Heat Exchanger
Heat Exchanger
What happens to the heat transfer process between heat exchanger and boiling water....?
What happens to the heat transfer process when the temperature difference between a heat exchanger and boiling water in contact with increases from a few degrees, to say 100C ? What are the regions of temperature difference most suitable for practical applications?
all other things being equal greater temperature difference equals greater heat transfer, BUT If you "force" the heat conduction too hard you can get pockets of steam forming next to the wall with water that's still got a bit to boil. This is because steam is a far worse conductor of heat than liquid water, and small bubbles of the steam can sometimes end up sticking to the boiler tube.
This is a lot like what happens if you take a RED hot skillet, and place a droplet of water on it. The water in contact with the hot surface will "flash" into steam, but that lifts the bulk of the water away from the heat surface, increasing the thermal resistance. If the skillet is a little cooler the water will often boil away faster, becasue the average heat transfer through the material might well be better
Within a boiler the bubbles can sometimes collapse in on themselves, causing boiler rumble which is much the same as cavitation(and erodes the tubes/plates), and because steam passes less heat they can cause local overheating of the tube. Likelihood of this happening can be reduced by increasing the turbulence so that the water heats up more evenly.This is done quite frequently in plate heat exchangers, by forming patterns on the water side of plates
The following was written by a model engineer, but it might be of some interest...
http://www.flysteam.co.uk/steaming.htm
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Heat Exchanger Principles - Sample
