LYDON OVEN UNIVERSITY
Junior Curriculum

Evaporation Factors

OVEN DRYING: The process of evaporation of liquids from a mixture of liquids and solids using a forced convection oven chamber to gently supply the heat of vaporization, local velocity and removal (or ventilation) of the heated mixture of process air and vapor without disturbing the wetted (or dry) solids. Usually the process is well below the boiling temperature of the liquid and the wetted solids are contained in such a way as will arrange the greatest surface area for contact with the process air. Oven drying of amorphous wetted solids is often called “tray drying” after the container of the material. Note also that oven drying is truly evaporation and not boiling. Using an oven to boil a liquid from a mixture of liquids and solids tends to violently disturb the mixture and also waste energy.

EVAPORATION RATE: The mass per surface area per time of a liquid’s vaporization.

PRINCIPAL EVAPORATION RATE FACTORS: Several factors can have a major effect upon the rate of evaporation.

Area: The rate of evaporation varies directly with the quantity of the wetted surface area in contact with air. The greater the size of the wetted area, the greater the rate of evaporation.

Temperature: Within limits, the rate of evaporation varies directly with the temperature of the liquid. More accurately, the greater the heat content of the liquid, the greater the rate of evaporation. [Limits might exclude non-useful temperatures at or below the temperature of the liquid and solid mixture, or temperatures above the boiling point of the liquid.]

Vapor pressure of the liquid: The rate of evaporation varies directly with the vapor pressure of the liquid.

Vapor pressure of the air: The rate of evaporation varies inversely with the vapor pressure of the air in contact with the liquid.

Air velocity: Within limits, the rate of evaporation varies directly with the velocity of the adjacent air. [Limits might exclude non-useful velocities above which the velocity pressure moves the liquid and solid mixture.]

SECONDARY EVAPORATION RATE FACTORS: There are additional factors that affect the rate of evaporation arising from the materials. Inter-related factors [such as liquid and vapor diffusion, surface tension, molecular adhesion, particle surface uniformity, capillarity, liquid viscosity, equilibrium moisture level] may allow the liquids to move freely in the liquid-solid mixture and be easily vaporized at the point of contact with air, or to hold tightly to one another with minimal fluidity and potentially be trapped beneath a "skin" layer of dry solids at the air contact surface. If the solids tend to be hydrophilic with the liquids adsorbed, then the equilibrium moisture level at the drying conditions may be quickly attained and the drying process slowed. Hygroscopic or hydrophillic materials absorb or adsorb atmospheric or liquid moisture and retain it in such a way as to diminish the rate of evaporation.

ALTITUDE: The vertical distance above mean sea level. Average sea level and actual altitude location of a process is significant due to variations in “standard pressure”. At sea level, the pressure per unit surface area exerted by the atmosphere is often called“one atmosphere”. This pressure is approximately 14.69 pounds per square inch, 406.9 columnar inches of water, 10340 columnar millimeter of water, 1014 millibar, 760.2 columnar millimeter of mercury, 29.93 columnar inches of mercury, 101.4 kiloPascals. The atmospheric pressure diminishes at increased altitude. Fan performance varies with atmospheric pressure, therefore altitude.

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