Equipment Heat Dissipation

[Zeroman]

I was told a while back from this very useful forum that you shouldn't take the tag value of a piece of equipments heat dissipation when calculating the total cooling capcatiy of an AHU. I was told to that the air handling unit would never have to handle the full 100% load, instead take 60% of the tag heat dissipation value. Now I try to tell my supervisors that "the guys in this forum" have all the answers (I consider you very credible sources, considering I am soo green). Can you give me a better reason why I choose to take 60% of a switchgears (or any piece of equipment) heat dissipation instead of taking 100%?

Thanks a lot.

 

[grim900]

dnmb

Not sure specifically about switchgear but as I understand it, if a piece of equipment along with any associated motors is housed within a room you are tryin to condition, then eventually all the energy drawn by the motor will be converted to heat. Depending on motor efficiency, approx 15% of the energy would be lost directly as heat. The remaining energy would be used to power the machinery, however this in turn would end up as heat through friction etc. Therefore, I would take the 100% figure providing the motor is within the space, and the machine itself is within the space.

Having said that, the nameplate rating of the motor will be somewhat higher than the actual power absorbed.This second figure would give you a more accurate figure for the heat dissipated from the motor / equipment.Maybe this is where the 60% figure has come from but this is low and would be determined by the manufacturers motor selection.

As an example we have a customer who specifies that air handling unit motors must be selected with 30% higher rating than the absorbed power. Even this would mean that you would need to allow for 70% of the rated load and I would expect that with most equipment this figure would be higher.

Sorry if I've rambled - its a bad habit, hard to break.

 

[lilliput1]

You should use the full tag heat dissipation value plus allowance for future expansion. Some switchgear records the highest temperature it has experience. Usually the maximum allowable is 104°F. Include the lights in the room & other heat producing equipment. Assume equivalent solar/outdoor temperature is 130°F and include conduction through walls & roof. If you can locate exhaust pickup directly over the heat producing equipment, you would be able to reduce effective heat gain to the room.