الرحمن الله بسمالرحيم

ALEXANDRIA UNIVERSITYFACULTY OF ENGINEERINGSPECIALIZED SCIENTIFIC

PROGRAMSGPE 336 HEAT TRANSFER

OPERATIONS

Design and Operation of Heat Exchangers

Outline

Introduction and purpose of the course

Importance of heat transfer

General revision on heat transfer fundamentals

Classification of heat transfer equipments

Types of heat exchangers

Design procedure of heat exchangers

Operation of heat exchangers ( start up and shutdown)

Problems and testing of heat exchangers

Maintenance of heat exchangers

Direct contact heat exchangers ( fired heaters)

Fired heaters (Boilers)

Introduction & Purpose of the course

Importance of heat transfer

1-To control the rate of chemical reactions [exothermic and endothermic reactions]

2-To control mass transfer operations [distillation, evaporation……]

3-Power generation [steam boilers]

Types of heat transfer equipments

1.Indirect heat transfer equipments [heat exchangers, heaters, coolers, vaporizers,

condensers, evaporators, cooler condenser, boilers, waste heat boiler, chillers….]

2.Direct heat transfer equipments [cooling towers]

What is the difference?

Classification of heat exchangers

The principal Types of heat exchangers

1.Double pipe heat exchangers

2.Shell and tube heat exchangers

3.Plate and frame heat exchangers

4.Plate fin heat exchangers

5.Spiral heat exchangers

6.Air cooled heat exchangers

7.Fired heaters [boilers]

1-Double pipe heat exchanger

Construction

1-Double pipe heat exchanger

Hairpin unit

Flow patterns1-Co current flow [parallel flow]

2 -Counter current flow

3-Cross flowIntermediate effectiveness between parallel flow and counter flow exchangers. [not used with double pipe]

Why is counter current flow more efficient than co-current flow?

Because counter current flow can have the hottest cold fluid temperature greater than the coldest hot fluid temperature

How to improve the efficiency of the double pipe H.X?

• If a fluid with a poor heat transfer coefficient [oil or air] is to be

cooled, an axially finned pipe can be placed inside of the larger pipe.

• Hairpin units can be connected in series or parallel to give the desired capacity.

Double pipe H.X in seriesDouble pipe H.X in parallel

Advantage of double pipe H.X

1. No expansion loops are required

2. Modular design allows for the addition of sections at a later time or the rearrangement of sections for new services.

3. Simple construction leads to ease of cleaning, inspection, and tube element replacement.

4. Can handle high pressure

Disadvantages

1. Multiple hairpin sections are not always economically competitive with a single shell and tube heat exchanger.

2. Proprietary closure design requires special gaskets.

II -Shell and tube heat exchanger

The most common used type….why?

1.The configuration gives a large surface area in a small volume (i.e. compact)

2.Can be constructed from a wide range of materials

3.Well established design procedure

Types of shell and tube heat exchangers:

I. Fixed tube sheet (plate)

II. U- tube

III. Internal floating head without clamp ring

IV. Internal floating head with clamp ring

V. External floating head

VI. Kettle re-boiler with U- tube bundle

I. Fixed tube sheet (plate) H.X

Main components

WHEN TO USE THIS TYPE?

1-Used where the shell side fouling is limited …….why?

2-Used where small temperature difference is required

[up to 80 ◦C]….why?

Heat exchanger standard and codes

TEMA standards [tubular heat exchanger manufacture association] are universally used.

1.Tubes

A.Dimensions

i. Tube diameter

Tube diameters in the range of 16 to 50 mm

When to use small diameters and when to use large diameters?

ii.Tube thicknessThe tube thickness depends on two parameter

• Internal pressure

• Corrosion rate

Standard dimensions for steel tubes

Wall thickness (mm) Outside diameter (mm)

-- -- 2.0 1.6 1.216

-- 2.6 2.0 1.6 -- 20

3.2 2.6 2.0 1.6 -- 25

3.2 2.6 2.0 1.6 -- 30

3.2 2.6 2.0 -- -- 38

3.2 2.6 2.0 -- -- 50

iii.Tube lengthThe preferred lengths of tubes for shell and tube heat exchangers are

6, 8, 12, 16, 20 and 24 ft.As the length increase, the shell diameter decrease, this will result in

a lower cost H.X, but at the expense of pressure drop … why???

