Return Air

[henegg]

Hi
I am designing a MAU for the first time
I am trying to understand how to determine the amount of return air to supply a mixing box before the heater.

The outside winter design conditions are -15f at 70% humidity. inside design is 75f at 50% humidity.

I need to supply 10,000cfm of fresh air which is need to meet my air change rate per hour .

The room needs 1000 cfm to maintain the pressure needed.

I would like to mix some return air with the outside air to save some energy.

But want minimize the size of my supply fan. How do I determine the amount of return air to use.

Attach is a general free body diagram.


.

 

[houseman20]

Outside air and "air change rate" are not usually the same. Minimum fresh air is calculated per ASHRAE 62.1 or code and is often a function of floor area and occupancy. Since this affects size of heating significantly, you should minimize the outside air as much as you can. Air change is usually a summer cooling requirement and is equal to supply air flow. Also, by definition a make up air unit doesn't recirculate air.

 

[KiwiMace]

Student

 

[henegg]

thanks for the info.

the air change per hour ACH,in this case is dictated by the purpose of this room, not the occupancy of the room. The ach requirement is larger than the heat loss so I am using the ACH for this design.

so if I need to use 100% fresh air(finished food product area) to meet my ACH how would one determine how much return air to use. I tried using 25% of the mass flow rate of the fresh supply air. but at -15f fresh air 1200lb/min and 75f return air 400lb/min( the rates are approximate), the mixed air is at the left of the saturation line.
I assume this is not a good situation.

Please straighten me out.

where

 

[houseman20]

With
OA=10,000cfm
leakage=1,000cfm

EX = SA - RA - Leakage
RA + OA = SA

2 equations, 3 unknowns

if you want to minimize SA, set RA=0
EX = 9,000 cfm
SA = 10,000 cfm

Recirculating air won't save heating energy, so there is no mixing problem.

 

[henegg]

Thanks that helps

 

[ChasBean1]

You need 1,000 cfm supply and 10,000 cfm of outdoor air. Go with -9,000 cfm return...

Return air quantity is not something you design around. You just maximize it when conditions permit (very hot or very cold out).

For help, you might need to clarify your problem.

 

[lma1]

As ChasBean1 (Mechanical) suggested, you need to clearly define your problem. The following might help:
1. I understood that you need to supply 10,000 CFM O/A to the room. And this is based on your ACH rate required?
2. You need also 1,000 CFM S/A to pressurize the room?
3. Is the 1,000 CFM the leakage air as houseman20 (Mechanical) suggested?
4. I’ve trouble with the statement “The ach requirement is larger than the heat loss so I am using the ACH for this design.” ACH gives you an idea of what ventilation air (fresh air) is required. Heat loss or gain determines the heating or cooling (air-conditioning) supply air (S/A) required. I think you mean the ACH criterion requires more air than that based on the heating or cooling calculation and you therefore based the supply air on 10,000 S/A which is required by the ACH estimate?
5. Since S/A = O/A + R/A & your S/A = O/A, R/A must be 0? You lose 1,000 CFM thru leakage and another 9,000 thru other exhaust opening(s). So S/A = 10,000 CFM = E/A = (1,000 + 9,000) CFM.
6. No R/A is needed or desired in your case as houseman20 (Mechanical) demonstrated if we understand what you’ve been asking.