When the maximum temperature of cooling water slowly increases with temperature changes and shifting climate patterns, smaller LMTD’s (log mean temperature differences) for the CCW’s to meet the same performance heat rejection. Making the issue more critical is that the peak cooling water temperatures will usually occur at the same time as peak summer load demand.
A smaller LMTD means a larger heat exchanger and more effective tubing surface area. More surface, means more tubing or smaller diameter tubing. If the original LMTD was 12 °F, a 1 degree change may mean an increase of 9%. To maintain the same nozzle locations on a replacement exchanger means a smaller tube outside diameter and/or a larger shell. Such increases are necessary for the high summer load conditions with the highest inlet water temperatures. At lower water temperatures, the amount of excess thermal capability can become a performance and corrosion issue as the water flows are modulated to meet temperatures. To help reduce these problems, a design which allows operation with reduced surface at low temperatures is appropriate.
The temperature approach (Cooling Water Out – Service Water In) based on the higher inlet cooling water temperature can be significantly smaller than when the CCW was originally designed. This paper will address a design configuration that will work with both higher summer temperature cooling water with the flexibility of using less water for cooler winter operation. The overall affect is less pumping power during colder months, more consistent tube velocities which will help with heat transfer, and minimization of sediment settling in the tubes due to lower velocities.