Chiller - Vapor-compression Chiller Technology

Vapor-compression Chiller Technology

There are four basic types of compressors used in vapor compression chillers: Reciprocating compression, scroll compression, screw-driven compression, and centrifugal compression are all mechanical machines that can be powered by electric motors, steam, or gas turbines. They produce their cooling effect via the "reverse-Rankine" cycle, also known as 'vapor-compression'. With evaporative cooling heat rejection, their coefficients-of-performance (COPs) are very high; typically 4.0 or more.

COP

Current vapor-compression chiller technology is based on the "reverse-Rankine" cycle known as vapor-compression. See the attached diagram which outlines the key components of the chiller system.

Key components of the chiller:

Refrigeration Compressors - are essentially a pump for refrigerant gas. The capacity of the compressor, and hence the chiller cooling capacity is measured in kilowatts input (kW), Horse Power input (HP), or volumetric flow (M3/hr ft3/hr). The mechanism for compressing refrigerant gas differs between compressors, and each has its own application. Common refrigeration compressors include Reciprocating, Scroll, Screw, or Centrifugal. These can be powered by electric motors, steam turbines or gas turbines. Compressors can have an integrated motor from a specific manufacturer, or be open drive - allowing the connection to another type of mechanical connection. Compressors can also be either Hermetic (welded closed) or semi-hermetic (bolted together).

In recent years, application of Variable Speed Drive (VSD) technology has increased efficiencies of vapor compression chillers. The first VSD was applied to centrifugal compressor chillers in the late 1970s and has become the norm as the cost of energy has increased. Now, VSDs are being applied to rotary screw and scroll technology compressors.

Condensers can be air cooled, water cooled, or evaporative. The condenser is a heat exchanger which allows heat to migrate from the refrigerant gas to either water or air. Air cooled condenser are manufactured from copper tubes (for the refrigerant flow) and aluminium fins (for the air flow). Each condenser has a different material cost and they vary in terms of efficiency. With evaporative cooling condensers, their coefficients-of-performance (COPs) are very high; typically 4.0 or more.

The expansion device or refrigerant metering device (RMD) restricts the flow of the liquid refrigerant causing a pressure drop that vaporizes some of the refrigerant; this vaporization absorbs heat from nearby liquid refrigerant. The RMD is located immediately prior to the evaporator so that the cold gas in the evaporator can absorb heat from the water in the evaporator. There is a sensor for the RMD on the evaporator outlet side which allows the RMD to regulate the refrigerant flow based on the chiller design requirement.

Evaporators can be plate type or shell and tube type. The evaporator is a heat exchanger which allows the heat energy to migrate from the water stream into the refrigerant gas. During the state change of the remaining liquid to gas, the refrigerant can absorb large amounts of heat without changing temperature.