Outside Air Design

[filippo56]

I am in the process of troubleshooting an O/A duct distributing air to four AHUs. The most remote AHU is modulating the return air dampers to give a constant 900CFM of outside air. Due to the return air damper modulating down the total air is low. I am getting a varing consensus of opinion among my coworkers and the TAB on this, from bringing in a separate O/A duct, to adding a fan in line for the specific unit, to adding a fan on the main duct(about 2500 CFM total O/A demand)with backdraft dampers in the O/A duct near each unit. I would like to solve this by adding an in-line O/A fan at the specific AHU with backdraft dampers at the other units but I am not shure how to calculate the fan size or if this will work at all.

 

[BronYrAur]

Is the OA ductwork sized to handle the OA requirements of all AHU's. Sounds like they are fighting each other for the air.

 

[filippo56]

No, I don't think they are fighting for air. This unit is the most remote, longest distance from the Louver.

 

[MintJulep]

What is the actual problem that you need to solve? It is not clear in your original post.

I'll guess that the problem is that "the total air (supply?) is low (from the most remote AHU)".

If that is it, I would guess that moving 900 cfm through the OA duct has used up most of the available static pressure from the AHU's fan.

The simplest way to to solve this would be to simply increase the AHU fan speed, provided that the motor has sufficient unused capacity.

The lowest cost (life cycle wise) would be a separate OA duct, sized for low pressure drop.

Adding a booster fan means you need to pay to run the fan forever. Adding dampers at the other AHU's increases the pressure drop at each, making their fans costlier to operate.

 

[filippo56]

Agree. When the TAB opened the return air damper manually to 100% the total air increased to near design. The O/A decreased below design. We do have some margin in the motor to increase the fan RPM but this thing is already outside of the fan curve envelope. The static pressure is below where the fan curve would have it at the lower CFM when I replotted it. I think the re-ducting of the O/A is the only realistic, cost effective solution. We have some restrictions on where the O/A louver can be placed to complecate things.

 

[trashcanman]

Increasing the speed of the AHU will also directly increase the AHUs cfm. Will this be a problem? Also HP increases as cube of speed.

Why not consider a separate O/A fan for each AHU? That fan could be interlocked with its respective AHU. Yes, you will face operating expense of the O/A fan, but it will work.

 

[quark]

I agree with MintJulep. The reduction in total flowrate is not due to the control of return damper but because the outside air is consuming the fan static.

Check the fan total static when the return damper is fully open (as this gives you the design flowrate). Disconnect the OA duct, control the return damper to get the required flowrate of OA and again check the total static pressure of fan. If the difference in static pressures is positive then design your duct as per the available static. If the difference is negative, then select the fan for the new ducted OA system + the difference.

 

[AbbyNormal]

It is always tough I think, when you have a VFD and you want to try and have return air dampers close off and outside air dampers to open up, to try and keep the outside air contant under lower airflows when the supply blower is ramped down.

Maybe get an inline fan with a pressure independent VAV box to contol the amount of outside air you want.

Take the "V" out of HVAC and you are left with a HAC(k) job.