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Heat Pump Basics: Part 2

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Published
July 9, 2009
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What are the other system components that make up a heat pump system?

A heat pump system has four major system components. In addition to the evaporator and condenser, the heat pump system also has a compressor and a metering device. These two system components maintain the flow of refrigerant through the heat pump system and also maintain the heat transfer coils at the desired temperatures and pressures.


What does the heat pump compressor do?

The compressor is the system component that is responsible for creating a pressure difference in the system that allows refrigerant to flow through the system. It is a vapor pump, so there must be vapor at its inlet as well as vapor at its outlet. The vapor refrigerant that enters the compressor is at a low temperature and a low pressure. At the outlet of the compressor, the vapor refrigerant is at a high temperature and a high pressure. The pressure difference that is created allows the refrigerant to flow from the outlet, or discharge,of the compressor through the heat transfer coils and then back to the inlet of the compressor where the pressure is once again lifted to the desired level. It can be concluded, therefore, that the compressor acts as a dividing point between the high pressure side and the low pressure side of the heat pump system.


What does the metering device do?

Like the compressor, the metering device is a system component that acts as a dividing point between the high and low pressure sides of the heat pump system. The metering device acts as a blockage in the refrigerant circuit and restricts flow as the refrigerant flows between the two heat transfer coils. The temperature and pressure of the refrigerant at the inlet of the metering device is high, while the temperature and pressure of the refrigerant at the outlet of the metering device is low.

Consider a car accident on a four-lane highway. Traffic builds before the accident, as a number of the travel lanes are blocked. This creates a “high pressure” of sorts before the accident.Now, consider what happens after the point of the collision is passed.The concentration of cars is low and the “pressure” is greatly reduced.




 
How are all these components connected?

The four major system components that were previously discussed make upwhat is called the basic refrigeration cycle. These components are connected to each other as shown in this figure.




How does the system operate?

Let’s start at the inlet of the compressor. At the inlet of the compressor,the state of the refrigerant is 100% vapor and is at a low pressure and a low temperature. As the refrigerant flows through the compressor, the refrigerant is compressed and the temperature and pressure of the vapor is increased. At the outlet of the compressor, the refrigerant is at a high temperature and a high pressure. Remember, because the refrigerant is 100% vapor, it does not follow the pressure/temperature relationship for that particular refrigerant.

From the compressor the refrigerant flows to the condenser coil. This is where system heat is rejected. As heat is transferred from the refrigerant to the surrounding medium, the refrigerant begins to condense into a liquid.Now the refrigerant is a mixture of liquid and vapor and it follows the pressure/temperature relationship for that refrigerant.

At the outlet of the condenser, the refrigerant is 100% liquid and then flows onto the metering device. This refrigerant is still at a high pressure and a high temperature. The metering device controls the flow of refrigerant into the evaporator by acting as a blockage. At the outlet of the metering device, the refrigerant is now at a low pressure and a low temperature. The low pressure, low temperature liquid then flows into the evaporator.

The evaporator is the point in the system where heat is introduced to the refrigerant. As heat is added to the refrigerant in the evaporator coil, the refrigerant begins to evaporate or boil. As the liquid refrigerant begins to boil, it once again becomes a saturated refrigerant, or a mixture of liquid and vapor.Since it’s saturated, it follows the pressure/temperature relationship for that refrigerant. At the outlet of the evaporator, the refrigerant is 100% vapor and ready to enter the compressor once again.


How does the heat pump system provide comfort cooling?

When the air in the conditioned space passes over or through the evaporator,heat transfers from the warmer air to the cooler refrigerant that is flowing through the evaporator. At near design conditions, the air passing over the evaporator, or cooling coil, is at a temperature of about 75°F with a relative humidity of about 50%. After passing through the coil, the air temperature is about 55°F.




How does the heat pump system provide heating?

The heat pump heats the air in the occupied space when the air passes over or through the condenser coil. Since the temperature of the condenser coil is higher than the temperature of the air passing through it, heat is transferred from the warm refrigerant in the condenser coil to the air, thus warming it.


        

How does the heat pump know when to heat and when to cool?

The heat pump system is equipped with a device called the reversing valve.This valve directs the refrigerant to the correct coils at the correct time to provide the desired mode of operation.

What does the reversing valve look like?


Here is a photo of a typical heat pump reversing valve. The valve has the ability to change the direction of refrigerant flow depending on the mode the system is operating in.


