Heat Exchanger

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A Model of Air Conditioning Heat Exchanger A heat transfer device is a system that transfers heat between two or more fluids. The fluids can be single-phase or two-phase and, depending on the exchanger type, may be separated or in direct contact. Devices involving energy sources such as nuclear fuel rods or fired heaters are not normally regarded as heat transfer devices although many of the principles involved in their design are the same. In order to discuss heat transfer devices it is necessary to provide some form of categorization. There are two approaches that are normally taken. The first considers the flow configuration within the heat transfer device, while the second is based on the classification of equipment type primarily by construction. Both are considered here. Classification of Heat Transfer Devices by Construction In this section heat transfer devices are classified mainly by their construction, Garland (1990), (see Figure 5). The first level of classification is to divide heat transfer device types into recuperative or regenerative. A Recuperative Heat Transfer Device has separate flow paths for each fluid and fluids flow simultaneously through the exchanger exchanging heat across the wall separating the flow paths. A Regenerative Heat Transfer Device has a single flow path, which the hot and cold fluids alternately pass through. Heat transfer device classifications. Figure 5. Heat transfer device classifications. Regenerative heat transfer devices In a regenerative heat transfer device, the flow path normally consists of a matrix, which is heated when the hot fluid passes through it (this is known as the "hot blow"). This heat is then released to the cold fluid when this flows through the matrix (the "cold blow"). Regenerative Heat Transfer Devices are sometimes known as Capacitive Heat Transfer Devices. A good overview of regenerators is provided by Walker (1982). Regenerators are mainly used in gas/gas heat recovery applications in power stations and other energy-intensive industries. The two main types of regenerator are Static and Dynamic. Both types of regenerator are transient in operation and unless great care is taken in their design there is normally cross-contamination of the hot and cold streams. However, the use of regenerators is likely to increase in the future as attempts are made to improve energy efficiency and recover more low-grade heat. However, because regenerative heat transfer devices tend to be used for specialist applications recuperative heat transfer devices are more common.