Heat exchanger

• The word exchanger really applies to all types of equipment in which heat is exchanged but

• is often used specially to denote equipment in which heat is exchanged between two process

• Streams.

These heat exchanger may be classified according to:

• Transfer process• 1. Direct contact• 2. indirect contact• (a) Direct transfer type• (b) Storage type• (c) Fluidized bed

Surface compactness

• 1. Compact (surface area density ¸ 700m2=m3)

• 2. non-compact (surface area density < 700m2=m3)

Construction

• 1. Tubular– (a) Double pipe– (b) Shell and tube– (c) Spiral tube

• 2. Plate– (a) Gasketed– (b) Spiral plate– (c) Welded plate

• 3. Extended surface– (a) Plate fin– (b) Tube fin

• 4. Regenerative– (a) Rotory

• i. Disc-type• ii. Drum-type

– (b) Fixed-matrix

Flow arrangement

• 1. Single pass(a) Parallel flow(b) Counter flow(c) Cross flow

• 2. Multipass(a) Extended surface H.E.

i. Cross counter flowii. Cross parallel flow

(b) Shell and tube H.E.i. Parallel counter flow (Shell and fluid mixed, M shell pass, N Tube pass)

ii. Split flowiii. Divided flow

(c) Plate H.E. (N-parallel plate multipass)

Number of fluids

• 1. Two-fluid• 2. Three fluid• 3. N-fluid (N > 3)

Transfer mechanisms

• 1. Single phase convection on both sides• 2. Single phase convection on one side, two-

phase convection on the other side• 3. Two-phase convection on both sides• 4. Combined convection and radiative heat

transfer

Classification based on service

• single phase (such as the cooling or heating of a liquid or gas)• two-phase (such as condensing or vaporizing).• Since there are two sides to an STHE, this can lead to several

combinations of services. Broadly, services can be classified as follows:

• single-phase (both shellside and tubeside); • condensing (one side condensing and the other single-phase); • vaporizing (one side vaporizing and the other side single-

phase); and • condensing/vaporizing (one side condensing and the other

side vaporizing). The following nomenclature is usually used:

• Heat exchanger: both sides single phase and process streams (that is, not a utility).• Cooler: one stream a process fluid and the other cooling water or air. Dirty water can be

used as the cooling medium. The top of the cooler is open to the atmosphere for access to tubes. These can be cleaned without shutting down the cooler by removing the distributors one at a time and scrubbing the tubes.

• Heater: one stream a process fluid and the other a hot utility, such as steam or hot oil.• Condenser: one stream a condensing vapor and the other cooling water or air.• Chiller: one stream a process fluid being condensed at sub-atmospheric temperatures and the other a boiling refrigerant or process stream. By cooling the falling film to its freezing point, these exchangers convert a variety of chemicals to the solid phase. The most common application is the production of sized ice and paradichlorobenzene. Selective freezing is used for isolating isomers. By melting the solid material and refreezing in several stages, a higher degree of purity of product can be obtained.

Reboiler: one stream a bottoms stream from a distillation column and the• other a hot utility (steam or hot oil) or a process stream.

Evaporators: These are used extensively for the concentration of ammonium nitrate, urea, and other chemicals sensitive to heat when minimum contact time is desirable. Air is sometimes introduced in the tubes to lower the partial pressure of liquids whose boiling points are high.

These evaporators are built for pressure or vacuum and with top or bottom vapor removal.

Absorbers: These have a two-phase flow system. The absorbing medium is put in film flow during its fall downward on the tubes as it is cooled by a cooling medium outside the tubes. The film absorbs the gas which is introduced into the tubes. This operation can be cocurrent or countercurrent.

