Chiller Plant

Introduction

This is a 3D model of a refrigeration chiller.

3D Model Annotations

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 utilise 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.

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