Heat transfer and its applications

5.1 Heat transfer mechanism

In the simplest of terms, the discipline of heat transfer is concerned with only two things- temperature, and the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place.

On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The greater the material’s temperature, the greater is the thermal agitation of its constituent molecules (manifested both in linear motion and vibrational modes). It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy. Several material properties serve to modulate the heat transferred between two regions at differing temperatures. Examples include thermal conductivities, specific heats, material densities, fluid velocities, fluid viscosities, surface emissivities, and more. Taken together, these properties serve to make the solution of many heat transfer problems an involved process.

5.2 Heat exchangers

Heat exchangers play an important role in process industries. A heat exchanger is a device, which is used for transferring heat from one fluid to another through a separating wall. This can be classified according to the process of heat transfer, mechanical design and principal material of construction.

 

Classification of heat exchangers

The following types of heat exchangers are presently used in process industries:

• Double pipe heat exchangers
• Shell and tube heat exchangers
• Fixed tube sheet
• Outside packed floating head
• Internal floating head
• U-Tube type
• Re-boilers

Special types of exchangers

• Pipe coils
• Spiral heat exchanger
• Plate type exchanger
• Finned tube exchanger
• Graphite heat exchanger
• Air-cooled exchanger

5.2.1 Double pipe heat exchangers

Straight tubes, internal bolted floating head cover, removable tube bundle. No special provisions needed for expansion.

Applications

For heating or cooling chemical or hydrocarbon fluids, condensing air or gases.

Construction

Removable bundle, pull-through bolted internal floating head cover.

Figure 5.1
Double pipe exchanger

Advantages

• Allows for differential thermal expansion between the shell and tubes
• Bundle can be removed from shell for cleaning or repairing without removing the floating head cover
• Provides multi-tube pass arrangements
• Provides large bundle entrance area
• Excellent for handling flammable and/or toxic fluids

Limitations

• For a given set of conditions, it is the most costly of all the basic types of heat exchanger designs
• Less surface per given shell and tube size than C500

5.2.2 Shell and tube heat exchangers

They are intended for heating or cooling process fluids, they are for example suitable for closed circuit cooling of electrical equipment using dematerialized water and for cooling water soluble oil solutions in quenching tanks.

The shell side usually contains the process fluid and the tube side water from the town mains or a cooling tower, or an ethylene glycol solution from a chiller unit.

5.2.3 Fixed tube sheet

Generally the most economical type design; however, the shell side fluid must be non-fouling since the tube bundle cannot be removed. Either designing with can offset this sufficient fouling allowance or providing for chemical cleaning. Inside of tubes easily cleaned by removing heads.

Figure 5.2
Straight tube fixed bundle

5.2.4 Internal floating head

Straight tubes, internal bolted floating head cover, removable tube bundle. No special provisions needed for expansion.

Applications

For heating or cooling chemical or hydrocarbon fluids, condensing air or gases.

Construction

Removable bundle, pull-through bolted internal floating head cover.

Figure 5.3
Internal floating head

Advantages

• Allows for differential thermal expansion between the shell and tubes.
• Bundle can be removed from shell for cleaning or repairing without removing the floating head cover
• Provides multi-tube pass arrangements
• Provides large bundle entrance area
• Excellent for handling flammable and/or toxic fluids

Limitations

• For a given set of conditions, it is the most costly of all the basic types of heat exchanger designs
• Less surface per given shell and tube size than C500

5.2.5 Outside packed floating head

External packed floating head allows differential thermal expansion between shell and tubes. No packing is exposed to tube side fluid. Large entrance area enables easier maintenance of removable tube bundle. 1, 2, 4, or 6-pass models.

Figure 5.4
Outside floating head

Applications

For tube side circulation of corrosive liquids, high-fouling fluids, or gases and vapors.

Advantages

• Floating end allows for differential thermal expansion between the shell and tubes
• Shell side can be steam or mechanically cleaned
• Bundle can be easily repaired or replaced
• Less costly than full internal floating head-type construction
• Maximum surface per given shell and tube size for removable bundle designs

Limitations

• Shell side fluids limited to non-volatile and/or non-toxic fluids, i.e., lube oils, hydraulic oils
• Tube side arrangements limited to one or two pass
• Tubes expand as a group, not individually (as in U-tube unit); therefore, sudden shocking should be avoided
• Packing limits design pressure and temperature

5.2.6 U-tube type

Perhaps the most inexpensive type of heat exchanger because only one end of the tube bundle is restrained, the unit is free to expand and contract on severe temperature differential service. Most common use is for heating applications where steam is the heating medium. Tube side fluid must be non-fouling since inside tube surfaces cannot be manually cleaned but bundle is removable for cleaning of the outside tube surfaces.