Falling-Film Exchangers: Falling-film shell-and-tube heat exchangers have been developed for a wide variety of services and are described by Sack The fluid enters at the top of the vertical tubes. Distributors or slotted tubes put the liquid in film flow in the inside surface of the tubes, and the film adheres to the tube surface while falling to the bottom of the tubes. The film can be cooled, heated, evaporated, or frozen by means of the proper heat-transfer medium outside the tubes. Tube distributors have been developed for a wide range of applications. Fixed tube sheets, with or without expansion joints, and outside-packed-head designs are used.

Principal advantages are high rate of heat transfer, no internal pressure drop, short time of contact (very important for heat-sensitive materials), easy accessibility to tubes for cleaning, and, in some cases, prevention of leakage from one side to another.

Classification by construction

• The principal types of heat exchanger are listed again as

• 1. Tubular exchanger• 2. Plate exchanger• 3. Extended surface• 4. Regenerative

2.1.1 Tubular heat exchanger

Tubular heat exchanger are generally built of circular tubes. Tubular heat exchanger is

further classified into:• Double pipe heat exchanger• Spiral tube heat exchanger• Shell and tube heat exchanger

Double pipe heat exchanger

Constructon :- This is usually consists of concentric pipes. One fluid flow in the inner pipe and the other fluid flow in the annulus between pipes.

The two fluid may flow concurrent (parallel) or in counter current flow configuration; hence the heat

exchanger are classified as: counter current double pipe heat exchanger cocurrent double

pipe heat exchanger Advantages :-• Is Easily by disassembly, no cleaning problem• ii Suitable for high pressure fluid, (the pressure

containment in the small diameter pipe• or tubing is a less costly method compared to a large

diameter shell.)Limitation• The double pipe heat exchanger is generally used for the

application where• the total heat transfer surface area required is less than or

equal to 20 m2 (215 ft2) because• it is expensive on a cost per square meter (foot) basis.

Spiral tube heat exchanger

• Spiral tube heat exchanger consists of one or more spirally wound coils fitted in a shell . Heat transfer associated with spiral tube is higher than that for a straight tube .

• In addition, considerable amount of surface area can be accommodated in a given

• space by spiraling. Thermal expansion is no problem but cleaning is almost impossible.

Advantages Inexpensive True countercurrent or co-current flow Easily designed for high pressure service

Disadvantages Difficult to clean on shell side. Only suitable for small sizes. They are generally not economical if UA > 50,000 Btu/hr-oF. Thermal expansion can be an issue.

• Typical Applications • Single phase heating and cooling when the required heat transfer

area is small. • Can be used for heating using condensing steam if fabricated with

elbows to allow expansion.

HAIRPIN HEAT EXCHANGERS

The hairpin heat exchanger design is similar to that of double pipe heat exchangers with multiple tubes inside one shell. The design provides the flexibility of a U-tube design with an extended shell length that improves the exchanger’s ability to achieve close temperature approaches.

Advantages • Good countercurrent or co-current flow – good temperature approach. • Can be designed with removable shell to allow cleaning & inspection. • Use of finned tubes results in compact design for shellside fluids with low heat transfer coefficients. • Easily designed for high pressure service. • Able to handle large temperature difference between the shell and tube sides without using expansion joints. • All connections are at one end of the exchanger. Disadvantages • Designs are proprietary – limited number of manufacturers. • Relatively expensive. • Limited size – Not economical if UA > 150,000 Btu/hr-oF.

Applications Single phase heating and cooling when the required heat transfer area is relatively small.

Often found in high pressure services and where there is a large temperature difference between the shell and tubeside fluids.

Shell and tube heat exchanger

• Shellside Flow Out

• Tubeside Flow In

• Tubeside Flow Out

• Shell

• Tube Bundle

• Shellside Flow In

Shell and tube heat exchanger is built of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell. One fluid flow inside the tube, the other flow across and along the tubes. The major components of the shell and tube heat exchanger are tube bundle, shell, front end head, rear end head, baffles and tube sheets

• The shell and tube heat exchanger is further divided into three categories as

• 1. Fixed tube sheet• 2. U tube• 3. Floating head

Fixed tubesheet

• A fixed-tubesheet heat exchanger has straight tubes that are secured at both ends to tubesheets welded to the shell. The construction may have removable channel covers , bonnet-type channel covers , or integral tubesheets.

