Basic concepts of the heat pump. In this article we will see how heat pumps work. There are a few variations of heat pumps, but in this article we are going to look at a typical air source heat pump. These use a reversing valve t
Scroll down for YouTube video tutorial on heat pump basics
The main parts that you will find in any system such as the indoor unit, the outdoor unit, and some pipes that connect the two.
The indoor unit can be installed in a duct system, which is very common in North America. Either way, they all basically work the same way.
When removing the covers, we will find the following components. Inside the indoor unit we find the heat exchanger, then we have the expansion valve and the check valve as well as a small fan to circulate the ambient air.
Inside the outdoor unit we have the compressor. It is the driving force of the refrigerant around the system. We will find in this device a heat exchanger accompanied by a fan, a reversing valve, a check valve and an expansion valve.
cooling mode
Thus, in cooling mode, the heat pump works like a normal air conditioner. The outdoor unit acts as a condenser and the indoor unit as an evaporator.
The refrigerant leaves the compressor as superheated vapor at high pressure and high temperature. This goes directly to the top of the reversing valve. The reversing valve will decide which way it can flow. In this case, it is bypassed and sent directly to the outdoor heat exchanger, where the refrigerant will condense, giving up part of its energy.
The outside air is colder than the temperature of the refrigerant inside the tubes. A fan inside the outdoor unit blows this air through the heat exchanger, which will remove energy from the refrigerant and cause it to condense. Remember that the refrigerant always stays in the tubes, it never mixes with the air. Thermal energy is simply transferred through the tube wall because there is a temperature difference and hot always flows to cold.
The refrigerant temperature at the compressor outlet is much higher than the outside air temperature. This is to allow heat exchange. If the refrigerant and the air were at the same temperature, there would be no heat transfer and the air conditioning system would not work. The greater the temperature difference between the refrigerant and the outside air, the greater the heat exchange.
Thus, when this refrigerant enters and passes through the outdoor heat exchanger, it condenses and becomes liquid. When it comes out and passes through the check valve, it will be a medium temperature liquid, saturated but still at high pressure. The force of all the refrigerant behind it coming out of this compressor is what pushes the refrigerant towards the other components in this system.
In this configuration, the outdoor unit, acting as a condenser, is there to reject heat from the system. Any unwanted heat that the system picks up from the indoor unit will be sent to the compressor and this will push it to the condenser, where it will reject the unwanted heat to the outside air. The fan is only there to help speed up this heat exchange.
As the liquid refrigerant is forced into the indoor unit, it reaches another check valve. This check valve is inverted and does not let refrigerant through. Instead, it will bypass this and go through the regulator. The expansion valve expands the refrigerant.
Learn how regulators work work here
When the refrigerant leaves the expansion valve, it is a low-pressure, low-temperature liquid-vapor mixture. This will push around the indoor heat exchanger and begin to capture some of the room's unwanted heat energy. A fan inside the indoor unit circulates the air in the room to draw warm air into the unit and exhaust cool air into the room.
Here the indoor unit acts as an evaporator. The refrigerant enters the indoor unit as a mixture of liquid and vapor, but the refrigerant has a very low boiling point. The air temperature inside the room is sufficient to boil this refrigerant into a slightly superheated saturated vapor. This is always pushed out by all the refrigerant that is pumped up behind it from the compressor.
B when it leaves the indoor unit, the refrigerant will come out as saturated vapor at low pressure and low temperature. This will return directly to the reversing valve, which is currently set to redirect refrigerant to the compressor. This refrigerant will then pass through the compressor and repeat the entire cycle.
heating mode
If we now look at how the heat pump would work in heating mode, you can now see that the outdoor unit acts as an evaporator and the indoor unit acts as a condenser.
So the refrigerant leaves the compressor, again, as high-temperature, high-pressure superheated vapor, and that goes directly to the reversing valve. In this configuration, the reversing valve diverts the refrigerant to the indoor unit instead of the outdoor unit.
The refrigerant enters the indoor unit, which is now the condenser, in the form of high pressure, high temperature, and superheated steam.
The room fan circulates ambient air through the heat exchanger. This air will be at a lower temperature than the temperature of the refrigerant in the tubes of the heat exchanger. Therefore, the refrigerant will transfer part of the energy to the air. This condenses the refrigerant and turns it into a liquid.
The refrigerant leaves the indoor unit and goes to the check valve and expansion valve. In this case, it will go through the check valve because the regulator won't let it through there.
The refrigerant flows along the liquid line to the outdoor unit. The check valve inside the outdoor unit will not allow it to pass in that direction, so it will pass through the expansion valve.
The expander expands the refrigerant, and when the refrigerant passes through the expander, it will come out as a low-pressure, low-temperature liquid-vapor mixture.
Now the outdoor unit fan blows air through the heat exchanger that is at a temperature above the boiling point of the refrigerant. Therefore, the refrigerant boils and captures some of the heat from this air.
This will convert this refrigerant to a saturated/slightly superheated vapor at low pressure and low temperature. This will then go directly to the compressor, where it will then go through the compressor and repeat the entire cycle again.
Thus, the refrigerant collects this thermal energy from all the outside air and takes it to the compressor. The compressor compresses it, compresses it together, which increases the temperature as the refrigerant particles collide more and expels it into the indoor unit.
Now, a lot of people say, “Well, what about winter? The air is cold. There is no power there. So how do you take energy from the air? Well actually that air still has a lot of energy, and certainly enough to be extracted by a heat pump. As long as the air is above -273 degrees Celsius, there is energy in that air that can be drawn from it. Obviously, we do not go down to this temperature in our climates. However, the lower the temperature, the harder it is to extract power, and at some point, the system will use more energy than it draws.
The reason it can extract heat energy from air based on temperature is that the boiling point of refrigerant is very low.
If we compare water and some common refrigerants. We know that water boils at about 100°C (212°F) and at this temperature it turns into steam, the steam leaves the body of water and takes the heat with it. Coolant, on the other hand, will boil at extremely low temperatures by comparison. Therefore, as long as the air temperature is above this boiling temperature, it can accumulate heat.
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