Air-cooled chiller design data. In this tutorial, we'll look at air-cooled chillers to find out how they work in a more advanced way. This time we'll look at refrigerant and pressures, temperature, enthalpy, entropy, flow rates, and heat transfer.
Scroll down for YouTube Tutorial of Air Cooler Design Data
In our previous tutorial on air cooled chillers (click here to view),
The values used in the example are taken from actual design data, but you should not compare chiller performance to these values. They are specific to a certain model and operational design. If you need to compare chiller performance, contact your chiller manufacturer and request their design data for that model. This model uses R134a refrigerant.
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| Air-cooled-chiller-main-parts jcool |
The first component we start with for the refrigeration cycle is the compressor which compresses the refrigerant into a smaller volume to increase the temperature and pressure and then pushes this compact refrigerant throughout the system. From there it flows to the condenser where the unwanted heat is expelled into the atmospheric air. Then we have an expansion that expands the refrigerant to lower pressure and temperature, and finally we have the evaporator that captures unwanted heat from the building. At the top of the chiller we have fans that will move air through the condenser coils.
We will now look at the behavior of the coolant at four key locations around the cooler.
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| Pressure-enthalpy-charts-air-cooled-chiller- jcool |
Above you can see the refrigeration cycle of the chiller plotted on two graphs with the reference points numbered and displayed on the chiller along with the graphs so you can follow them.
For the first Point : the refrigerant just picked up unwanted heat from the evaporator and turned into a vapor. We know that the refrigerant must be a low pressure, low temperature, saturated vapor, and the refrigerant at this point would be about 350 kPa or (3.5 Bar) at 5°C (41°F) an enthalpy of 250 kJ/kg ( 107 Btu/lb) and an entropy of 0.916 kJ//kg/K (0.219 Btu/lbm/F).
Point two: Point two: It's right after the compressor, so we know it's going to be high-pressure, high-temperature superheated steam. From the graphs we can also see that it will be in the superheated region. The pressure is now 1500 kPa (15 bar), the temperature is 60 °C (140 °F), with an enthalpy of 280 kJ/kg (120 Btu/lb) and an entropy of 0.916 kJ/kg/K ( 0.219 Btu/lbm/F). At this point we can see that the pressure, temperature, and enthalpy have increased, but the entropy has stayed the same.
Third point: It's right after the condenser, so the refrigerant has just given up some of the unwanted heat to the atmosphere. We know that refrigerant must always be a saturated liquid at high pressure and medium temperature, and it will surely be very close to the saturation line on the graphs. Here we can see that the pressure remained the same at 1500 kPa (15 Bar). The temperature decreased as it lost thermal energy and is now 55°C (131°F). Enthalpy also decreased to 129 kJ/kg (56 Btu/lbm) and entropy also decreased to 0.458 kJ/kg/K (0.109 Btu/lbm/F).
Fourth point: After the expansion valve, the refrigerant has expanded, so it will have dropped in pressure and temperature. So we know it's going to be somewhere in the steam dome. The pressure dropped to 350 kPa (3.5 bar). The temperature also dropped to 5°C (41°F); It's because it's enlarged. The enthalpy remained the same at 129 kJ/kg (56 Btu/lbm) and the entropy increased slightly to 0.483 kJ/kg/K (0.115 Btu/lbm/F).
Unwanted heat is removed from the condenser by fans that blow cooler ambient air through the condenser coil. in this example, the fans will have a total volumetric flow rate of 30.75 cubic meters per second (30,750 liters per second). The inlet air temperature is approximately 30°C (86°F) and the outlet air temperature will be approximately 44°C (111°F). This would translate into a heat transfer of 496 kW. You can verify this from the enthalpy difference between points 2 and 3. So this enthalpy difference multiplied by the mass flow of refrigerant. The mass flow in this system is 3.3 kg/s (7.2 lbm/s). It uses R134a refrigerant and the compression power is 98.9 kW. You can find the compression power by finding the difference between the enthalpy at point 2 minus the enthalpy at point 1 and then multiplying that by the mass flow rate of the refrigerant.
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