Outside Air Design
Outside Air Design
(OP)
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.





RE: Outside Air Design
RE: Outside Air Design
RE: Outside Air Design
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.
RE: Outside Air Design
RE: Outside Air Design
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.
RE: Outside Air Design
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.
RE: Outside Air Design
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.