Modeling and Simulation of the Heat Transfer Behaviour of a Shell-and-Tube Condenser for a Moderately High-Temperature Heat Pump

Abstract

For many countries, increasing energy efficiency and reducing greenhouse gas emissions are key countermeasures to cope with the energy crisis and climate change. Of the various heating equipment such as heat pumps, gas-fired boilers, oil-fired boilers, or electric boilers, heat pumps are widely adopted for space heating, water heating, and process heating by households, business, and industry due to high energy efficiency and effective reduction of greenhouse gases. The main components in a heat pump include the compressor, the expansion valve and two heat exchangers referred to as evaporator and condenser. Condenser is the essential component for heat pump transferring heat from refrigerant-side to water-side. Among many types of heat exchangers, shell-and-tube condensers are probably the most common type for using in heat pump as a heat exchanger for heating water, because of their relatively simple manufacturing and adaptability to various operating conditions. When heat pumps are used for residential and commercial hot water supply, the outlet water temperature of condenser is generally kept below 60°C. However, when heat pumps are used for process heating, the hot water outlet water temperature of condenser can range anywhere from 60 to 95°C. Such heat pumps are referred to as a moderately hightemperature heat pump. Under such operating conditions, the shell-and-tube condensers may face several issues: (1) the temperature difference between hot water inlet and outlet temperatures is high; generally above 40K and even up to 60K; (2) the condensing temperature increases as the outlet temperature increases contributing to the increase in the sensible heat ratio of refrigerants during the condensing process by as much as 25%; (3) as condensing temperature increases, the discharge temperature of compressor also increases, resulting in a decrease in energy efficiency of heat pump units. Therefore, under conditions of large temperature difference, high sensible heat ratio, and high outlet water temperature, how to effectively recover sensible heat, reduce condensing temperature, and thus improve the performance of heat pump units have become key challenges for this type of shell-and-tube condenser design. Because the factors that can influence heat transfer performance of heat exchangers are many, including flow arrangement, tube size, geometric configuration, and inlet/outlet conditions on the water