Analysis of internal heat recovery capability in air-cooled indirect fired GAX-based absorption chiller in part-load operation

Abstract

Single-effect absorption chillers are the most popular because of their low cost, despite their low COP. Among them, GAX-based AirCooled absorption chillers are very interesting because they have improved COP because of their ability to recover internal heat at low thermal lifts. This workshop analysed the ability of these types of chillers to recover internal heat at high thermal lifts by changing the pressure drop of an extra valve, a feature of the Robur absorption chiller when operating in subzero applications. The complete differential mathematical model analyses the components involved in heat supply and recovery and provides information on their operation. A thorough assessment of exergy destruction in the absorption refrigeration system was carried out. The main results show that when the chiller is driven at a temperature of 210 °C and an ambient temperature of 40 °C, a rise in the pressure drop of the additional valve, ΔP_val1, from 175 to 700 kPa causes the extension of the vapour purification process to be reduced by 13.7% in the column of distillation and by 70.6% in the rectifier. Despite the adverse effect of ΔP_val1 increase on the cooling capacity, there is no risk that the distillation column operates in weeping mode. However, this adjustment increases the internal thermal load of the generator by 26.9%. Furthermore, the mass fraction of the refrigerant flow is very similar. In addition, the occurrence of a two-phase solution flow at the input of a solution cooling absorber is the practical upper limit of ΔP_val1. The refrigerant flow in the solution cooling absorber is reduced by 21.3%. Finally, the contribution of the heat recovery loop to the total exergy destruction in the refrigeration system rises slightly by 2.7% when ΔP_val1 increases due to the contribution of the vapour purification system, which increases at the expense of that of the re-boiler and the SolutionCooled absorber. The results of this study show to what extent the modified GAX-based cycle can function effectively at high thermal lifts.

Keywords: Absorption; Ammonia-water; Gax; Hot climate; Part-load; Thermodynamics.