How is the heat transferred?
The energy transfer in a heat exchanger having liquid on both sides of a metal plate occurs in three stages (Fig. 7.4):
· Transfer of heat from the liquid with the highest temperature to the fixed material, the plate
· Transfer of heat through the fixed material Transfer of heat from the fixed material to the liquid of lower temperature
· Transfer of heat from the liquid to the fixed mate-rial is normally via convection and conduction.
Close to the fixed material there will be a layer with laminar flow through which heat is transferred by conduction more slowly than for convection. It is advantageous to reduce the thickness of the laminar layer and, to improve convection, have tur-bulent conditions in the flowing liquids. Heat transfer through the fixed material is by conduc-tion; a material with good conductivity (e.g. metal) improves this. Conduction and convection cause heat transfer into the liquid on the other side of the fixed material.
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