MED 담수설비용 열압축기의 흡입부 형상에 관한 연구

Abstract

A thermal vapor compressor is the equipment which compresses a vapor to a desired discharge pressure. Since it was first used as the evacuation pump for a surface condenser, it has been widely adopted for energy saving systems due to its high working confidence. The primary advantages of the thermal vapor compressor are simplicity of an operation, no mechanical driving device and have no moving parts.

Thermal vapor compressor is constructed of three basic parts a jet nozzle, a suction chamber and a diffuser. The high pressure steam is supplied to a steam chest and then expanded through the jet nozzle which is the shape of converging and diverging nozzle and makes the steam accelerating up to Mach number 2.5 to 4.0. The accelerated motive steam is injected into a suction chamber, which entrained the surrounding vapor.

The suction chamber has the lowest static pressure in the thermal vapor compressor, which is approximately equivalent to the suction pressure. The suction fluid enters into the suction chamber and is mixed with the motive steam in the diffuser inlet. The kinetic energy of the motive steam is transferred to the suction fluid through the diffuser throat. By the mixing process of the motive steam and the suction fluid, the motive steam is decelerated while the suction steam is accelerated and their velocity energy is converted to the pressure energy. It means that the mixed steam is re-compressed to the discharge pressure through diffuser outlet.

In the present study, the geometrical analysis of the shape between the jet nozzle and the diffuser inlet, the drag force was calculated by means of the integrated equation of motion and the computational fluid dynamic (CFD) package called FLUENT. The computer simulations were performed to investigate the effects by the various suction flow rates, the distance from jet nozzle outlet to the diffuser inlet and the dimensions of the diffuser inlet section through the iterative calculation. In addition, the results from the CFD analysis on the thermal vapor compressor and the experiments were compared for the verification of the CFD results.

In the case of a jet nozzle, the results from the CFD analysis showed a good agreement with the experimental results. However, a fairly large deviation was found in the static temperature corresponding to saturation temperature which is determined by the suction pressure. The reason for the disagreement in the temperature might result from the fact that all flow was considered with an isentropic process in the CFD analysis while the actual flow was not isentropic. Furthermore, in this study, a special attention was paid on the performance of the thermal vapor compressor by varying the diffuser convergence angle of 1.0°, 4.0° and 7.0°. With the increase of the diffuser convergence angle, the suction capacity was improved up to the degree of 4.0° while it was decreased over the degree of 4.0°. However, the discharge pressure was increased when the diffuser convergence angle was reduced.

From this study, it was found by comparing with an entrainment ratio and the discharge pressure that the thermal vapor compressor has three operating regions, which are called a chocked flow, an un-chocked flow, and the reversed flow of a suction flow, respectively. When comparing with the CFD analysis and the experimental results on the thermal vapor compressor, the former showed a larger value of 25 % than the latter.