Phase of refrigerant when entering evaporator?

Question

In the diagrams for refrigeration cycle(vapour-compression), the refrigerant at the inlet to the evaporator lies under the vapour dome i.e partly vapour and partly liquid. Now i have been studying thermodynamic panels, which are essentially heat pumps but with the evaporator being modified such that the refrigerant also absorbs the radiant energy of the sun, apart from the ambient heat. Now from what i have read about them, they use a liquid refrigerant that enters evaporator at - 22°C which then boils at - 15°C, this means there is sensible heating of refrigerant also involved, whereas in your regular refrigerators and maybe heat pumps, the refrigerant would only absorb energy through latent heat. Is there a chance that this information is wrong? I need the information for my seminar report on thermodynamic panels. Also, if there are systems where sensible heating is involved, are they at any advantage over usual refrigeration systems that use latent heat.

Answer 1

Assuming that your system is a normal refrigerant system, the refrigerant exists in the evaporator under saturated conditions, because the high pressure, hot refrigerant goes through an expansion valve, and "flashes" at the lower pressure. This means that there is excess heat in the high pressure refrigerant when it experiences lower pressure conditions, and the refrigerant boils and loses heat and temperature until the refrigerant reaches the temperature that corresponds to the new pressure that it is seeing, according to the Antoine equation. The only way to experience several degrees of subcooling involves some type of heat transfer to the environment immediately after the expansion valve. This isn't going to happen, for at least two reasons. No chemical engineer (my background) would try to design a process to do this because extraneous (and expensive) equipment would be involved, and the heat transfer to the environment immediately downstream of the expansion valve would have to be very rapid (big heat exchanger area involved). There is a very high probability that your information has an error in it, so it is very unlikely that you need to worry about the 7 degrees of subcooling mentioned in your problem statement.

In the unlikely event that your process actually does deal with subcooled refrigerant, you would need to post a process drawing so others on this site could determine how the stated process conditions are being achieved.