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Old 05-16-20, 04:18 PM   #4
jeff5may
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so here's a generic diagram of a heat pump:


The cooling coil is the evaporator. It's attached to the suction side of the compressor, so the working pressure is low. High pressure liquid refrigerant is forced through a restriction, and the resulting pressure drop forces the refrigerant to boil off (evaporate) at constant temperature until it becomes gaseous. The phase change requires energy, so it is absorbed from the surrounding environment. Not a big deal to refrigerate glycol mixture, just make sure it can't freeze.

The condenser heat exchanger is fed by the hot high pressure output of the compressor. The hot gas is cooled down below it's boiling point and changes to the liquid phase. The heat gained from boiling is released at constant temperature until the gas has all condensed into the liquid phase.

Both of these phase change processes happen at a constant temperature, depending on the refrigerant pressure. This is known as the "latent" portion of the heat flow, and maximizing it will increase your efficiency. Once all the refrigerant has completely vaporized or condensed, a little heat flow forces a temperature change towards ambient temperature and the heat transfer is done for the most part. This is known as "sensible" heat transfer. You need a little of it to keep the gas gas and the liquid liquid (compressors don't like pumping liquid and expansion valves don't like gases), but not very much.

The thing with the EV heat pump systems is that they have more than two heat exchangers. Outside the cabin, they have one for air to flow through like most cars, plus a transmission cooler and maybe a battery heater/cooler. Inside, they have a separate evaporator and condenser, plus maybe a dehumidifier as well. As with most modern cars, everything has sensors and valves, plus a brain box controlling the system.

Last edited by jeff5may; 05-17-20 at 11:22 PM..
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