

 

[AnotherEllis]

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.

http://www.ellisconsultingengineers.com

 

[MechEngNCPE]

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.

 

[AnotherEllis]

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."

http://www.ellisconsultingengineers.com

 

[scottlsu]

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.

 

[AnotherEllis]

Use the fan laws. You could probably estimate your parameters faster than Revit can load a file.

Here's a link:

http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

http://www.ellisconsultingengineers.com