• Advantage The fixedtubesheet construction is its low cost because of its simple construction. In fact, the fixed tubesheet is the least expensive construction type, as long as no expansion joint is required.

tubes can be cleaned mechanically after removal of the channel cover or bonnet, and that leakage of the shell side fluid is minimized since thereare no flanged joints.

Disadvantage This design is that since the bundle is fixed to the shell and cannot beremoved, the outsides of the tubes cannot be cleaned mechanically. Thus, its application is limited to clean services on the shell side.

• However, if a satisfactory chemical cleaning is designed can be employed, fixed-tubesheet construction may be selected for fouling services on the shell side.

• In the event of a large differential temperature between the tubes and the shell, the tubesheets will be unable to absorb the differential stress, thereby making it necessary to

• Incorporate an expansion joint. This takes away the advantage of low cost to a significant extent.

U-tube

• As the name implies, the tubes of a U-tube heat exchanger are bent in the shape of a U.• There is only one tubesheet in a Utube heat exchanger. However, the lower cost for the

single tubesheet is offset by the additional costs incurred for the bending of the tubes and the somewhat larger shell diameter (due to the minimum U-bend radius), making the cost of a U-tube heat exchanger comparable to that of a fixed tubesheet exchanger.

Advantage • U-tube heat exchanger as one end is free, the bundle• can expand or contract in response to stress differentials.• In addition, the outsides of the tubes can be cleaned, as the

tube bundle can be removed.

Disadvantage U-tube construction is that the insides of the tubes cannot becleaned effectively, since the U-bends would require flexible-end drill shafts for cleaning. Thus, U-tube heat

exchangers should not be used for services with a dirty fluid inside tubes.

Floating head

• The floating-head heat exchanger is the most versatile type of STHE, and also the costliest.

• In this design, one tubesheet is fixed relative to the shell, and the other is free to ”float” within the shell. This permits free expansion of the tube bundle, as well as cleaning of both the insides and outsides of the tubes. Thus, floating-head SHTEs can be used for services where both the shell side and the tube side fluids are dirty-making

The standard construction type used in dirty services, such as in petroleum refineries. There are various types of floating- head construction. The two most common are the pull-through with backing device and pull through without backing service designs. The design with backing service is the most common configuration in the chemical process industries (CPI). The floating-head cover is secured against the floating tubesheet by bolting it to an ingenious split backing ring. This floating-head closure is located beyond the end of the shell and contained by a shell cover of a larger diameter. To dismantle the heat exchanger, the shell cover is removed first, then the split backing ring, and then the floating-head cover, after which the tube bundle can be removed from the stationary end.

• In the design without packing service construction (Figure 2.8), the entire tube bundle, including the floating-head assembly, can be removed from the stationary end, since the shell diameter is larger than the floating-head flange. The floatinghead cover is bolted

• directly to the floating tubesheet so that a split backing ring is not required.

• The advantage of this construction is that the tube bundle may be removed from the shell without removing either the shell or the floatinghead cover, thus reducing maintenance time. This design is particularly suited to kettle reboilers having a dirty heating medium where Utubes cannot be employed. Due to the enlarged shell, this construction has the highest

• cost of all exchanger types.

Plate heat exchangers

• These exchangers are generally built of thin plates. The plate are either smooth or have

• some form of corrugations and they are either flat or wound in exchanger. Generally

• theses exchanger cannot accomodate high pressure/temperature differential relative the

• tubular exchanger.