Figure 5.5
U tube exchanger

5.2.7 Re-boilers

A re-boiler is used to turn the liquid leaving the bottom of a column into a vapor by transferring the heat of vaporization to the liquid.

Thermosyphon re-boilers

Thermosyphon re-boilers are generally heat exchangers used to provide vapor boil-up to a distillation column. They can be provided in either a vertical position with the boiling fluid in the tubes or in the horizontal position with the boiling fluid in the shell side. In normal operation, between 10% and 33% of the liquid entering the unit is boiled.

Figure 5.6
Thermosyphon re-boilers

Kettle re-boilers

Kettle re-boilers are used in services where 20% to 100% boil-up is required. The enlarged outer shell provides a separation between the boiling fluid and the exit nozzle reducing the amount of liquid entrainment in the exiting vapor stream. In cases where the amount of liquid entrainment must be minimal, a demister pad is provided inside the shell.

Figure 5.7
Kettle reboilers

5.2.8 Spiral heat exchanger

Spiral heat exchangers are custom built both in the thermal design & construction. They work efficiently for many applications in different industries. Spiral heat exchangers are designed to operate in a counter current mode and have low fouling.

Figure 5.8
Spiral exchangers

Advantages

• Optimal Use of Pressure Drop
• Differing Flow Rates in Each Channel
• High Heat Transfer Coefficients
• Minimum Fouling & Self Cleaning Effect
• Compactness of a Spiral
• Utilize the Smallest of Temperature Differences

Applications

• Chemical Industry
• Municipal/Sewage Treatment Plants
• Sterilization Plants
• Alcohol Plants
• Textile Mills
• Cellulose Industry
• Aluminum Plants
• Sugar
• Food Industry

5.2.9 Plate type exchanger

The plate heat exchanger, often called the plate-and-frame heat exchanger, consists of a frame, which holds heat transfer plates. A plate pack of corrugated metal plates with portholes for the media is aligned in a frame and compressed by tightening bolts. The plates form a series of channels for the two media. The channels are sealed by gaskets, which direct the fluid into alternate channels. The fluids normally flow in countercurrent flow, one in the odd number channels and one in the evenly numbered channels.

Plate heat exchangers are characterized by:

• Heat transfer efficiency up to five times that of other types of heat exchanger
• Compact dimensions which fit in a small space and are low weight
• Low investment and operating costs
• Flexible construction which can be extended or rearranged to meet new process conditions
• Low fouling as a result of turbulence created by the plate pattern

5.2.10 In-line plate heat exchangers

They have been designed as a low-cost alternative to our shell and tube types. They consist of numerous 316 stainless steel heat transfer plates, two outer covers and four connections copper vacuum-brazed together to form an integral unit.

Unlike other plate heat exchangers, they have a unique internal flow arrangement, which enables the inlet and outlet connections to be axially in line. This means that they can be installed directly in pipe work without any change of direction. Each fluid stream flows in series through alternate plates. As a consequence, the plate spacing is larger and internal velocities are higher than is normally the case with this type of heat exchanger, thus rendering them less prone to fouling.

Figure 5.9
Inline exchangers

5.2.11 Finned tube exchanger

Finned tube heat exchanger can be installed in a different two-phase flow media or low-density flow media heat exchanging process.

Figure 5.10
Fined tube exchangers

Typical applications include:

• Fluid Cooling and Heating Coils
• Compressor Intercoolers and After coolers
• Gas Turbine Inlet Air Chilling
• Exhaust Gas Waste Heat Recovery
• TEWAC Generator/Motor Coolers
• Transformer Oil Coolers
• Steam Distributing Heating Coils
• Food Dryer and Cooling Coils
• Furnace/Oven Heating and Cooling

5.2.12 Air-cooled exchanger

Air-cooled heat exchangers are generally used where a process system generates heat which must be removed, but for which there is no local use. A good example is the radiator in your car. The engine components must be cooled to keep them from overheating due to friction and the combustion process. The water/glycol coolant mixture carries the excess heat away. The car’s radiator to heat the interior may use a small amount of the excess heat. Most of the heat must be dissipated somehow. One of the simplest ways is to use the ambient air. Air-cooled heat exchangers (often simply called air-coolers) do not require any cooling water from a cooling tower. They are usually used when the outlet temperature is more than about 20 deg. F above the maximum expected ambient air temperature. They can be used with closer approach temperatures, but often become expensive compared to a combination of a cooling tower and a water-cooled exchanger.

