EVAPORATION AND BOILING CLASS NOTES FOR ENGINEERING

  EVAPORATION AND BOILING

CLASS NOTES FOR ENGINEERING

Definition of boiling

It is a temperature at which a liquid starts changing into vapors and it remains constant till the whole liquid is changing into vapors. Normal boiling point is with respect to one atmospheric pressure. Boiling point and melting points increase with the increase of pressure and decrease with the decrease of pressure. For example, normal boiling point of water is 1000C at one atmospheric pressure. Water boiling point becomes 1200C at 2 atmospheric pressure.

Evaporation

It takes place at all temperatures in the atmosphere as long as water vapor pressure in air is less than the saturated pressure at the prevailing temperature.

Sr. No.

Evaporation

Boiling

1.
Evaporation takes place at all temperatures
Boiling takes place at a fixed temperature corresponding to the pressure.
 2.
 It takes place at the common interface of liquid and vapor when the vapor pressure over the free surface of the liquid is less than the saturation pressure
 It starts from the bottom heated surface and then it spreads in the entire body of the liquid.
 3.
 There is no bubble formation Or no bubble movement appears in it.
 Bubbles are formed, these detach the heated surface and travel towards the free surface and ultimately leave the free surface and  thus bubble movement appears in it.
4.
It takes place only at the free surface of a liquid. For example from the floor surface or from a river.
It takes place in the entire body of the liquid.
5.
Vapors are not visible.
Vapors are visible.
6.
It is a quiet process.
It is a noisy process.
7.
Evaporation causes cooling effect.
Boiling causes heating effect.
8.
It is a slow process.
It is a fast process.
 9.
 It does not require a surface at a temperature higher than the saturation temperature
 It requires a surface at a temperature higher than the saturation temperature.
 10.
 Law applicable is Dalton’ s Law of Partial pressures.
 Newton’s Law of heating is applicable. q. boiling =h(ts – tsat)
where ts is the temperature of the hot surface,
tsat is the saturation temperature of the liquid
 11.
 There is no further types of evaporation.
 Types of boiling are sub-cooled boiling, saturated boiling, pool boiling, flow boiling,  nucleate and film boiling, external and internal boiling.
 12.
 No heat source is required. Heat is available from the surrounding atmosphere.
 A strong heat source is required.