What is a Heat Pump?

Basic explanation:

The Enerflow heat pump uses an air source vapour compression cycle to transfer heat from the outside air into the water stored inside a geyser. The cycle can be seen as a magnifying glass for heat. It can, therefore, transfer heat by collecting it from the air and then compressing or concentrating it to ultimately heat the water to a temperature higher than the outside air temperature.

The air source vapour compression cycle normally uses the following four main components to heat the water:

  1. Evaporator to collect heat from the ambient air
  2. Compressor to drive the cycle and pump the energy from the evaporator to the condenser
  3. Condenser to transfer heat into the water
  4. Expansion Valve to reset the cycle for collection of heat on the evaporator side

Detailed explanation:

Generally speaking a heat pump transfer heat from the outside ambient air to the water in the storage vessel with the aid of a fluid called a refrigerant. As the refrigerant is pumped through the vapour compression system it undergoes variations in temperature and pressure at different points in the cycle and furthermore alternates between liquid and vapour states. Let’s now examine the four stages of one complete cycle.

For the explanation, we select the point between the expansion valve and the evaporator (the bottom right corner in the picture above) as the start of the cycle. At this point, the refrigerant is in a low-pressure, low-temperature two-phase liquid-gaseous state, that’s colder than the ambient air.

The low-temperature refrigerant now passes through the tubes of a finned coil heat exchanger, known as the evaporator. The evaporator is a heat exchanger much like a typical car radiator with many fins to increase the surface area for better heat transfer. Because the refrigerant is at a lower temperature than the ambient air, heat is transferred from the air to the refrigerant within the pipes (Fourier’s law).

The transfer of heat from the air to the refrigerant cools the air as it heats the refrigerant. A fan is used to further improve the heat transfer on the air-side by increasing the air flow rate over the evaporator. The heat added to the refrigerant, as it flows through the evaporator, causes its temperature to rise and the fluid to boil until all liquid becomes a warm low-pressure gas.

After the evaporator process, the refrigerant now contains the energy absorbed from the air but the problem is that it is at a temperature that is still not hot enough to heat the water. The next step is then to concentrate this energy to a state where it can actually heat the water, and this is where the compressor comes in. The compressor serves as a pump and compresses the gas into a smaller space and in doing so raises the pressure and temperature thereof (Boyle's law, Charles’s Law and Gay-Lussac’s Law) to a state that is warmer than the water to be heated. It then pumps the now high-temperature high-pressure discharge gas into another heat exchanger called the condenser.

The main purpose of the condenser is to transfer the energy from the high-temperature gas into the cooler water that is circulated from the storage vessel. The refrigerant now transfers its heat to the cooler water (Fourier’s Law). As the heat transfers from the refrigerant to the water, it condenses thereby undergoing a change from a high-pressure gas to a high-pressure liquid. With the energy now transferred the high-pressure refrigerant needs to be returned to a low-pressure condition to be able to again extract energy from the ambient air. This can be accomplished by passing the refrigerant through an expansion valve.

The expansion valve in a refrigerant cycle does the exact opposite of the compressor. Where the compressor compresses the gas, increasing the temperature and pressure thereof; the expansion valve relieves the compression causing the temperature and pressure to reduce dramatically (Boyle's law, Charles’s Law and Gay-Lussac’s Law). After the expansion valve, the refrigerant is therefore in a cool two-phase state and the cycle can be started all over again to extract heat in the evaporator.    

The refrigeration cycle design found in a heat pump works on four fundamental laws or concepts referenced in the example above. These laws can be researched individually for further clarification and a fundamental understanding of “what is a heat pump and how a heat pump works”.

About Enerflow

The Enerflow range is a proud brand of M-Tech Industrial (Pty) Ltd. We specialise in residential, industrial, commercial, as well as, swimming pool heat pumps.

+27(0) 18 297 0326


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