Jon Elson fired this volley in
:


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

Jon, I agree with most of what you said, but it's not true about
cycling the compressor. That's what accumulators (high liquid and low
gas) are for. Think about the automotive CCOT systems. They don't
have expansion valves at all, but still cycle the compressor, based
upon the high-side pressure at the accumulator.


Now, I'll agree that you may need some pretty large low-side capacity
to keep the low-side pressure low enough, but it certainly can be - is
- done that way on many systems.

It's not a simple setup in terms of design criteria, but it's simple
mechanically.

I wholeheartedly agree that -40 is a tough spec.

LLoyd