B.Tube arrangements (layouts)

Exchanger tubes can be installed in a variety of patterns Triangular (higher heat transfer rate but at the expense of higher

∆P)

Square (used when the conditions require low ∆P and for heavily fouling fluids but low heat transfer rate is obtained)

Rotated square (as triangular layout but at a lower heat transfer rate since low number of tubes are used in a given area)

Tube layouts

pitchtriangular Square

Rotatedsquare

The recommended tube pitch is 1.25 times the tube outside diameter

ExampleSquare layout

C.Tube joints and tube sheet

The tube joint is the connection between the tube and the tube sheet

The better the fit at the tube joint, the less the possibility that there is leakage

Tube joints are either rolled press fit or welded

Welded Rolled press fit

• Used for severe service (high pressures)

• An exchanger is likely to be more expensive if the tube joints are welded. Why?

(cost of welding- a wider tube spacing will be needed)

• Used with the metals that can not be welded

• Make a very good seal• Can be used in a reasonably

higher pressure up to 2000 psi.

The tube sheet forms the barrier between the shell and tube fluids.

It is essential for safety or process reason to prevent any possibility of intermixing due to leakage at the tube sheet joint

To reduce the possibility for leakage, double tube sheets can be used

with the space between the sheets vented. To allow sufficient thickness to seal the tubes, the tube sheet thickness

should not be less than the tube outside diameter. Recommended minimum tube sheet thickness are given in the

standards

The tube sheet

Tube side passes

The fluid in the tube is usually directed to flow back and forth in a number of passes through groups of tubes arranged in parallel, to increase the length of the flow path.

The number of the passes is selected tom give the required tube design velocity

ndv

2

4

passes

total

n

nn

Examples

►Single Pass

►Double Pass

►Multipass

ShellsShells are fabricated from steel pipes for small

diameters or rolled steel plates for large diameters.

The shell diameter is ranged from 6 in. to 60 in.

For pressure applications the shell thickness would be sized according to the pressure vessel design standard

The shell diameter must be selected to give as close

a fit to the tube bundle as is practical to reduce bypassing round the outside of the bundle.

Shell types[ passes]

Shell types [passes]

Shell side flow arrangements are generally one of the following illustrated in the table

One pass shell [E shell]The most commonly used type in which the shell side fluid enters one end of the exchanger, flow through the exchanger, and exits through the opposite end of the exchanger.

Two pass shell or double pass shell [F shell]Require that fluid enters and exits through the same end of the exchanger. This type is used where the shell and tube side temp difference will be unsuitable for a single pass. The flow arrangement can be achieved by using two shells

Split flow [G shell] Divides incoming shell fluid into two separate streams

Double split flow [H shell]Divides incoming shell fluid into four separate streams

Divided flow [J shell]Shell fluid enters at the center or middle of the exchanger rather than at the end.

NOTE:The divided flow and split flow arrangements are used to reduce the shell side pressure drop.

The kettle type re-boiler [K shell]Has divided flow and a dome outlet for vapors

Example Divided flow type

Factors affecting the choice of the shell arrangements

The amount of cooling and heating required

The pressure drop

The type of service [for instance the shell arrangement that provides space for vapors to accumulate is the kettle type re-boiler]

Kettle type Re-boiler

Shell

Tubes

Baffle

Baffles

Why are baffles used?

To support the tube

To direct the fluid stream across the tube

To improve the rate of heat transfer

Types of baffles

(a) Segmental

(b) Segmental and strip

(c) Disc and doughnut (Disc and ring)

(d) Orifice

Segmental bafflesThe most common used type.

Segmental baffles are drilled plates with heights ranged from 55 to 85 percent of the inside shell diameter.

If the height of the baffle is 85 percent of the shell inside diameter

this is known as 15 percent cut baffles

The baffle cut is the height of the segment removed to form the baffle expressed as a percentage of the baffle disc diameter

The optimum baffle cut was found to be in the range of 20 to 25 % (which give good heat transfer without excessive pressure drop).

Segmental baffles may be arranged to give up and down flow [horizontal baffles] or may be rotated 90◦ to provide side by side flow [vertical baffles]

Up and down flow

Side by side flow

Baffle pitch or baffle spacing

The center to center distance between baffles is called the baffle pitch or baffle spacing, which affect the fluid velocity through the shell.

The baffle spacing range from 0.2 to 1.0 times the shell inside diameter.

