Heat Exchanger Parts

Heat Exchanger Parts Blog detailed study contains

1. Shell
2. Shell Side Pass Partition Plate
3. Baffles
4. Tube
5. Tube Side Pass Partition Plate
6. Tie Rods
7. Spacers
8. Tube Sheet
9. Sealing Strip
10. Expansion Joint

1. Shell

Shell is the costliest part of the heat exchanger.

Cost of shell and tube heat exchanger sensitively changes with change in the diameter of shell.

As per the TEMA standard, shell size ranges from 6 in (152 mm) to 60 in (1520 mm).

Standard pipes are available up to 24 in size (600 mm NB).

If shell size is greater than 24 in, it is fabricated by rolling a plate.

Shell diameter depends on tube bundle diameter.

For fixed tube sheet shell and tube heat exchanger, the gap between shell and tube bundle is minimum, ranging from 10 to 20 mm.

For pull through floating head heat exchanger, it is maximum, ranging from 90 to 100 mm.

2. Shell Side Pass Partition Plate

Single pass shell is used in the most of the cases.

Two pass shell is rarely used and is recommended where shell and tube temperature difference is unfavorable for the single shell side pass.

For such cases, normally two or more smaller size 1-1 heat exchangers, connected in series, are recommended.

Shell side pass partition plate is not provided to improve shell side heat transfer coefficient but it is provided to avoid the unfavorable temperature difference or to avoid the cross of temperatures between hot fluid and cold fluid.

3. Baffles

There are two functions of baffles:

1. Baffles are used in the shell to direct the fluid stream across the tubes to increase the velocity of shell side flow and thereby to improve the shell side heat transfer coefficient.

In other words, baffles are used in shell to increase the turbulence in shell side fluid.

Baffles indirectly support the tubes and thereby reduce the vibrations in tubes. If shell side liquid velocity is higher, say more than 3 m/s, vibration analysis calculations should be carried out to check whether baffle spacing is sufficient or not.

Similarly, for very high velocity of gas or vapour and also for the higher baffle spacing (higher than shell ID), vibration analysis calculation must be carried out to check the baffle spacing. Vibration analysis calculations are given in TEMA standard.

Different types of baffles are used in shell and tube heat exchangers:

(i) Segmental baffle,

(ii) Nest baffle.

(iii) Segmental and Strip baffle.

(iv) Disk and Doughnut baffle.

(v) Orifice baffle,

(vi) Dam baffle, etc.

Figure shows various types of baffles.

Most widely used type of baffle is segmental baffle.

Other types of baffles like nest baffle,

segmental and strip baffle and disk and doughnut baffle provide less pressure drop for the same baffle spacing but provide lower heat transfer coefficients as compared to segmental baffle. Situations when other types of

1. For shell and tube heat exchangers shell side pressure drop controls the overall design.

For example, intercoolers of compressors and heat exchangers used in very high vacuum system.

In intercoolers of compressors maximum allowable shell side pressure drop may baffles might be used could be as given below be as low as 3.45 kPa.

For such cases different types of shells are also used. Split flow (G shell) or divided flow (I shell) designs provide low shell side pressure drop compared to commonly used single pass (E shell as shown in Fig.

2. For shell and tube heat exchanger in which boiling or condense out on shell side.

In such a case baffles are required only to reduce the vibrations in tubes.

For other type of baffles (other than segmental baffle) correlations for finding heat transfer coefficient and pressure drop are not easily available in open literature.

Helical baffles are also developed by some heat exchanger manufacturers.

These are claimed to substantially alter the shell side flow pattern by inducing a swirling pattern with a velocity component parallel to the tubes.

Optimum helix angle of 40° is recommended. This arrangement is expected to improve the heat transfer and reduce the pressure drop. However, its design procedure is proprietary sation is carried

4. Tube

Tube size range from 1/4 in (6.35 mm) to 2.5 in (63.5 mm) in shell and tube heat exchanger. Data for standard tubes are given in TEMA standard and in Ref.

For the standard tubes, its size is equal to outer diameter of tube.

Thickness of standard tubes are expressed in BWG (Birmingham Wire Gauge).

Increase in the value of BWG means decrease in tube thickness.

For no phase change heat exchangers and for condensers, 3/4 in (19.05 mm) OD tube is widely used in practice.

While for reboiler 1 in (25.4 mm) OD tube size is common.

Tubes are available in standard lengths like

6 ft (1.83 m),

8 ft (2.44 m),

12 ft (3.66 m),

16 ft (4.88 m) and

6 meter.

5. Tube Side Pass Partition Plate

Tube side passes are provided to decrease the tube side flow area and to increase tube side fluid velocity thereby to improve the tube side heat transfer coefficient at the expense of pressure drop.

