Refrigeration can be accomplished by various means. Typically, there are 3 components - the evaporator, compressor and condenser.

An expansion valve controls the amount of high-pressure liquid refrigerant entering an evaporator. As the refrigerant passes through the orifice of the valve, the pressure falls, causing it to vaporize and absorb heat.

The compressor then takes the low-pressure vapor and increases both its pressure and temperature. This hot, high-pressure gas is pumped through the compressor discharge valve into the condenser, where it is cooled by either air or water. As heat is removed, it condenses to a liquid and the cycle starts again.

Main Types of Chillers: Chillers are typically categorized by size - compact or central (measure in tons of refrigeration capacity) - and by the type of condenser - air or water.

Air Cooled Chillers: Compared to water, air is a poor conductor of heat and therefore air-cooled chillers are larger and less efficient. The typical condensing temperature for an air-cooled chiller is 120°F as opposed to a 105°F in a comparable water-cooled chiller. Air-cooled chillers also operate at higher compressor ratios – which means less cooling per watt of energy consumption.

For the smaller, portable chillers, the air-cooled condenser is normally integrated into the chiller box and the heat is expelled into the area around the chiller. (This heat can be reclaimed to supplement a building’s heating system in the winter.) In the summer, the expelled will cause extra load on the building’s air conditioning system unless the chiller and/or condenser is located outdoors.

There two types of air-cooled chillers:

• Split systems: With a split system or “remote condenser” the chiller is indoors and the condenser outdoors.
• Integrated Systems: With an integrated system you can put a small unit indoors since the load on the air-conditioner will not be great, or you can install a condenser indoors and duct the heated air outdoors. You can always locate the entire chiller outdoors.

Water Cooled Chillers: Water-cooled chillers use condensers which are essentially heat exchangers. There are three basic designs; tube in tube, shell in tube or brazed plate.

Water-cooled condenser units are more efficient than air condensers, often operating in the range of 15 SEER or better (SEER is the Seasonal Energy Efficiency Ratio or BTU/hr/ Watt of energy used). Water cooled chillers require a source of cooled water, such as cooling tower water, to extract heat from the refrigerant at the condenser and expel it to the environment. The typical condensing temperature in a water-condensed chiller is 105°F.

Evaporative Condensers: An alternative to air or water-cooled condensers is an evaporative condenser, which is like a cooling tower with a built-in heat exchanger. Refrigerant passes through a copper tube bundle in the evaporator, while water cascades over its outer surface. The air-flow which is in the oppostive direction to the water-flow causes some of the water to evaporate. This results in efficient cooling of the refrigerant.

There is a sump in the bottom of the condenser to store water which is sprayed over the coils. In Winter, the pump is shut off and the air flowing across the coils is sufficient to cool the refrigerant.

Types of Compressors: The compressor is the heart of a chiller. Compressors are used to increase the pressure of the refrigerant. There are a variety of compressors available, but the most common are the reciprocating, scroll and screw compressors.

Reciprocating Compressors: Driven by an electric motor, reciprocating compressors use pistons and valves to compress the refrigerant. These compressors are normally available as hermetic, semi-hermetic or external versions.

In a hermetic unit, the motor and compressor are enclosed in a sealed housing. Because the components are not accessible for repair, the entire compressor unit must be replaced if it fails. In a semi-hermetic unit the motor is also part of the unit, however it is not sealed, so it can be serviced.

In a direct drive unit, the motor and compressor are separated by a flexible coupling. These types of units utilize older technology and are not commonly used today.

Scroll Compressors: Scroll compressors perform at higher efficiency levels than reciprocating compressors. The compressors operate without cylinders, pistons or valves so it offers:

• Low noise and vibration
• Low space requirements
• Low maintenance and high reliability

Inside the scroll compressor, two helical spirals fit together with spaces between them. As the helical spirals rotate, refrigerant is forced toward the center of the scroll, developing increasingly higher pressures.

With several spaces compressing simultaneously, there is a nearly continuous compression cycle.

Screw Compressors: A screw compressor also has significant benefits:

• Fewer compressor parts
• Low noise and vibration levels
• Low maintenance and high reliability

The screw compressor’s suction, compression and discharge all occur in one direction. The screw-like motion compresses the refrigerant where the gas discharges to the condenser.

This design minimizes wear and the lubricating oil performs both as a lubricant and a sealer. Motor cooling is provided by the refrigerant flowing through holes in the screw – which also operates as a “built-in” liquid separator. The screw design allows higher evaporating temperatures, which are good for all applications except those requiring lower temperatures.