Heat Exchanger- Heat Transfer

A device used for transferring heat from one fluid to another is called a heat exchanger. Its use is made in radiators in automobiles, intercoolers, and preheaters, condensers, and boilers in steam plants, condensers, and evaporators in refrigerators and air conditioning units. The following two types of heat exchangers are common in use:

heat exchanger

  1.  Parallel flow heat exchanger.
  2.  Counter-current flow heat exchanger.

Parallel flow heat exchanger: 

In parallel-flow heat exchangers, the fluids flow in the same direction. The temperature difference is maximum at the inlet and consequently, the rate of flow of heat and the rate of decrease of temperature is maximum here.

Counter-current flow heat exchanger:

In a counter-current flow heat exchanger, the fluids flow in the opposite direction. The heat transfer takes place between the fluids at the moment when each is in its coldest state or when each is in its hottest state. The average temperature between the two fluids is greater than the parallel-flow heat exchanger.

In terms of construction type, heat exchangers are classified as compact, shell and tube, and regenerative heat exchangers. Compact heat exchangers are specifically designed to obtain large heat transfer surface areas per unit volume. The large surface area in compact heat exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids. Shell and tube heat exchangers contain a large number of tubes packed in a shell with their axes parallel to that of the shell. Regenerative heat exchangers involve the alternate passage of the hot and cold fluid streams through the same flow area. In compact heat exchangers, the two fluids usually move perpendicular to each other.

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