• This type of exchanger is further classified as:• Gasketed plate• Fixed plate• Spiral plate

Gasketed plate heat exchangerGasketed plate heat exchanger consists of a series of corrugated alloy material channel plates, bounded

by elastomeric gaskets are hung off and guided by longitudinal carrying bars, then compressed by large-diameter tightening bolts between two pressure retaining frame plates (cover plates)

Construction The frame and channel plates have portholes which allow the process fluids to enter

alternating flow passages (the space between two adjacent-channel plates) Gaskets around the periphery of the channel plate prevent leakage to the atmosphere and also prevent process fluids from coming in contact with the frame plates. No inter fluid leakage is possible in the port area due to a dual-gasket seal.

Expansion of the initial unit is easily performed in the field without special considerations.The original frame length typically has an additional capacity of 15-20 percent morechannel plates (i.e. surface area). In fact, if a known future capacity is available duringfabrication stages, a longer carrying bar could be installed, and later, increasing thesurface area would be easily handled. When the expansion is needed, simply untighten the carrying bolts, pull back the frame

plate, add the additional channel plates, and tighten the frame plate.

Applications:

Most PHE applications are liquid-liquid services but there are numerous steam heater and evaporator uses from their old ages in the food industry.

Industrial users typically have chevron style channel plates while some food applications are washboard style.

Fine particulate slurries in concentrations up to 70 percent by weight are possible with standard channel spacing's.

Wide-gap units are used with larger particle sizes. Typical particle size should not exceed 75 percent of the single plate (not total

channel) gap. Close temperature approaches and tight temperature control possible with

PHE’s and the ability to sanitize the entire heat transfer surface easily were a major benefit in the food and pharmaceutical industry.

Advantages: - Easily assembled and dismantledEasily cleaned both chemically and mechanicallyFlexible (the heat transfer can be changed as required) Can be used for multiple service as required Leak is immediately deteced since all plates are vented to the atmosphere, and thefluid split on the floor rather than mixing with other fluid Heat transfer coefficient is larger and hence small heat transfer area is required than STHEThe space required is less than that for STHE for the same dutyLess fouling due to high turbulent flowVery close temperature approach can be obtainedlow hold up volumeLMTD is fully utilized More economical when material cost are high

• Disadvantages: - Low pressure <30 bar (plate deformation) Working temperature of < (500 F) [250 oC]

(maximum gasket temperature)

Welded- and Brazed-Plate exchanger• To overcome the gasket limitations, PHE

manufacturers have developed welded-plate• exchangers. There are numerous approaches

to this solution: weld plate pairs together• with the other fluid-side conventionally

gasketed, weld up both sides but use a horizontal

• nickel brazing, diffusion bond then pressure form plates and bond etched, passage plates

• Typical applications include district heating where the low cost and minimal maintenance

• have made this type of heat exchanger especially attractive.

Most methods of welded-plate manufacturing do not allow for inspection of the heattransfer surface, mechanical cleaning of that surface, and have limited ability to repair or plug off damage channels. Consider these limitations when the fluid is heavily fouling,has solids, or in general the repair or plugging ability for severe services.

PLATE & FRAME HEAT EXCHANGERS A plate and frame heat exchanger is a compact heat exchanger where thin corrugated plates are stacked in contact with each other, and the two fluids flow separately along adjacent channels in the corrugation.

The closure of the stacked plates may be by clamped gaskets, brazed (usually copper brazed stainless steel), or welded (stainless steel, copper, titanium), the most common type being the first, for ease of inspection and cleaning.

Advantages Very compact design High heat transfer coefficients (2 – 4 times shell & tube designs) Expandable by adding plates Ease of maintenance Plates manufactured in many alloys All connections are at one end of the exchanger Good temperature approaches Fluid residence time is very short No dead spots Leakage (if it should occur) is generally to the outside – not between the fluids Low fouling due to high turbulence

Disadvantages • Designs are proprietary – limited number of manufacturers • Gaskets limit operating pressures and temperatures & require

good maintenance • Typical maximum design pressures are 150-250 psig.• Gasket compatible with fluids are not always available• Poor ability to handle solids – due to close internal clearances • High pressure drop• Not suitable for hazardous materials• Not suitable in vacuum service.Typical Applications • Low pressure and temperature single phase heating and

cooling when fluids are not hazardous, a high pressure drop can be tolerated and alloys are required for the fluids being handled.

