Absorption chiller design and cycle calculation

The thermodynamic cycle of an absorption chiller is characterised by the working fluids used and the temperatures of generation (desorption), condensation, evaporation and ab­sorption. These temperatures are determined by the external temperatures of the heating, the cooling and the cold water and by the design of the heat exchangers (desorber, con­denser, evaporator, absorber). In high capacity absorption chillers flooded tube bundle heat exchangers are used as generator. This causes a relatively high difference between the temperature of the external heating water and the solution temperature reached inside the desorber. The limit for the solution temperature is the heating water outlet temperature. For the low capacity absorption chiller that was developed a special heat exchanger de­sign is used for the main components (desorber, condenser, evaporator, absorber). They consist of spiral copper tubes over which the solution (generator, absorber) or water (evaporator) is dripped. In the generator there is a cross current flow of the solution trick­ling down the tubes and the heating water inside the tubes. The limit for the solution tem­perature is the hot water inlet temperature.

To reach a high efficiency a plate heat exchanger is used as solution heat exchanger.

The design parameters of the small capacity absorption chiller are:

— Condensation temperature of 40 °C with a cooling water inlet into the chiller of 32 °C and an outlet temperature of 38 °C. The cooling water flows in series first through the ab­sorber and then through the condenser. The absorption temperature is 36 °C.

— Evaporation temperature of 13 °C with a cold water inlet temperature of 21 °C and a cold water outlet temperature of 15 °C.

Figure 2: Absorption chiller with state points

Temperature of the weak solution after generation of 77 °C with a hot water temperature of 90 °C/80 °C inlet/outlet.

Figure 2 shows the configuration of the absorption chiller and the location of the state points of the cycle. Table 1 contains the results of the calculation of the absorption chiller cycle.