Sizing of Pumps, Compressors, and Control Valves

To get the fluid to flow from a low-pressure vessel into a high-pressure vessel. 

To overcome pressure drops in equipment such as heat exchangers. 

To provide pressure drops through control valves. 

The first item is self explanatory. The pressures are set by the operating pressures in the vessels. Reactors and distillation columns, for example, mn at different operating pressures so as to optimize the performance of each unit. 

Since the flow through most heat exchanges is turbulent, the pressure drop over the heat exchanger (and any other equipment with fixed resistance) will vary with the square of the flowrate. If the flowrate through the unit is doubled, the pressure drop over the unit will increase by a factor of four. These large changes in pressure drop have a strong effect on how control valves, pumps, and compressors should be sized. 

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