Spiral Plate Exchanger (SPHE)SPHEs offer high reliability and on-line performance in many

severely fouling services such as slurries.

CONSTRUCTION :-The SHE is formed by rolling two strips of plate, with welded-on

spacer studs, upon each other into clock-spring shape and This forms two passages. Passages are sealed off on one end of the SHE by welding a bar to the plates; hot and cold fluid passages are sealed off on opposite ends of the SHE.

• A single rectangular flow passage is now formed for each fluid, producing very high shear rates compared to tubular designs. Removable covers are provided on each end to access and clean the entire heat transfer surface.

• Pure countercurrent flow is achieved and LMTD correction factor is essentially = 1.0.

• Since there are no dead spaces in a SHE, the helical flow pattern combines to entrain any solids and create high turbulence creating a self-cleaning flow passage. There are no thermal-expansion problems in spirals. Since the center of the unit is not fixed, it can torque to relieve stress. The SHE can be expensive when only one fluid requires high alloy material.

• Since the heat-transfer plate contacts both fluids, it is required to be fabricated out of the higher alloy. SHEs can be fabricated out of any material that can be cold-worked and welded. The channel spacings can be different on each side to match the flow rates and pressure drops of the process design. The spacer studs are also adjusted in

• their pitch to match the fluid characteristics. As the coiled plate spirals outward, the plate thickness increases from a minimum of 2 mm to a maximum (as required by pressure)

• up to 10 mm. This means relatively thick material separates the two fluids compared to tubing of conventional exchangers.

Applications: • The most common applications that fit SHE are slurries. The

rectangular channel provides high shear and turbulence to sweep the surface clear of blockage and causes no distribution problems associated with other exchanger types.

• A localized restriction causes an increase in local velocity which aids in keeping the unit free flowing. Only fibers that are long and stringy cause SHE to have a blockage it cannot clear itself.

• As an additional antifoulant measure, SHEs have been coated with a phenolic lining. This provides some degree of corrosion protection as well, but this is not guaranteed due to pinholes in the lining process.

There are three types of SHE to fit different applications:

Type I is the spiral-spiral flow pattern It is used for all heating and cooling services and can accommodate temperature crosses such as lean/rich services in one unit. The removable covers on each end allow access to one side at a time to perform maintenance on that fluid side. Never remove a cover with one side under

• pressure as the unit will telescope out like a collapsible cup.

Type II units are the condenser and reboiler designs One side is spiral flow and the other side is in cross flow. These SHEs provide very stable designs for vacuum condensing and reboiling services. A SHE can be fitted with special mounting connections for reflux-type ventcondenser applications. The vertically mounted SHE directly attaches on the column or tank.

Type III units are a combination of the Type I and Type II where part is in spiral flow and part is in cross flow. This SHE can condense and subcool in a single unit. The unique channel arrangement has been used to provide on-line cleaning, by switching fluid sides to clean the fouling (caused by the fluid that previously flowed there) off the surface. Phosphoric acid coolers use pond water for cooling and both sides foul; water, as you expect, and phosphoric acid deposit crystals. By

• reversing the flow sides, the water dissolves the acid crystals and the acid clears up the organic fouling. SHEs are also used as oleum coolers, sludge coolers/ heaters, slop oil heaters, and in other services where multiple flow- passage designs have not performed well.

SPIRAL PLATE HEAT EXCHANGERS Spiral plate heat exchangers are fabricated from two metal plates that are wound around each other. One process fluid stream enters the exchanger at the centre and flows outwards while the second fluid enters on the outside and flows inward. This creates almost a true countercurrent flow.

