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bersaga
21-01-2003, 09:42 AM
Need some views on condensor selection :

The system is using compound 2-stage compressors ( T evap = -35 deg and Tcond = 40 deg C ) Its an NH3 system. (Ambient is 35 deg C) Calculated heatload is 160 kW. Thinking of using two compressors - MYCOM N62WB that gives a capacity of 92.4kW and absorbed power of 54.4kW each at the said operating conditions. In order to cater for pulldown load and defrost cycles, I have selected a water-cooled condensor of about 600kW .

Normally, I would look at the compressor operating envelope and check both the refrigerating capacity and the absorbed power at the highest evaporating temperature - in the case of a Bitzer 2-stage , it is -20deg C - sum both and select the condensor accordingly higher.

MYCOM catalogues or software do not give operating envelopes. Taking pulldown load as 3 times the normal operating load gives the following : ( 92.4 + 54.4 ) x 3 + ( 92.4 + 54.4) = 587.2 kW ( select 600 kW)

Is the condensor oversized ? Learned views deeply appreciated!:)

Andy
21-01-2003, 09:16 PM
Hi, bersaga:)

in the case of a Bitzer 2-stage , it is -20deg C - sum both and select the condensor accordingly higher.
That is how I would select any compressor at -35 Deg C evaporation. A similar Grasso will have the capacity of 180kW at -20 deg C and an absorbed power of 70kW giving a required THR (Total Heat Rejection) of 250kW each. Allowing two compressors your condenser should be sized at about 500kW with the appropiate TD. You are a little big, but better safe than sorry.
The next part is restriction of compressor and evaporator capacity to keep the compressor within these bands. Iwould hold off the evap fans to -25 deg C witha differential on the pressure switch of 5 K (may require an electronic pressure switch ffor such a small diff). Then I would fit an other pressure switch to restrict compressor loading to say 50% until the pressure is below -27 (same diff). This will be essential if you size your drive motor at the -20 deg C evap as you will overload the motor if you allow it to go outside the operating envelope.
Hope this helps, Regards. Andy.

bersaga
22-01-2003, 11:12 AM
Thanks Andy.

I think it's oversized too!

Is MYCOM's operating envelope similar to Grasso's i.e. does pulldown evaporating temperature begin at -20 deg C for all 2-stage compound compressors ?

Using -20 deg C for the MYCOM, the refrigerating load becomes 188.7kW and the absorbed power becomes 72.2kW. This totalled is 260.9 say 261 kW. And for both , it becomes 522kW ; so 600kW is still OK , huh ?

( if you're familiar with MYCOM , the selected model is N62WB)

Thanks for the help!

Bernard

Andy
22-01-2003, 10:40 PM
Hi, Bersaga:)
-20 deg C is a rule of thumb used by most manufactures, Grasso does not state that but generally it is a good idea to select on this. I have a friend who works for a commercial refrigeration wholesaler (condensing units and such like) who uses -10 deg C for condenser selection, but the equipment selected has no capacity reduction by unloading so this counteracts this.
I might go for a little smaller condenser, but if theirs little difference on price I would use the same condenser with a lower condensing temperature (and hence smaller dt) and go for the effeciency thing.
Regards. Andy.

bersaga
23-01-2003, 01:17 AM
Thanks for the advice , Andy