How Do Water Chillers Work?

A chiller or chilled water system is a type of refrigeration system that uses water as a secondary refrigerant. They are used in larger and more complex heating, ventilation, air conditioning and refrigeration (HVACR) applications. Typical applications for chillers are listed below.

ㆍDistrict cooling

ㆍCentral air conditioning

ㆍHydroponics

ㆍFood and beverage processing

ㆍPharmaceutical and medical

ㆍCold Storage

ㆍThermal Energy Storage (TES) systems

ㆍMachining, waterjet cutting, laser cutting, welding, etc.

ㆍPlastic processing   

Water Cooled Screw Chiller

As opposed to chillers, direct expansion (DX) refrigeration systems cool air directly from the room or enclosed space without the use of secondary refrigerants. The DX type is more suitable for smaller applications, such as residential space cooling and small freezers and refrigeration units.

Principle of operation

Chiller systems consist of two main loops or circuits. These are the refrigeration circuit and the chilled water circuit. The refrigeration circuit is the subsystem that provides the cooling. This is where the thermodynamic processes take place. The chilled water circuit, on the other hand, is a distribution system where the chilled water is supplied to the consumer unit. The processes involved in this system are mainly heat transfer.

Vapour compression cycle

The refrigeration circuit works on the principle of a vapour compression refrigeration cycle. In this cycle, a chemical substance called refrigerant is alternately changed from liquid to gas and back again from gas to liquid by means of a heat exchanger. In addition to changing the phase of the refrigerant, the compressor and expansion valve are used to pressurise and depressurise the fluid. The steps of a typical vapour compression cycle are explained in detail below.

Air Cooled Screw Chiller

Compression

At the start of this part of the cycle, the refrigerant is in a low-pressure vapour phase at the same temperature as the ambient air. It carries with it the heat absorbed from the evaporator.

The compressor pressurises the vaporised refrigerant and discharges it to the high-pressure side of the system. At this pressure, the temperature of the refrigerant becomes higher than the temperature of the surrounding air or ambient.

To increase the pressure of the refrigerant, mechanical energy is consumed. The compressor uses the shaft power provided by the electric motor to increase the pressure of the vaporised refrigerant. The power input to the system occurs in this step.

Condensation

The condenser is the high-pressure side of the refrigeration unit. It is a heat exchanger that is used to transfer heat from the refrigerant to the environment. Heat transfer occurs because of the thermal gradient in the temperature difference between the refrigerant and the environment. The heat absorbed from the evaporator and the heat generated by the compressor is discharged into the environment.

The environment acts as a heat absorber and absorbs the heat discharged from the system. This heat sink can be outdoor air for air-cooled chillers and water for water-cooled chillers.

As the refrigerant cools in the condenser unit, it returns to its liquid state. This is because, at this pressure, the saturation temperature of the refrigerant is higher or almost the same as its current temperature. The saturation temperature is the point at which a substance begins to vaporise or liquefy when the temperature is raised or lowered further. It is an inherent characteristic of refrigerants.