Design data of the heat pump. Earlier, we looked at how heat pumps work, but in this article we're going to dig deeper and look at some heat pump design data. We will see how the pressure, temperature, enthalpy, and entropy change throughout the system as the refrigerant passes between the major components of the compressor, condenser, expander, and evaporator.
Scroll down for Heat Pump Design Data YouTube Tutorial
I just want to point out that the numbers shown in this tutorial are used as an example only, to help you understand what's going on throughout the system. You should not compare these numbers to an actual heat pump system. Instead, you should check with the manufacturer specifically for your heat pump model and ask and compare it to their design data.
Heat pumps can work in two modes, either heating or cooling. The evaporator and condenser will reverse their functions when changing modes. Notice how in heating mode the evaporator is in the open air while the condenser is inside, and in the cooling mode the condenser is in the open air and the evaporator is inside.
Above you can see a schematic representation of this circuit. In both cases, the refrigerant passes from the compressor, through the condenser, then the expansion valve, and then the evaporator before returning to the compressor. The direction is changed using a reversing valve.
We can plot this cycle using two graphs, a temperature enthalpy graph and a pressure enthalpy graph. Above is a simplified version of this profile. You may notice the different points, which are different colors and numbers, and these correspond to a specific point in the system depending on whether the system is running in heating or cooling mode.
The first point is after the evaporator, so the refrigerant will be a low pressure, low temperature saturated vapor as it has just absorbed heat energy and expanded.
The second point is after the compressor, so we know that the refrigerant will be superheated steam at high pressure and temperature.
Point three is just past the condenser where the refrigerant gave up its thermal energy and began to condense into a liquid. It will therefore be a saturated liquid at high pressure and medium temperature.
Point four is just after the expansion valve and before the evaporator. We know that it will now be a mixture of liquid and vapor at low pressure and low temperature. From there, the coolant returns to point one.
Example of a heat pump
We start at point one. As we know, the first point is saturated steam at low temperature and low pressure. We'll start with a temperature of around 2.5 degrees Celsius (36 degrees Fahrenheit). It will have a pressure of 260 kilopascals -260 kPa (2.6 bar).
The entropy is 0.9 kilojoules per kilogram per Kelvin - 0.9 kJ/kJ/K (0.45 BTU per pound per Fahrenheit - 0.45 BTU/lb/F). The enthalpy will be 246 kilojoules per kilogram - 246 kJ/kg (105 BTU per pound - 105 BTU/lb).
Now, second point, we can see from the graphs that the temperature and the pressure will increase. There will also be an increase in enthalpy. These increase because the compressor works on the system, compressing the energy into a smaller package. The pressure rose to 1600 kPa (16 bar). This results in an increase in temperature ti of 63 degrees Celsius (149 degrees Fahrenheit). The entropy will stay about the same, but the enthalpy will also have increased, so it is now 282 kJ/kg (121 BTU/lb).
At the third point, there will be some reduction in heat and the pressure will drop due to the resistance of the pipes and fittings affecting the flow of refrigerant. As the pressure drops and you can see it, there will be a drop in entropy and enthalpy, so it dropped to 56°C (133°F). There was a slight reduction in pressure to 1550 kPa (15.5 bar). Entropy has also decreased. It is nearly halved to 0.46 kJ/kg/K (0.11 BTU/lb/F). Then the enthalpy also dropped to 134 kJ/kg (57 BTU/lb).
At point four you can see on the graph that there has been a large drop in pressure and temperature. It's the regulator's fault. Entropy increased slightly. This is because the gas has expanded, it was in a liquid state before the expander and is now a mixture of liquid and vapor. As it expands, the entropy increases, but note that there is no or very little change in enthalpy. The coolant temperature has dropped to -1.23 °C (29 °F). The pressure dropped to 280 kPa (2.8 bar), the entropy increased to 0.55 kJ/kg/K (0.13 BTU/lb/F) and the enthalpy remained the same at 134 kg/kg (57 BTU/lb ).
The refrigerant will then make its way from point four to point one, passing through the evaporator. This will lead to a certain increase in temperature. There is also a slight head loss due to fittings and pipes. Since the refrigerant has recovered thermal energy here, the entropy and enthalpy return to their value at point one.
The numbers will be slightly different depending on whether the unit is in heating or cooling mode, as it will be drawing heat energy from the building or from outside ambient air. These will be different with wind, sun, moisture content, heat sources, etc.
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