A/C System Design - Fluctuating Load 3

[DoggyDog69]

Hi All,

Sorry if this is a silly question, but I'm an electrical person with a problem I've never had to think about before....

We have an electrical control room that has a large heat load (44kW) for 30 seconds every hour. The question is; if we have an A/C system rated at (say) 8kW sensible load, will it be handle to cope with this large heat load? The A/C will be designed for 20C, but the electrical equipment can handle up to 35C. I assume that the temperature will rise, but what will this rise to?

Or do we just add the "average" heat load to the other heat loads of the room?

Thanks in advance for your reply.

Cheers,
DoggyDog.

 

[SAK9]

That is only 0.36kw in an hour and rather insignificant as a percentage of the air conditioner's cooling capacity.Temperature rise caused by this load would depend on the volume of air around it ie.if the heat rejection happens within a closed space,temperatue rise will be inversely proportional to the enclosure volume.

0.36kw = m.cp.delta T of air where delta T is the temperature rise,cp specific heat of air and m mass of air in the enclosure volume.If the temperature rise is too high,increase m either by increasing enclosure volume or moving air using a cooling fan.

 

[KiwiMace]

Yes, you can generally just apply the average heat load for such a short cycle time. Use SAK9's formula to determine the influence on your space, but this is the equivalent having a couple of computers running continuously and I doubt there is going to be a large effect on anything other than a cupboard.

 

[EnergyProfessional]

I think you'll be fine. You also know that if your equipment is rated for 35°C, it will survive 50°F for 30 seconds.

using the air volume (and rack heat capacity etc.) likely isn't worth of doing since you can't assume an average temperature within that 30 second window. the heat-generating equipment will be 80°C, and the air 5 feet away still will be almost the same temperature as before unless you have a good circulation fan running.

What you could do is to add a fan that turns on whenever that device turns on (if you have room and can interlock it).

the AC unit won't help you at all for the 30 seconds. I doubt it will start up fast enough. first the thermostat needs to heat up, then it has some PID loop, then the compressor starts and needs to build up pressure.... it will take 5 minutes till you have really cold air coming out of that unit. A fan dispersing the heat away from your equipment would be much better.

 

[DoggyDog69]

Thanks for all of the replies!