Chiller

Absorption Chillers

In absorption systems, a thermal compressor—comprising a generator, absorber, pump, and heat exchanger—is used to boil the refrigerant within a lithium bromide (LiBr) solution and to compress the refrigerant vapor to a higher pressure. Increasing the refrigerant pressure also raises its condensing temperature, meaning the refrigerant vapor condenses into liquid at a higher temperature and pressure. Since this condensing temperature is higher than the ambient temperature, heat is rejected from the condenser to the surroundings.

The high‑pressure liquid then passes through an expansion device, where its pressure is reduced, and as the pressure continues to drop, its boiling point decreases. This low‑pressure liquid then enters the evaporator and begins to boil. Because the boiling temperature is now lower than the air temperature, heat is transferred from the air to the refrigerant—causing the refrigerant to evaporate while cooling the air itself.

The refrigerant vapor then re‑enters the absorber, where it is absorbed and condensed by the lithium bromide solution. As the LiBr absorbs the vapor, it releases heat, becoming diluted, and is pumped to the generator. Since the solution does not boil, heat is applied to separate a portion of the solution as vapor, leaving pure lithium bromide, which is returned to the absorber—and the cycle repeats continuously.

In central heating and HVAC systems, the boiler already used for building heating is typically employed to serve as the generator for these chillers.

Advantages of absorption chillers

• Lower electricity consumption
• Reduced noise and vibration
• Longer service life
• Minimal need for major maintenance and overhauls
• No use of ozone‑depleting refrigerants

One of the main drawbacks of absorption systems is the risk of crystallization of the concentrated lithium bromide solution within the refrigeration cycle. When the LiBr solidifies, the cycle stops. To restart the system, the crystallized area must be heated to melt the solid and restore liquid flow. Causes of crystallization include improper purging of non‑condensable gases, excessive fluctuations in chilled water temperature, and incorrect LiBr solution concentration.

Vapor Compression Chillers

In vapor compression chillers, the refrigerant gas is compressed by a compressor, then enters the condenser, where it is cooled by either air or water and condenses into a liquid. This liquid then passes through an expansion valve or capillary tube and enters the evaporator, which operates at a lower pressure. The pressure drop causes the liquid to evaporate, and as the refrigerant evaporates, it absorbs heat from the surrounding medium, thereby cooling it. The resulting vapor is then drawn back to the compressor.

The condensers of chillers can be either air‑cooled or water‑cooled. A condenser is a heat exchanger that allows heat to be transferred from the refrigerant to the surrounding air or water. Air‑cooled condensers typically consist of copper tubes (for the refrigerant) and aluminum fins (for airflow).

The properties of the refrigerant used in vapor compression cycles play a crucial role in determining compressor power, cycle efficiency, and application range.

Key Features of refrigerants include

• Suitable condensing and evaporating pressures
• Low boiling point at atmospheric pressure
• High latent heat of vaporization
• Low specific volume
• High thermal conductivity
• Non‑toxic and with no ozone depletion potential

In compression systems, additional components such as filter‑dryers, strainers, solenoid valves, sight glasses, pressure gauges, and others are also used.

Advantages of vapor compression chillers

• Higher coefficient of performance (COP)
• Lower initial cost
• No risk of crystallization