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# Hi How to determine the return a2

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## Hi How to determine the return a

(OP)
Hi

How to determine the return air flow rate and exhaust air flow rate of AHU? OA & supply air shall be calculated. all I know return has to be same as supply and OA shall be subtracted, but I found in other designs that return is taken 2-10% less the the supply rate ,can anyone please tell explain Why?

### RE: Hi How to determine the return a

There's no situation where your return air + outdoor air won't equal your supply air, that's simple conservation of mass. I haven't seen it but maybe some designs account for some sort of leakage or changes in air density?

### RE: Hi How to determine the return a

I don't understand what you are asking. The numbers come out valid -- 7544-2200+3180 = 8524 approximately 37% OA. Based on your numbers, the capacity is sized for ~65 ft.sq. room with 15 air changes per hour.

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### RE: Hi How to determine the return a

As IRstuff said, the numbers add up. You also have to remove the exhaust airflow from the supply: RA - EA + OA = SA

The air that isn't returned will leave the room through openings to other spaces and as exfiltration.

### RE: Hi How to determine the return a

(OP)
Thanks Gents,

I would like to know the EA and RA rates, I mean I the designer came up with those numbers?
is there any rules to size the return flow or is it from the equipment data?

### RE: Hi How to determine the return a

Your drawing says it there:
It will be controlled by the CO2 sensor.
If you have an increase on occupancy, your CO2 will rise and it will trigger the opening of the motorized damper to exhaust the air (you don't want to recirculate air with excessive CO2).
At the same time, to compensate your additional extracted air your motorized damper in the fresh air side will open as well.
Not sure exactly what is your doubt.

### RE: Hi How to determine the return a

#### Quote (Miko123)

I would like to know the EA and RA rates, I mean I the designer came up with those numbers?
is there any rules to size the return flow or is it from the equipment data?

The designer is allowing a 11% of the air that is supplied into the Cafeteria to leak out (and/or to be exhausted), avoiding infiltration of non-conditioned air through exterior doors and/or windows and/or elevator doors.

He could follow a general requirement of keeping the space 10% pressurized or he could calculate how much air actually leaks out (and/or is exhausted), using an estimate volume of air per door or window.

Physically, he is using the blower of the roof package unit to achieve that slight pressurization of the Cafeteria.
The extreme of this situation would be a 100% fresh air unit with zero return: all OA is forced to leak out and/or it is removed by exhaust fans.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

### RE: Hi How to determine the return a

(OP)
Thank you all for the replies,

Lnewqban, yes the infiltration should have been taken into account. either +/- pressure, we can calculate certain amount of air to compensate.
could you please help me to calculate the infiltration rate?

### RE: Hi How to determine the return a

Infiltration happens due to wind pressure and stack effect (for tall buildings).
Buildings should blow, not suck, keeping humid hot air out.
Exhaust and leakage flow rates should be less than intake (OA) in order to maintain positive pressure (except for kitchens).

Take a look at this:

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

### RE: Hi How to determine the return a

Calculating infiltration is more art than science and depends greatly on the type and quality of construction; also a factor is the age of the building. It is also important to know that it is impossible to balance air flows to the design values perfectly. To accommodate this tolerance, it is common practice to provide 10-25% more air into a space than is exhausted to ensure the building is positive to the outside at all times.

In your scenario with the cafeteria, code requires so much OA that to not have an EF would create over-pressure problems. Such problems include whistling doors/windows, doors that never close/'heavy' doors (depending on swing), or doors that fly open.

It is likely the RA was calculated to provide about 10% positive pressure, then EA selected to match an EF's curve, finally RA recomputed to provide balance. You really need to ask your designer his thought process on how he came up with the exact numbers.

### RE: Hi How to determine the return a

dont forget 15-20 CFM of fresh air per people in the building.

### RE: Hi How to determine the return a

We usually allow about 0.05 CFM/SF or 100 CFM per door for pressurization.
Usually we want the pressurization to come from AHU/space around the cafeteria so if the cafeteria pressure is + the surrounding indoor areas would be ++ so food odors do not get transmitted to the other occupied spaces.
The air to the cafeteria should be transferred to the kitchen to provide makeup air to the kitchen hoods & dishwasher hood.
There should be control interlocks to vary the amount of outdoor air based on the hoods operating. The dishwasher hood should run all the time to avoid odor.

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