Jon,
Good info. The cold plates I have bought are used units and they all have
the expansion valves with them. I have assumed they also have integral spray
orifices. I suppose I could apply compressed air to determine approximate
flow rates. How else would I test these, as they are not able to be
disassembled? The cold plates are used marine units and I want to use them
for a freezer, not a refrigerator, hence the -40 requirement. I am aware of
the requirement to change the medium in the cold plate itself in order to
reduce the temperature of the state change. The goal is to freeze the plates
down once a day or less when in use. How do I estimate the heat load? The
cold plates are aproximately 24" x 18" x 4".
Steve

"Jon Elson" wrote in message
...


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