2 chillers, 1 generator, sizing help..

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cyriousn

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I am working for a client who wants to add a generator to power two 30-ton trane chiller units. Each chiller has a Minimum Circuit Ampacity Rating of 146A and a Max Fuse size of 200A. I figured I could feed off of a 400A Breaker in the MDP with 400A rated conductors into a 400A exterior rated ATS then hitting a 400A exterior rated fused disconnect. From there have 200A rated feeders go into each of the Chillers. Oh yeah, and the chillers are at 1Kva per ton so i was figuring around a 75kva Generator.

146 + 146 = 292

292 * 1.25 = 365

400A Rating for the equpiment listed above. Do these numbers make sense?
 
How are you starting these chillers? Across line, wye delta, VFD?

Cause I doubt 75 KVA will put a dent in this. You need additional information to determine engine/generator size. Additionally I would involve a generator vendor and let him do the calculations.
 
I found a nice lil Generac Generator Sizing Guide and used their Load Summation Method.

2 30 Ton (15x2) Chillers

Motor Load running total (minus largest motor): 30kw
Starting Load from largest cycling motor: 30kw
Total electrical load: 60kw

Residentially add 10 to 20 percent to total kw = 72kw
 
You have 2 - 30 kw chillers starting and cycling during operation. You do not have 2 - 15 kw chillers for a total of 30 kw. Assuming that is correct
My program indicates that you will need 400 kw gen set. To size this unit, it
must be able to start the first chiller and have it running while the second chiller starts. The starting load is what increases the gen size. You need to get with a gen mfg and let him size it for you. Let us know what happens.
 
cyriousn said:
I found a nice lil Generac Generator Sizing Guide and used their Load Summation Method.
2 30 Ton (15x2) Chillers
Motor Load running total (minus largest motor): 30kw
Starting Load from largest cycling motor: 30kw
Total electrical load: 60kw
Residentially add 10 to 20 percent to total kw = 72kw
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cyriousn said:
They are across the line starting. Two compressor motors each with a RLA of 56.9A and LRA of 409A.
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Your starting load is not 30 kw. You have stated that the LRA is 409 amps.
Thats 340 kva.
 
Thanks

Thanks

Bob and Brian John

Thanks for the help on this. I have left out the details that I am working with a residence with 208 3phase.

I am still trying to work through with the math on it.

The chillers are 30 ton units. Each chiller has 2 motors in each. There is already control gear on the chillers so that they both cannot start at the same time so the sequence is 1,2,3,4. So the worst case scenrio seems to be that the first three motors have already started and the fourth one kicks in. So at this point the first three will have their running load amperage of....56.9A a peice. That is 59.6*3 = 178.8 A. So when the 4th motor kicks there will be an inrush current of 409A. So the total amperage will be 178.8 + 409 = 587.8A. So to get the Generator size is 587.7*208*1.73 = 587.7*360 = 211608. Divide by 1k = 211.608 kVA. I hope I got it this time. :)

Brian, will sticking VFD's on these chillers possible reduce the generator size? Thanks again guys you are awesome.
 
A soft start would help but there are trade offs, The soft starts may cause lead to excessive THD and mess the frequency and voltage up.
 
A soft start will a bypass contactor will not increase THD whereas a VFD will definitely do. Moreovere a VFD ia an overkill for a residentila chiller application.



I would suggest sofstarter instead of a VFD, and then a 150 Kva 208 volts 3 phase generator would be sufficient.



Cheers.
 
Last edited:
cyriousn said:
Bob and Brian John

Thanks for the help on this. I have left out the details that I am working with a residence with 208 3phase.

I am still trying to work through with the math on it.

The chillers are 30 ton units. Each chiller has 2 motors in each. There is already control gear on the chillers so that they both cannot start at the same time so the sequence is 1,2,3,4. So the worst case scenrio seems to be that the first three motors have already started and the fourth one kicks in. So at this point the first three will have their running load amperage of....56.9A a peice. That is 59.6*3 = 178.8 A. So when the 4th motor kicks there will be an inrush current of 409A. So the total amperage will be 178.8 + 409 = 587.8A. So to get the Generator size is 587.7*208*1.73 = 587.7*360 = 211608. Divide by 1k = 211.608 kVA. I hope I got it this time. :)

Brian, will sticking VFD's on these chillers possible reduce the generator size? Thanks again guys you are awesome.
Click to expand...

Generally gensets are capable to upstand starting currents up to 3 times their rated capacity depending on the exitation method. Ask the manufacturer about it to avoid unnecesary oversizing
 
Back to it.....

Back to it.....

The lower end of the operting voltage range is 180V. Would this mean that the lowest voltage drop the chiller could handle would be 13.5%? 1-(180/208). If that is the case then the smallest Gen the manufacturer recommended was a 205kW. Also what's the deal with Diesel being able to handle voltage dips and frequency dips better then the same size kW gas? And can someone please explain to me what a frequency dip is and what it can do to equipment if sever enough? Thanks in advance.
 
cyriousn said:
I asked a generator mfg and his program sized it out at 200kw with soft starters.
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Obviously the manufacturer of the Genset gave you no credit for the soft-start.

The harminic content on startup of the SSRV starters can mess up the voltage regulator of the generator, so you have to clear that up with the Genset manufacturer. You also have to talk to the chiller manufacturer to see what minimum starting current is necessary to turn the chiller compressors over and how long the run-in of the chiller compressor is expected to be. You have to give the SSRV starter manufacturer the current limit ADN the expected frequency of starts ofr the compressor so they can adequaltely size it.

As a guess; a 125kVA unit should be able to handle the chillers with SSRV's and cycling interlock between the two compressors.
 
chaterpilar said:
A soft start will a bypass contactor will not increase THD whereas a VFD will definitely do. Moreovere a VFD ia an overkill for a residentila chiller application.

I would suggest sofstarter instead of a VFD, and then a 150 Kva 208 volts 3 phase generator would be sufficient.

Cheers.
Click to expand...

I agree with your ASD comments.

The harmonic interference from the SSRV's will exist, even with the bypass contactors, until the SCR's are bypassed, eg. during the startup when the voltage regulation is the most crucial.
 
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