IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
The heat ratio and the COP thermal for the various applications of an absorption refrigeration
process depend on the kind of working fluid and the application of the process.
2.2.3.2 Design of an absorption refrigeration process
The design of an absorption process will be demonstrated for the working fluid pair ammonia
as refrigerant and water as the absorbent. The thermodynamic properties for the ammonia
water mixture are well known. The tool for process configuration is the lgp,1/T-diagram for
the basic process definition and the table of Merkel-Bosnjakovic for process design at a
higher accuracy. A basic process definition is shown with the help of the lgp,1/T-diagram in
Figure 5.
Figure 5: Basic design of a single stage, continuous absorption refrigeration process in the
lgp,1/T-diagram for working fluids with positive heat of the chemical reaction.
The following items have to be determined for the basic design of a refrigeration process:
- Working pair (NH3/H2O; H2O/LiBr)
- Evaporation temperature (To)
- Temperature of cooling water (heat rejected at Ta,Tc)
The process starts in Figure 5 at point 4. The working fluid, an ammonia/water mixture, is
pumped out of the absorber (30 … 40°C) at a pressure of about 3 to 4 bars to a pressure of
10 to 13 bars into the generator with the temperature T g,in . In the generator the temperature
rises to T g,st and the working fluid starts boiling. The refrigerant (NH3) is separated from the
working fluid by boiling from point 1 (T g,st ) to 6 (T g,e ). At point 6 the WF boils at the highest
temperature T g,e and the concentration reaches ξ w . With the help of a pressure reduction
valve (WFCV) the working fluid leaves the generator at point 6 and is reduced to the low
pressure stage at point 5. On its way from 5 to 4 the weak WF absorbs the refrigerant, which
comes from the evaporator 3 in the absorber 4. The absorber has to be cooled continuously.
The refrigerant (NH3) is separated from the working fluid in the generator between point 1
and 6, and is condensed in the condenser (point 2) on cooled surfaces at the high pressure
stage. After condensing, a pressure reduction valve reduces the pressure of the liquid
refrigerant before entering the evaporator at 3. The refrigerant receives heat at the low
temperature (+5 … -30 °C) in the evaporator 2 and can now boil and evaporate at low
temperature. The vapour of the refrigerant enters the absorber at point 4 and meets the weak
working fluid. Further information on that matter is offered in Niebergall [2] and Bogard [3].
page 9