Copressor capacity/evaporation capacity - calculation

[czamara2010]

Hello Guys,

I created this thread because I am really curious about compressor cooling capacity and I cannot find an answer in the Internet, meybe you will clarify this issue...

Recently I used Copeland selection software to calculated ZS21k4E compressor capacity.
The program showed 2.97 kW at evap. temp. -25 C and cond. temp. 40 C on R404A then decided to check it manually.

I assuemd compressor capacity as:

c=q*NRE

where
c= compressor capacity (kW)
q= refrigerant circulated (kg/s)
NRE= net refrigeration effect (kJ/kg)
NRE= hi-he (where hi- enthalpy of vapour leaving evaporator, he - enthalpy of vapour entering evaporator)
so if the hi of R404 at -15 C is 357,94 kJ/kg and he at -25 C is 352,33 kJ/kg,
NRE is (357,94-352,33)= 5,61 kJ/kg
q=is constant and is 22,6 g/s -> 0,0226 kg/s
So, compressor cooling capacity is
c=0,0226*5,61=0,126 kW
0,126 kW and 2,97 kW from where this difference?
I know that the above equation doesn't contain condensing issue, but how to calculate compressor capacity in a proper way?


Regards
Bartlomiej

[AlexG]

What do you mean q=const? It depends on evap T and suction superheat.
Cheers

[sandybapat]

Every compressor has fixed capacity while operating at a particular speed. That is it's swept volume.

The mass flow handled by compressor varies with evaporation temperature and the condition of gas leaving the evaporator (saturated or superheated).

You have specified the refrigeration effect correctly. However the inlet point may vary depending up on the sub cooling of refrigerant before expansion valve.

Therefore from swept volume you have to arrive at mass flow handed and then multiply by RE you will get refrigeration capacity of compressor. In practice you will have to consider the volumetric efficiency of compressor which depends upon the clearance volume. in case of piston comp and on probable leak back in case of screw compressor.