Before buying an air-cooled chiller, it is important to know what the term "air-cooled chiller" means. Air-cooled chillers, like all chillers, require cooling - the term 'air-cooled' simply refers to how the chiller discharges heat. Air-cooled chillers transfer heat from one medium to another by using surrounding air to cool a finned tube refrigerant-air heat exchanger.
The design of air-cooled chillers varies depending on where the equipment is used. Some chillers are designed for outdoor or indoor applications. Other air-cooled designs, called split systems, allow for the cooling section of the unit to be installed indoors and the hot air discharge section or condenser to be installed outdoors.
When considering your options, remember that air-cooled chillers produce more heat than they absorb. The total heat discharge is equal to the heat discharge rating of the chiller plus the electrical input of the compressor. If you are using an air-cooled chiller indoors, the room in which the chiller is installed must be well ventilated, have adequate operating space and a consistent ambient temperature of 65 to 95°F (18 to 35°C).
If the room where the chiller is to be installed is poorly ventilated, has limited operating space or is air-conditioned, the chiller should be installed outside or in a split system. Installing the cooling section indoors and the condenser outdoors offers several advantages over installing the entire chiller outdoors. They include
Smaller space requirements. Without the air-cooled section, the chiller has a much smaller footprint.
Lower service costs. With the cooling section indoors, the unit is easier to use.
Lower operating costs. Split systems installed in northern climates do not require antifreeze if the process water temperature is above 48°F (9°C).
For any chiller application, understand the chiller's operating limitations. Process cooling chillers must be able to operate at temperatures of -20°F (-29°C) in northern climates and 0°F (-18°C) in southern climates. These design constraints distinguish HVAC units from process chillers: HVAC chillers do not typically operate at temperatures below 32°F (0°C), whereas process chillers are expected to operate at a variety of temperatures throughout the year.
When choosing an outdoor air-cooled chiller, look for a design that is less prone to refrigerant leakage. Refrigerant leaks are the main cause of chiller shutdowns.
Remember that the compressor or pump that delivers and compresses the refrigerant is the heart of the chiller. Chillers using rotary technology compressors are less likely to fail. These rotary technology units are called scroll or screw compressors and have no connecting rods, pistons or valves. screw compressors are used above 40 hp and scroll compressors are used below 30 hp.
Air-Cooled Screw Chiller
Look for indoor air-cooled chillers with washable, reusable filters. Plastic dust, usually found in plastic forming environments, can be drawn into the chiller's air intake, reducing cooling efficiency. This will eventually shut down the chiller if left unattended. Simply cleaning the filter can save money and is cheaper than cleaning the finned tube heat exchanger.
How the waste heat is discharged is another consideration. If you choose to deliver chiller discharge air throughout the plant, ensure that the chiller fan can withstand sufficient static pressure to overcome the frictional losses caused by duct work.
Accurate control of the discharge water temperature is essential. Look for chillers that use off-the-shelf PID temperature controllers. This type of controller offers an auto-tuning feature that allows you to tailor the control to your process. Proprietary controls do not offer this. As with outdoor chillers, the compressor is the heart of the indoor chiller and rotary compressor technology offers the same advantages.
Finally, remember that a rusty system can impede heat transfer and lead to costly repairs. Consider using chillers with non-ferrous water circuits. Pump tanks, pumps, evaporators (refrigerant-water heat exchangers), pipes, valves and fittings should be made of non-ferrous materials such as brass, stainless steel, rubber or plastic. When purchasing any type of equipment, knowledge is key. These tips on indoor and outdoor air-cooled chillers will help ensure that the next air-cooled chiller you buy will be the right chiller for your situation.
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