Basic Design Procedure

Rating of the Heat Exchanger Design

Shell Diameter

Insufficient Thermal Rating

Excessive Pressure Drop Rating

Note: There is always a compromise process between the thermal and Pressure drop ratings

Fluids Allocation

Cooling water is usually placed in the tubes due to its tendency to corrode carbon steel and to form scale, which is difficult to remove from the exterior tube surfaces.

It is generally less expensive to confine a high-pressure stream in the tubes rather than in the shell.

In summary

Tubing selection

– The most frequently used tube sizes are 3/4 and 1 in.

– For water service, 3/4 in., 16 BWG tubes are recommended.

– For oil (liquid hydrocarbon) service, 3/4 in., 14 BWG tubes are recommended if the fluid is non-fouling.

– For fouling fluids, I in., 14 BWG tubes should be used.

– Tube lengths typically range from 8 to 30 ft, and sometimes longer depending on the type of construction and the tubing material.

– A good value to start with is 16 or 20 ft.

Number of tubes

Tube layout – Triangular and square layouts are the most common with a

pitch of 1.0 in. (for 3/4-in. tubes) or 1.25 in. (for 1-in. tubes). Rotated square pitch is also used but rotated triangular pitch is seldom used.

– However, the clearance between tubes is typically the larger of 0.25 in. and 0.25 Do, and with triangular pitch this is not sufficient to allow cleaning lanes between the tube rows.

– Although chemical cleaning may be possible, triangular pitch is usually restricted to services with clean shell-side fluids.

– Rotated square pitch provides some enhancement in the heat-transfer coefficient (along with higher pressure drop) compared with square pitch, while still providing cleaning lanes between the tubes.

– This configuration is especially useful when the shell-side Reynolds number is relatively low (less than about 2000).

Tube layout

(c) triangle pitch

Tube Pitch

Tube Length

Tube & Header Plate Deformation

Thermal expansion of tubes needs to be taken into account for heat exchangers operating at elevated temperatures

Tube elongation due to thermal expansion causes: Header plate deformation Shell wall deformation near the header plate

Fatigue strength of the tube, header plate and shell joint needs to be considered when using Longer tubes High operating tube side temperatures Cyclic thermal loads

Tube passes – For typical low-viscosity process streams, it is highly desirable to

maintain fully developed turbulent flow in the tubes since turbulent flow provides the most effective heat transfer.

– Once the tube size and number of tubes have been determined, the number of tube passes can be chosen to give an appropriate Reynolds number, i.e.,

– Except for single-pass exchangers, an even number of tube passes is always used so that the tube-side fluid enters and exits at the same header.

– Fluid velocity can also be used as a criterion for setting the number of tube-side passes. It is desirable to maintain the liquid velocity in the tubes in the range of about 3-8 ft/s. Too low a velocity can cause excessive fouling, while a very high velocity can cause erosion of the tube wall.

Maximum Recommended Velocities for Water in Heat-Exchanger Tubes

– Maximum velocities for water are given in the Table shown below. For liquids other than water, multiply the values from the table by the factor:

– For gases flowing in plain carbon steel tubing, the following equation can be used to estimate the maximum velocity:

– For tubing materials other than

plain carbon steel, assume the maximum velocities are in the same ratio as given in Table

Fouling Shell and tubes can handle fouling but it can be reduced by

keeping velocities sufficiently high to avoid deposits

avoiding stagnant regions where dirt will collect

avoiding hot spots where scaling might occur

avoiding cold spots where liquids might freeze or where corrosive products may condense for gases

Flow-induced vibration Two types - RESONANCE and INSTABILITY Resonance occurs when the natural

frequency coincides with a resonant frequency

Fluid elastic instability Both depend on span length and velocity

-

Velocity Velocity

Resonance Instability

Tube

dis

plac

emen

t

Avoiding vibration Inlet support baffles - partial baffles in first few tube

rows under the nozzles

Double segmental baffles - approximately halve cross flow velocity but also reduce heat transfer coefficients

Patent tube-support devices

No tubes in the window (with intermediate support baffles)

J-Shell - velocity is halved for same baffle spacing as an E shell but decreased heat transfer coefficients (Information about J- and E- types will be given in the next lecture about Shell and Tube HE.

Inlet support baffles

Double-segmental baffles

No tubes in the window - with intermediate support baffles

Tubes Windows with no tubes

Intermediate baffles

Avoiding vibration

Baffles and tubesheets – Single segmental baffles are standard and by far the most widely used. In order

to provide good flow distribution on the shell side, the spacing between baffles should be between 0.2 and 1.0 shell diameters (but not less than 2 in.).

– However, the maximum baffle spacing may be limited by tube support and vibration considerations to less than one shell diameter.

– Although baffle spacing and baffle cut are apparently independent parameters, in practice they are highly correlated.

Recommended baffle cut, Bc , as a function of baffle spacing. SBC, for single-phase flow; CV, for condensing vapors

Baffle Cut, Bc

Nozzles – Nozzles can be sized to meet pressure drop limitations and/or to match process

piping.

– The fluid entering the shell through the inlet nozzle impinges directly on the tube bundle. If the inlet velocity is too high, excessive tube vibration and/or erosion may result.

– TEMA specifications to prevent tube erosion are given in terms of the product of density (lbm/ft3) and nozzle velocity (ft/s) squared

Sealing Strip • The function of the sealing strips is to reduce the effect of the

bundle bypass stream that flows around the outside of the tube bundle.

• They are usually thin strips of metal that fit into slots in the baffles and extend outward toward the shell wall to block the bypass flow and force it back into the tube bundle.

Sealing Strip

• Typically, one pair is used for every four to ten rows of tubes between the baffle tips. Increasing the number of sealing strips tends to increase the shell side heat-transfer coefficient at the expense of a somewhat larger pressure drop.

• They are placed in pairs on opposite sides of the baffles running lengthwise along the bundle. Sealing strips are mainly used in floating-head exchangers, where the clearance between the shell and tube bundle is relatively large.

Estimated Sizes of the HE • The initial size (surface area) of a heat exchanger can be estimated

from

• Tm Mean temperature

• Tlm,cf Log Mean Temperature difference based on cross flow HE

The overall heat transfer coefficient can be estimated from:

Typical Film Heat Transfer Coefficients for Shell-and-Tube Heat Exchangers

Total number of tubes

Total number of tubes

Shell Diameter