Essay on Heat Exchangers

Heat Exchangers: An Introduction
Heat exchangers provide a simple purpose that is controlling a system or substance’s temperature by adding or removing thermal energy. While there are many different sizes, levels of sophistication, and types of heat exchangers, they all use a thermally conducting element generally in the form of a tube or plate—to separate two fluids, such that one can transfer thermal energy to the other. Home heating systems use a heat exchanger to transfer combustion - gas heat to water or air, which is circulated through the house. Power plants use locally available water or ambient air in quite large heat exchangers to condense steam from the turbines. Many industrial applications use small heat exchangers to create or maintain a required temperature. In industry, heat exchangers do many tasks, ranging from cooling lasers to establishing a controlled sample temperature prior to chromatography. Anyone who wants to use a heat exchanger deals with a basic challenge: fully defining the problem to be solved, which requires an understanding of the thermodynamic and transport properties of fluids. Such knowledge can be in addition to some simple calculations to define a specific heat-transfer problem and select a suitable heat exchanger.
How heat gets transferred from one fluid to another depends mainly on the physical characteristics of the fluids involved, particularly their density, specific heat, thermal conductivity, and dynamic viscosity.

Density is a fluid’s mass per unit volume, measured as lb m/ft (where lb m represents pounds of mass) or kg/m. Density can be used to convert a measurement from a mass-flow rate, such as lb m/hr, to the more common volumetric units, such as gallons per minute for liquids, or cubic feet per minute for gases. All through a heat exchanger, the mass-flow rate remains constant, however changes in temperature and pressure can change the volumetric flow rate, mainly for a gas.............

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