Selectiom compressor

[19730206]

Hello experts
Let me ask you some questions.

I need choose compressor set for 3 low temperature island showcase (evaporation temperature is -35 C).
I have evaporator cooling capasity 2500W C for each island showcase.
Is it right, if I make next:
2500+2500+2500=7500W (total cooling capasity)
7500+15%= 8625W ( cooling capasity reserve)
Then I choose compressor with cooling capasity 8625W
(evaporation temperature is -35 C, condensing temperature is +45 C)?
What should be:
useful superheat ?
liquid subcooling ?
suction gas temperature ?

Single or multiple compressor set will better for my purpose?

Thanks for any advice

 

[sterl]

Useful Superheat is your condition at the exit of the evaporator...Excess of temperature over the saturated state of the refrigerant. Providing you are using a TXV for liquid management the steady state number should be around 6-7 Deg. C....At the compressor superheat will be considerably higher. If the compressor is a long way from the cases, the actual suction temperature tends to be higher and the suction pressure tends to be lower. Exactly what it will be at the entry to the compressor has a lot to do with how big the suction pipe is, how it is insulated, whether or not it needs to accomodate rise and turned down flows and so on.

Multiple display cases are not often connected to a single compressor, and oil (lubricant) return is a big consideration...Particularly at -35 C E.T. The piping and lubricant management involve a number of detail considerations if the duty cycle cannot be considered steay state.
For a single stage compressor with no Low Side to High Side heat exchange the Liquid Subcooling for design purposes will be 5-7 Deg. C., maybe a little more depending on how you select your condenser.

We design very large systems for these temperatures and flows...But each one has its own design criteria and is devised to suit the installation and operating situation. Would not undertake this without specific experience and detailed understanding of the operating and installation parameters.