Baffled Heat Exchangers and Compact Heat Exchangers. Classification of Heat Exchangers.

 

The flow of the shell - side fluid in baffled heat exchangers is partly perpendicular and partly parallel to the tubes. The heat transfer coefficient on the shell side in this type of unit depends not only on the size and spacing of the tubes and the velocity and physical properties of the fluid but also on the spacing and shape of the baffles. In addition, there is always leakage through the tube holes in the baffle and between the baffle and the inside of the shell, and there is bypassing between the tube bundle and the shell. Because of these complications, the heat transfer coefficient can be estimated only by approximate methods or from experience with similar units. According to one approximate method, which is widely used for design calculations, the average heat transfer coefficient calculated for the corresponding is multiplied by 0.6 to allow for leakage and other deviations from the simplified model.

In some heat exchanger applications, the heat exchanger size and weight are of prime concern. This can be especially true for heat exchangers in which one or both fluids are gases, since the gas-side heat transfer coefficients are small and large heat transfer surface area requirements can result.

Compact heat exchangersrefer to heat exchanger designs in which large heat transfer surface areas are provided in as small a space as possible. Applications in which compact heat exchangers are required include an automobile heater core in which engine coolant is circulated through tubes and the passenger compartment air is blown over the finned exterior surface of the tubes and refrigerator condensers in which the refrigerant is circulated inside tubes and cooled by room air circulated over the finned outside of the tubes. Another application is an automobile radiator. The engine coolant is pumped through the flattened, horizontal tubes while air from the engine fan is blown through the finned channels between the coolant tubes. The fins to the coolant tubes help transfer heat from the exterior surfaces of the tube into the airstream. Experimental data are required to allow one to determine the gas-side heat transfer coefficient and pressure drop for compact heat exchanger cores. Fin design parameters that affect the heat transfer and pressure drop on the gas side include thickness, spacing¹, material, and length.

Given the heat exchanger requirements, the designer can estimate the performance of several candidate heat exchanger cores to determine the best design.

Given the large variety of applications and structural configurations of heat exchangers, as just discussed, it becomes important to provide a classification scheme to help in their selection process.

1. The type of heat exchanger: (a) recuperator and (b) regenerator. A recuperator, as discussed earlier, is the conventional heat exchanger in which heat is recovered by the cold fluid stream from the hot fluid stream. The two fluid streams flow simultaneously, possibly in a variety of flow arrangements, through the heat exchanger. In a regenerator, the hot and cold fluids alternately flow through the exchanger, which essentially acts as a transient energy storage and dissipation unit.

2. The type of heat exchange process between the fluids: (a) indirect contact, or transmural, and (b) direct contact. In a transmural heat exchanger, the hot andcold fluids are separated by a solid material, which is typically of either tubular or plate geometry. In direct contact heat exchanger, as the name suggests, boththe hot and cold fluids flow into the same spacewithout a partitioning wall².

3. Thermodynamic phase or state of the fluids: (a) single phase, (b) evaporation or boiling, and (c) condensation. This criterion refers to the state of phaseof the hot and cold fluids, and the three categories refer to cases where both fluids maintain single - phase flow and one of the two fluids undergoes flow evaporation or condensation.

4. The type of construction or geometry: (a) tubular, (b) plate, and (c) extended or finned surface. A typical example for each of the first two categories,respectively, is the shell - and - tube heat exchanger and the extended - or finned - surface exchanger. The extended - or finned - surface exchanger could eitherhave a tubular (tube - fin) or plate (plate - fin) geometry. It is often referred toas a compact heat exchanger, especially when it has a large surface area density,i.e., relatively large ratio of heat transfer surface area to volume.

Thus, based on this simple scheme, an automobile radiator, for example would be classified as a transmural recuperator with single-phase fluid flows and a finned (tube - fin type construction) surface. This heat exchanger is often also characterized as a compact heat exchanger because of its large area density. Likewise, a boiler feed-water heater, which is a shell - and - tube heat exchanger would be classified as a transmural recuperator of a tubular construction with condensation in one fluid (feed - water is heated by the condensation of steam extracted from a power turbine). Students should bear in mind, however, that classification schemes serve only as guidelines and that the actual design and selection of heat exchangers may involve several other factors.

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¹Spacing – зд. расположение, размещение.

²Partitioning wall – зд. разделительная перегородка.