A hot, viscous process stream is cooled using cooling water (cw). The exchanger is a double-pipe design in which the cooling water flows in the annulus and the viscous process fluid flows in the inner pipe. The T-Q diagram for the exchanger at design conditions is shown in Figure E20.17 along with an image of a double-pipe exchanger. At design conditions, the cw is in the turbulent regime, but the process fluid is in the laminar flow regime. The process is to be scaled down such that the process flow is reduced to 72% of its current value, but the inlet temperature is maintained at 95°C. It is desired to keep the exit temperature of the process at the current value of 55°C by reducing the cooling water flow rate. However, the maximum temperature of the cooling water is set at 45°C because, if it gets hotter than this value, then excessive scaling of the heat exchanger will result.

Can this operation be accomplished? Determine the cw flow rate and exit temperature of cw.

If the desired reduction in process flow rate cannot be achieved, determine the minimum process flow rate and the cw flow rate to accomplish this.

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-95°C 40°C (a) (b) 55°C 30°C