Close baffle spacing will give higher heat transfer coefficient

but at the expense of higher pressure drop Optimum spacing was found in the range of 0.3 to 0.5 times

the shell inside diameter.

How are the baffles held securely?

By means of baffle spacer which consists of through- bolts screwed into the tube sheet (tie rods) and a number of smaller lengths of pipe which form shoulders between adjacent baffles.

The number of rods required will depend on the shell diameter

Baffles are held by baffle spacer

U-tube heat exchanger or U bend exchanger

Isometric view for U-tube exchanger

U-tube heat exchanger or U bend exchanger

As the drawing shows, a U-tube exchanger has only one tube sheet integrated with the tube bundle [the tube bundle in the form of hairpin tubes and the tube sheet form one unit]

The tube bundle can be removed from the shell for cleaning [outside cleaning].

A tube side header and a shell with integral shell cover which is welded to the shell are provided.

Each tube is free to expand or contract without any limitations

being placed upon it by the other tube.

When to use this type?

It is used where the temperature difference between the shell side and tube side fluids is quite great….WHY?

[Because the tubes are free to expand since the tube bundle is fastened to only one tube sheet].

Flow patterns in a U-tube exchanger:

The baffle dividing the channel (pass partition) directs incoming tube side fluid through only the upper half of the tube openings.

Tube side fluid flow through the tubes around the bend and through the lower chamber.

Tube side flow in this case is two pass flow, while the shell side flow is one pass flow

Advantages and disadvantages of U-tube exchanger

Advantages

The U-tube has the advantage of providing minimum clearance between the outer tube limit and the inside of the shell for any of the removable tube bundle constructions.

The U-tube design offers the advantage of reducing the number of joints. In high pressure constructions this feature becomes of considerable importance in reducing both initial and maintenance cost.

The tube bundle in a U-tube exchanger is free to expand [i.e. no thermal expansion problem]

Disadvantages

The bend in the tube inhibits cleaning and inspection inside of the tube [makes it difficult]

The number of tube holes in a given shell is less than that for a fixed tube sheet exchanger because of limitations on bending tubes of a very short radius [i.e. low area of heat transfer for the same volume of fixed tube sheet heat exchanger]

III. Internal floating head without clamp ring

In this exchanger two tube sheets are used, one tube sheet is bolted between the channel and the shell in a fixed position while the other tube sheet with a cover float inside the shell

This design allows the following

1. The tubes are free to expand or contract [no stresses caused by thermal expansion]

2. The exchanger can be used for high temperature differences

3. All parts of the exchanger can be inspected and cleaned [can be used with fouling fluids]

III. Internal floating head without clamp ring [Pull through type]

Disadvantages

The clearance between the shell and the tube bundle is large which leads to:

1. No tubes can occupy this space, so the space is wasted

2. Fluid is likely to move through the space rather than past the tube

[i.e. reduce the exchanger efficiency]

IV. Internal floating head with clamp ring

More efficient but expensive

In this type a split backing ring is used to held the tube sheet to the tube cover

The use of the split ring allows the use of more tubes and reduce the space between the shell and the tube bundle

NoteThe diameter of the shell cover is greater than the diameter of the rest of the shell holding the tubes

IV.Internal floating head with clamp ring

Comparison between pull through type and split backing ring

•Split backing ring •Pull through type•Expensive•Small clearance

•More tubes can be used

•More efficient•More parts•Harder to disassemble

•Cheap•Clearance between the shell and the tube bundle is large•Less tubes can be used in the same space•Less efficient•Less parts form the H.X•Easy to disassemble

TEMA type designation for shell and tube heat exchanger

Front head types

Allocation of fluidsTube side

1. Put dirty stream on the tube side [fouling fluid] - easier to clean inside the tubes

2. Put high pressure stream in the tubes to avoid thick, expensive shell

3. When special materials required for one stream, put that one in the tubes to avoid expensive shell

4. Put corrosive fluid in the tube tom reduce the cost of expensive shell

5. Put toxic fluid in the tube to minimize leakage

Shell-Side1. Viscous fluid to increase (generally) the value of "U“

[Cross flow gives higher coefficients than in plane tubes, hence put fluid with lowest coefficient on the shell side]

2. Fluid having the lowest flow rate

3. Condensing or boiling fluid

NoteIf no obvious benefit, try streams both ways and see which gives best design.

