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Typically cooling tower systems do not have flow control on individual exchangers, most systems I have encountered just don't spend the money, they set the flow and forget it. By changing the flow, there are significant changes to the flow regimes of exchangers elsewhere on the system, exchangers located well above grade become starved of water as the pump delivers less flow. The other aspect of reducing the flow is the velocity of water in the exchangers is less, and fouling continues with sediment that begins to settle out in exchangers, when tube velocities drop below 5 f/s, exchangers begin to plug! Cooling water tends to be some of the most dirtiest water in the plant. Exchanger scaling is a very big issue, most cooling water systems don't use soft water, they tend to use barely treated raw water. There are many more issues with the approach of varying flow than any with controlling the return temperature. We have seen excellent results from all processes connected VFD on a cooling tower system network when the return temperature is controlled using the fan speed, including reduced icing in extreme cold weather. Many of the process other than the reactor will benefit from a controlled return water temperature. This has been the result I have had experience with and witnessed. I have also installed variable frequency drive closed loop cooling tower systems, that use ethylene glycol and water, with excellent results on reactor effluents, and there again, we control fan speed to manage heat load, and return temperature. In my experience, altering the return temperature is the same as hitting the process with a large disturbance, and the process control aspects become even more of an issue for tight process control. When glycol and distilled water are used as the heat transfer, controlling the flow is less of an issue because of the quality of the stream.  Raw cooling water tends to be a thorn in many sides for operations. more...