Heat exchanger The word exchanger really applies to all types of equipment in which heat - PowerPoint Presentation

Slide1

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.Slide2

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 bedSlide3

Surface compactness

1. Compact (surface area density

¸ 700m2=m3)

2. non-compact (surface area density

< 700m2=m3)Slide4

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-matrixSlide5

Flow arrangement

1. Single pass

(a) Parallel flow

(b) Counter flow

(c) Cross flow

2.

Multipass

(a) Extended surface H.E.

i

. Cross counter flow

ii. 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 flow

iii. Divided flow

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

multipass

)Slide6

Number of fluids

1. Two-fluid

2. Three fluid

3. N-fluid (

N > 3)Slide7

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 transferSlide8

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:Slide9

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.Slide10

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. Slide11

Classification by construction

The principal types of heat exchanger are listed again as

1. Tubular exchanger

2. Plate exchanger

3. Extended surface

4. RegenerativeSlide12

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 exchangerSlide13

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.Slide14

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.Slide15

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. Slide16

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.Slide17

Shell and tube heat exchangerSlide18

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 sheetsSlide19

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

1. Fixed tube sheet

2. U tube3. Floating headSlide20

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.Slide21

tubes can be cleaned mechanically after removal of the channel cover

or bonnet, and that leakage of the shell side fluid is minimized since there

are no flanged joints.

Disadvantage

This design is that since the bundle is fixed to the shell and cannot be

removed, 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.Slide22

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.Slide23

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 be

cleaned 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.Slide24

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-makingSlide25

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.Slide26

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.Slide27
Slide28

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.Slide29

This type of exchanger is further classified as:

Gasketed

plate

Fixed plate

Spiral plateSlide30

Gasketed

plate heat exchanger

Gasketed

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)Slide31

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 more

channel plates (i.e. surface area). In fact, if a known future capacity is available during

fabrication stages, a longer carrying bar could be installed, and later, increasing the

surface 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.Slide32

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.Slide33

Advantages: -

Easily assembled and dismantled

Easily cleaned both chemically and mechanically

Flexible (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 the

fluid split on the floor rather than mixing with other fluid

Heat transfer coefficient is larger and hence small heat transfer area is required than

STHE

The space required is less than that for STHE for the same duty

Less fouling due to high turbulent flow

Very close temperature approach can be obtained

low hold up volume

LMTD is fully utilized

More economical when material cost are highSlide34

Disadvantages: -

Low pressure <30 bar (plate deformation)

Working temperature of < (500 F) [250

oC

] (maximum gasket temperature)Slide35

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.Slide36

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.Slide37

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 Slide38

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. Slide39

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.Slide40

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. Slide41

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.Slide42
Slide43

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. Slide44

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.Slide45

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 Slide46

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. Slide47

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 Slide48
Slide49

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. Slide50
Slide51

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. Slide52
Slide53

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.Slide54

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.Slide55

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