Thumb rule 6。 Preferably use fixed head type。 Shells 

The  British  standard  BS  3274  covers exchan-

gers from 6 in。 (150 mm) to 42 in。 (1067 mm) dia- meter; and the TEMA standards, exchangers up to 60 in。 (1520 mm)。 Up to about 24 in。 (610 mm) shells are normally constructed from standard, close tolerance,  pipe; above 24 in。 (610 mm) they  are  rolled  from  plate。 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。 

The design process is to fit the number of tubes into a suitable shell to achieve the desired shell side velocity 4 ft/s (1。219 m/s) subject to pressure drop  constraints。 Most efficient conditions for heat transfer are to have the maximum number of tubes possible in the shell to maximize turbulence。 

Preferred tube length to shell diameter ratio is in the range 5 to 10。 

Using the maximum number of tubes, subject to adequate provision for inlet nozzle, for a given shell  size will ensure optimum shell side heat transfer in minimizing tube bundle bypassing。 The heat transfer area required design margin is then achieved by adjusting the tube length subject to economic consi- derations。 On low cost tube materials it may be more economical to use standard lengths and accept the  increased design margin。 It is a common practice to  reduce the number of tubes to below the maximum  allowed particularly with expensive tube material。 In  these situations the mechanical design must ensure suitable provision of rods, bar baffles, spacers, baffles to minimize bypassing and to ensure mechanical  strength。 

Thumb rule 7。 Preferably use shell diameter to obey the constraints tube length  to  shell  diameter  ratio is in the range 5 to 10。 The smaller, the better。 

Baffles 

Baffles are used in the shell to direct the fluid  stream across the tubes, to increase the fluid velocity and so improve the rate of transfer。 The most com- monly used type of baffle is the single segmental baffle。 

Baffles serve  two  functions:  support  the tubes for structural rigidity, preventing tube vibration and sagging and pert the flow across  the  bundle  to  obtain a higher heat transfer coefficient。 

Baffle cut 

The term “baffle cut” is used to specify the di- mensions of a segmental baffle。 The baffle cut is the  height of the segment removed to form the baffle, expressed as a percentage of the baffle disc diame- ter。 Baffle cuts from 15 to 45% are used。 The upper  limit ensures every pair of baffles will support each tube。 Generally, a baffle cut of 20 to 25% will be the  optimum, giving good heat-transfer rates, without ex- cessive drop。 There will be some leakage of  fluid round the baffle as a clearance must be allowed for  assembly。 The clearance needed will depend on the shell diameter。 The minimum thickness to be used for baffles and support plates are given in the standards。 

Thumb rule 8。 Preferable use 25% baffle cut。 Baffle spacing 

The baffle spacing used range from 0。2 to 1。0  shell diameters。 A close baffle spacing will give higher heat transfer coefficients but at the expense of higher pressure drop。 The optimum spacing will usually be between 0。3 to 0。5 times the shell diameter。 

Minimum spacing (pitch)。 Segmental baffles nor- mally should not be closer than 1/5th of shell diameter (ID) or 50。8mm (2 in。) whichever is greater。 

Maximum spacing (pitch)。 Spacing does not nor- mally exceed the shell diameter。 

Tube support plate spacing determined by me- chanical considerations e。g。 strength and vibration。