Four Factors That Affect Evaporation

THEORY

Evaporation is the removal of solvent as vapour from a solution. There are four factors that affect evaporation, the area of exposed water surface the larger the area, the greater the loss; the temperature of the water and air above it a higher temperature increases loss; the movement of air above the water surface a greater movement carries off more vapour; and the relative humidity dry air absorbs moisture more readily than moist air. According to the kinetic theory of evaporation, molecules of water are in a constant state of random motion, colliding with one another, and rebounding. In the liquid state, these molecules are held in balance bycohesive force; however, some molecules possess sufficient kinetic energy to overcome this

In general, the greater the temperature difference, the better the driving force of the steam. Additionally, having a high temperature difference will increases the flow rate of the liquid and vapour within the tube. This increase in flow rate causes higher turbulence which results in an increase of the heat transfer coefficient. However, the overall temperature difference has to be within the range of boiling points of the two components as it could affect the quality and the purity of the products. Although rising film evaporator have many advantages for example rising film evaporator has low residence time this means that it allows the use of high temperature and still produce products of high quality irrespective of whether the product is heat sensitive, but it has low efficiency compared to other evaporators. Rising film evaporation is a continuous process.

Total mass balance:

m1=

The evaporator is a special type of heat exchanger. Here evaporation takes place inside the vertical tubes, but there are also cases where the process fluid evaporates on the outside of horizontal or vertical tubes. In this evaporator the solution that is to be evaporated flows downwards by gravity. The solution creates a film along the tube walls, progressing downwards (falling). A suitable distribution unit is necessary in order to achieve an even liquid distribution.

• Multiple effect evaporator

Is the type of evaporator that use heat from steam to evaporate water from a solution. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the boiling temperature of water and pressure are inversely proportional to each other that as the temperature increases the pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel requires an external source of heat.

• Forced circulation