Diffusion absorption refrigerator

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A diffusion absorption chiller (DAK) is a modification of the absorption chiller , which in turn is counted as a refrigeration machine .

construction

The diffusion absorption chiller (DAK) consists of the following components:

  1. Expeller - expels the refrigerant from the solution by heating
  2. Dehumidifier - also called a rectifier, separates solvent residues from the vaporous refrigerant
  3. Condenser - liquefies the refrigerant
  4. Evaporator - evaporates the refrigerant, the energy required for this is perceived externally as cooling capacity
  5. Absorber - dissolves the gaseous refrigerant again in the solvent
  6. Heat exchanger for the inert gas - reduces energy losses
  7. Heat exchanger for the solvent - reduces energy losses

In contrast to the compression refrigeration machine (classic refrigerator), a DAK uses three different circulating substances:

  1. Solvent - for example, water with ammonia
  2. Refrigerant - for example ammonia (NH 3 )
  3. Auxiliary gas - for example helium or hydrogen

These only partially run through separate cycles.

functionality

Diffusion absorption refrigerator
Abbreviations:
NiAmWs low-concentration ammonia-water solution
HoAmWs highly concentrated ammonia-water solution
NH 3 ammonia
NiAmHe low concentration ammonia-helium mixture
HoAmHe highly concentrated ammonia-helium mixture
(1) NH 3 (gas) + water vapor
(2) NH 3 (gas)
(3) NH 3 (liquid)
(4) Mixture NiAmHe + NH 3 → HoAmHe
and evaporation.
(5) HoAmHe cold
(6) HoAmHe warm
(7) NiAmHe warm
(8th) NiAmHe cold
(9) Separation HoAmHe → NH 3 + NiAmHe and
Mixture of NH 3 + NiAmWs → HoAmWs
(10) HoAmWs cold
(11) HoAmWs warm
(12) NiAmWs hot
(13) NiAmWs cold

Coolant circuit

The drive that leads to the circulation corresponds to the pump principle of the mammoth pump , i.e. it works without moving parts. The coolant circuit is driven by the heat supply in the expeller.

  • A highly concentrated ammonia-water mixture is strongly heated in the expeller. Some of the ammonia escapes from the solution and high pressure is created. What remains is a low-concentration ammonia-water mixture (12). The ammonia (1) now flows into the dehumidifier.
  • In the dehumidifier, the water vapor that is also generated in the expeller is removed by condensation. Ammonia vapor remains in the circuit (2). This now enters the condenser.
  • The ammonia vapor liquefies in the condenser and heat is dissipated. Part of the pressure generated in the expeller is retained (3).
  • The liquid ammonia enters the evaporator. There it is fed to the low-concentration ammonia-helium mixture coming from the absorber at approx. 9 bar (a highly concentrated ammonia-helium mixture is created) and then the ammonia evaporates and expands (out of the mixture) with absorption of Warmth (4). The desired cooling effect is thus achieved on this component. The ammonia has a low partial pressure of 5 bar. The helium ensures pressure equalization.
  • This highly concentrated ammonia-helium mixture now flows through a heat exchanger (see below) into the absorber (5 and 6). Here the ammonia is absorbed (9) by the low-concentration ammonia-water mixture coming back from the expeller (13). What remains is a low-concentration ammonia-helium mixture (7), which is returned to the evaporator via the heat exchanger (8). The highly concentrated ammonia-water mixture (10) is now returned to the expeller (11) via a solvent heat exchanger.

Solution cycle

The solvent cycle is also driven by the heat supply in the expeller.

The solvent passes through the expeller and absorber, with a heat exchanger between these components ensuring that the energy of the hot, low-concentration solvent coming from the expeller is used to preheat the highly concentrated solvent coming from the absorber.

Auxiliary gas circuit

The auxiliary gas runs through the evaporator and absorber, with a gas heat exchanger between these components ensuring better use of energy. The auxiliary circuit is driven by the difference in density between highly concentrated, cold and low-concentration, warm gas.

Conclusion

advantages

  • No mechanical parts (pump, compressor, ...)
  • Maintenance free
  • Self-regulating
  • Can be built without complex components.
  • Works almost noiselessly.
  • Any heat source (e.g. gas flame or solar energy) is sufficient for operation, electricity is then unnecessary.
  • Excess heat that is already available can be used for cooling (e.g. waste heat from power plants) so that there is no additional energy requirement.

disadvantage

  • Moderate efficiency for absorption refrigerators between 0.1 and 0.2 due to simple construction without rectification (for reasons of cost)
    • Gas-powered absorption refrigerators, however, come very close to the efficiency of the combination of a gas-fired power plant plus an electric compressor refrigerator.
  • Improved efficiency with indirectly thermally heated diffusion absorption chiller (DAKM) between 0.3 and 0.5
  • Difficult construction and layout
  • High tightness requirements against helium loss

Areas of application

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