6. MECHANICAL DESIGN CONSIDERATIONS

Temperature and pressure are the two important factors that affect the mechanical design of evaporator systems. Many other factors like startup, shutdown, upset, dryout, external loading from supports, pulsating pressure, wind loading, earthquake load etc. also significantly affect the evaporator operation. Various factors are considered that affect the mechanical design of equipment and their affect is detailed in heat exchanger design ( module #1 ). Here the temperature and pressure factors are outlined below in brief.

Operating temperature and pressure: The operating temperature is the temperature that is maintained for the specified operation of the metal vessel suitably selected during design. The operating pressure is the pressure at the top of a pressure vessel. However if it's a tall vessel static hydraulic head even during mal-operation needs to be consider.

Design temperature and pressure: It is important to determine both minimum and maximum anticipated operating temperature and pressure in order to obtain the design temperature and pressure. The design pressure is generally is the sum of the maximum allowable pressure and the static head of the fluid in the pressure vessel. The combination of temperature and pressure affect the mechanical design of the equipment. Much of design considerations are also related pressure design too.

Maximum allowable working pressure: The maximum allowable working pressure is the maximum pressure to which the equipment can be safely operated. Generally, it should not be less than the maximum anticipated operating pressure divided by a factor of 0.90 [ 4 ].

Thermal expansion: Differential thermal expansion between various parts of equipment has a significant effect on the mechanical design. There may be a significant difference of expansion between the shell and the tube side because of temperature difference of two fluids. Thermal expansion may also determine the way in which tubes are fixed to the tube sheet. Usually a suitable expansion joint is centrally placed between two segments of the shell when the differential expansion may be large.