Evaporation Rate

Definition - What does Evaporation Rate mean?

Evaporation is an endothermic process that changes liquid into a gaseous state of matter before reaching the boiling point of the liquid. Evaporation rate is linked with many factors such as surface area, temperature of the liquid and surrounding air. A system’s evaporation rate slows down once the vapor pressure achieves equilibrium.

Corrosionpedia explains Evaporation Rate

The easiest way to measure the evaporation rate of a liquid is to record the volume at two points in time and calculate the difference.

Calculating the evaporation rate by a formula is less straightforward due to numerous variables. One such formula to determine the evaporation from a swimming pool or similar container under similar conditions is:

7.4 x P x A x (0.447W)0.78

E = -----------------------------------------

T + 459.67

Where:

  • E = Evaporation rate (gallons/day)
  • A = Pool surface area (ft2)
  • W = Wind speed above pool (mph)
  • P = Water's vapor pressure (mmHG) at ambient temperature
  • T = Temperature (°F)

Unlike boiling, evaporation does not occur at a fixed temperature. Factors that can affect evaporation rate include:

  • Temperature
  • Humidity
  • Surface area

Temperature and surface area are directly proportional to the rate of evaporation. Higher temperatures give the liquid molecules more kinetic energy, which helps to overcome intermolecular forces. As evaporation only occurs at the surface of a liquid, more surface area increases evaporation. Additionally, evaporation requires heat and dry air. Thus, humidity decreases its rate.

Apart from the factors explained above, evaporation rate also depends on the heat supplied and the strength of intermolecular forces in a liquid. Therefore, liquids having hydrogen bonding have slower evaporation rates than those with van der Waal's intermolecular forces.

Controlling evaporation rate is very important in industries where the products are thermo-sensitive. For that, evaporators are used mainly in making pharmaceutical products, coffee extracts, fruit juices, and for treating oil and water.

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