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exbm t1_j18uzsj wrote

Reply to comment by incredibolox in Why do we use phase change refrigerants? by samskiter

The way this was explained in HVAC/refrigeration school was a typical refrigerant cycle deals with two phases of matter liquid and gas. When a material changes phase from liquid to gas it requires extra energy to complete the change. The molecule will absorb that energy from the surrounding area. This is called latent heat (heat is energy). This lack of energy makes everything cold. Because cold is really the absence of energy.

The reason you compress the gas on the high side is because in a gas temperature and pressure correspond. Increasing the pressure of the gas increases it's temperature. By increasing the temperature to higher than ambient air you allow the latent heat to to flow into the ambient air. Heat/energy flows from hot to cold. Like water flows downhill. Once this latent heat has been absorbed into the ambient air the gas will phase change back to a liquid. Now you can slowly let the liquid back in the lowside of the cycle for it to be evaporated.

Source: AS in environmental control technology

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zebediah49 t1_j18x0wm wrote

Note that this only works because of vapor pressure differences.

It is necessary that the compressor lower the pressure on the cold side such that its boiling point is below the cold reservoir temperature, and raise the pressure on the hot side such that that boiling point is above the hot reservoir temperature.

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St1cks t1_j190ivz wrote

The compressor doesn't cause the lower pressure side on its own. We use a type of metering device to achieve a flash off within the suction line. Ideally a 25/75 mix of liquid and gas should be present at the start of the evaporator coil. Metering devices can be fixed or adjusting, which generally use a sensing bulb attached at the inlet of the evaporator.

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zebediah49 t1_j191who wrote

This is true; you need some type of pressure drop device.

I give disproportionate credit to the compressor, due to it being the part that does the Work.

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exbm t1_j19rjrr wrote

You need to use a compressor some types of systems use external heat but all systems need a flow restricting device

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zebediah49 t1_j1a0qqf wrote

Strictly speaking many external-heat systems don't. They use an absorption refrigeration cycle, and while the partial pressure of the refrigerant components changes around, the total pressure of the system is approximately constant.

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bob0979 t1_j199pol wrote

This latent heat for the phase change is also energy that doesn't require as much temperature difference, allowing more efficient energy extraction because some of the energy being transferred isn't received 'in temperature' but in physical state change. The Temps don't equalize as quickly because some of this energy is stored elsewhere than temperature. The whole system just gets to be a higher energy process by including the buffer of state changes.

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