changer from Table 4。 These guidelines are chosen from variety of literature [14,15]。 Some thumb rule is formulated to aid the user to choose the best among  various alternatives。 

Tubes 

Tube diameter 

Tube diameters in the range 5/8 in。 (16 mm) to 2 in。 (50 mm) are used。 The smaller diameters 5/8 to 1 in。 (16 to 25 mm) are preferred for most duties, as they will give more compact, and therefore cheaper, exchangers。 Larger tubes are easier to clean by me- chanical methods and would be selected for heavily fouling fluids。 The tube size is often determined by the plant maintenance department standards, as clearly it is an advantage to reduce the number of sizes that  have to be held in stores for tube replacement。 

Thumb rule 1。 Use smallest diameter for greater heat transfer area with a normal minimum of 3/4”OD  tube due to cleaning considerations and vibration。 1/2”OD tubes can be used on shorter tube lengths, say < 4 ft。 For chemical cleaning, smaller sizes can  be used provided that the tubes never plug completely。 

Tube length 

Tube length affects the cost and  operation of  heat exchangers。 

– Longer the tube length (for any given surface  area); 

- Fewer tubes are needed, requiring less compli- cated header plate with fewer holes drilled; 

- Shell diameter decreases resulting in  lower  cost。 The optimum tube length to shell diameter will  usually fall within the range of 5 to 10。 

Mechanical cleaning is limited to tubes 20 ft and shorter, although standard exchangers can be built with tubes up to 40 ft。 

Maximum tube length is dictated by: 

- architectural layouts; 

- transportation (to about 30 m); 

-Long tube lengths with few tubes may give rise to shell side distribution problems。 

Thumb rule 2。 The preferred lengths of tubes for heat exchangers are: 6 ft。 (1。83 m), 8 ft (2。44 m), 12 ft (3。66 m), 16 ft (4。88 m) 20 ft (6。10 m), 24 ft (7。32 m)。 The shorter, the better, as it will require  less floor  space in plant。 

Tube-side passes 

The fluid in the tube is usually directed to flow  back and forth in a number of “passes” through   groups of tubes arranged in parallel, to increase the length of the flow path。 The number of passes is selected to give the required tube-side design velocity。 Exchan- gers are built with from one to up to about sixteen tube passes。 The tubes are arranged into the number of  passes required by piding up the exchanger headers (channels) with partition plates (pass partitions)。 

The number of tube passes depends on the available pressure drop。 

Higher velocities in the tube result in higher heat transfer coefficients, at the expense of increased   pressure drop。 Therefore, if a higher pressure drop is acceptable, it is desirable to have fewer but longer  tubes (reduced flow area and increased flow length)。 

Long tubes are accommodated in a short shell exchanger by multiple tube passes。 

The number of tube passes in a shell generally range from 1 to 16。 

The standard design has one, two, or four tube passes。 

An odd number of passes is  uncommon  and may result in mechanical and thermal problems in fabrication and operation。 

Thumb rule 3。 Select the number of tubes per tube side pass to give optimum velocity 3-5 ft/s (0。9- 1。52 m/s) for liquids and reasonable gas velocities are 50-100 ft/s (15-30 m/s) 。If the velocity cannot be achieved in a single pass consider increasing the number of passes。 Less number of tube passes is preferable as it will make the fabrication simple。