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Heat Pump Basics: Part 1

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Published
July 9, 2009
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What is a heat pump?


A heat pump is a piece of air conditioning equipment that has the ability to provide both comfort cooling in the warm summer months as well as comfort heating in the cooler winter months.

Some heat pumps are designed to heat water instead of air. These heat pumps are used in conjunction with spas, pools and hydronic/radiant heating systems.


Can you give me an example of the heat pump concept?

If you think in terms of a window air conditioner you might have in your home, the appliance provides you with cooling during the warm summer months. If you were to walk outside and feel the air coming out from the back of the unit while it is operating, it will be warm. So, we can say that the air conditioner takes heat from inside the structure and moves it to the outside of the structure.



An air conditioner moves heat from the inside of a structure to the outside.

So, what would happen if we took the window air conditioner and installed it with the inside portion on the outside and the outside portion on the inside? Well, instead of blowing cold air into the occupied space, we would be blowing warm air. Instead of having hot air blow from the back of the unit, we would be blowing cold air. So, in this case, we would be taking heat from outside and blowing it into the structure.



If an air conditioner was turned end-for-end, heat would be transferred from the outside of the structure to the inside.


So, without having to constantly rotate the air conditioner in the window, the heat pump mechanically reverses itself to perform both heating and cooling functions.


How can a heat pump provide both heating and cooling?

There are two heat transfer surfaces in a typical heat pump system. One coil or heat transfer surface is located inside the structure, while the other is located outside the structure. These surfaces are referred to as the condenser and the evaporator. The evaporator absorbs heat, while the condenser is responsible for rejecting heat. The function of the heat transfer surfaces can be changed to produce the desired mode of system operation. So, the indoor and outdoor coils can function as either the condenser or the evaporator, depending on the mode it’s operating in.


How does a heat pump system operate in the cooling mode?

In the cooling mode, the indoor coil functions as the evaporator and the outdoor coil functions as the condenser. Air from the occupied space passes over the evaporator, or cooling coil, and heat energy is transferred from the air to the coil. This heat is ultimately transferred to the outdoor coil, which is acting as the condenser. At the condenser, the heat is then rejected to the outside.


How does a heat pump system operate in the heating mode?

In the heating mode, the indoor coil functions as the condenser and the outdoor coil functions as the evaporator. Air from outside the structure passes over the outdoor coil (the evaporator) and heat is transferred from the air to the coil. This heat is ultimately transferred to the indoor coil, which is acting as the condenser. At the condenser, the heat from outside is transferred to the air passing over the indoor coil.


How does the evaporator absorb heat?

In order for the evaporator to absorb heat from a substance, it must be at a temperature that is lower than the medium we are transferring heat from. For example, to cool 75 degree air in an occupied space, the surface temperature of the evaporator must be lower than 75 degrees.

A heat transfer medium, known as a refrigerant, is circulated through the coil to facilitate the desired heat transfer.


How does the condenser reject heat?

In order for the condenser to reject heat, the medium that is surrounding the condenser coil must be cooler than the temperature of the condenser coil itself. In other words, the condenser must be maintained at a temperature that is well above the temperature of the surrounding medium.

A heat transfer medium, known as a refrigerant, is circulated through the coil to facilitate the desired heat transfer.


What is a refrigerant?

A refrigerant is a chemical substance that has the ability to absorb and reject large amounts of heat energy quickly. By controlling the pressure of the refrigerant at different points in the system, we can determine whether a particular heat transfer surface will operate as the condenser or as the evaporator.

In a heat pump system, refrigerant can be present in any one of three states. These states are 100% liquid, 100% vapor or a mixture of liquid and vapor. When the refrigerant is a mixture of liquid and vapor, the refrigerant is said to be saturated. Saturated refrigerants follow a specific relationship between pressure and temperature. Each refrigerant has its own pressure/temperature relationship.

By utilizing the laws of physics that govern the behavior of saturated refrigerants, we can control the temperature of the refrigerant by controlling its pressure.


What does a pressure/temperature chart look like?

Here is a portion of the pressure/temperature chart for R-22, which is a refrigerant commonly found in many heat pump and air conditioning systems.



From this chart we can see that saturated R-22 will be at a temperature of 40°F if it is at a pressure of 69 psig. (Refer to the yellow highlighted portion of the chart). We can also see that saturated R-22 will be at a temperature of 120°F if it is at a pressure of 260 psig. (Refer to the green highlighted portion of the chart).


How is the evaporator coil kept cool?

The evaporator is the heat exchange surface that absorbs heat into the heat pump system so it must be cooler than the medium being cooled. Since we want the evaporator to be at a low temperature, the pressure of the refrigerant must also be low. We use other system components, namely the compressor and the metering device, to maintain the desired refrigerant pressures in the heat transfer coils.

For cooling applications, when the medium being cooled is the air in the space, a near design evaporator temperature is about 40°F. So, for R-22, the pressure in the evaporator needs to be about 69 psig in order to maintain this temperature. This assumes near design conditions, which are a 75°F indoor air temperature with a relative humidity of 50%, and an outside ambient temperature of 95°F.

For heating applications, the temperature of the evaporator will vary depending on the temperature of the outside air. If the temperature of the outside air is 40°F, then the refrigerant in the evaporator will be at a temperature of about 15°F.


How is the condenser coil kept warm?

The condenser is the heat exchange surface that is responsible for rejecting heat from the heat pump system so it must be warmer than the surrounding medium. Because of this, we want the condenser to be at a high temperature. This is accomplished by maintaining the pressure of the refrigerant in the condenser at a high level. We use other system components, namely the compressor and the metering device, to maintain the desired refrigerant pressures in the heat transfer coils.