Heat Transfer/Heat Transfer with Phase Change
Heat Transfer with Phase Change
In general, means heating thus increasing the temperature of the substance to its boiling temperature and above. The heat required to change a mass m of a pure substance from a liquid to a vapor is generally given in terms of a latent heat of vaporization as:
Latent heats are in general functions of pressure; they are usually given at 1 standard atmosphere (or 1 bar) in standard texts. For many applications the pressure dependence of latent heats is neglected.
Condensation means lowering the temperature of the vapour substance to its boiling/vaporising temperature. For example, water at 120 deg. Celsius, at atmospheric pressure, will be in vapour state. when its temperature is lowered below 100°C, at the same pressure, vapour gets condensed to liquid state.
The heat released when one condenses a liquid is the same magnitude as the amount required to vaporize the same mass of the liquid. It will just have a negative sign rather than a positive one (because when something condenses, it releases heat).
Freezing and melting
Freezing is the act of lowering the temperature of a system enough to change a substance from a liquid to a solid. Melting is the change from a solid back to a liquid.
The heat required to melt or solidify a pure substance is calculated in the same manner as that required to vaporize or condense that substance, except a different latent heat called the latent heat of fusion is used. it is used to freeze things
At a certain range of pressures, rather than melting, a solid will be transformed directly into a gas, and vice versa. The latent heat of sublimation is the sum of the heats of vaporization and fusion, assuming the effect of pressure changes is negligible: