What to Know Before Buying an Air-Cooled Chiller
Before you buy an air-cooled chiller, be sure you understand what the term "air-cooled chiller" means.
Indoor and Outdoor Air-Cooled ChillersAir-cooled chiller designs vary depending upon where the unit is going to be used. Some chillers are designed for dedicated use in outdoor or indoor applications. Other air-cooled designs, known as split systems, allow indoor installation of the cooling portion of the unit and outdoor installation of the hot air discharge portion, or condenser.
When considering your options, remember that an air-cooled chiller will generate more heat than it absorbs. The total heat rejection is equal to the chiller's heat removal rating plus the compressor's electrical energy input. If you are using the air-cooled chiller indoors, the room in which the chiller will be housed must have good ventilation, adequate space for operation and a consistent ambient temperature of 65 to 95°F (18 to 35°C). If the room in which the chiller is to be housed has poor ventilation, limited space for operation or is air-conditioned, the chiller should be installed outdoors, or a split system should be employed. Installing the cooling portion indoors and the condenser outdoors offers several advantages over installing the entire chiller outdoors. They include:
- Smaller Space Requirements. The chiller's footprint is much smaller without the air-cooled section.
- Lower Service Costs. With the cooling portion indoors, the unit is more serviceable.
- Lower Operating Costs. Split systems installed in Northern climates do not require antifreeze if the process water temperature is above 48°F (9°C).
Outdoor Air-Cooled ChillersWith any chiller application, know the operating limitations of your chiller. A process cooling chiller must be able to operate in temperatures to -20°F (-29°C) in Northern climates and 0°F (-18°C) in Southern climates. These design limitations differentiate an HVAC unit from a process cooling chiller: An HVAC chiller typically does not operate at temperatures below 32°F (0°C) while a process cooling chiller is expected to operate year-round at various temperatures.
Process cooling chillers offer individually baffled air sections with horizontally mounted coils that prevent cold winds from whistling through the unit. This design keeps air from being drawn through the fans, which have been cycled off, and helps maintain consistent refrigeration pressures. For temperatures below 0°F, this design also will modulate the speed of the last fan motor in response to refrigerant pressure. This allows the chiller to operate below 0°F.
When selecting an outdoor air-cooled chiller, look for a design that is not prone to refrigerant leaks. Refrigerant leaks are the main reason chillers shut down. Air-cooled heat exchangers use copper tubes that are mechanically expanded into aluminum fins and tube sheets. Often, leaks form at the tube sheet where the weight of the heat exchanger is supported. Seek out a design that does not expand the refrigeration tubes into the tube sheet. Such a design uses a blind or nonrefrigeration tube to support it through the tube sheet. The copper tubes that circulate the refrigerant pass through a hole in the tube sheet that is made larger than the tube, thereby eliminating any chance of wear caused by vibration.
Remember that the compressor, or pump that conveys and compresses the refrigerant, is the heart of the chiller. Chillers that use rotational technology compressors have a lower probability of failure. Known as scroll or screw compressors, these rotational technology devices have no connecting rods, pistons or valves. Screw compressors are used above 40 hp while scroll compressors are used below 30 hp.
Indoor Air-Cooled ChillersLook for an indoor air-cooled chiller that has washable, reusable filters. Plastic dust, typically found in a plastic molding environment, can be drawn into the chiller's air intake, reducing cooling efficiency. If left unattended, this eventually will shut down the chiller. Simply washing the filter can save money and is less expensive than having to clean a finned-tube heat exchanger. Also, look for a unit that has a variable speed fan motor. A discharge fan controlled by a variable speed drive will vary the fan speed with respect to the surrounding air temperature and process heat load. Because a chiller rarely operates at full load, the variable speed drive will lower the fan speed, reducing energy consumption and noise level.
How you will discharge waste heat is another consideration. If you choose to duct the chiller discharge air throughout the plant, make sure the chiller fan is capable of enough static air pressure to overcome frictional losses caused by the duct work.
Accurately controlling the leaving water temperature is vital. Find a chiller that uses an off-the-shelf PID temperature controller. This type of controller provides an autotune capability that customizes the control to your process. A proprietary control does not offer that. As with outdoor chillers, the compressor is the heart of the indoor chiller, and rotational compressor technology offers the same advantages.
Finally, remember that rust in the system can hinder heat transfer and lead to expensive repair costs. Consider using a chiller that has a nonferrous water circuit. The pump tank, pump, evaporator (refrigerant-to-water heat exchanger), piping, valves and fittings should be made of nonferrous materials such as brass, stainless steel, rubber or plastic.
When purchasing any type of equipment, being informed is the key. These hints about indoor and outdoor air-cooled chillers will help ensure that the next air-cooled chiller you purchase will be the appropriate chiller for your situation. PCE