Begin with the thermal duty
A useful specification includes both fluid flow rates, inlet and required outlet temperatures, physical properties, phase-change information, allowable pressure drop and design pressure and temperature. Missing property data or an unrealistic temperature approach can dominate the selection more than the exchanger type itself.
Plate heat exchangers
Plate units provide compact area and close temperature approaches for many clean liquid-to-liquid duties. They can be expanded by adding plates and opened for inspection in gasketed designs. Gasket compatibility, pressure and temperature limits, port velocity and fouling behaviour must be reviewed.
Shell-and-tube heat exchangers
Shell-and-tube construction provides broad flexibility for pressure, temperature, materials, phase change and mechanical cleaning. Tube diameter, layout, passes, baffles and removable bundle arrangement can be selected for the service. The unit is generally larger and requires space for bundle or tube maintenance.
Fouling and cleanability
A high theoretical heat-transfer coefficient is not useful if the selected passages foul quickly or cannot be cleaned. Consider particle size, viscosity, crystallisation, polymerisation, biological growth and cleaning chemicals. Velocity must be high enough to control deposits without creating erosion or excessive pressure drop.
Mechanical and material conditions
Corrosion, thermal expansion, cyclic operation, external loads and the governing mechanical design standard influence materials and construction. For shell-and-tube units, tube-to-tubesheet joints, expansion accommodation, channel design and inspection access are part of the final design.
Preliminary selection checklist
Provide fluid names and compositions, flow rates, temperatures, operating and design pressure, allowable pressure drop, fouling tendency, cleaning method, materials, utility data, installation limits and required inspection or certification scope.