On the performance of a desorber for absorption heat pumps with a thermosyphon and a surface-flame burner

Abstract

Experiments were carried out to study the operation characteristics of a desorber for a lithium bromide (LiBr)-water absorption heat pump. In order to reduce the size and enhance the thermal efficiency of air-cooled absorption heat pump, a thermosyphon and a surface-flame burner were used. The generation rate of refrigerant, circulation rate of solution and concentration difference in LiBr solution were used as an indication of the performance, and the relationship between these variables were studied. With the heat supply increasing from 11.7 to 21.2 kW, the solution circulating rate was found to increase at a rate that is proportional to the square root of the heat supply. The refrigerant generating rate also increased linearly with the heat supply. The difference in LiBr concentration between the strong and the weak solution was increased with, and linearly proportional to, the heat supply. Also, the vapor-solution circulating ratio was linearly proportional to the increasing heat supply. So the difference in LiBr concentration is proportional to the vapor-solution circulating ratio. The discharge limit, defined as the discharge height where the circulation of solution stops, was shown to increase with the heat supply from 61 cm at 19.3 kW to 64 cm at 23.1 kW. The effect of the solution level in a heat exchanger tube and the effect of initial LiBr concentration of solution was also studied. With the constant heat supply, the circulation rate of solution is decreased with the discharge height and rapidly drops to zero at the discharge limit. The generation rate of refrigerant remains unchanged over the wide range of the discharge height. Therefore, the vapor-solution circulation ratio will increase quickly with discharge height. So the concentration difference was increased as the discharge height was raised. The required concentration difference will be obtained by determining the initial solution level in tubes and the adequate circulating rate of solution. The generation rate of refrigerant was nearly constant between 55.5 to 59.5% initial solution concentration. This means that the generation of refrigerant is not strongly dependent on the initial concentration of solution.