HPAC Magazine

What is a Heat Pump?

November 28, 2022 | By Dave Demma

Here are some of the fundamentals of heat pump operations.

So, where did the term “Heat Pump” come from anyway???

The invention of the heat pump has been credited to American inventor Robert C. Webber, and it was quite by accident that the concept for the heat pump was discovered. In the late 1940’s Webber was experimenting with his deep freeze and, get this, as legend goes he accidentally touched the “outlet pipes of the cooling system” (the discharge line) and burned his hand. You can almost see the light bulb going off in his mind.

Webber decided to see if the mechanics could be reversed. Some minor modifications were in order for the ol’ deep freeze unit, as sources on the internet explain: “He connected the outlet piping from a freezer to a hot water heater and, since the freezer was producing a constant excess heat, he hooked up the heated water to a piping loop.” A small fan was used to transfer the heat from the hot water to the air, and voila the heat pump was born.

According to Lord Kelvin’s Second Law of Thermodynamics, heat will always travel from a warmer area to a colder area. Webber saw this as “pumping” heat from a warmer area to a colder area, hence the term “heat pump”.

After he saw that his invention was successful, he built a full-size heat pump to provide heat for his entire home. His design used copper tubing buried in the ground through which he ran refrigerant to gather the ground heat. The gas was condensed in his cellar, providing heat for the entire house.

Now, taking Webber’s initial idea and applying it to a typical air-conditioning unit—with the addition of a few modifications—you have the modern residential/commercial heat pump.

Utilizing Kelvin’s Second Law of Thermodynamics, the process of blowing warm air through a fin-tube coil, with a cold fluid (refrigerant) flowing through the tubing, will result in heat transferring from the air to the fluid. This lowers the temperature of the air in the conditioned space and the result is what we know as “cooling”.

The goal of the vapour compression cycle used in a refrigeration/air-conditioning system is to provide a continuous source of cold liquid refrigerant to a fin-tube coil (evaporator), which will result in a continuing ability to transfer heat from the refrigerated space.

A basic review of the cycle:

  1. Low pressure superheated refrigerant vapour, containing the heat from the refrigerated space, flows from the evaporator into the compressor.
  2. The compressor “compresses” the vapour into a high-pressure vapour.
  3. The compression process adds heat to the refrigerant vapour, resulting in a high temperature (superheated) high-pressure vapour leaving the compressor.
  4. The superheated refrigerant vapour exits the compressor and flows into a fin-tube coil (condenser). Air flows through the condenser, transferring vapour’s heat content to the air.
  5. The temperature of the superheated high-pressure vapour is reduced and experiences a phase change into a warm liquid.
  6. The warm liquid flows to the expansion device and experiences a pressure drop. This lowers its temperature to the saturation temperature corresponding to the new lower liquid pressure.
  7. This low pressure/low temperature saturated liquid flows through a fin-tube coil (evaporator) located in the refrigerated space. Air in the refrigerated space is transferred to the liquid refrigerant, causing a change of state into a vapour. All of the liquid should change state to a vapour prior to exiting the evaporator tubing, resulting in a cool vapour flowing to the compressor inlet.

Simply put, the cycle transfers heat from the refrigerated space to the refrigerant. In order for the vapor compression cycle to be an endlessly repeating cycle, the heat from the conditioned space has to be transferred from the refrigerant. This occurs at the condenser, located in a space where the temperature is of no concern (outdoors). The refrigerant can then again start the cycle to allow it to be used to transfer heat from the refrigerated space, over and over.

A standard air conditioning system transfers heat from the conditioned (refrigerated) space, lowering the temperature in the space. That heat transferred to the refrigerant, plus the heat added to the refrigerant during the compression process, is transferred to the outdoor air via the condenser.

The heat pump also transfers heat from the conditioned space, but in a heat pump application the conditioned space is now outdoors. So, the evaporator is now located outdoors. The heat transferred to the refrigerant in that process, plus the heat added to the refrigerant during the compression process, is transferred to the air in the conditioned space via the condenser.