Advantages • •Single flow paths reduce fouling rates associated with fluids

containing solids. • Ability to handle two highly fouling fluids • No dead spots for solids to collect inside exchanger • Countercurrent flow • Manufactured in many alloys • Very low pressure drop

• Disadvantages • Designs are proprietary – limited number of

manufacturers • Generally more expensive than shell & tube designs

Typical Applications • 1. Liquid/liquid heating, cooling or heat recovery,

where one or both of the fluids may cause fouling. • 2. Vapour/liquid condensing, particularly at very low

pressure and/or high-volume flow.

SPIRAL TUBE & HELIFLOW HEAT EXCHANGERS Spiral tube type heat exchangers are fabricated from coiled tubing. In some cases the tubing is installed inside a fabricated bundle to provide a compact stand alone heat exchanger. These exchangers are used primarily for small services such as pump seal fluid and sample coolers. See attached article "Graham Spiral Flow Heat Exchangers.pdf" for a more detailed description.

Advantages Compact very inexpensive exchanger for small applications Can handle high pressures

Disadvantages Designs are proprietary – limited number of manufacturers

AIR COOLED HEAT EXCHANGERS • locations where there is a shortage of cooling water. • Air-cooled heat exchangers are usually used when the heat exchanger outlet

temperature is at least 20 oF above the maximum expected ambient air temperature. They can be designed for closer approach temperatures, but often become expensive compared to a combination of a cooling tower and a water-cooled exchanger.

• Air cooled heat exchangers use electrically driven fans to move air across a bank of tubes. There are two basic arrangements:

• • • Induced draft Fans draw air through the tube banks. • • • Forced draft Fans blow air through the tube banks.

• Air cooled exchangers are expensive compared to water cooled exchangers due to their large size, low heat transfer coefficients on the air size, and structural and electrical requirements. In addition air cooler exchangers require large plot areas and must be designed to handle diurnal and seasonal changes in air temperature.

• The very low heat transfer coefficient associated with air on the outside of the tubes is partially overcome through extensive use of finned tubes to increase the outside surface area.

Changes in ambient air temperatures are often handled by using variable speed or pitch fans to adjust the air flow. In cold climates, it may be necessary to design in the ability to recirculate air to prevent freezing in the process.

Smaller units (similar to radiators) are available and commonly used for small duty applications.

Advantages Do not use water for cooling

Disadvantages Requires large plot area Expensive Fins can plug in "dirty" environments Fans can be noisy

Typical Applications Cooling and condensing where cooling water is unavailable or is uneconomical to use.

Extended surface• The tubular and plate exchangers described previously are all prime surface heat

exchangers. The design thermal effectiveness is usually 60 % and below and the heat transfer area density is usually less than 300 m2/m3. In many application an effectiveness of up to 90 % is essential and the box volume and mass are limited so that a much more compact surface is mandated.

• Usually either a gas or a liquid having a low heat transfer coefficient is the fluid on one or both sides. This results in a large heat transfer area requirements. for low density fluid (gases), pressure drop constraints tend to require a large flow area. so a question arises how can we increase both the surface area and flow area together in

a reasonably shaped configuration. The surface area may be increased by the fins. The flow area is increased by the use of thin gauge material and sizing the core property.

There are two most common types of extended surface heat exchangers.

Plate fin

• Plate -fin heat exchanger has fins or spacers sandwiched between parallel plates (refereed to as parting plates or parting sheets) or formed tubes.

• While the plates separate the two fluid streams, the fins form the individual flow passages. Fins are used on both sides in a gas-gas heat exchanger. In gas-liquid applications fins are used in the gas side.

Tube fin

• In tube fin heat exchanger, tubes of round, rectangular, or elliptical shape are generally used. Fins are generally used on the outside and also used inside the tubes in some applications. they are attached to the tube by tight mechanical fit, tension wound, gluing,

soldering, brazing, welding or extrusion. Tube fin exchanger