Heat transfer evaporator

Heat Transfer

This is the most important single factor in evaporator design, since the heating surface represents the largest part of evaporator cost. Other things being equal, the type of evaporator selected is the one having the highest heat-transfer cost coefficient under desired operating conditions in terms of J/s.K (British thermalunits per hour per degree Fahrenheit) per dollar of installed cost. When power is required to induce circulation past the heating surface, the coefficient must be even higher to offset the cost of power for circulation. 

Vapor-Liquid Separation

This design problem may be important for a number of reasons. The most important is usually prevention of entrainment because of value of product lost, pollution, contamination of the condensed vapor, or fouling or corrosion of the surfaces on which the vapor is condensed.Vapor-liquid separation in the vapor head may also be important when spray forms deposits on the walls, when vortices increase head requirements of circulating pumps, and when short circuiting allows vapor or unflashed liquid to be carried back to the circulating pump and heating element. Evaporator performance is rated on the basis of steam economy kilograms of solvent evaporated per kilogram of steam used. Heat is required to raise the feed from its initial temperature to the boiling temperature, to provide the minimum thermodynamic energy to separate liquid solvent from the feed, and to vaporize the solvent.

The first of these can be changed appreciably by reducing the boiling temperature or by heat interchange between the feed and the residual product and/or condensate. The greatest increase in steam economy is achieved by reusing the vaporized solvent. This is done in a multiple-effect evaporator by using the vapor from one effect as the heating medium for another effect in which boiling takes place at a lower temperature and pressure. 

Another method of increasing the utilization of energy is to employ a thermocompression evaporator, in which the vapor is compressed so that it will condense at a temperature high enough to permit its use as the heating medium in the same evaporator. Selection Problems Aside from heat-transfer considerations, the selection of type of evaporator best suited for a particular service is governed by the characteristics of the feed and product. Points that must be considered are crystallization, salting and scaling, product quality, corrosion, and foaming. In the case of a crystallizing evaporator, the desirability of producing crystals of a definite uniform size usually limits the choice to evaporators having a positive means of circulation.

Salting, which is the growth on body and heating-surface walls of a material having a solubility that increases  with increase in temperature, is frequently encountered in crystallizing evaporators. It can be reduced or eliminated by keeping the evaporating liquid in close or frequent contact with a large surface area of crystallized solid. Scaling is the deposition and growth on body walls, and especially on heating surfaces, of a material undergoing an irreversible chemical reaction in the evaporator or having a solubility that decreases with an increase in temperature.
Scaling can be reduced or eliminated in the same general manner as salting. Both salting and scaling liquids are usually best handled in evaporators that do not depend on boiling to induce circulation. 

Fouling is the formation of deposits other than salt or scale and may be due to corrosion, solid  matter entering with the feed, or deposits formed by the condensing vapor. Product Quality Considerations of product quality may require low holdup time and low-temperature operation to avoid thermal degradation. The low holdup time eliminates some types of evaporators, and some types are also eliminated because of poor heat-transfer characteristics at low temperature. Product quality may also dictate special materials of construction to avoid metallic contamination or a catalytic effect on decomposition of the product.  

Corrosion may also influence evaporator selection, since the advantages of evaporators having high heat-transfer coefficients are more apparent when expensive materials of construction are indicated. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high liquid and vapor velocities used, the frequent presence of solids in suspension, and the necessary concentration differences.

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.

               

Evaporation

What is Evaporation ?

Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. Evaporation will be understood as the physical process of concentrating solutions by removing part of the water from them by vaporization

 

This is factors influencing the rate of evaporation

1. Concentration of the substance evaporating in the air. When the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
2. Concentration of other substances in the air. When the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
3. Concentration of other substances in the liquid (impurities). When the liquid contains other substances, it will have a lower capacity for evaporation.
4. Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of then boundary layers at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.
5. Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by the entalphy of vaporation
6. Pressure. Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.
7. Surface area. A substance that has a larger surface area will evaporate faster, as there are more surface molecules that are able to escape. Temperature of the substance If the substance is hotter, then its molecules have a higher average kinetic energy, and evaporation will be faster.
8. Density. The higher the density the slower a liquid evaporates. 

Examples of evaporation :
Evaporation water from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces. The enthalpy of vaporization, also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas at a given pressure (often atmospheric pressure).  the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere.