Compressor selection from process requirement

[alchara]

This is my situation:

I have the 10 equation coefficients that represent compressor performance (C1,C2,...,C10) for the polynomial equation specified in the ANSI/ARI Standard 540-1999 (Positive Displacement Refrigerant Compressors and Compressor Units) for each compressor.

X=C1+C2*(S)+C3*(D)+C4*(S²)+CF*(S*D)+C6*(D²)+C7*(S³)+C8*(D*S²)+C9*(S*D²)+C10*(D³)

where:

C=Equation Coefficients
S=Suction Dew Point Temperature
D=Discharge Dew Point Temperature

I also have the process requirement variables:

Refrigerant
Cooling (Required) Capacity
Evaporator Temperature
Condenser Temperature
Liquid Subcooling
Suction Gas Temperature or Suction Gas Superheat

My question is how do I go from the process requirements to the compressor selection. What formula/process is needed?

I know this is usually done with a compressor selection program, but this is not an option in my case.

Thank you for any help that you can give me.

 

[ChrisConley]

I hate to ask but... Why can't you use compressor selection software? Statements like that always make me think I'm doing someone's homework. If I'm wrong, I humbly apologize.

 

[alchara]

Chris et al,

It would be nice to be back in school, but no, this is a real-life situation.

We have received a beta compressor selection utility and it does not seem to give us the correct results.

To check the results I wanted to know how to select the compressor by hand.

I extracted the compressors' polynomial coefficients from the program's database.
I went to the ARI web page and downloaded the pertinent standard for the polynomial equation.

I do not know how to compare the process requirements and the polynomial coefficients with the ARI Standard polynomial equation.

Thanks for any light that you can shed on my situation.

 

[alchara]

Ok, this is what I have worked out so far, please correct me if I have gone down the wrong path.

1. Input Refrigerant
2. Input Required Capacity
3. Input Evaporator Temperature & Condenser Temperature
4. Input Evap. Temp. & Cond. Temp. into the polynomial equ.
5. Goto database of compressor polynomial coefficients and plug them into the polynomial equ.
6. Select the compressor that gives the closest calculated capacity (from polynomial equ.) to the inputed Required Capacity.

I get this far, but I think my problem is that I don't know how to factor in the suction gas superheat and/or liquid subcooling. Any help here would be great.

Thank you in advance.

 

[alchara]

Ok, this is what I have come up with:

When calculating the compressor's cooling capacity, you need the suction and discharge temperatures.

This means that you cannot necessarily use the evaporator and condenser temperatures.

If there is subcooling or superheat in the system you have to take them into consideration.

Suction Temperaure = Evaporator Temperature + Superheat
Discharge Temperature = Condenser Temperature - Subcooling

Now you just have to plug the suction and discharge temperatures along with the polynomial coefficients into the compressor polynomial equations.

Select a compressor that has a cooling capacity >= your required capacity.

My question now is, is this a valid method?

Any help or comments would be great.

Thanks in advance.