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Balance and effectiveness
The characteristics of fluids contribute to a basic property of heat exchangers—the heat-transfer rate. The heat transferred to the colder fluid must equal that transferred from the hotter fluid. So the heat transferred per unit time equals the product of mass flow per unit time, specific heat, and the temperature change. This quick calculation should be done before specifying any heat exchanger. Although heat exchangers are normally specified only with desired temperatures, m QV
Plate heat exchangers have high heat-transfer coefficients and area. The heat-transfer rate is the main criterion. An exchanger’s effectiveness is the ratio of the actual heat transferred to the heat that could be transferred by an exchanger of infinite size. Efficiency is the best way to compare different types of heat exchangers. For example, a hot fluid stream being cooled by a cold-fluid stream in a counter flow heat exchanger. When the hot stream exits the exchanger, it must be warmer than the inlet temperature of the cold stream. In an ideal heat exchanger, the outgoing hot stream’s temperature equals the incoming cold stream’s temperature. Besides, this heat exchanger’s cold stream exits at a temperature lower than the inlet temperature of the hot stream. Given that the temperature drop on the hot stream is greater than the temperature gain in the cold stream in this example, the product of the mass-flow rate and the specific heat of the hot stream must be less than that of the cold stream, because of the required heat-transfer rate balance.