Getting the Most, part 4: Cooling Tower Optimization

Thu, August 08, 2018

In the last part of this series, we talked about getting the most out of your Plate & Frame Heat Exchanger.  One of the critical decisions to make about sizing a HX is the approach temperature.  The approach temperature is the difference between the inlet temperature of one fluid and the outlet temperature of the other fluid. The same holds true for cooling towers.  A cooling tower is really a heat exchanger that uses the highly efficient concept of evaporating a fluid in order to cool it.  Like a water to water heat exchanger, it doesn’t have a set capacity.  Instead, it can increase or decrease the amount of cooling based on the conditions it receives.  If we increase the approach temperature, we can decrease the size of the tower, the fan horsepower, or both. SPX Marley has an incredible tower selection tool called Update that can size the tower based on specific conditions AND give you the ability to see what would be required to reduce the footprint and/or energy usage if you made a change to the approach temperature or the range. Range?  That is the change in temperature of the water being evaporated.  Most tower selections are based on a 10F degree range, with an approach of 7 degrees.  This is why we often see selections made base on 95-85F for the water with a 78F wet bulb.  The range is 10 degrees (95F-85F), and the approach is 7 degrees (85F leaving water temp – 78F wet bulb). Want to see if the tower size or fan horsepower can be reduced?  Start by adjusting the approach temperature.  What if we sized the tower based on a higher set of water temperatures, such as 97F-87F?  The approach is now 9F rather than 7F. This could reduce the footprint of the tower and/or reduce fan HP. What if we made the selection based on 100F-85F?  The range is now 15F and the fan Hp is likely to be smaller. Any number of combinations of approach and range may possibly lead to the right balance for your system.

 Marley cooling tower install at 35 W. Wacker Drive in Chicago (work by AMS Mechanical Systems)

Naturally, you need to look at the impact of these changes on your chillers, coils, etc.  An increase in the range means a reduction in flow and a higher mean temperature in your condenser water loop.  The pump size may be reduced, but the condenser may become larger or require more horsepower. Your Bornquist sales engineer can walk you through all of this.  Feel free to reach out to us.  This is what we love to do! Next time: Centrifugal pumps in the real world.

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Tags: cooling tower , Education , engineering , HVAC , optimization , spx marley