Heat Exchanger

What is Heat Exchanger?

     Heat exchangers, metal shells and tubes, work by transferring heat from one place to another. The hot flue gas heats the metal as the gas makes its way to the exhaust outlet of the furnace. As this is happening, the hot metal heats the air circulating over the exterior of the heat exchanger.

Classification of Heat Exchanger

Feature of Heat Exchanger - 

Shell and Tube Exchangers	Advantages	Limitations
Fixed tube sheet	-Provides maximum heat transfer area for a given shell and tube diameter. -Provides for single and multiple tubes passes to assure proper velocity. -Less costly than removable bundle designs	-Shell side/outside of the tubes are inaccessible for mechanical cleaning. -No provision to allow for differential thermal expansion developed between the tube and the shell side. This can be taken care of by providing an expansion joint on the shell side.
Floating head	-Floating tube sheet allows for differential thermal expansion between the shell and the tube bundle. -Both the tube bundle and the shell side can be inspected and cleaned mechanically.	-To provide the floating-head cover it is necessary to bolt it to the tube sheet. The bolt circle requires the use of space where it would be possible to place a large number of tubes. -Tubes cannot expand independently so that huge thermal shock applications should be avoided. -Packing materials produce limits on design pressure and temperature.
U-tube	-U-tube design allows for differential thermal expansion between the shell and the tube bundle as well as for individual tubes. -Both the tube bundle and the shell side can be inspected and cleaned mechanically. -Less costly than floating head or packed floating head designs.	-Because of the U-bend some tubes are omitted at the center of the tube bundle. -Because of U-bend, tubes can be cleaned only by chemical methods. Due to U-tube nesting, an individual tube is difficult to replace. -No single tube pass or true counter-current flow is possible. -Tube wall thickness at the U-bend is thinner than at the straight portion of the tubes. -Draining of tube circuit is difficult when positioned with the vertical position with the head side upward.

Different parts of Heat Exchanger -

1. Shell

-Shell is the costliest part of the heat exchanger.

-The cost of shell and tube heat exchanger sensitively changes with change in the diameter of the 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 the 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 most 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 smaller size 1-1 heat exchangers, connected in series, are recommended.

-Shell side pass partition plate is not provided to improve the 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 improve the shell side heat transfer coefficient.

In other words, baffles are used in shells to increase the turbulence in the 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 the very high velocity of gas or vapor and also for the higher baffle spacing (higher than the shell), vibration analysis calculation must be carried out to check the baffle spacing. Vibration analysis calculations are given in the 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.

The figure shows various types of baffles.

The most widely used type of baffle is the 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 For shell and tube heat exchangers shell side pressure drop, control the overall design.

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

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. 

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 types of baffles (other than segmental baffle) correlations for finding heat transfer coefficient and pressure drop are not easily available in the 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 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, their size is equal to the outer diameter of the tube.

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

- Increase in the value of BWG means a 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 meters.

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 improving the tube side heat transfer coefficient at the expense of pressure drop.

This is true only if there is no phase change on the tube side. Hence, more number 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 is 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 an increasing number of tube side passes from 1 to 2, nearly becomes 1.74 times.

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

An increase in hi means a decrease in heat transfer area required and a decrease in fixed cost.

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

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

Many 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 > 1m/s.

This is essential if tube-side fluid is cooling water, otherwise, the 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. The 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 bars.

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

- Diameter of the tie rod is less than the diameter of the tube. The diameter and number of tie rods required for a 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 most 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 the spacer is equal to the space between the baffles.

8. Tube Sheet

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

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

- In the 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 the tube sheet is exposed to tube-side fluid and another surface is exposed to shell-side fluid.

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

In the majority of cases tube to tube sheet joints are two types; (an Expanded joint, and (b) Welding joint as shown in fig.

- In an 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.       The roller is slightly tapered.

- On application of the roller, the 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 to the inside surface of the shell as shown in Fig. throughout the length of the 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

An expansion joint is attached to the shell wall. In this case, the shell is made from two pipe pieces. Two pipe pieces are joined together by an expansion joint.

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