So, the heat pump is nothing more than the basic vapour compression cycle utilized in an air conditioning system, with added controls and valving to allow the system to either remove heat from the conditioned space (and transfer it to the outdoors), or remove heat from the outdoors (and transfer it to the conditioned space).

As such, rather than a distinct evaporator and condenser, we now have two dual purpose coils…an “indoor” coil and an “outdoor” coil.

Same vapour compression process, but the evaporator and condenser have changed places. We’re removing heat from outdoors and transferring it to the indoor space.

Figure 1: Four-way reversing valve.

When the conditioned space requires cooling, the indoor coil functions as the evaporator, and the outdoor coil functions as the condenser, and when the conditioned space requires heating the refrigerant flow is reversed, allowing the discharge from the compressor to flow to the indoor coil, where it functions as the condenser. Reversing the refrigerant flow is accomplished with a four-way reversing valve (see Figure 1).

The reversing valve is located in the discharge line between the compressor outlet and the outdoor coil inlet. A solenoid coil (not shown), when energized, allows the valve to “shift” from one position to another.

Figure 2: Reversing valve in the cooling mode.

In the de-energized mode the refrigerant flows from the compressor discharge port to the inlet of the outdoor coil. The other two ports allow the refrigerant vapour from the indoor coil to flow to the compressor suction port (see Figure 2).

When the temperature in the conditioned space falls below the minimum heating temperature setting of the thermostat it will cause the following sequence of events to occur:

  1. With the thermostat set in the heating mode the reversing valve will be energized.
  2. The “Y” terminal on the thermostat will supply power to the compressor contactor, starting the compressor and outdoor fan.
  3. The “G” terminal on the thermostat will supply power to the indoor coil blower motor, starting the motor.

Figure 3: Reversing valve in heating mode.

The unit is now in the heating mode, and the refrigerant flow through the reversing valve is shown in Figure 3.

There’s More

There are several other modifications required in the refrigerant circuit to allow for trouble free reverse flow in a heat pump:

Liquid Filter-Drier: The filter-drier should be mounted in the common liquid line between the indoor coil and the outdoor coil. Given the nature of a heat pump, liquid refrigerant will flow from the outlet of the outdoor coil to the inlet of the indoor coil during the cooling mode, and from the outlet of the indoor coil to the inlet of the outdoor coil in the heating mode.

As such, a standard filter-drier cannot be used in this application. It will need to be a special filter-drier capable of removing system contaminants regardless of which direction the refrigerant is flowing—a bi-directional filter drier (see Figure 4).

Bi-directional flow is accomplished with a series of check valves at each end of the filter-drier housing. They allow refrigerant to enter from either fitting,
directing it to flow from the outside of the core to the inside, and then exiting the shell through the opposite fitting.

Suction Filter-Drier: In the case of a highly contaminated system, where a suction filter-drier is needed to assist in the removal of contaminants, the only location for this would be between the outlet of the four-way reversing valve and the inlet of the compressor.

Given the limited space between these two components, a standard suction filter-drier is too large to be piped in. A special “pancake” style suction filter-drier must be used (see Figure 5). (note the Schrader access fittings on the inlet and outlet, as these are present to monitor the pressure drop through the filter-drier.)

Expansion Device: The indoor coil and outdoor coil both require an expansion valve. Since standard thermostatic expansion valves (TEV) are not suitable for reverse flow, they must be piped in parallel with a check valve.

This piping arrangement allows (1) liquid refrigerant to enter the TEV when a coil is used as an evaporator, and (2) condensed liquid to exit the coil (through the check valve) when a coil is used as a condenser.

There are also special “reverse flow” TEVs with internal check valves available. The internal check valve provides a reverse flow path (liquid entering the valve’s outlet, flowing around the TEV port via the check valve, then flowing into the common liquid line) when the coil is used as a condenser.

In package heat pumps, where there is a minimal distance between the indoor and outdoor coil, an electronic expansion valve (EEV) can be used in the common liquid line.