This is true only if there is no phase change on tube side. Hence, more number of tube side passes are recommended only if there is no change in the phase of tube side fluid.

For example, at the design stage, if the number of tube side passes are increased from one to two, then for the given volumetric flow rate.

Flow area becomes half and velocity becomes double.

Since, tube side heat transfer coefficient,

hi ∝ ut 0.8 (where ut is tube side fluid velocity), on increasing number of tube side passes from 1 to 2, hi nearly becomes 1.74 times.

But Δpi ∝ ut 2.8 , so the pressure drop increases by 6.96 times.

Increase in hi means decrease in heat transfer area required and decrease in fixed cost.

Increase in Δpi means increase in power required for pumping the tube side fluid and increase in operating cost.

Hence, ideally optimum number of tube side passes must be decided.

Many a times, tube side velocity in a single pass could be calculated very low (say 0.3 to 0.5 m/s).

Under such circumstances, two passes or four passes could be beneficial. It is recommended that Ut > 1 m/s.

This is essential if tube side fluid is cooling water, otherwise rate of fouling will be higher.

Tube side passes are very common and are advantageously used for improving

Tube side heat transfer coefficient. These passes can be achieved in many ways by locating partition plates in channel covers. Figure gives different designs

For achieving desired tube passes.

6. Tie Rods

Baffles are supported by tie rods. Tie rods are made from solid metal bar.

Normally four or more tie rods are required to support the baffles.

Diameter of tie rod is less than the diameter of tube. Diameter and number of tie rods required for given shell diameter are specified by TEMA standard and IS:4503.

7. Spacers

Spacers are used to maintain the space between baffles.

Spacers are the pieces of pipes or in the most of the cases they are the pieces of extra available tubes.

Spacers are passed over the tie rods and because of them baffles do not slide over tie rods under the effect of the force of fluid.

Hence, spacers fix the location of baffles and maintain the space between them.

Length of spacer is equal to space between the baffles.

8. Tube Sheet

Tubes and one end of tie rods are attached to tube sheet (also called tube plate).

Hence, entire load of tube bundle is transferred to one or two tube sheets.

In U tube shell and tube heat exchanger only one tube sheet is used.

While in fixed tube sheet shell and tube heat exchanger, two tube sheets are used.

One surface of tube sheet is exposed to tube side fluid and other surface is exposed to shell side fluid.

This point is very important in the selection of material for tube sheet and also in determining tube sheet thickness.

In majority cases tube to tube sheet joints are two types; (a Expanded joint, and (b) Welded joint as shown in fig.

In expanded type joint, tube holes are drilled in a tube sheet with a slightly greater diameter than the tube OD.

Two or more grooves are cut in the wall of each hole.

The tube is placed inside the tube hole and a tube roller is inserted into the end of the tube. Roller is slightly tapered.

On application of the roller, tube expands and tube material flows into grooves and forms an extremely tight seal.

Welding joint is used only for the cases where leakage of fluid can be disastrous.

9. Sealing Strip

It is a shell side component. Sealing strips are attached on the inside surface of shell as shown in Fig. throughout the length of shell.

There are two functions of sealing strips:

1. Sealing strips reduce the amount of bypass stream of shell side fluid flowing through the clearance between shell inside diameter and tube bundle diameter and thereby improve the shell side heat transfer coefficient. (This is valid only if there is no phase change of shell side fluid).

2. Sealing strips also make the removal of tube bundle from the shell easy. Hence, they are also known as sliding strips.

10. Expansion Joint

Expansion joint is attached to shell wall. In this case, shell is made from two pipe pieces. Two pipe pieces are joined together by an expansion joint as shown In fig.

Expansion joint is used in fixed tube heat exchanger to permit the differential thermal expansion or contraction between shell and tubes which otherwise is not permitted by fixed tube sheet heat exchanger.

Differential thermal expansion between shell and tube is significant if there is a large temperature difference between temperature of shell material and temperature of tube material during operation or if tube material and shell material are different.

For the given case of fixed tube sheet heat exchanger, whether an expansion joint is required or not can be determined by calculations given in TEMA.

If expansion joint is not provided in fixed tube sheet heat exchanger, then fixed tube sheet does not permit the unequal expansion or contraction between shell and tube and it can result in the development of thermal stress across the tube sheet.

If this thermal stress is higher than permissible value, then it may develop a crack in tube sheet, and can result in leakage at tube to tube sheet joint.

Other options, available to avoid the development of thermal stress in the tube sheet, are use of either U tube heat exchanger or floating head heat exchanger.