Uncertain about cooling req.

[yoleven]

Hi.
I need to replace an air conditioning unit for an MCC room. Currently there is a 10 ton unit and it cools sufficiently. The fins are corroding, etc.
I have calculated the area of the room at approx. 23 ft2
I have transformers with the following kVA rating..
45, 1250, 1250, 15, 50, 30, 45, 1250, 1250, 150.

The heat output for no load on the transformers I got from a table at 14000 watts.
I guessed the change in temp. to be 20 deg. F.
Using the equation:
(watts * 3.413) + (1.25 * 23ft2 * 20deg F) = 48357 Btu/hr
48357/12000 = 4 tons of cooling.

If I do the same calculation with a heat loss with a 50% load I get 13 tons of cooling.

What am I doing wrong. This doesn't seem correct because a 10 ton unit is keeping the room cool.

Thank you

 

[tys90]

First, are you sure this is a 23 sq. ft. room? Awfully small for all those transformers...Second, I would try to find manufacturer provided heat loss tables at 100% load. From my guesstimation you need a 15-ton unit at a minimum, maybe 20-ton unit. I'm not sure what you are calculating with your equation there, if you just take your watts of heat loss, multiply it by 3.413 to convert to BTU/hr you will have your load.

 

[MintJulep]

Currently there is a 10 ton unit and it cools sufficiently.

So your motivation to change from what works is...

 

[yoleven]

I originally wanted to see if the 10 ton was sufficient. It cools sufficiently but I wanted to make sure it was neither over sized nor undersized and over working.
When I make a recommendation for replacement, I need to justify it.
The more I looked into the heat load, the more it appears that the 10 ton isn't sufficient but subjectively it seems to work

 

[DRWeig]

Can you hook up a wattmeter to the 10-ton unit and datalog it for a while? If you can take steps to create maximum-electrical-load through the equipment in the room, you'll be able to see how hard the 10-Ton unit is working. If the room has an outside wall or is othewise adjacent to unconditioned space, you may want to consider calculating added load due to conduction/radiation/infiltration. If not, the internal gains are all you need other than ventilation that the unit provides already.

Good on ya,

Goober Dave

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[SAK9]

It appears your transformers are loaded approx 50% or less.With total KVA at 5335 at 50% loading approx heat rejection is around 40 KW or 11.4 tons.Air conditioning is preferred only when transformers are loaded beyond 75%.You have not mentioned what temperature is being maintained presently(that will decide what you are getting out of the unit).If the power consumption is not likely to go up in the near future, a 10 tonner seems adequate.If the loading really goes up,install an additional unit.That will introduce some redundancy as well.

 

[317069]

you said (I guessed the change in temp. to be 20 deg. F.Using the equationwatts * 3.413) + (1.25 * 23ft2 * 20deg F) = 48357 Btu/hr48357/12000 = 4 tons of cooling.)
what does 1.25 mean?