(C) 2006
Harry R Cripps

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 (DTLM)
Thus total heat transferred Q = UADTLM
  • 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
Return to main heat recovery article