Start up and shut down procedure

On initial start up and shut down the heat exchanger can be subjected to damaging thermal shock, over pressure or hydraulic hammer

This can lead to leaky tube to tube sheet joints, damaged expansion joints because of excessive thermal expansion of the tubes or the shell

Excessive shell side flow rates can cause tube vibrations and catastrophic failure

Start up procedure1. Check all parts of the heat exchanger [no loose bolts, all valves inthe shut position]

2. Testing the heat exchanger for leakage Hydrostatic test Soap bubble test

3. Purging of the heat exchanger [before adding a liquid or a gaseous hydrocarbon to an exchanger inert gas is used to remove air or liquids from the exchanger to avoid the possibility of explosion]

4. Any temperature change should be made slowly because the shell and the tubes are made of different materials and do expand at different rates causing the tubes to be loosened from the tube sheet or may be broken or ruptured so during start up cold fluid is introduced first, then hot fluid is gradually added and the heat exchanger is brought to the operating temperature.

Note about testing for leaks

There are another tests that can be performed while the heat exchanger in service [online maintenance without

dismantling] Such tests can be run on either the tube or the shell side

namely;

1. Physical test [visual test] If the two fluids in the H.X have different physical

properties, the easiest way to test for leaks is to take a sample from the lower pressure fluid, then it is easy

to see if there is a leak by just looking to the sample.

2. Chemical test used if the fluids are very similar.

Shut down procedure

During shutdown, the flow of hot fluid is stopped first. With no input of the hot fluid the heat exchanger gradually cools. Then the flow of cold fluid is stopped

The heat exchanger should not be valved closed while it is full of fluids….WHY?

Just like a solid, a liquid expands when it is heated and its volume increase. If the expanding liquid is enclosed, it exerts force or pressure on its container. Therefore a filled exchanger which is valved closed can be damaged by expanding fluid.

Recommended general start up and shut down procedure

Heat exchangers problems

Exchanger fouling

Corrosion

vibration

Exchanger fouling

You have to know the following

Definition of fouling

Types of fouling

Effect of fouling on the H.X performance

Troubles that indicate the presence of fouling

Factors affecting the kind and degree of fouling

How to handle the problem of fouling

Definition of fouling Build up of various kinds of deposits on the parts of an

exchanger

Types of fouling

1. Salt deposit [as Ca and Mg deposits in case of hard water]

2. Chemical fouling [as corrosion products]

3. Biological fouling [as growth of algae which form insulating layer]

4. Coking

Effect of fouling on the H.X performance

1. Increase the thermal resistance and reduce the rate of heat transfer [decrease the efficiency of the H.X]

2. Increase the surface roughness [the flow of the fluid is restricted] and increase the pressure drop

Troubles that indicate the presence of fouling

1. Change in temperature or pressure

2. Change in flowrate [outlet flow rate]

Factors affecting the kind and degree of fouling

1. The materials used in the heat exchanger

► Some materials corrode faster than others providing corrosion products which decrease heat transfer

► Rough surface provides cavities for the build up of deposits

2. Fluid velocity

Affect the fouling rate [as the velocity increase the fouling rate decrease]

How to handle the problem of fouling

►Antifoulants prevent the formation of deposits

►Inhibitors [as corrosion inhibitors] prevent chemical reactions

which might cause deposits to build up

►Frequent cleaning of the H.X [maintenance]

Corrosion of heat exchangers Another series problem in heat exchangers is corrosion

Severe corrosion can and does occur in tubes and very often withcommon fluids such as water

To avoid corrosion►Proper material selection based on full analysis of the operating

fluids, velocities and temperatures is a must

►Heavier gauge tubing is specified to offset the effect of corrosion followed by proper start up operating and shut down procedure

►Protection of the heat exchanger from corrosion [e.g.cathodic protection]

►Treatment of the cooling water used and using of inhibitors

Heat exchangers vibration

Vibration of the tubes as a result of the flow of the shell side past them is important phenomena specially when the H.X size and flow quantities of flow are increased

Vibration effects

►Vibration has a bad effect on both tubes and shell

►The joints between the tubes and tube sheet can fail due to vibration causing leakage

►It causes leakage in the joints between shell and tubes

►Increase the shut down time to repair the H.X

Factors affecting tube vibration

Tubes geometry [layout]

Material of construction

Means of support

Heat exchanger size

Flow quantities

How to avoid vibration

Using inlet support baffles

Using double segmental baffles [improve tube support]