Typically, an air-cooled exchanger for process use consists of a finned-tube bundle with rectangular box headers on both ends of the tubes. One or more fans provide cooling air. Usually, the air blows upwards through a horizontal tube bundle. The fans can be either forced or induced draft, depending on whether the air is pushed or pulled through the tube bundle. The space between the fan(s) and the tube bundle is enclosed by a plenum chamber, which directs the air. The whole assembly is usually mounted on legs or a pipe rack.

The fans are usually driven be electric motors through some type of speed reducer. The speed reducers are usually V-belts, HTD drives, or right angle gears. The fan drive assembly is supported by a steel mechanical drive support system. They usually include a vibration switch on each fan to automatically shut down a fan, which has become imbalanced for some reason.

Figure 5.11
Air-cooled exchangers

5.2.13 Graphite heat exchanger

Graphite heat exchanger is used wherever corrosive fluids or gases must be handled.

• Heating - corrosive fluids with steam, gases or hot fluids.
• Cooling - corrosive fluids with cold water, brine or other fluids.
• Condensing - vapors, gases - straight or reflux.
• Evaporating - boiling liquids in evaporation or distillation plants, etc.
• Reaction - including hydrolysis, polymerization or condensation reactions, where heat must be removed or supplied rapidly, during the reaction.

Figure 5.12
Graphite exchangers

5.3 Boilers

The boiler’s job is to apply heat to water to create steam. There are two approaches: fire tube and water tube.

5.3.1 Fire-tube boilers

Fire-tube boilers are generally similar to Scotch marine or locomotive boilers. In this type of boiler, the gases of combustion pass through tubes that are surrounded by water.

A fire-tube boiler was more common in the 1800s. It consists of a tank of water perforated with pipes. In a fire-tube boiler, the entire tank is under pressure, so if the tank bursts it creates a major explosion. The hot gases from a coal or wood fire run through the pipes to heat the water in the tank. Figure 5.13 illustrates a cutaway view of the fire-tube boiler.

Figure 5.13
Fire tube boiler

• Used primarily for space heating and smaller industrial processes
• Range in size from small steam outputs of 600,000 Btu/hr to very large steam output of 50 million Btu/hr
• Not suitable for high capacity/ high pressure steam generation; upper limit is typically 250psig
• Relatively low first cost
• Compact
• Easy to clean
• Ability to handle sudden and very big peak steam loads
• Not suitable for high capacity/ high pressure steam generation

5.3.2 Water tube boilers

Water-tube, natural-circulation boilers consist basically of a steam drum and a water drum connected by a bank of generating tubes. The two drums are also connected by a row of water tubes, which forms a water-cooled sidewall opposite the tube bank. The water-wall tubes pass beneath the refractory furnace floor before they enter the water drum. In natural-circulation boilers, several tubes of larger diameter, called down comers or water tubes, connect the steam and water drums. These tubes are positioned away from the flow of hot gases of combustion. Refractory is also used to protect these down comers from contact with the combustion gases.

The operating principle of a natural-circulation boiler is quite simple. It relies on the difference in density (weight) between the cooler (heavier) water in the water tubes (or down comers) and the hot, less dense (lighter) water in the steam-generating tubes. This is the force that causes the hot water and steam mixture to rise in the tubes in the generating bank, from the water drum to the steam drum, where the steam is separated from the water and rises to the top of the steam drum. The flow of water up the tubes of the steam-generating bank must be maintained; otherwise, the tubes would quickly melt.

Figure 5.14
Water tube boiler

A constant flow of water and steam up the tubes is required to carry away heat at the proper rate. If the flow from natural circulation is allowed to stop, such as when the water level in the steam drum falls below the openings of the bank of tubes for the water wall, the tubes of the generating bank will be severely damaged and the boiler will need major repairs. (Replacing boiler tubes is an expensive operation.)

