How do absorption chillers work?

The original design of an absorption chiller is credited to the likes of Ferdinand Carré, Carl Munters and Baltzar Von Platen, renowned scientists who did their works between the 1850s and 1920s. While the products were first produced commercially in 1923, it was only in the 60s that serious production started due to rising demand for caravan refrigerators.

The legendary contributions of these men are still in visible today —they are playing a crucial role in district cooling systems—. Recently, research and development in absorption cooling technology has increased due to higher interest in decentralized energy systems and the ever-tightening energy efficiency regulations.

Absorption chillers have proven to be ideal replacements for compressor chillers in places where power is unreliable, unavailable or costly, where waste heat is available, or where restrictions on noise deem compressor chillers useless. Comparing the chillers with compression chillers may give more insight into how do absorption chillers work.

 

Comparison with compression chillers

An absorption chiller has a very straightforward operation. Its operation is fundamentally similar to what happens in a vapor compression chiller in that both processes involve condensation and evaporation of the refrigerant within the system. However, while an absorption chiller uses a thermo-chemical process, a conventional chiller relies on mechanical energy.

Simply put, the absorption chiller does not compress refrigerant vapor; instead, it dissolves the vapor in an absorbent, and transfers the resulting product to higher-pressure environment using a pump with a very low electricity consumption. Of course, this only a description of the basic absorption cycle —there are more complex cycles that even have extra components—.

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Working principle of absorption chiller

Some substances have the peculiar property of having affinity for other substances at certain pressure and temperature conditions, only for this affinity to change if the conditions are altered. Michael Faraday came up with the idea of the absorption chiller based on this concept in 1824.

The principle behind an absorption process is to separate and recombine to fluids (refrigerant and absorbent) to create a cooling effect. Usually, absorption chillers are either NH3-H20 (ammonia-water) cycle or LiBr (Lithium bromide) cycle.

In the first cycle, water acts as the absorbent while ammonia water solution acts as the refrigerant. In the latter cycle, lithium bromide is the absorbent and water is the refrigerant. Most industrial chillers use the ammonia-water vapor absorption system because of the following benefits:

  • High solubility of ammonia in water.
  • Ammonia water absorption chiller works with positive pressure (Li-Br works with negative pressure) reducing maintenance problems and making the machine to be more robust
  • Ammonia water absorption chiller can work with extreme conditions (high condensation temperature and low evaporation temperature).
  • Able to cool glycol at negative temperatures.
  • Compatible with air cooled condenser (zero water consumption).

 

How do absorption chillers work: step by step explanation

absorption chillers

Fig 1: Representation of a Simple Absorption Cycle

 

  • Generator: in the generator, a heat source produces ammonia vapor from a strong ammonia solution. Before the ammonia (refrigerant) vapor enters the condenser, it passes through a rectifier for dehydration.
  • Condenser: the now dehydrated and high-pressure ammonia enters the condenser where it is condensed. After cooling, it goes through a throttle valve (expansion valve) and pressure and temperature is reduced. The new values must be below what the evaporator (next stage) maintains.
  • Evaporator: the evaporator, which is essentially the cold refrigerated space, appears now. The cooled ammonia enters the evaporator, absorbs heat and then leaves as saturated ammonia vapor.
  • Absorber: as the vapor enters the absorber, it is exposed to a spray of weak ammonia-water solution. The weak solution in turns becomes a strong solution. The pump directs the new solution to the generator through the regenerator (may also be referred to as heat exchanger). By the time the solution arrives, it has already attained generator/condensing pressure. The process starts again.

 

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Reasons an absorption chiller would be preferable

We touched on this slightly at the start of this post. Going by the description of the operation and requirements of absorption chillers, the following are scenarios where the chillers would be preferable.

  1. High electricity costs and low fuel costs. Make sure the differential is huge enough.
  2. There is no enough electrical power available.
  3. Waste  heat is available (for example from exhaust flow or hot water from engine jacket).
  4. Availability of enough hot water or low grade waste steam.

It will also suit areas where a quiet environment is the priority —an absorption chiller is quiet, wear-free system due to lack of moving parts— minimal maintenance requirements.

 

The bottom line

Are you still wondering, “how do absorption chillers work?”. ARANER has been doing cooling and heating load analyses for many years, so they can help. If you are considering installing an absorption chiller, it is worthwhile to conduct a feasibility study before injecting your money into a huge project. Such a study shows whether indeed there are any economic advantages. Absorption chillers work in a manner that justifies them where peak electricity demands are high. Using heat recovery is another opportunity for cost savings for facility owner. Get in touch with the team for guidance and support.

 

 

 

 

 

 

 

 

 

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