Principles of heat exchangers

Simplified heat exchanger concepts

Heat exchangers work because heat naturally flows from higher temperature to lower temperatures. Therefore if a hot fluid and a cold fluid are separated by a heat conducting surface heat can be transferred from the hot fluid to the cold fluid.

Figure 1 Simplified Heat Exchanger

The rate of heat flow at any point (kW/m2 of transfer surface) depends on:

Heat transfer coefficient (U), itself a function of the properties of the fluids involved, fluid velocity, materials of construction, geometry and cleanliness of the exchanger
Temperature difference between hot and cold streams

Total heat transferred (Q) depends on:

Heat transfer surface area (A)
Heat transfer coefficient
Average temperature difference between the streams, strictly the log mean (DT_LM)

Thus total heat transferred Q = UADT_LM

But the larger the area the greater the cost of the exchanger

Therefore there is a trade-off between the amount of heat transferred and the exchanger cost

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