5.3.3 Tubeless boilers

These high quality systems are ideal for small space installations where volume, simplicity and reliability are critical. All units are test-fired at the factory before shipping, ensuring top performance upon installation.

Figure 5.15
Tubeless boiler

5.3.4 Circulating fluid bed (CFB) boiler

The CFB boiler is designed for high reliability and availability with low maintenance, while complying with stringent emissions regulations.

In a circulating fluidized-bed boiler, a portion of air is introduced through the bottom of the bed. The bed material normally consists of fuel, limestone and ash. Water-cooled membrane walls with specially designed air nozzles support the bottom of the bed, which distributes the air uniformly. The fuel and limestone (for sulfur capture) are fed into the lower bed. In the presence of fluidizing air, the fuel and limestone quickly and uniformly mix under the turbulent environment and behave like a fluid. Carbon particles in the fuel are exposed to the combustion air. The balance of combustion air is introduced at the top of the lower, dense bed. This staged combustion limits the formation of nitrogen oxides (NOx).

The bed fluidizing air velocity is greater than the terminal velocity of most of the particles in the bed and thus fluidizing air elutriates the particles through the combustion chamber to the U-beam separators at the furnace exit. The captured solids, including any unburned carbon and unutilized carbon oxide (CaO), are re-injected directly back into the combustion chamber without passing through an external re-circulation. This internal solids circulation provides longer residence time for fuel and limestone, resulting in good combustion and improved sulfur capture.

5.3.5 AFBC boilers

One of these advanced technologies is Atmospheric Fluidized Bed Combustion (AFBC), which promises to provide a viable alternative to conventional coal fired boilers for utility and industrial application.

The principal features are:

• Drum Natural circulation design
• Baffle less boiler bank (Cross flow)
• Bottom Supported boiler block
• Machine welded membrane wall - gas Tight Furnace
• Underbed fuel feed system using drag chain feeders for smooth flow of high moisture fuel to feed lines
• Fuel feeding lines of higher thickness for longer life
• Hot air for feeding fuel inside combustion for smooth flow fuel in feed lines
• No bends on the fuel transport lines - preventing choking of fuel
• Distributor plate 16-mm. thickness Carbon Steel
• High-density rounds studs welded on inbed surfaces to minimize erosion

5.3.6 Solid fuel fired boiler

The steam boilers fired with wood-waste (sawdust, bark, chopped wood) are designed to raise technological steam in thermal power stations.

They are natural circulation flue boilers, with the heat exchange surfaces arranged on three gas paths, and negative pressure in the furnace.

Figure 5.16
Solid fuel boiler

5.3.7 Thermal oil heating boilers

The heating boilers using thermal oil (mineral oil) are complex units, forming parts of the “Thermal oil heating plants”. They are designed to provide for an outlet temperature ranging between 290-300 °C, at a low pressure (6 bar).

The boilers are used in any application required a temperature of max. 290-300 °C for the heat carrier.

Figure 5.17
Thermal boiler

5.4 Evaporators

Evaporation refers to the process of heating liquid to the boiling point to remove water as vapor. Because milk is heat sensitive, heat damage can be minimized by evaporation under vacuum to reduce the boiling point.

The basic components of this process consist of:

• Heat-exchanger
• Vacuum
• Vapor separator
• Condenser

The heat exchanger is enclosed in a large chamber and transfers heat from the heating medium, usually low-pressure steam, to the product usually via indirect contact surfaces. The vacuum keeps the product temperature low and the difference in temperatures high. The vapor separator removes entrained solids from the vapors, channeling solids back to the heat exchanger and the vapors out to the condenser. It is sometimes a part of the actual heat exchanger, especially in older vacuum pans, but more likely a separate unit in newer installations. The condenser condenses the vapors from inside the heat exchanger and may act as the vacuum source.

The driving force for heat transfer is the difference in temperature between the steam in the coils and the product in the pan. The steam is produced in large boilers, generally tube and chest heat exchangers. The steam temperature is a function of the steam pressure. Water boils at 100° C at 1 atm., but at other pressures the boiling point changes. At its boiling point, the steam condenses in the coils and gives up its latent heat. If the steam temperature is too high, burn-on/fouling increases so there are limits to how high steam temperatures can go. The product is also at its boiling point. The boiling point can be elevated with an increase in solute concentration. This boiling point elevation works on the same principles as freezing point depression.

