How does a chiller system work?

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Chiller System Overview

Chillers use either a vapor-compression or absorption refrigerant cycle to cool a fluid for heat transfer. Both chiller types rely on three basic principles.

  • First - When a liquid is heated it vaporizes into a gas, and when a gas is cooled it condenses into a liquid

  • Second - Lowering the pressure above a liquid reduces its boiling point and increasing the pressure raises it

  • Third - Heat always flows from hot to cold.


Basic Cooling Cycle

The basic cooling cycle is the same for both vapor-compression and absorption chillers. Both systems utilize a liquid refrigerant that changes phase to a gas within an evaporator which absorbs heat from the water to be cooled.

The refrigerant gas is then compressed to a higher pressure by a compressor or a generator, converted back into a liquid by rejecting heat through a condenser and then expanded to a low- pressure mixture of liquid and vapor that goes back to the evaporator section.  The cycle is repeated.

Vapor Compressor Chiller

Vapor Compressor Chiller

A vapor-compression chiller consists of four primary components of the vapor-compression refrigeration cycle. They include a compressor, evaporator, condenser and a metering device.

Vapor-compression chillers typically utilize HCFC or CFC refrigerants to achieve a refrigeration effect. Compressors are the driving force in a vapor-compression chiller and act as a pump for the refrigerant.

Compressed refrigerant gas is sent from the compressor to a condenser unit that rejects the heat energy from the refrigerant to cooling water or air outside of the system.

The transfer of heat allows the refrigerant gas to condense into a liquid which is then sent to a metering device.

The metering device restricts the flow of liquid refrigerant which causes a drop in pressure. The drop in pressure causes the warm refrigerant liquid to change phase from liquid to gas and in doing so absorbs heat from the water to be cooled due to adiabatic flash evaporation.

The metering device is positioned so that the expanding refrigerant gas is contained within the evaporator, transferring the heat energy from the water to be cooled into the refrigerant gas. The warm refrigerant gas is then sent back to the compressor to start the cycle over again and the newly chilled water in the separate loop can now be used for cooling.

Absorption Chiller

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An absorption chiller uses a heat source to drive the refrigeration cycle in place of a mechanical compressor. Absorption chillers utilize two fluids which include a refrigerant, typically water and an absorbent, typically a lithium bromide solution or ammonia.

These liquids are separated and recombined in the absorption cycle where due to the low pressure conditions the water can more easily change phase than it normally would, and the high affinity of the two liquids promotes easy absorption.

The cycle begins with a mixture of liquid refrigerant water and absorbent that is heated at a higher pressure to boil the water out of solution. The refrigerant water vapor is then sent past a condenser coil where heat rejected and it is condensed into a high pressure liquid. The liquid refrigerant water is then sent to the lower pressure evaporator where through adiabatic flash evaporation returns to a gas, absorbing the heat from the water to be chilled. The concentrated liquid absorbent from the generator is sent back to be recombined with the low-pressure refrigerant vapors returning from the evaporator starting the cycle over again.

Cooling Towers

The heat energy absorbed by the chiller needs to be rejected out of the system and into the atmosphere. Evaporative heat rejection devices called cooling towers are typically used to lower the water temperature in large chiller systems.

Author: Ryan.Shea@siemens.com