How to Design Refrigeration Systems

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US_Iceman did a very good job with this post, let’s give it a new push!

Several design considerations are at play when the manufacturer chooses an application, US_Iceman mentioned the main one, note that in all these the torque the motor develops is not constant.

A high temperature compressor will have a smaller displacement with a larger motor provided.

It is hard to generalize so I’ll stick to reciprocating (possibly semi hermetic) compressors.

The main design objective here is high efficiency because you spend more energy and is the most widespread application but also you get the highest refrigerant flow and density therefore:

1. Compressors should minimize internal pressure drops including the valve plate because of high density and high flow. If you compare a valve plate you’ll see bigger (or more) bores.
2. Cooling the motor is easier in semi hermetics so you can divert internal flow not superheat it so much.
3. Thermal losses are of less concern because temperature differences are less.
4. Thermal stress is of less concern for the same reasons.
5. Gas re-expansion is of less concern due to low compression ratio.
6. Lubrication temperature is less critical.
7. Mechanical loads in general are less due to low compression ratio.

This also means some drawbacks if you go to the extremes of the range:

1. High suction and low condensing pressures can really overload the motor.
2. Quickly looses efficiency and capacity if suction pressure is low due to high gas re-expansion.

A medium temperature compressor will have an average compressor displacement for the size of motor fitted to the compressor.

In general these compressors extend the high temperature range improving on volumetric efficiency.

The problem is most manufacturers fit large motors in order to extend their range, sacrificing efficiency in high temperatures to have better efficiencies in the lower range.

If you compare capacity of a medium temperature compressor is generally less than a high temperature compressor from a given temperature and higher and has more capacity from that point down. This is the crossover point that would make you decide if you have to choose.

A low temperature compressor will have a larger displacement and a relatively smaller motor.

Well here the list is almost the opposite of above:

1. Internal pressure drop is of little concern due to low density low flow of refrigerant.
2. Cooling the motor is a MAYOR concern because you may not get enough refrigerant to do it well under all working conditions. You may need external cooling at some conditions.
3. Thermal losses are of big concern because temperature differences between suction and discharge are very high. So you try to separate both flows as much as you can.
4. Thermal stress is of concern you get thicker metal.
5. Gas re-expansion is a MAYOR concern due to high compression ratio.
6. Lubrication temperatures are critical and need external cooling.
7. Mechanical loads in general are high due to high compression ratio.

These compressors used at high suction can be more efficient than medium range compressor but that is because the motor can be nearly overloaded.
More capacity in the high end of its temperature range than medium range compressors due to higher volumetric efficiencies so there is also a crossover point here.

This also means some drawbacks if you go to the extremes of the range:

1. High suction can overload the motor and produce motor protector trips.
2. High discharge temperatures can limit the application severely.
3. Reliability is a MAYOR concern.

There are many more (of course) feel free to add to the list!

As I have posted elsewhere you should use P-H diagrams to look closely on where the problems of your application may be, densities, discharge temperatures, superheat, subcooling.

Use T-S diagrams if you want to study or compare efficiencies because it has to do with the area of the cycle.

Use equilibrium diagrams to see how the system will behave under varying external conditions in ambient and the cold room.

US_Iceman will gladly reply to all your posts on this!