Supercritical:
There is nothing “supercritical” about a typical CO2 mechanical refrigeration process used for creating a refrigeration temperature. I designed, fabricated, installed, and operated CO2 systems for many years – most of them had a refrigeration requirement for storing, handling, and transporting the liquefied CO2 at low saturation temperatures (normally 250 psig & -8 oF). However, note that I use the word “typical” for refrigerant condensers working with the refrigerant condensed at ambient (70 – 90 oF) temperatures.
You ask for help in the heat transfer and pressure drop characteristics of a mirco-channel gas cooler within an automobile air conditioning system using CO2. Therefore, since you haven’t described your refrigeration cycle (conditions at the compressor suction, the condenser, and the evaporator) I have to assume all these things as I conceive them for an A/C system:
Condenser outlet = 1,200 psig and 115 oF (supercritical state);
Evaporator outlet = 490 psig and 32 oF (saturated state);
Compressor suction = 487 psig and 37 oF (superheated vapor).
The above conditions require a compression ratio in the compressor of 2.42, which is OK and a good design for a reciprocating compressor. However, note that the absolute values of the suction and discharge pressures are relatively high and demand a very heavy and robust compressor cylinder, piston, and valves. The tubing using will be heavy gauge.
I have assumed an air-cooled condenser and therefore, can only condense at approximately 115 oF. The necessary Liquid CO2 receiver will require a pressure vessel with an MAWP of approximately 1,500 psig – a relatively heavy steel vessel.
The heat transfer in the air-cooled condenser and the evaporator will not be a difficult or challenging task. Both exchangers will, of course, have the CO2 in the tube side (because of the relatively high pressures) and the tubing employed will have to be of a heavy gauge. The same robust quality will also apply to the expansion device or valve.
The expansion device will carry out an isenthalpic expansion that can be easily followed on any Mollier Diagram or a CO2 database. CO2 in the pressure range I have described will behave as any other refrigerant – as long as you stay away from the Triple Point (-70 oF & 60.5 psig).
Of course, you must be aware that the thermodynamic efficiency of the CO2 refrigeration cycle is rather low when compared with other conventional refrigerants – such as Ammonia. This means that you are going to consume more energy than a conventional refrigerant cycle. That inefficiency and the higher capital cost of the high pressure equipment (compressor, condenser, receiver, evaporator, and tubing) are the main trade offs when using the CO2 refrigerant.
