Tube arrangements 

The tubes in an exchanger are usually  arranged in an equilateral triangular, square, or rotated square pattern。 The triangular and rotated  square patterns  give higher heat-transfer rates, but at the expense   of a higher pressure drop than the square pattern。 A  square, or rotated square arrangement, is used for heavily fouling fluids, where it is necessary to mecha- nically clean the outside of the tubes。 

Triangular pattern provides a more robust tube sheet construction。 Square pattern simplifies cleaning and has a lower shell side pressure drop。 

For the identical tube pitch and flow rates, the tube layouts in decreasing order of shell-side heat transfer coefficient and pressure drop are: 30, 45, 60 and 90°。 

The 90° layout will have the lowest heat transfer coefficient and the lowest pressure drop。 

Thumb rule 4。 The square pitch (90 or 45°) is used when jet or mechanical cleaning is necessary on the shell side。 In that case, a minimum cleaning   lane of 1/4 in。 (6。35 mm) is provided。 The square pitch is generally not used in the fixed header sheet design because cleaning is not feasible。 The triangular pitch provides a more compact arrangement, usually result- ing in smaller shell, and the strongest header sheet for a specified shell-side flow area。 It  is  preferred  when the operating pressure difference between the two fluids is large。 

Tube pitch 

The selection of tube pitch is a compromise  between: 

- close pitch (small values of Pt/do) for increased shell-side heat transfer and surface compactness; 

- open pitch (large values of Pt/do) for decreased shell-side plugging and ease in shell-side cleaning。 

Tube pitch PT is chosen so that the pitch ratio is 1。25 < Pt/d0 < 1。5。 

When the tubes are too close  to  each  other  (Pt/do less than 1。25), the header plate (tube sheet) becomes too weak for proper rolling of the tubes and cause leaky joints。 

Tube layout and tube locations are standardized for industrial heat exchangers。 

However, these are general rules of thumb and can be “violated” for custom heat exchanger designs。 

Thumb rule 5。 Prefer to use Pt/d0 = 1。25 unless  otherwise specified。 

Head type 

The simplest and cheapest type  of  shell  and  tube exchanger is the fixed tube sheet  design。 The main  disadvantages  of  this  type  are  that  the  tube 

bundle cannot be removed for cleaning and there is no provision for differential expansion of the shell and tubes。 As the shell and tubes will be at different tem- peratures, and may be of different materials, the diffe- rential expansion can be considerable and the use of this type is limited to temperature differences up to  about 80 °C。 In the other types, only one end of the  tubes is fixed and the bundle can expand freely。 

The U-tube (U-bundle) type requires only one  tube sheet and is cheaper than  the  floating-head types; but is limited in use to relatively clean fluids as the tubes and bundle are difficult to clean。 It is also  more difficult to replace a tube in this type。 

Exchangers with an internal floating head are  more versatile than fixed head and U-tube exchan- gers。 They are suitable for high-temperature differen- tials and, as the tubes can be rodded from end to end and the bundle removed, are easier to clean and  can be used for fouling liquids。 A disadvantage of the pull- through design is that the clearance between the  outermost tubes in the bundle and the shell must be made greater than in the fixed and U-tube designs to accommodate the floating head flange, allowing   fluid  to bypass the tubes。 The clamp ring (split flange de- sign) is used to reduce the clearance needed。 There  will always be a danger of leakage occurring from the internal flanges in these floating head designs。 In the external floating head designs the floating-head   joint is located outside the shell, and the shell sealed   with a sliding gland joint employing a stuffing box。  Be- cause of the danger of leaks through the gland, the shell-side pressure in this type is usually limited to about 20 bar, and flammable or toxic materials should not be used on the shell side。