Figure 5.18
Principle of operation

 

Evaporator designs

Types of single effect evaporators:

• Batch Pan
• Rising film
• Falling film
• Plate evaporators
• Scraped surface

5.4.1 Batch pan

These evaporators are the simplest and oldest. They consist of spherical shaped, steam jacketed vessels. The heat transfer per unit volume is small requiring long residence times. The heating is due only to natural convection; therefore, the heat transfer characteristics are poor. Batch plants are of historical significance; modern evaporation plants are far-removed from this basic idea. The vapors are a tremendous source of low-pressure steam and must be reused.

They are currently used in making jams and jellies but mostly outdated by more efficient means of evaporation. A batch pan evaporator is one of the oldest methods of concentrating.

Figure 5.19
Batch pan evaporator

5.4.2 Rising film

They consist of a heat exchanger isolated from the vapor separator. The heat exchanger, or calandria, consists of 10 to 15 meter long tubes in a tube chest, which is heated with steam. The liquid rises by percolation from the vapors formed near the bottom of the heating tubes. The thin liquid film moves rapidly upwards. The product may be recycled if necessary to arrive at the desired final concentration. This development of this type of modern evaporator has given way to the falling film evaporator.

Dating back to the early 1900s, this equipment uses a vertical tube with steam condensing on its outside surface. Liquid on the inside of the tube is brought to a boil, with the vapor generated forming a core in the center of the tube. As the fluid moves up the tube, more vapors are formed resulting in a higher central core velocity those forces the remaining liquid to the tube wall.

Figure 5.20
Rising film evaporator

5.4.3 Falling film

These types of evaporators are the most widely used in the food industry. They are similar in components to the rising film type except that the thin liquid film moves downward under gravity in the tubes. A uniform film distribution at the feed inlet is much more difficult to obtain. This is the reason why this development came slowly and it is only within the last decade that falling film has superceded all other designs. Specially designed nozzles or spray distributors at the feed inlet permit it to handle more viscous products. The residence time is 20-30 sec. as opposed to 3-4 min. in the rising film type. The vapor separator is at the bottom which decreases the product hold-up during shut down. The tubes are 8-12 meters long and 30-50 mm in diameter.

Figure 5.21
Falling film evaporator

5.4.4 Falling film tubular

Similar to the rising film unit, this unit has several advantages. First, because the vapor is working in the same direction as gravity, these units are more efficient. Second, to establish a well-developed film, the rising film unit requires a driving film force, typically a temperature difference of at least 25 degrees across the heating surface whereas the falling film evaporator is not limited by this permitting many more multiple effect stages of evaporation. Therefore, with this technology, it is feasible to have as many as ten effects in a process.

5.4.5 Forced circulation

The forced circulation evaporator was developed for processing liquors, which are susceptible to scaling or crystallizing. Liquid is circulated at a high rate through the heat exchanger, boiling being prevented within the unit by virtue of a hydrostatic head maintained above the top tube plate. As the liquid enters the separator where the absolute pressure is slightly less than in the tube bundle, the liquid flashes to form a vapor.

The main applications are in the concentration of inversely soluble materials, crystallizing duties and in the concentration of thermally degradable materials, which result in the deposition of solids.

Figure 5.22
Forced circulation evaporator

5.4.6 Plate equivalents of tubular evaporators

Plate type evaporators were initially developed by APV in 1957 to provide an alternative to tubular systems to meet the growing challenges of the process industries. The plate evaporator offers full accessibility to the heat transfer surface. It also provides flexible capacity merely by adding more plate units, shorter product residence time resulting in a quality concentrate, a more compact design with low headroom requirements and low installation cost.

Figure 5.23
Tubular evaporator

5.4.7 Scrapped surface thin film evaporators

They are designed for evaporation of highly viscous and sticky products, which cannot be otherwise evaporated. This type of evaporators have been specially designed to provide high degree of agitation, effecting heat transfer as well as scrapping the walls of the evaporator to prevent deposition and subsequent charring of the product.

In circumstances in which a scale is formed the tubes of the calandria can be scraped to remove the scale. Such evaporators are called Scrapped surface tube evaporators. Inside the tubes are rotating devices, which scrape off any scale formed with spring-loaded blades. These scraping devices can be used in long tube or short tube evaporators, however, particularly in long tube evaporators there can be problems of poor liquid distribution. The scale that is removed must be separated from the product later in the process. Other types of evaporator are often more efficient for use with liquids that form scale.

