The absorption refrigeration cycle



Simplified block diagram of a single stage ammonia absorption refrigeration plant with the main components.


  1. evaporator
  2. absorber
  3. desorber
  4. condenser
  5. rectification column
  6. solution heat exchanger
  7. subcooler
  8. high pressure pump
  9. solution expansion valve
  10. ammonia expansion valve

Single stage ammonia absorption refrigeration cycle


An ammonia absorption refrigeration plant consists of two circuits, the ammonia circuit described with the red lines and the water-ammonia solution circuit with the blue ones.

Ammonia changes state from liquid to gas in the evaporator (1) producing the refrigeration capacity at a temperature from a few degrees down to – 60 deg C. The natural tendency of water to absorb ammonia permits it to collect the ammonia vapor by the weak solution in the absorber (2).  This absorption of ammonia vapor in water is exothermic. The generated heat has to be dissipated to grant a continuous and high absorption capacity of the solution. The enriched solution coming from the absorber is pumped (8) to the desorber (3). In the desorber the solution boils and due to its higher volatility mainly ammonia evaporates. To maintain a high efficiency ammonia must be as pure as possible, therefore it’s distillated in the rectification column (5). This almost pure ammonia vapor liquefies in the condenser (4) and is ready to evaporate again (1).

For efficiency consideration, two heat exchangers are added to the system. The solution heat exchanger (6) is used to recuperate the heat from the weak solution and to pre-heat the strong solution. In the subcooler (7) the cold ammonia vapor leaving the evaporator (1) is used to pre-cool the liquid ammonia before it enters the evaporator.

Absorber & desorber


ABSORBER

This is where the absorption of the ammonia vapor by the ammonia water solution is taking place. The solution is poor in ammonia and gets enriched by the ammonia vapor coming from the evaporator. This absorption reaction is exothermic, the heat generated must be rejected in order to maintain the performance of the absorber. Generally, this heat is moved from the absorber via a heat exchanger to a cooling tower which dissipates it to the ambient temperature. In some configurations the solution could flow directly into a cooling tower. The strong solution is then pumped to the desorber.


DESORBER

This component, also called generator, does the opposite of the absorber. The desorber is heated in order to boil out the solution and release the ammonia vapor. Ammonia absorption plants are typically driven by hot water or steam but with an adapted design, it is possible to get the required heat directly from any hot gases or liquids. The lower the desired refrigeration temperature is, the higher the required heat at the desorber must be.

Condenser & evaporator


These components do not differ much from their equivalents in conventional compression systems.


CONDENSER

In the condenser the high-pressure ammonia vapor is liquefied, it can be water-cooled, air-cooled or evaporative condenser. The liquid ammonia flows through the expansion valve into the evaporator.


EVAPORATOR

The evaporator is the heat exchanger where the refrigeration capacity is produced, cooling down the side medium. It might be one single evaporator or many evaporators in parallel distributed through the factory.

Double stage evaporation


It’s common to have the need for more than a single cooling demand, often different temperature levels and various refrigeration capacities are necessary.

For example, a split of absorption plant in two stages of absorption / evaporation permits you to generate two separate temperature levels and two refrigeration capacities. The solution leaving the low temperature absorber (2a) has an intermediate concentration and is capable of absorbing more ammonia vapor in the high temperature absorber (2b). In this way the overall efficiency is improved. 

Absorption plant integration


There are different ways to transfer the refrigeration capacity from the absorption plant to the process or equipment to be cooled.


  1. Direct evaporation: The ammonia of the absorption plant leaving the subcooler is collected in a vessel and pumped to the different evaporators (e.g. air coolers).
  2. Brine cooling: The evaporator of the absorption plant is a heat exchanger where a cooling brine is pumped through and cooled down.
  3. Cascade ammonia cooling: The ammonia from a compressor refrigeration plant can not be mixed with the ammonia of an absorption plant. Both systems can be installed in parallel using an evaporator-condenser heat exchanger, also called cascade. In this heat exchanger, on the side of the absorption plant the ammonia evaporates while on the compressor side the ammonia condensates. The absorption plant is doing the job of a compressor with the condenser but returns the condensate at a low temperature.
COP


On an absorption refrigeration processes the COP (coefficient of performance) is used to define the efficiency of the process. It is the ratio between the produced refrigeration capacity and the hot temperature heat input.

The COP of an ammonia absorption refrigeration plant depends on the three temperatures that define the process:

  1. The heat source temperature, usually between 75°C and 200 °C, may be steam, hot water or any other fluid with a sufficiently high temperature.
  2. The heat sink temperature, usually the cooling water temperature is in a range of 10°C to 35°C.
  3. The evaporation temperature, as the process uses ammonia as refrigerant, is normally between 0°C and -60°C.

The electric energy needed to drive the solution pump is in the range of 1 – 2 % of the heat consumption and is not taken into account on the calculation of the COP.

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