# Air Mixture Calculation

1. Apr 2, 2013

### yeswan

Hi all,

I have a question here. If I have a room with size of 3m x 7m x 3.5m with equipment of heat dissipation of 4kW. A blower fan is installed in the room to draw in the ambient air into the room to reduce the room temperature. At extreme condition the ambient temperature is around 33C. What is the calculation of the maximum air flow is if we want to maintain the room temperature below 36C?

Thanks.

2. Apr 2, 2013

### Staff: Mentor

Welcome to physics forums.

You need to make sure that the air in the room is well mixed. Then,

$WC_p(T - T_{amb}) = 4$ kilowatts

3. Apr 3, 2013

### yeswan

By the way, how to consider the room is well mixed?

And for your formula, what is the W and Cp stand for?

Since this is related to a room volume, doesn't it the room volume have to be considered as well??

4. Apr 3, 2013

### Staff: Mentor

Well, you have a fan blowing air in, so that does some mixing, but the room is pretty big. Maybe another fan would help. You also need to provide an opening where the air can exit the room, and, by supplying the mixing, you want to make sure that the intake air does not channel directly to the exhaust fan so that it avoids mixing with the other air in the room. You can check to see if more mixing is needed by moving a thermometer around the room.

In the formula, W is the mass rate of flow of air through the fan and exit port, and Cp is the heat capacity of air.

The room volume only has to be considered if you are trying to estimate how long it takes for the system to reach steady state. That amount of time will be on the order of the room volume divided by the volume rate of flow of air through the room.

5. Apr 4, 2013

### yeswan

Mmm... What is the unit for heat capacity? How do we know that the heat capacity is? Is it get from Psychometric chart or there actually is a constant?

If we also want to calculate the time to steady the room temperature, what is the formula like?

Thanks.

6. Apr 4, 2013

### Staff: Mentor

It is close enough to constant. You can google specific heat capacity of air...

7. Apr 4, 2013

### Staff: Mentor

I'm beginning to get the feeling that you are not so much interested in learning Physics as you are in getting some free consultation on a specific system you are dealing with. If you want some consultation, many of us can accommodate you, but not for free.