2. Design considerations
2.1. Design pressure and temperature
Design pressure of a heat exchanger is the gage pressure at the top of the vessel. This pressure is used to determine the minimum wall thickness of the various pressure parts. The IS: 4503 species that the design pressure should at least 5% greater than the maximum allowable working pressure. Usually a 10% higher value is used. The maximum allowable working pressure is the gage pressure for a specified operating temperature that is permitted for the service of the exchanger units. According the IS: 4503, the shell and tube sides pressure should be specified individually. The design pressure specification is at 250, 120 and 65ºC for carbon steel, stainless steel and non-ferrous metals respectively. The maximum permissible stresses for various heat exchanger components should not be exceeded at the allowable pressure.
The design temperature is used to determine the minimum wall thickness of various parts of the exchanger for a specified design pressure. It is normally 10ºC greater than the maximum allowable temperature.
2.2. Materials of construction
All materials used construction of shell and heat exchangers for pressure parts must have the appropriate specification as given in IS: 4503 Appendix C. The materials of construction should be compatible with process fluids and others parts of materials and also should be cost effective. The maximum permitted operating fluid temperatures should not exceeds for the various pressure-retaining components as specified by IS:4503 given in Table 2.2 [1]. High chrome-Mo-Ni alloys (Cr content 12-27%) can be used for high temperature services upto 2100ºC. Use of any carbon or low alloy steel is not recommended for the construction of heat exchangers for the service below 0ºC.
Table 2.2 . Materials of constructions [ 1 ].
Materials of construction |
Allowable fluid temperature, °C (°F) |
Carbon steel |
540 (1004) |
C-Mo steel |
590 (1094) |
Cr-Mo steel |
650 (1202) |
Low alloy steel (< 6 % Cr) |
590 (1094) |
Alloy steel (<17 % Cr) |
590 (1094) |
Austenitic Cr-Ni steel |
650 (1202) |
Cast iron |
200 (392) |
Brass |
200 (392) |