Exhausting heat from electrical room - check my calcs?

[nuuvox000]

I'm trying to figure out how much exhaust CFM I would need to provide to cool an electrical room with transfer air from surrounding spaces. It has a 115 kVA transformer inside; it looks like these are typically about 95% efficient so 5% would be given off as heat in the room. So I'm looking at about 5.75 kW, which is about 19,600 BTU/hr. I'd like it to turn on at 90 °F and turn off at 85 °F. Air would be coming from the corridors at approximately 75 °F.

My equation for sensible gain is CFM*1.1*▲T*AF

CFM = cubic feet per minute
1.1 = constant in BTUh/CFM-°F
▲T = temperature difference
AF = altitude factor (0.85836 at my altitude)

So CFM = q_s/(AF*1.1*▲T)
19600/(0.85836*1.1*(90-75)) = 1385 CFM

Am I right in using this equation? Thanks in advance.

 

[LittleInch]

You wouldn't get down to 85F though.

I calculated about 1800 CFM.

That's a lot of air....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[nuuvox000]

Thanks for the response, I'd love to know how you calculated that if you have time. Yes, and there are three of these rooms so it's 3 times a lot of air. My plan is to recommend mini splits for these spaces but I just wanted to make sure I had calcs to back it up too.

 

[Snickster]

The equation you are using is correct. I don't know about the altitude correction factor but the basic equation is correct.

I have a spreadsheet attached from the previous company I worked for that estimates heat loads from electrical equipment. Attached is spreadsheet for transformers. For 112.5 kVA transformer heat load is 11,604.2 btu/hr.

Based on this and the equation assuming the correct altitude correction factor I calculate the air flow required = 820 CFM for 90 F resulting temperature.

The actual maximum design temperature for electrical equipment is 104 F, based on this the actual air flow required = 420 CFM approx.

 

[Snickster]

Here is the file.

 

[nuuvox000]

Thanks, Snickster! I think you're right that I could take up the temperature in the room. It's still a lot of air and we're going to be doing a heat recovery VRF system anyway, so I'm going to push them to fan coils unless they really push back and want exhaust.