How does a chiller plant work?

The vapour compression refrigeration cycle (VCRC) is the most widely used refrigeration cycle in the world. It's used to cool buildings, vehicles, refrigerators, and freezers, as well as to provide cooling for many industrial processes. But how does it work?

The vapour-compression cycle is a process that uses a refrigerant to cool and remove heat from a space. The refrigerant circulates through four main components: a compressor, condenser, expansion valve (metering device), and evaporator.

In the compressor, the refrigerant vapour is compressed to a higher temperature and pressure. The high pressure refrigerant vapour is then discharged to a condenser, where heat is transferred from the refrigerant to an external medium, this causes the refrigerant temperature to decrease and it changes state to become a high pressure liquid.

The liquid refrigerant then flows through an expansion valve, which reduces its pressure, causing it to partially evaporate; this results in a mixture of both liquid and vapour refrigerant being discharged from the expansion valve.

The mixture then flows into an evaporator and is heated by an external medium (usually water). The refrigerant absorbs heat from the external medium, which causes its temperature to increase until it evaporates and changes state from a low pressure liquid to a low pressure vapour. The absorption of heat by the refrigerant causes the external medium's temperature to decrease, thus a cooling effect occurs. Once the external medium has been cooled, it can be pumped to a different location and used to remove heat from a space.

After the evaporator, the low pressure refrigerant vapour returns to the compressor and the cycle repeats.

 What are the main parts of a chiller?

The main parts of a chiller are listed below.

Chilled Water Circuit

Heat is rejected from a process or building and discharged to the evaporator. The evaporator uses the heat from the chilled water to boil the liquid refrigerant contained within the evaporator. As the refrigerant evaporates, the cooling water is cooled (evaporative cooling).

Evaporator 

The evaporator is a shell and tube heat exchanger. Liquid refrigerant submerges the tubes and is referred to as the ‘shell side’ fluid. Chilled water flows through the tubes and is referred to as the ‘tube side fluid’. Low pressure liquid refrigerant is heated by the incoming chilled water until it boils and becomes a low-pressure gas. The chilled water is cooled as the refrigerant evaporates.

Chilled water is often taken from comfort heating areas (inside a building), an industrial process, or server rooms, but applications are numerous.

Compressor

The compressor increases the density of the evaporated refrigerant gas. As the refrigerant gas density increases, so too does its temperature. Refrigerant compressors for large chillers are usually of the centrifugal compressor, reciprocating piston, or gear design; smaller compressors use the lobe or scroll compressor designs.

Motor

The prime mover for the compressor is an electric motor. Small compressors can be driven by single phase motors, but larger chiller compressors are driven by 3-phase alternating current (AC) motors.

Condenser

High-pressure refrigerant gas from the compressor is discharged to the condenser. The condenser is usually of the shell and tube heat exchanger design. Refrigerant gas is the shell side fluid whilst cooling water (condenser water) is the tube side fluid.

The high-pressure refrigerant gas is cooled by the cooling water circuit, which causes the gas to condense into a high-pressure liquid; liquid refrigerant is then discharged to the expansion valve, then evaporator.

Expansion Valve

High pressure liquid refrigerant is discharged to the evaporator via an expansion valve. Expansion valves decrease the pressure, decrease the temperature, and increase the volume, of the liquid refrigerant. Due to the reduction in pressure, the high-pressure liquid refrigerant enters the evaporator in a vapour state.

Control Box

The electronics (and usually wiring diagrams) required to control the chiller unit are housed within the control box. Temperature, pressure, flow, and level sensors, relay information to the control box. The control box controls when the compressor motor is turned on/off, as well as acting to alarm or shutdown the unit should an operational problem arise.

Modern motors utilize variable frequency drives (VFD); varying the frequency varies the speed of the motor and thus the compressor. VFDs are useful because it is not necessary to cycle (turn on and off) the compressor as frequently, thus prolonging its useful working life whilst also giving better control over the chiller plant cooling capacity.