Effects of metal mass on the performance of adsorption heat pumps utilizing zeolite 4A coatings synthesized on heat exchanger tubes

The effects of the wall thickness of stainless steel heat exchanger tubes on the performance of adsorption machines, employing zeolite 4A coatings synthesized on metal heat exchanger tubes, are investigated. A recently developed mathematical model is used to determine the cycle durations when various wall thicknesses of the heat exchanger tubes as well as different zeolite layer thicknesses are utilized. For each case, the power and the COPcycle values of the system are estimated. In general, very high power and quite low COPcycle values are obtained when the proposed arrangement is utilized in the adsorption heat pumps. The zeolite layer thicknesses that may result in obtaining high COPcycle values are generally much higher than the optimum layer thickness value that maximizes the power and the utilization of layers thicker than the optimum value may lead to significant extensions in the cycle durations and hence to a decrease in the power obtained from the system. Decreasing the wall thickness of the heat exchanger tubes increases both the power and the COPcycle values when the optimum zeolite layer thickness for each wall thickness is taken into account. The possibility of such an enhancement will most probably be limited by the minimum wall thickness value that can actually be obtained by the available technology. The COP values of adsorption heat pumps may also be increased by using regenerative processes. Due to the generally low COP values obtained, the proposed arrangement seems especially suitable to be employed in adsorption machines utilizing energy sources of low economical value, such as waste heat. An optimum compromise between the COP value, which is closely related to the operating costs, and the power of the system should be provided, in case more valuable energy sources are utilized.