Maximizing Your Chiller's Cooling Performance
Several guidelines should be followed during and after installation of a portable chiller to ensure that it operates within the manufacturer's published specifications and provides you with the cooling capacity needed for your application. Heat transfer fluid selection, chiller placement and simple maintenance procedures can improve your chiller's cooling performance, extend its service life, and prevent downtime and costs due to excessive maintenance.
Selecting a Heat Transfer FluidThe boiling and freezing points of the heat transfer fluid are important, but several other fluid properties can also affect the equipment's cooling performance. For example, if your process requires cycling of the chiller temperature, the specific heat of the heat transfer fluid selected will affect the performance of the chiller. As the chiller's bath temperature changes, the chiller must not only cool the external process, but it must also cool the heat transfer fluid within its own bath and within the piping to the heat exchanger.
Also keep in mind that a chiller's cooling capacity is limited for a given internal temperature (the temperature inside the chiller's bath). A suitable heat transfer fluid with a relatively low specific heat and low density (such as silicone oil, which has a CP around 1.5 kJ/kg-°C) will require less capacity for cooling the heat transfer fluid, thereby reserving more cooling capacity for the external system during a temperature change or during startup. The typical specific heats of some common heat transfer fluids are shown in table 1.
Locating the Chiller Within the PlantCareful consideration also must be given to the portable chiller's location. In general, the chiller should be located as close as possible to the heat exchanger to decrease the overall length of piping required and thus minimize the ambient heat gain to the heat transfer fluid through the piping. Locating the chiller close to the heat exchanger will also reduce the volume of heat transfer fluid required to fill the circuit, which will allow for faster temperature changes during equipment startup and operation. Additionally, shorter piping will improve the heat transfer in the heat exchanger by decreasing the pressure drop in the circuit and increasing the flow rate of the heat transfer fluid.
Air-cooled chillers should be located away from walls and other equipment to ensure that the inlet and outlet for the airflow remains clear. The chiller also should be located away from any sources of hot air. When insufficient or hot air is flowing through the condenser coils, the condensing temperature in the chiller's refrigeration system must increase to maintain a certain amount of heat transfer to the air, and the chiller's compressor must work harder to maintain the higher condensing temperature (figure 1).
Maintaining the CondenserIf your condenser is dirty, the dirt or dust will insulate the condenser coils, reducing the heat transfer from the refrigerant to the air. As with a high ambient temperature, a dirty condenser causes the condensing temperature of the refrigerant to increase and forces the compressor to work harder.
The frequency of cleanings required depends on the amount of particles in the air flowing through the chiller. To clean the condenser, it is best to vacuum the coils and fins rather than blowing them with compressed air. Blowing the condenser clean will blow the dirt and dust into the chiller, where it can contaminate other components.
Preventing Ice in the BathWater that gets into the heat transfer fluid will freeze on the evaporator coils of the chiller's bath at temperatures near and below freezing. Because ice is a good insulator (with a thermal conductivity kice = 2.2 W/m-K at 0∞C, compared to ksteel = 40 W/m-K or more), any ice that forms on the evaporator coils could considerably limit the cooling capacity of the chiller.
If the chiller is located in an area with humid air, or if a hygroscopic heat transfer fluid is used (e.g., certain silicon oils), you might need to apply a dry-gas purge to the bath tank. Many portable chillers have a small port on the bath cover; applying a small positive pressure of dry gas (dry nitrogen or air) through this port will help exclude the ambient air, along with its attendant humidity, from the tank. Just be sure that the gas is dry enough so that its dewpoint is below the bath temperature. If a dry-gas purge is not possible, you might need to replace the heat transfer fluid more often or periodically run the fluid above 212°F (100°C), if possible, to boil off the water.
Optimizing your chiller's cooling performance doesn't have to be complicated. By following these basic guidelines, you can ensure that your chiller operates properly for the longest possible service life with minimal maintenance requirements. PCE