The engineer making the material selection must know all aspects involved in the construction maintenance and operation of heat exchangers. The importance of this is illustrated with the following examples; an operator may isolate a heat exchanger with raw water for sufficient time to initiate a pitting corrosion; partial blockage of tubes, specially of small diameter, would result in stagnant condition that may cause pitting in alloys that are so prone; fouling may result in operating the heat exchangers in throttled or part load condition.
The general conditions for material selection are as follows
Define the heat exchanger requirements.
Establish a strategy for evaluating candidate materials.
Identify candidate materials.
Evaluate materials in depth
Select the optimum material
For the first step the engineer must consider the normal operating parameters ( eg. Nature of fluids of both the tube and the shell side, flow rate, temperature and pressure), start up and shut down conditions, upset conditions and special conditions like product purity requirements, hazardous effects of inter mixing of shell and tube side fluids, radioactivity and associated maintenance etc. The applicable codes and safety regulations must also be considered. The heat exchanger designer would also identify the tube attachment method as this also affects the material selection is being done by someone other than the heat exchange designer, there must be close consultation between these individuals.
While establishing strategy for evaluating candidate materials, the main factors to be considered are cost and reliability. The minimum cost strategy would mean use of less expensive materials and rectifying the problems as they show up. Maximum reliability strategy would mean going for most reliable material regardless of its cost. Both strategies have to be weighed against initial cost, loss due to possible shutdowns, repair cost, indirect loss to other industries etc.
In identifying candidate materials, it is desirable to narrow the field to a comparatively small number of materials for extensive evaluation. The initial identification and selection procedure, if done properly will eliminate those materials which are unsuitable and those which are excessively expensive. This calls for use of operating experience, use of handbook data and literature of advanced materials under development and judgement.
Special consideration on material selection include.
Physical Properties
High heat transfer coefficient ( requiring high thermal conductivity for tube material)
Thermal expansion coefficient to be low and as compatible as possible with those of the materials used for tube sheet, tube support and shell to provide resistance to thermal cycling.
Mechanical Properties
Good tensile and creep properties ( high creep rupture strength at the highest temperature of operation and adequate creep ductility to accommodate localised strain at notches are important)
Good fatigue, corrosion fatigue and creep-fatigue behaviour.
High fracture toughness and impact strength to avoid fast fracture.
Corrosion Resistance
Low corrosion rate to minimise the corrosion allowance ( and radioactivity control in heat exchangers for nuclear industry)
Resistance to corrosion from off normal chemistry resulting from leak in upstream heat exchanger or failure in the chemistry control.
Tolerance to chemistry from mix up of shell and tube fluids.
Manufacture
Ease of fabrication is an important aspect for selection of materials. The usual manufacturing steps involved for heat exchangers are bending of tubes, joining of tube to tube sheet by rolling, welding or rolling and welding of shell plates and shell to nozzle and the heat treatment associated with the welding steps.
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