HVAC Glossary: Absorption Chiller

An absorption chiller is a type of refrigeration system that uses a heat source to drive the refrigeration cycle, rather than the mechanical compressor found in traditional vapor-compression chillers. This makes them an energy-efficient alternative in settings where waste heat is readily available, such as industrial plants, hospitals, and large commercial buildings. This guide provides a comprehensive overview of absorption chiller technology for HVAC professionals.

Principle of Operation

The fundamental principle behind an absorption chiller is the use of a refrigerant and an absorbent solution. The most common combinations are water as the refrigerant and lithium bromide as the absorbent, or ammonia as the refrigerant and water as the absorbent. The cycle leverages the fact that the refrigerant\'s boiling point is dependent on pressure, and that the absorbent has a strong affinity for the refrigerant.

The Absorption Cycle

The absorption cycle consists of four main components: the generator, condenser, evaporator, and absorber.

1. Generator: A heat source (such as steam, hot water, or exhaust gas) is applied to a solution of refrigerant and absorbent. This heat boils the refrigerant out of the solution, creating a high-pressure refrigerant vapor.
2. Condenser: The high-pressure refrigerant vapor moves to the condenser, where it is cooled by a separate cooling water loop (typically from a cooling tower). This cooling causes the refrigerant to condense back into a liquid.
3. Evaporator: The high-pressure liquid refrigerant then flows through an expansion valve into the evaporator, which is at a much lower pressure. The sudden drop in pressure causes the refrigerant to flash-evaporate, absorbing heat from the chilled water loop and creating the cooling effect.
4. Absorber: The low-pressure refrigerant vapor from the evaporator is then drawn into the absorber, where it is absorbed by the concentrated absorbent solution from the generator. This absorption process releases heat, which is removed by the cooling water loop. The now-diluted absorbent solution is then pumped back to the generator to repeat the cycle.

Types of Absorption Chillers

Absorption chillers are primarily categorized by the number of generators they employ, which affects their efficiency.

Comparison of Absorption Chiller Types

Chiller Type	Description	Typical COP	Heat Source Temperature
Single-Effect	The simplest design, with one generator. Best suited for applications with low-temperature waste heat.	0.6 - 0.8	180°F - 250°F (82°C - 121°C)
Double-Effect	Features two generators, allowing for a more efficient cycle. Requires a higher-temperature heat source.	1.0 - 1.4	300°F - 400°F (149°C - 204°C)
Triple-Effect	The most efficient type, with three generators. Requires a very high-temperature heat source and is less common due to its complexity and cost.	> 1.4	> 400°F (> 204°C)

Applications and Benefits

Absorption chillers are ideal for a variety of applications where both cooling and a source of waste heat are present. For more information on chillers, visit our chillers category page.

• Industrial Facilities: Manufacturing plants, refineries, and power plants often have abundant waste heat that can be used to power absorption chillers.
• Commercial Buildings: Large commercial buildings, such as hospitals, hotels, and universities, can benefit from absorption chillers, especially when combined with a cogeneration (Combined Heat and Power) system.
• District Cooling: Absorption chillers are a key component of many district cooling systems, providing chilled water to multiple buildings from a central plant.

The primary benefits of absorption chillers include:

• Energy Efficiency: By utilizing waste heat, absorption chillers can significantly reduce electricity consumption compared to vapor-compression chillers.
• Environmental Benefits: The use of natural refrigerants with low GWP and the reduction in electricity demand contribute to a lower carbon footprint.
• Quiet Operation: With no major moving parts like a compressor, absorption chillers operate very quietly.
• Reliability and Low Maintenance: The simple design with few moving parts results in high reliability and lower maintenance costs. For maintenance supplies, check out our HVAC parts.

Frequently Asked Questions (FAQ)

What is the primary advantage of an absorption chiller over a vapor compression chiller?

The primary advantage is that absorption chillers can utilize waste heat as their primary energy source, which can lead to significant energy savings and reduced operating costs, especially in facilities with a readily available source of waste heat.

What are the common absorbents and refrigerants used in absorption chillers?

The most common pairs are Lithium Bromide (absorbent) and water (refrigerant), and Ammonia (refrigerant) and water (absorbent).

Are absorption chillers environmentally friendly?

Yes, they are considered more environmentally friendly than traditional chillers because they use natural refrigerants like water and ammonia, which have a low Global Warming Potential (GWP). They also reduce the demand for electricity from fossil fuels by using waste heat.

What is the function of the generator in an absorption chiller?

The generator is where the refrigerant is boiled out of the absorbent solution using a heat source. This process separates the refrigerant from the absorbent, allowing the refrigeration cycle to continue.

Can absorption chillers be used for residential applications?

While technically possible, absorption chillers are most commonly used in large commercial and industrial applications where there is a significant and consistent source of waste heat. Their size, complexity, and initial cost make them less practical for most residential settings.