Using j shell type [ divided flow type to reduce the shell velocity]

Inlet support baffles

Double-segmental baffles

Air cooled heat exchangerUsed for cooling and condensation and used when cooling water is

in short supply or expensive

They can also be competitive with water cooled units even when water is plentiful

Most common used in petroleum and gas processing industries

Main components

Air cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubeIf the fan is mounted below the tubes the unit is termed forced draft unit and if the fan is mounted above the tubes the unit is termed induced daft

Air cooled heat exchanger

Forced draft air cooled heat exchanger[cross flow]

Finned tubes

Some design dataThe height of the bundle aboveground must

be one half of the tube length

The standard air velocity passing through the tube bundle generally ranges from 1.5 to 3.6 m/s

The 25.4 mm outside diameter tube is most commonly used. Fin heights vary from 12.7 to 15.9 mm, fin spacing from 2.3 to 3.6 mm , and tube triangular pitch from 50.8 to 63.5 mm

Tube lengths vary and may be as great as 18.3 m

When tube length exceed 12.2 m three fans are generally installed

Forced draft unit

Forced draft unit

Less power is required

Offer better accessibility to the fan for on stream maintenance

Structural costs are less than induced draft since the fan is not exposed to the hot air

Mechanical life is longer

Induced draft unit

Induced draft unit

Provide more even distribution of air across the bundle, since air velocity approaching the bundle is relatively low

This design permits close approach of the product temperature to ambient air temperature

In service in which sudden temperature change would cause upset and loss of product, the induced draft unit gives more protection in that only a fraction of the surface is exposed to rainfall

Advantages of air cooled exchanger

Simple mechanical in design

The cooling medium [air] is always available and require no treatment

Air side fouling is ignored [less fouling problem]

Low maintenance cost

Disadvantage of air cooled exchanger

Noise

The performance is affected by the weather fluctuations

High capital cost

Spiral heat exchanger

Spiral heat exchanger

A spiral heat exchanger can be considered as a plate heat exchanger in which the plates are formed into a spiral

The fluids flows through the channels formed between the plates

Spiral heat exchangers are compact units, the maximum operating pressure is limited to 20 bar and the temperature to 400 oC

For a given duty the pressure drop over a spiral heat exchanger will usually be lower than that for the equivalent shell and tube exchanger

Can be used with dirty process fluids and slurries since they are easily cleaned and the turbulence in the channels is high

Spiral heat exchangers give true counter current flow and can be used where the temperature correction factor for a shell and tube exchanger would be too low

Advantages of spiral H.X over shell and tube H.X

Can be used for cooling and heating of slurries or sludge and liquid containing fibrous materials

Gives good fluid distribution

Foul at much lower rates than shell and tube H.X and if fouling does occur, it can be effectively cleaned chemically

Spiral plates avoid the problem of differential expansion

Enables close temperature approaches and precise temperature control

The spiral heat exchanger is compact

The possibility of leakage is much less than shell and tube H.X [because the plates are thicker than the tube wall]

Disadvantages

The maximum pressure is limited [20 bar]

A leak can not be plugged as in the shell and tube H.X

Repairs of the inner parts of the plates is complicated

Flow patterns in plate heat exchanger

The plate-and-frame heat exchanger has emerged as a viable alternative to shell and-tube exchangers for many applications throughout the chemical process industries.

Such units are comprised of a series of plates, mounted in a frame and clamped together.

Space between adjacent plates form flow channels, and the system is arranged so that hot and cold fluids enter and exit through flow channels at the four comers

Within the exchanger, an alternating gasket arrangement diverts the hot and cold fluids from each inlet into an alternating sequence of flow channels.

In this arrangement, each cell of heat transfer media is separated by a thin metal wall, allowing heat to transfer easily from one media to the other.

Advantages of plate heat exchanger

High heat transfer rate [gives efficient counter current flow typically yieldheat transfer coefficient three to five times other types of H.X

More compact design is possible for a certain capacity

Can be used for more than one duty by adding or remove plates [flexible]

Low maintenance cost since the plates can be easily removed

Can be easily opened for inspection and cleaning [accessibility]

Disadvantages

High pressure drop because of the narrow passageways in the plate heat exchanger [making the H.X incompatible high volume gas applications

Cant be easily fouled

Cant not be used with all fluids [some fluids affect the gasket material as organic solvents