Basic Compression

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​Once super-heated refrigerant gas leaves the compressor it travels through the suction line to the compressor. The compressor in this example is a reciprocating compressor and the cycle can be referred to as the vapor compression cycle.
To compress the vapor, it must be put in an enclosed container, then, the container must be made smaller, forcing the molecules of the gas closer together, or, compressing them.
A few things change when you compress a gas, the volume goes down, the pressure and the temperature go up.
If I took a frictionless compressor and put 1 cubic foot of R404A in it and compressed that gas to .33 cubic foot the following would be the outcome:

​Compressing the volume in a ratio of 3 to 1 (1/.33=3), the pressure increased 110.5 psi (pound per square inch), and the temperature increased 72.5 degrees Fahrenheit!
This is a pressure ratio of 3.17 and a temperature ratio of 2.65
Considerable change in the properties for a relatively small compression ratio.
Notice the ratios are not the same, volume = 3, pressure = 3.16, and temperature = 2.65. These properties change based on the physical aspects of the individual refrigerant and will be different for every compression process.
A basic reciprocating compressor will have the piston and cylinder just like the example above except with a few more components to put work into the system.
The compressor needs a suction valve to open when gas is coming into the cylinder, and a discharge valve that opens when gas is leaving the cylinder.
The valves are reeds or spring loaded disks, reeds being more common due to cost. Below is a sketch of the example compressor with the suction and discharge reed valves.

​Reed is a flat piece of spring steel which lays flat over the suction and discharge ports. As the picture indicates, the valves open when there is a difference in pressure, they spring closed when the pressure is equal.
If you lose the suction or discharge valve, the suction and discharge pressure will run about the same.
If you lose the rings in a compressor, but not the valves, oil sump pressure spike, discharge pressure will be low.