heat of evaporation

Process of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat of evaporation transfer, vapor-liquid separation, and efficient utilization of energy. On this article will be explained about heat of evaporation.

The literature for heat of evaporation transfer generally recognizes three distinct modes of heat transfer : conduction, radiation, and convection. Strictly speaking, only conduction and radiation should be classified as heat transfer processes, because only these two mechanisms depend for their operation on the mere existence of a temperature difference. The last of three, convection, does not strictly  comply with the definition of heat transfer because it depends for its operation  on mechanical mass transport also. But since convection also accomplishes transmission of energy from regions of high temperature to regions of lower temperature, the term “ heat transfer by convection” has become generally accepted. In most situations heat flows not by one, but by several  of these mechanisms simultaneously.

Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of the body. Conduction can occur in solids, liquids, or gases.

Radiation is the transfer of heat from one body to another, not in contact with it, by means of electromagnetic wave motion through space, even when a vacuum exist between them.

Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temperature differences; in forced convection, the motion is produced by mechanical means. When the forced velocity is relatively low, it should be realized that “free-convection” factors, such as density and temperature difference, may have an important influence.

In the solution for heat of evaporation transfer problems, it is necessary not only to recognize the modes of heat transfer which play a role, but also to determine whether a process is steady or unsteady. When the rate of heat flow in a system does not vary with time-when it is constant—the temperature at any point does not change and steady-state conditions prevail. Under steady-state conditions, the reate of heat input at any point of the system must be exactly equal to the rate of heat output, and no change in internal energy can take place. The majority of engineering heat-transfer problems are concerned with steady-state systems. The heat flow in asystem is transient, or unsteady, when the temperatures at various points in the system change with time.

 

Types heat of evaporation transfer operations

There are two types heat of evaporation transfer operations :  sensible heat and change of phase. Sensible heat operations involve heating or cooling of a fluid in which the heat transfer results only in a temperature change of the fluid. Change of phase heat transfer results in a liquid being changed into a vapor or a vapor being changed into a liquid. Boiling or vaporization is the convection process involving a change in phase from liquid to vapor. Condensation is the convection process involving a change in phase from vapor to liquid. Many applications involve both sensible heat and change of phase heat transfer.

In the next article it will explained about  sensible heat transfer inside tube, condensation fallin film heat transfer, gasketed plate evaporators, patterned plate evaporators that related to heat of evaporation transfer operations. So keep up on us.