A study of a desuperheater heat recovery system complete with a reversibly used water cooling tower (RUWCT) for hot water supply

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A study of a desuperheater heat recovery system complete with a reversibly used water cooling tower (RUWCT) for hot water supply


Author: Tan, Kunxiong
Title: A study of a desuperheater heat recovery system complete with a reversibly used water cooling tower (RUWCT) for hot water supply
Degree: Ph.D.
Year: 2000
Subject: Heat recovery
Hot-water supply
Air conditioning
Refrigeration and refrigerating machinery
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxiii, 192 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1532079
URI: http://theses.lib.polyu.edu.hk/handle/200/472
Abstract: Recovering heat rejected from the condenser in a refrigeration system to generate service hot water for buildings is commonly seen in both tropics and subtropics. This has the advantages of increasing the overall energy efficiency of a refrigeration system and reducing heat pollution to the environment. It is also advantageous in minimizing the total installation and operating costs, because the need for a boiler plant may be eliminated. This study included a critical literature review on heat recovery from air-conditioning / refrigeration systems, with particular emphasis on the direct condenser heat recovery and its related mathematical simulation models. The review identified many applications of desuperheaters to small-scaled residential air-conditioning or heat pump units. There is, however, a limitation to the application of this method in commercial and hotel buildings during cold seasons in subtropics, as there may not be sufficient heat from the buildings to be recovered due to the absence of a substantial cooling load. Consequently, backup heating provisions, usually by electricity, are often required. In order to provide a year-round service hot water supply without requiring backup electric water-heaters, an existing standard water cooling tower may be used in a reverse mode, as part of a desuperheater heat recovery system, to extract heat from ambient air in subtropics when necessary. This reversibly used water cooling tower (RUWCT) can be an idle cooling tower in an air-conditioning plant. A number of RUWCTs have been successfully installed and operated in subtropics in southern China. The heat and mass transfer characteristics of a RUWCT have been studied in detail, which is based on the theory of direct contact heat and mass transfer between moist air and water. The thesis reports on the differences in the heat and mass transfer process that takes place in a RUWCT, a standard water cooling tower and a spray room. A corrective factor that accounts for the change of chilled water mass flow rate is incorporated into the theoretical analysis of a RUWCT. The condensation of water vapor from moist air causes the change in chilled water flow rate. The introduction of the ratio of latent heat to total heat transferred helps to complete the theoretical analysis, and to better understand the heat and mass transfer characteristics in a RUWCT. The algorithms developed from the theoretical analysis are capable of predicting the heat exchange capacity of a RUWCT at any operating conditions. This theoretical analysis is the first of its kind. Extensive field experimental work on the heat and mass transfer characteristics of a RUWCT has been carried out in a hotel building in Haikou, Hainan province of China, where the RUWCT is installed. Results from the experimental work indicate that the theoretical analysis can represent the heat and mass transfer characteristics in a RUWCT with an acceptable accuracy. Furthermore, a procedure for calculating the thermal performance of a RUWCT has been developed based on the theoretical analysis and the field experimental work. A numerical analysis for a RUWCT is undertaken to determine both air and water states at intermediate horizontal sections along the tower height. A set of simultaneous differential equations has been developed to describe the correlation of fluid parameters and their variations within the tower. These differential equations are solved by a fourth-order Runge-Kutta method. The numerical analysis can provide better understanding of the variations in the air and water states inside a RUWCT. Field experimental data confirm that the predicted air and water conditions at the tower inlet and outlet are of acceptable accuracy. A steady-state mathematical model is developed to simulate the operational performance of a water chiller plant complete with a desuperheater heat recovery system and a RUWCT. The simulation results based on the technical specifications of the same plant used in the RUWCT experimental work indicate that the model is stable and behaves as expected. With the aid of the model developed, a number of the operational parameters of such a heat recovery system have been studied. This model will be useful in future studies on the optimum design of a water chiller complete with a desuperheater and a RUWCT for heat recovery.

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