           
The Reversing Valve


How does the reversing valve operate?

The reversing valve is controlled by a solenoid coil that, when energized, causes the valve to change positions.


What exactly is a solenoid?

A solenoid is basically a coil of wire. When electric voltage is applied to the coil and current flows through it, a magnetic field is generated. This magnetic field causes the reversing valve to change position. When the solenoid is energized, the system will work in one mode, and in the other mode when the solenoid is d-energized. Depending on the manufacturer of the heat pump equipment, the system mode associated with the energized or de-energized solenoid coil varies.


The Solenoid Coil



Coil Mounted on the Reversing Valve

How does the reversing actually redirect refrigerant flow?

The reversing valve has four ports:

•    One port connected to the outlet of the compressor
•    One port connected to the inlet of the compressor
•    One port connected to one side of the indoor coil
•    One port connected to one side of the outdoor coil

The reversing valve has an internal slide that ultimately determines the mode in which the system will operate.

For example, in the diagram that follows, if the slide is to the left, the refrigerant will flow from the compressor to the outdoor coil. When such is the case, the system is operating in the cooling mode, since the refrigerant always flows to the condenser first upon exiting the compressor. It can also be seen that the “U-bend” directs the refrigerant from the indoor coil, in this case the evaporator, back to the compressor.



When the system operates in the heating mode, the indoor coils acts as the condenser. In this case, the hot gas from the compressor is directed to the indoor coil, which is functioning as the condenser. It can be seen that the refrigerant from the outdoor coil, in this case the evaporator, is directed back to the compressor via the “U-bend” in the reversing valve.



You should be able to note that, in the following two photos, the direction of refrigerant flow through the tube on the left side of the valve remains the same, even though the slide has changed position. The same can be said for the center pipe connection on the right side of the valve. As the slide position changes, the direction of refrigerant flow changes only in the top and bottom pipes of the right hand side of the valve.


The two positions of the reversing valve.


Where is the reversing valve located in the heat pump system?

The reversing valve is typically located between the compressor outlet (discharge) and the inlet of the outdoor coil. If the system is a split-type system, this location is within the outdoor portion of the system.


How does the refrigerant flow through the system in the heating mode?

The refrigerant (upon leaving the compressor via the red line in the following diagram) first flows through the reversing valve where it is directed to the indoor coil. Since the refrigerant always flows to the condenser first after leaving the compressor, the indoor coil is acting as the condenser. In this mode of operation, the heat from the refrigerant is rejected to the air in the occupied space. From the indoor coil, the refrigerant flows through the metering device and then to the outdoor coil, where the refrigerant picks up or absorbs heat from the outside air. The refrigerant then flows back to the compressor via the reversing valve and the cycle repeats itself.




How does the refrigerant flow through the system in the cooling mode?

The refrigerant (upon leaving the compressor via the red line in the following diagram) first flows through the reversing valve where it is directed to the outdoor coil. Since the refrigerant always flows to the condenser first after leaving the compressor, the outdoor coil is acting as the condenser. In this mode of operation, the heat from the refrigerant is rejected to the outside air. From the outdoor coil, the refrigerant flows through the metering device and then to the indoor coil, where the refrigerant picks up or absorbs heat from the air in the area being cooled. The refrigerant then flows back to the compressor via the reversing valve and the cycle repeats itself.




Do all heat pump systems have reversing valves?

No. Heat pumps that are intended to provide both heating and cooling are equipped with reversing valves. However, heat pump systems that are intended to provide only heating are not equipped with reversing valves.


Can you give me an example of a heat pump system that does not have a reversing valve?

The heat pump swimming pool heater does not use a reversing valve. The sole purpose of the heat pump pool heater is to heat the water in the swimming pool. This heat pump is never used to cool the pool water, so there is no need for the system to operate in both the heating and cooling mode. The heat pump swimming pool heater is an air-to-water heat pump and uses one air-to-refrigerant heat exchanger and one refrigerant-to-water heat exchanger.

The hot gas from the compressor is piped directly to the tube-in-tube heat exchanger and transfers heat to the pool water. The refrigerant leaves this heat exchanger as a high temperature, high pressure liquid and, after flowing through the metering device, flows to the outdoor coil as a low temperature, low pressure liquid. It is at the outdoor coil, the evaporator, that heat from the outdoor air is transferred to the refrigerant.




Simplified piping diagram of a heat pump swimming pool heater.