Steve Lusardi wrote:
I wish to use a single large compressor to service 3 cold plates and 3 air
conditioning evaporators, but I am not a refrigeration expert, so if my
thinking is incorrect please let me know. Each one of these (users) will
have their own mechanical temperature controlled evaporator valve. I assume
the compressor should be cycled by head pressure with an overtemp sensor for
safety. I would like to use a salt water cooled condensor on the high
pressure side and a freon gas that changes state around -40 C. Please
advise. Do I need an accumulator of some description on either side? If so,
what? Each one of the users have 3/8" in/out tubes. How do I size the
compressor? Any tips or books on this subject would be welcome.
Steve



You can't realy cycle the compressor off in an application like this.
When the compressor is off, the liquid in the evaporator continues to
evaporate, and very quickly the low-side pressure rises. Then, ALL the
evaporators stop working. With thermostatically-controlled metering
valves regulating the temperature of the separate evaporators, you can
probably run the compressor all the time (except for fault conditions)
and as long as the accumulator has sufficient volume, it should all
regulate itself pretty well. The compressor will unload when there's
little gas coming in. Are you really trying to achieve -40 (C or F,
doesn't matter) with a single stage? That is quite pushing it. R-22
will do it, but at low pressure, so a larger compressor and evaporators
will be needed.

You can't size the compressor until you know the heat flow. Knowing the
tube size is nearly worthless. You need orifices in the expansion
valves to prevent slugging the liquid in the evaporator all into the
compressor at once. These set the maximum capacity per evaporator.

Jon