Condensers and Boilers constitute an important and widely used type of heat exchanger with unique characteristics of heat transfer. Well, the analysis of heat transfer during condensing and boiling states is very complicated. To understand this phenomena we should start with the very basic steps.
If a vapor strikes a surface that is at a temperature below saturation temperature, the vapor will immediately converts into its liquid form. This incident is called condensation.
Thus we get filmwise condensation and dropwise condensation. When condensation takes place over a surface which is continuously being cooled by some cooling system and the condensed liquid is being removed by the motion produced by gravity then a thin layer of continuous liquid covers the whole condensing surface. This incident is called filmwise condensation.
Again, when condensation takes place over a surface and the condensed liquid is broken into droplets then it is called dropwise condensation.
Dropwise condensation usually occurs on those surfaces which are non-wettable or contains a good amount of impurities such as dust, oil, etc.
Heat transfer rate is between surface and vapor is very high for dropwise condensation because the film of condensed liquid produced in filmwise condensation process acts as a barrier. Thus dropwise condensation offers much less resistance to heat flow on the vapor side than the filmwise condensation.
The motion of condensate is laminar and heat is transferred from vapor to liquid interface by conduction unless the velocity of vapor is very high or the liquid film thickness is very large.
The rate of heat flow depends on the thickness of condensate. And, this thickness is dependent on the rate at which vapor is condensed and the rate at which vapor is removed.
On the vertical surface the film thickness increases continuously from top to bottom.
If a vapor strikes a surface that is at a temperature below saturation temperature, the vapor will immediately converts into its liquid form. This incident is called condensation.
Thus we get filmwise condensation and dropwise condensation. When condensation takes place over a surface which is continuously being cooled by some cooling system and the condensed liquid is being removed by the motion produced by gravity then a thin layer of continuous liquid covers the whole condensing surface. This incident is called filmwise condensation.
Again, when condensation takes place over a surface and the condensed liquid is broken into droplets then it is called dropwise condensation.
Dropwise condensation usually occurs on those surfaces which are non-wettable or contains a good amount of impurities such as dust, oil, etc.
Heat transfer rate is between surface and vapor is very high for dropwise condensation because the film of condensed liquid produced in filmwise condensation process acts as a barrier. Thus dropwise condensation offers much less resistance to heat flow on the vapor side than the filmwise condensation.
Heat Flow Facts:
The motion of condensate is laminar and heat is transferred from vapor to liquid interface by conduction unless the velocity of vapor is very high or the liquid film thickness is very large.
The rate of heat flow depends on the thickness of condensate. And, this thickness is dependent on the rate at which vapor is condensed and the rate at which vapor is removed.
On the vertical surface the film thickness increases continuously from top to bottom.
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