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Old 09-15-13, 04:22 AM   #3
jeff5may
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The simplest, most common heat pump is a refrigerator or deep freezer. It moves heat from inside the box to outside the box. A thermostat inside the box maintains a set temperature, so when the box gets warm, it starts pumping heat out of the box. When the box is cooled enough, the pumping stops until it warms again.

Large, industrial heat pump systems use either steam or ammonia water as a heat transfer fluid. These systems are not very energy efficient and fall out of the scope of this thread. On a massive sized installation, they usually capitalize on waste heat generated from another process (power generation, machine cooling, etc.), so the heat reclaimed is a "freebie". They function just as an automobile heater does: they use some of the engine heat that would otherwise be cooled in the radiator to heat the passenger compartment.

Most residential heat pump (and air conditioning) systems use what is called a "vapor compression cycle" or "phase change cycle". They use a volatile chemical (like freon, puron, propane, etc) for a heat transfer fluid. The chemical "refrigerant" is in a sealed plumbing loop, and a compressor is used to move the chemical by pressure differential. The process is very stable and very energy efficient.

This process takes advantage of the energies of evaporation and condensation or "boiling and distilling". As with water on the stove, it takes a lot less heat (sensible heat) to bring the fluid to boiling temperature than it does to actually boil all the water(latent heat). The heat pump takes advantage of this large latent heat transfer, illustrated below in the phase changes that occur when water is heated from ice to steam:
As you can see, it takes many more joules of heat energy to melt the ice and boil the water at constant temperatures than it does to raise it from -50 to 0, 0 to 100, and 100 to 150 degrees celsius.

In the sealed refrigerant loop, internal pressures determine the boiling point of the chemical rather than just raw temperature. So by changing the pressure inside the plumbing, the chemical can be forced to change from a liquid to a gas and vice versa at whatever temperatures we want or need them to. When the chemical changes from a liquid to a gas (evaporation), it must absorb heat to do so. When it changes from a gas to a liquid (condensation), it has to release the same amount of heat.

The device that separates the pressures is called a metering device. It is basically a highly engineered blockage in the plumbing. When the compressor runs, it builds up pressure on the discharge side due to this blockage. As the discharge pressure builds up, the chemical gets hot inside and releases heat through the piping wall. This release of heat at high pressure causes the chemical to condense into liquid form. The higher the pressure, the more the liquid builds up in front of the blockage. The longer the wait, the more the liquid is cooled in the plumbing. If the liquid refrigerant is cooled below its boiling temperature, we say it is "subcooled".

At some point, the built up liquid forces its way through the blockage. Making its way through, it encounters a massive pressure drop, which forces the liquid to boil violently and absorb heat in the process. The liquid has to seek a lower temperature in order to absorb heat, since at this lower pressure, the boiling temperature is many degrees lower than it was on the other side of the metering device. The longer the plumbing is between the blockage and the compressor intake, the more heat is absorbed along the way. When (and if) the liquid all boils off and begins to absorb sensible heat from the plumbing, we say the chemical has become "superheated".

In well-designed heat pumps, the plumbing in between the compressor and metering device is made so the heat flows easily as the chemical changes states of matter (phase change).This area of plumbing surface is known as a heat exchanger. Heat exchangers are designed so that the refrigerant can absorb or release all the latent heat it needs to make the change and then some. The heat extracted or released can be upwards of 5 times as much as the energy used by the compressor to actually move the refrigerant.

Below is a basic diagram of an air-to-air heat pumping system:




And a reversible heating / cooling system diagram:
Attached Images
 

Last edited by jeff5may; 05-14-20 at 07:38 PM.. Reason: Clarity + pics
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