5.4.8 Vertical agitated thin film evaporators (ATFE)

They contain a rotor designed to produce and agitate a thin film between the rotor and the heated wall of the evaporator. The agitation of the film on the heated surface promotes heat transfer and maintains precipitated or crystallized solids in a manageable suspension without fouling the heat transfer surface. This capability makes ATFEs particularly suited for volume reduction of radioactive wastes that contain suspended solids. The tests were conducted with surrogates that contained no radionuclides. The results of the tests indicated that a variety of products could be produced with the ATFE. It was possible to vary the consistency from a highly concentrated liquid to a completely dry powder. Volume reductions ranged from ~20 to 68% and decontamination factors in the range of 10,000 to 100,000 were achieved.

Figure 5.24
ATFE

5.4.9 Multiple effect evaporators

Two or more evaporator units can be run in sequence to produce a multiple effect evaporators. Each effect would consist a heat transfer surface, a vapor separator, as well as a vacuum source and a condenser. The vapors from the preceding effect are used as the heat source in the next effect.

There are two advantages to multiple effect evaporators:

• Economy - they evaporate more water per kg steam by re-using vapors as heat sources in subsequent effects
• Improve heat transfer - due to the viscous effects of the products as they become more concentrated

Each effect operates at a lower pressure and temperature than the effect preceding it so as to maintain a temperature difference and continue the evaporation procedure. The vapors are removed from the preceding effect at the boiling temperature of the product at that effect so that no temperature difference would exist if the vacuum were not increased. The operating costs of evaporation are relative to the number of effects and the temperature at which they operate. The boiling milk creates vapors, which can be recompressed for high steam economy. This can be done by adding energy to the vapor in the form of a steam jet, thermo compression or by a mechanical compressor, mechanical vapor recompression.

Figure 5.25
Multiple effect evaporators

5.4.10 Thermo compression (TC) evaporator

Involves the use of a steam-jet booster to recompress part of the exit vapors from the first effect. Through recompression, the pressure and temperature of the vapors are increased. As the vapors exit from the first effect, they are mixed with very high-pressure steam. The steam entering the first effect calandria is at slightly less pressure than the supply steam. There are usually more vapors from the first effect than the second effect can use. Usually only the first effect is coupled with multiple effect evaporators.

Figure 5.26
Double-effect thermal recompression evaporator

To reduce energy consumption, water vapor from an evaporator is entrained and compressed with high-pressure steam in a thermo compressor so it can be condensed in the evaporator heat exchanger. The resultant pressure is intermediate to that of the motive steam and the water vapor. A thermo compressor is similar to a steam-jet air ejector used to maintain vacuum in an evaporator.

Only a portion of the vapor from an evaporator can be compressed in a thermo compressor with the remainder condensed in the next-effect heat exchanger or a condenser. A thermo compressor is normally used on a single-effect evaporator or on the first effect of a double- or triple-effect evaporator to reduce energy consumption. As with mechanical recompression, thermal recompression is more applicable to low boiling-point rise liquids and low to moderate Delta-T’s in the heat exchanger to minimize the compression ratio.

5.4.11 Mechanical vapor recompression (MVR)

Whereas only part of the vapor is recompressed using TC, all the vapor is recompressed in an MVR evaporator. Radial compressors or simple fans using electrical energy mechanically compress vapors.

There are several variations; in single effect, all the vapors are recompressed therefore no condensing water is needed; in multiple effect, can have MVR on first effect, followed by two or more traditional effects; or can recompress vapors from all effects

Figure 5.27
MVR cycle

Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapor from an evaporator is compressed (with a positive-displacement, centrifugal or axial-flow compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger. Various combinations are possible, including single-effect recompression, multiple-effect recompression, multiple-stage recompression, and single-effect recompression combined with a multiple-effect evaporator.

A simplified flow sheet of a single-effect recompression evaporator illustrates why mechanical recompression is energy efficient

Mechanical recompression is not limited to single-effect evaporation. It is sometimes economical to compress vapor from the last effect of a double- or triple-effect evaporator so that the vapor can be condensed in the first-effect heat exchanger.