Jon,
Effectively, I am building an old fashioned ice box and these plates will be
atached to the walls of the well insulated box. The air flow inside the box
would be convective, just like in your freezer at home. All the plates would
be in parallel. These cold plates are holding plates and I suspect they are
filled with brine, as they were probably used as a refrigerator. In order to
use these plates in a freezer efficiently, the solution will have to be
changed to one that changes state at a lower temperature. You stated that
the expansion valve must have the same freon on both sides of the diaphragm.
I hadn't thought of that, but it makes sense. I know I must use the most
appropriate freon for the task, which means that I will have to buy new
expansion valves to match that choice. Good info. I understand the
difficulty in calculating the heat load, but I don't think the ice box
itself has much to do with it. I would think the volume of the cooled fluid
medium is the 90% answer. The rate that medium has in warming up is not very
relevent, because the system will not be operating then. It is also
unreasonable to think that these holding plates will freeze solid at he same
time, but if each has its own control valve it shoudn't matter.
Steve

"Jon Elson" wrote in message
...
Steve Lusardi wrote:
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.
Yes, they should. The orifice size should be able to be looked up from
the part number. Also, thermostatic expansion valves have to have the
same refrigerant in the sensing bulb as is used in the system.
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".
What are you cooling? Just the plates? A tank of brine? Making a block
of ice?

It gets messy. What is the starting temp of the plates when you begin to
cool them? Will you be cooling only one at a time? Would you be keeping
one or more plates cold while starting the cool-down process on a warm
plate? That's the worst-case scenario. To calculate the heat load, you
need to know the surface area of the tubing, and the thermal conductivity
of the heat source to the cold plate. If you are hanging the plates in
mid air and cooling the air in a room, the heat flow is very low. If the
cold plates are attached to metal walls or immersed in circulating brine,
the heat flow is maybe 100x higher. You really aren't telling us enough
to give you any help.

Anyway, to get to -40 C (or F) you need an evaporator pressure (suction,
low-side) of 105 kPa absolute, or just barely above atmospheric pressure.
This will cut the performance of the evaporator, as well as making the
compressor work a lot harder.

Jon