GENSET for HVAC System

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zohaibraja

Member
Location
Pakistan
Hope all of you people are fine.

Question:
HVAC company gives us a load of 900 KW(Only HVAC) for our building. Now we want to finalize the GENSET for that system. Fire pumps are currently excluded from this calculation. We use two transformer of 630KVA (Total : 1260 KVA) transformers to feed that above mentioned load. My suggestion to our company, buy a GENSET of 1500 KVA. That is enough for our system or NOt ? i dont know about load of Fire pumps so how we can estimate fire pumps. assistance in this regard really appreciated.

Thanks
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
HVAC Loads

HVAC Loads

Hope all of you people are fine.

Question:
HVAC company gives us a load of 900 KW(Only HVAC) for our building. Now we want to finalize the GENSET for that system. Fire pumps are currently excluded from this calculation. We use two transformer of 630KVA (Total : 1260 KVA) transformers to feed that above mentioned load. My suggestion to our company, buy a GENSET of 1500 KVA. That is enough for our system or NOt ? i dont know about load of Fire pumps so how we can estimate fire pumps. assistance in this regard really appreciated.

Thanks

I would insist on seeing all nameplate data for the HVAC loads to avoid anything being undersized.
 

kwired

Electron manager
Location
NE Nebraska
One important factor is what kind of surge must your source be able to deliver when starting?

Can be possible that you maybe only back up a portion of the total HVAC load, which would reduce the necessary source capacity, or even utilize load shedding to limit maximum load if you can get by with limited HVAC when other demands are high.
 

drktmplr12

Senior Member
Location
South Florida
Occupation
Electrical Engineer
Typically you need to be sure that when all loads which can run coincidentally are running, the largest motor starting kVA can be supplied by the engine-generator. Otherwise the engine can stall attempting to start the final load. If you have a single line diagram and a load calculation, you would take the demand kVA and subtract the largest motor. Then find the starting kVA of the largest motor and add it back in. Add another 10-15% for good measure.

edit: this method will always give you a generator with adequate capacity, but does not take load diversity into account. You would likely have a slightly over-sized gen due to diversity.

Another way is to let a manufacturer size it, but they might say you need a larger gen than is truly necessary. You can take their recommendation in consideration to your sizing. You don't want an severely over-sized gen because that leads to a different set of maintenance problems.

Powersuite is software available from Cummins to assist with gen set sizing.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
One important factor is what kind of surge must your source be able to deliver when starting?

Can be possible that you maybe only back up a portion of the total HVAC load, which would reduce the necessary source capacity, or even utilize load shedding to limit maximum load if you can get by with limited HVAC when other demands are high.

An important factor will be to interlock the thermostat circuits so all the A/Cs do not try to start at the same time when the generator is brought online after a power interruption. Especially if there is no interlock in the individual A/C units to allow the compressor head to bleed down before restarting. Otherwise you could get locked rotor amps on all the units simultaneously for several seconds (instead of just the shorter actual motor starting time) until the overload relays kick out.
 

kwired

Electron manager
Location
NE Nebraska
An important factor will be to interlock the thermostat circuits so all the A/Cs do not try to start at the same time when the generator is brought online after a power interruption. Especially if there is no interlock in the individual A/C units to allow the compressor head to bleed down before restarting. Otherwise you could get locked rotor amps on all the units simultaneously for several seconds (instead of just the shorter actual motor starting time) until the overload relays kick out.
That is a method of reducing the potential starting surge I mentioned;)
 
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