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Old 12-30-14, 05:45 PM   #32
jeff5may
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There are way too many differences between an internal combustion engine and a refrigeration compressor to make sense of anything in this topic, so I'll not even open that box here. But yes, the simple ones of each type do revolve and have constant displacement.

With respect to fridgie compressors, they are generally optimized to work with a single specific refrigerant, within a relatively narrow range of temperatures and pressures. They are also optimized to operate at relatively low rpm's. If you push the conditions out of the design "sweet spot", the compressor usually suffers.

The CR factors in here due to the fact that this is a closed system, with a relatively constant amount of refrigerant in circulation. I say relatively because all phase change systems have some sort of liquid receiver or suction accumulator in them somewhere. Without some space to "stash" extra liquid refrigerant in the system, the compressor would only operate within its design conditions over a very small range. In smaller rigs, this extra space is in the muffler (mini-accumulator) and the crankcase sump. This small buffer space doesn't affect pressures much, so the compressor is at the mercy of the delta pressure between the heat exchangers.

The saturation pressures of each heat exchanger follow the secondary side temperature. Let's talk about r-22 or propane here. In heating mode, at 60 degF outside, the most pressure the evaporator can see is around 100 psig, and normal saturation pressure will be around 80 psig or less. at 75 degrees inside, the least pressure the condenser can see is 132 psig, and normally pressure will be near 200 psig. This is less compression ratio than the compressor is rated to move, so it has a light load and works at high efficiency.

Suddenly, the temperature drops to 40 degrees outside. The evaporator follows this temperature drop and saturation pressure drops to 50 psig. With 2 atmospheres less worth of suction pressure, the compressor can no longer maintain 200 psig worth of discharge pressure, due to its limited displacement. At first, the CR will be high, due to the reduced suction pressure. As the compressor falls behind, discharge pressure will bleed off and the condenser will cool due to the reduced mass flow. The system will balance at a lower condensing pressure and CR, due to the constant volume displacement of the compressor.

This is one thing I had trouble wrapping my head around at first: isn't cooler gas more dense than hotter gas? The answer is yes, at the same pressure. But as far as the compressor is concerned, the pressure swings induced by the heat exchangers are many orders of magnitude higher in terms of gas density. Being 20 degrees cooler will make the same mass of gas 0.005% heavier but it can only exist at 30 less psi in the evaporator. Less intake (suction) pressure, same displacement = less mass flow and lower exhaust (discharge) pressure.

Last edited by jeff5may; 03-13-16 at 03:32 PM..
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