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I got sent to a job this week to look at a cascade system, I know how it works and everything but I cant understand the reason behind compressing the gas twice. I understand about re-expansion of gas and everything. Can someone explain it in simple terms or point into the direction of where to find the info. hope this is posted in the right section. Cheers chris.
You have to separate which type of cascade system you are asking about.
A mixed gas system called auto-cascade is different than a cascade system using two individual refrigerants where each is in a separate circuit.
The later is simply two refrigeration systems, one cooling the condenser of the other lower temperature system.
Twice compressing? Isn't it possible that you've seen a 2-stage compressor and not a cascade?
yeah thats what I saw it is a bitzer 6 cylinder 2 stage compressor. With the suction from the plate heat exchanger to the top of 4 pistons then another pipe going to the actual suction of the compressor which was compressed again by the 2 remaining pistons and that went off to the oil sep and condenser. It was explained to me as a cascade or two stage system.
Details of the compressor here....click to open the pdf.
http://www.bitzer.de/eng/product/lis...4/dcode/KP-150
Cascade systems and two stage systems are completely different in basic operation.
Although, it's not to say someone did not get creative and try something new either....
where they using vapour injection ?
2 stage compression basicly takes advantage of a twice as larger compression ratio, by compressing already compressed gases on the first stage. The resulting pressure lift is twice as much (or almost twice as much).
Booster systems are approximately the same, except that the two compressions are done by two compressors, discharge of one goes into the suction of the next one.
Cascade systems are made up of two or more refrigerant circuits, where the evaporator of the lower stage is used to condense refrigerant of the higher stage. The resulting lift is divided into two smaller lifts. Typically, these days the lowest stage is done with CO2 subcritical systems.
a lot of times (especially with low temp R22) single stage compression can lead to massive discharge temps which can in turn lead to breaking down of the oil that is carried over into the discharge lines. 2 stage compression offers a chance to desuperheat the discharge gas from the first stage thereby lowering the overall discharge temp and prolonging compressor life.