Heat exchange process, automatic control

When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Frank L. Rubin, Tower Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans, Oil Gas J., 165-167 (Oct. 11, 1982)]. Alternatively, when fans have two-speed motors, these dehver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition. 

The discharge pressure developed hy the compressor must he equal to the process gas s total system resistance, of control valves, hand valves, orifices, heat exchangers, and any other process-related devices through which the discharge gas from the compressor must flow. As this resistance changes, the gas flow through the compressor will automatically adjust itself to equal the new resistance. ... 

This demonstration plant will normally operate at a first-effect boiling point of 250° F., a last-effect boiling point of 120° F., and a discharge sea water concentration factor of 4. The primary control of the process is accomplished by automatic control of steam flow rate, sea water flow rate, and last-effect vacuum. No control is needed for temperature or pressure in the individual effects and heat exchangers, since these achieve their own levels, influenced only by the proportioning of the equipment. The demonstration plant is rather heavily instrumented to permit close surveillance of operating conditions and carrying out of special tests. 

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