Heat transfer through a composite plane wall @chemicaladda

Heat transfer through the sphere

TYPES OF EVAPORATOR

Types of Evaporator

Single effect evaporator

When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. This method is called single effect evaporation, and although it is simple, it utilizes steam ineffectively.

• The solution to be concentrated flows inside the tubes. 
• The heating medium is steam condensing on metal tubes. 
• Usually boiling liquid is enter under a moderate vacuum.
• This increases the temperature difference between the steam and boiling liquid.

Multiple effect evaporator

Increasing the evaporation per kilogram of steam by using a series of evaporators between the steam supply and the condenser is called multiple effect evaporator

• If vapor from one evaporator is fed into the steam chest of the second evaporator and the vapor from second is then sent to a condenser, the operation becomes the double effect. 
• The heat in the original steam is reused in the second effect, and the evaporation is achieved by a unit mass of steam in the same manner.

Once through evaporator

• In once-through evaporation, the feed liquor passes through the tube only once, releases the vapor, and leaves the unit as thick liquor. 
• All the evaporation is accomplished in a single pass. 
• The ratio of evaporation to feed is limited in single pass units: the evaporators are well adopted to multiple effect operation, where the total amount of operation can be spread over several effects. 
• Agitated film evaporators are always operated once- through  falling film and climbing film evaporators can also be operated in this way. 
• Once through evaporators are specially useful for heat-sensitive materials. 

Circulation evaporators

• In circulation evaporators a pool of liquid is held within the equipment. 
• Incoming feed mixes with the liquid from the pool, and the mixture passes through the tubes. 
• Unevaporated liquid discharged from the tubes returns to the pool so that only part of the total evaporation occurs in one pass. 
• All forced circulation evaporators are operated in this way; climbing- film evaporators are usually circulation units.
• These are adapted to single-effect evaporation. 
• These are not suited for heat-sensitive materials.

EVAPORATION

Evaporation

• Evaporation is when a liquid becomes a gas without forming bubbles inside the liquid volume.
• During evaporation, only the molecules near the liquid surface are changing from a liquid to vapor.
• Evaporation is the process that occurs when the surface of a liquid is converted into a gas
• For example, evaporation occurs when a glass of water is left out overnight and the water level is found to drop.
• Another example is when steam flows through a tube that is submerged in a pool of liquid, minutes bubbles of vapor form at random points on the surface of the tube. The heat is passing through the tube surface where no bubbles form enters the surrounding liquid by convention. Then some of the heat in the liquid then flows toward the bubbles, causing evaporation from its inner surface into itself.
• The objective of evaporation is to concentrate a solution consisting of a non volatile solute and volatile solvent.
• Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution of thick liquor.
• Normally, in evaporation thick liquor is the voluble product and the vapor is condensed and discarded.
• Hence, Evaporation is a widely used method for the concentration of aqueous solutions, involves the removal of water from a solution by boiling the liquor in a suitable vessel, an evaporator, and withdrawing the vapor. 
• If the solution contains dissolved solids, the resulting strong liquor may become saturated so that crystals are deposited. 
• Evaporation is achieved by adding heat to the solution to vaporize the solvent. 
• The heat is supplied principally to provide the latent heat of vaporization, and, by adopting methods for recovery of heat from the vapor, it has been possible to achieve a great economy in heat utilization. 
• Whilst the normal heating medium is generally low-pressure exhaust steam from turbines, special heat transfer fluids or flue gases are also used. 
• Evaporation is differ from drying in that the residue is the liquid, sometimes a highly viscous one rather than solid.
• Evaporation is differ from distillation in that vapor usually is a single component, and even when vapor is a mixture, separation of vapor into a smaller fraction is not carried out in evaporation.

• Single effect Evaporator
• Multiple effect evaporator
• once through and circulation evaporators
• Capacity
• Economy
• Steam consumption
• Heat transfer in Evaporators
• Boiling point elevation
• Boiling point rise
• Comparison of single and multiple effect evaporators 

MODES OF HEAT TRNSFER

Modes of heat transfer

Heat transfer may be defined as the transmission of the energy from one region to another region as a result of the temperature gradient.

Heat transfer takes place in three modes.

1. Conduction

Conduction is the transfer of heat from one part of substance to another part of same substance or from one substance to another which is in physical contact with it. 

• While transferring heat there is no appreciable displacement of molecules forming that substance is occur.
• In solid, the flow of heat is result of vibrational energy.
• In liquid, the flow of heat is result of kinetic energy.

2. Convection

Convection is the transfer of heat within a fluid which is arises due to mixing of one portion of the fluid with another. 

• In a fluid, the flow of heat results in the microscopic motion of a fluid.
• There are two types of Convection.

1. Natural Convection:- Heat transfer is caused by differences in density arising from a temperature gradient
2. Forced Convection:- Heat transfer is caused by use of external means of agency

3. Radiation

Radiation is the transfer of heat through space or matter by means other than conduction or convection.

• Transfer of heat is occurred without any material medium or through a perfect vacuum.
• Transfer of heat is in the form of electromagnetic waves.
• When this wave falls on another body, a part of the energy is reflected, transmitted through the body and remaining absorbed by the body.

Figure of Conduction Convection and Radiation