Subcooling, Vapor Binding, and Condensation

Effect of subcooling. When steam condenses at atmospheric pressure, it gives off 1000 Btu per pound of condensing steam. This is called the latent heat of condensation of steam. 

When water cools off from 220 to 120°F, it gives off 100 Btu per pound of water. This heat represents the sensible-heat content of water between 220 and 120°F. 

It takes less of the radiator s surface area to condense one pound of steam at 220°F than to cool off one pound of water from 220 to 120°F. And this is true, even though the condensation of steam generates 10 times as much heat as the cooling of hot water. 

Does this mean that it is a lot easier to condense steam than to cool water Yes. This also explains, then, why condensate backup reduces the rate of heat transfer and condensation. 

Mechanics of subcooling. As the condensed steam flow out of the radiator is restricted, the surface area of the radiator, available to cool the hot water, increases. Hence, the water temperature leaving the radiator decreases. To summarize, the effect of restricting the condensate flow from a radiator or condenser is to 

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