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Cooling – Vapor Compression Chillers

Vapor compression cooling is a very mature technology and widely used in large commercial buildings for air conditioning. It is also found in industry for refrigeration of foodstuffs and in the processes of the oil and chemical industries.

The electrically driven vapor compression cycle uses an electric motor driven compressor and a refrigerant to cool and dehumidify building air by absorbing heat from the area to be cooled and rejecting that heat, either via air or water. The vapor compression process can also be accomplished using a natural gas engine, which allows for variable-speed operating capability and greater efficiency when operating partial loads. Natural gas systems also offer the advantage of waste heat recovery from the engine jacket and exhaust gas, which can save energy and cost when used to supply thermal energy for domestic hot water, steam generation and other applications.

How It Works
In a vapor compression system, a circulating liquid refrigerant is used to absorb and remove heat from a stream of chilled water, which is sent to air coils to cool and dehumidify the space to be conditioned.

Single Stage Compression Cycle

The chiller consists of four main components: the compressor, condenser, expansion valve and evaporator. The compressor raises the pressure and temperature of the refrigerant to a vaporous level where it can be condensed and heat can be removed in the condenser. In the condenser, the refrigerant is condensed using either cool air or water and the rejected heat is carried away by either water or air. The expansion valve reduces the pressure and the temperature of the refrigerant, which absorbs heat from the chilled water in the evaporator. Because heat is removed from the chilled water stream, the chilled water is cooled. The refrigerant returns to the compressor where the cycle begins again.

Types of Compressors
Four types of compressors are commonly found in vapor compression chillers. Size, acceptable noise levels, efficiency and pressure issues determine the best option for a given application. Generally, smaller chillers (less than 100 tons) are reciprocating, screw or scroll. Reciprocating, rotary screw and scroll compressors are positive displacement compressors; centrifugal is a dynamic compressor.

  • Reciprocating – piston-style compressor; generally least efficient; uses multiple compressors to provide stages of cooling capacity (unloading)
  • Rotary Screw – two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor and reducing the volume of the refrigerant along the rotors to the discharge point; range in size from 10 to 1100 tons, but most common in 100 to 300 ton size; uses slide valve for unloading
  • Scroll – refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures; by the time the refrigerant is discharged, it is fully pressurized; uses multiple compressors for unloading
  • Centrifugal – raises the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller and converting it to pressure energy; generally most efficient at full and part loads; range in size from 70 to 9000 tons; uses inlet vanes for unloading

Peak Shaving and Hybrid Systems
Gas engine-driven chillers are more expensive to purchase than electric motor-driven chillers, but typically cost less to operate. Electric demand charges are an integral component of the cost savings potential of gas engine-driven chillers. Since cooling is generally a cause of peaks in electric usage, gas engine-driven chillers can contribute significantly to peak-shaving. Businesses with high electric costs and high peak demand charges can benefit from a hybrid system that combines electric chillers for base load with gas engine-driven cooling for peak loads.

More Information About Vapor Compression Systems
The Engine Drive module includes further information about the types of gas engine-drive and electric motor-driven vapor compression systems and the applications and considerations for each.