Page 102 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.6 Mechanical detailing 99

is preferred from the point of economy and common spares inventory holding. Industrial designs
mostly use tube length of 2.5 m and higher.
Choosing longer tube length (for a given surface area) requires fewer tubes and less complicated
tube sheet with fewer holes drilled. This also decreases the shell diameter resulting in lower cost.
However, mechanical cleaning is limited to 6 m (20 ft) tubes and shorter, although standard ex-
changers can be built with tubes up to 12 m (40 ft). Maximum tube length limit may also be dictated by
transportation limitation (up to 30 m typically).
Part of the tube length within the tube sheet (TS) and the portion within the dead space in the
floating head are not considered for heat transfer and baffle spacing cal-
culations. However, in case of U-tubes, a credit for the bend portion is
considered. The effective tube length (L e ), i.e., the length used for
Effective tube length
calculating heat transfer area and baffle spacing, is estimated as follows

Table 4.9 Materials for noncorrosive and corrosive service.
Service Material
Noncorrosive service
Any HE type, T < 100 C Al and austenitic Cr-Ni steel

1
Any HE type, 100 <T < 45 C 3 Ni steel

2
Any HE type, 45 <T < 0 C Carbon steel (impact tested)

Any HE type, 0 <T < 500 C Carbon steel

Shell and tube T > 500 C Refractory lined steel

Corrosive service
Mildly corrosive serve; tempered cooling water Carbon steel
Sulphur bearing oils at T > 300 C; hydrogen at Ferritic Co-Mo and Cr-Mo alloys

elevated temperature
Tubes for moderately corrosive service; cladding Ferritic Cr steel
for shell and channel in contact with corrosive
sulphur bearing oil
Corrosion resistance duties Austenitic Cr-Ni steel
Mildly corrosive fluids Aluminium
Freshwater cooling in surface condensers; Copper allow: admiralty brass, cupronickel
seawater cooling
Resistance to mineral acids and chloride- High Ni-Cr-Mo alloys
containing acids
Seawater cooler and condensers Titanium
Highly corrosive services Carbon/graphite
Exposure to sea and brackish water Coatings: aluminium, epoxy resins
Channels for seawater coolers Lining: lead, rubber
General corrosion resistance Lining: austenitic Cr-Ni steel