Heat Transfer/Heat Transfer with Phase Change

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Heat Transfer with Phase Change

Boiling[edit]

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:

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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[edit]

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[edit]

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

Sublimation[edit]

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: