Cost Saving for Screw Ammonia Chillers

[aida2011]

Hi all,

I have several ammonia chillers. Each chiller has a COP of 5.35, of course this is based on full load.

The temperatures (return and supply) are 11 deg.C/5 deg. C. The rated capacity is 1500 kWR with a drive power of 450 kW (COP was based on 1500 kWR and 280 kW power). In actual operation, the load (for the screw compressor) indicated on the panel was 495 A at 1554 kWR, and since the maximum rated amp is 790 A (no VSD), the motor was run at 63% and the chiller was run at 100% based on load.

I have three questions:

1. I have another NEW ammonia chiller (1500 kWR, 450 kW with VSD) which is not yet run, and we intend to put a new load of 560 kWR from a new plant. There is a centrifugal R134a chiller (old) which is run at 478 kWR. Is it going to be a cost saving if I combine these two loads (478 kWR + 560 kWR = 1038 kWR) and put in the new ammonia chiller (1500 kWR)? How do you calculate the saving?

2. 5.35 is the COP at 100% load, how to know the COP (or Integrated part load efficiency) at 75% and 50% load?

3. I have a cooling tower with a capacity of 800 ton/h. It supplies condensing water to the chillers at 28 deg. C and the return temperature is 32 deg. C. If I were to increase the supply temperature from 28 deg. C to 32 deg.C (for cost saving purpose), will the chillers do extra work resulting in more power required by the chillers.

Thanks for your kind attentions.

Best regards

Hisham

 

[EnergyProfessional]

If you have sufficient data, you could run an energy simulation. I've never used the chiller plan analysis in Trane Trace, but would think that would help.

My gut feeling is it would be good to hook up that new chiller, why would yo have bought it if you now think about if you actually use it?

 

[aida2011]

Thanks HerrKaLeun for your response.

Hooking up with the new chiller was my initiative, but the management is not convinced that combining the load would result in substantial savings. They asked me to prove if load (for the new chiller) the increase in power consumption if increased the load from 560 kWR to 1038 kWR results in substantial saving.

FYI, I have calculated the load of running the old chillers (478 kWR) due to actual running condition is 240 kW, based on motor amp for cooling towers fans and pumps, chiller's compressor (centrifugal), and primary and secondary pumps.

Thanks.

 

[EnergyProfessional]

Assuming management already approved the purchase of the new chiller, what is holding you up from installing? You could have the old ones as spare chillers if they really are in love with old chillers (not sure how old). How much exactly you save you need an energy simulation for. but think about this for thenew chiller:
- more reliable
- likley much more efficient at part load due to VSD
- likley more efficient at full load since it is newer

 

[Drazen]

in general you could say your new chiller will have better efficiency running on say 60% of nominal load than on 30%. the only way to prove that is to obtain part-load efficiency from manufacturer.

that also answers your second question. mere cop itself is more or less useless, it applies only to referent conditions which can occur be it 10% or 1% of overall usage time.

you can get combined or part load data only if manufacturers provide it. as is is still not mandatory, only manufacturers of the most efficient equipment are glad to provide such data, the others are prone to hiding it of obvious reasons.

for your last question, increasing supply temp from tower will reduce chiller capacity. if chiller can still cover consumption with reduced capacity, they might use the same electrical power, but you will have savings on tower's side.