Ground Source

For years, when the term “heat pump” was mentioned it was understood to mean a conventional heat pump as described above: that being a compressor, indoor and outdoor coils, and some form of expansion device for each coil, and a four-way reversing valve mounted between the compressor outlet and the inlet to the outdoor coil.

Over the years there have been advancements to heat pumps which have allowed them to operate more efficiently and over a broader range of temperatures in the winter. Aside from the air source options there are also “ground source” heat pumps (GSHP) that use either ground water or surface water as the outdoor coil’s heat transfer medium—a heat sink in the cooling mode or a heat source in the heating mode.

Ground source water a depth of 5- to 10-feet will remain at a fairly constant temperature year round. Likewise, water from subsurface aquifers and water from surface bodies will remain fairly constant in temperature (although at slightly greater depth would be required for surface bodies).

This is in contrast to the near 100F temperature difference that the outdoor ambient air temperature may experience between summer and winter conditions. This provides two benefits for heat pump operation:

Figure 6: Various closed-loop ground source options exist as heat sources and heat sinks for ground source heat pumps.

In the heating mode, the constant ground water temperature will provide a constant heat load for the outdoor coil, allowing sufficient load on the compressor to generate sufficient mass flow and heat of compression to provide a constant source of heat to the conditioned space.

In the cooling mode the comparably low water temperature used as the heat transfer medium for the outdoor coil (condenser) will result in lower discharge pressures as compared to an air cooled condenser in the dead of summer. This results in greater compressor capacity and reduced electrical consumption, and can be illustrated by the higher SEER ratings available with GSHPs.

There are various methods available for using the ground source water, can be categorized into closed loop and open loop systems.

Closed Loop: This is an application where the outdoor coil is buried in the earth below the frost line, with the earth or ground water being used as the heat source/heat sink. In essence, the outdoor coil is fashioned into either a vertical or horizontal heat exchanger, and buried in the ground (see Fig. 6).

Horizontal heat exchangers require significantly more land area, but given the fact that they are not buried nearly as deep as vertical heat exchangers, they are less costly to install.

Vertical heat exchangers are normally used on larger buildings where it would be impractical to dedicate the necessary land required for burying a horizontal heat exchanger. These will be constructed of polyethylene and buried in holes drilled approximately 100- to 400-feet deep, and located approximately 20-feet apart. Each hole will have two vertical pipes connected at the bottom with a U-bend, forming a loop. Each vertical loop is connected via a manifold, and then connected to the heat pump.

Surface water heat exchangers can be used if the location has an adequately sized body of water. Depending on the Btu capacity of the heat pump there will be minimum requirements for volume and depth of the water body (in colder climates the water will need to be of sufficient depth such that the heat exchanger can be located well below the freeze line. Additionally, the water quality would need to meet some minimum specifications.

Open Loop: Imagine a system with a water cooled condenser being fed by an endless supply of 60F water. Because it’s an endless supply of supply water, there is no need for a cooling tower to transfer heat from the condenser water.

Or imagine a chiller receiving an endless supply of 60F water at its inlet. The water sees a 10F reduction in temperature in the chiller heat exchanger, but because of the endless supply of water there is no need for a fan coil unit to absorb heat to the chilled water.

That is the essence of an open loop system, an endless supply of water available as a heat sink for cooling or a heat load for winter applications. A pump supplies water to the heat exchanger in the heat pump. Since it is an endless supply, it is simply pumped through the heat exchanger, and then onto another location separate from the source of the water.

One drawback of this method is that there might be an issue with fouling of the heat exchanger due to the condition of the water. As fouling increases, it will then cause a reduction in the efficiency of the process.

In larger commercial applications hybrid systems might be employed where the presence of refrigeration equipment using water cooled condensers (and the accompanying water tower) would provide a year round supply of water for the heat pump’s heat load needs in the winter, and supply water for the heat sink needs in the summer.

The embrace of heat pump technologies around the world is growing. With the shift to decarbonization and electrification, it’s best to learn more about how these systems operate and can be applied for your clients.  <>

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at ddemma@uri.com.



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