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Submitted Chris Haslego, Nov 21 2011 11:21 AM | Last updated Nov 21 2011 01:29 PM




Category: Equipment Design
Question: I would like you to explain to me whether I can apply the packed tower equations (Constant of Henry, L'/ G' ratio, pressure drops,diffusivities, Shulmann correlation, etc) in my design of the cooling tower?. Could you provide the correlation/equation of L/G vs KaV/L without resort to the diagram. How should I apply the value of KaV/L in my design?.
Keywords: designing,cooling,tower,packed,column
Answer: If you are going to design a cooling tower, forget the packed tower correlations, they're very differenttowers. Packed towers rely on increased surface area for mass transfer while cooling towers rely more on air velocity and contact time for mass transfer. Designing a cooling tower in an industrial setting (as described by my article) is different than doing the actual theory of the design. First, you have to answer the following questions:1. What is the highest web bulb temperature for the geography? (for example 75 degrees F)2. How close to the wet bulb temperature do you want to cool the water? Realistically, the best you can hope for is to cool water to 5 deg F above the wet bulb, in our case 80 degrees F. The closer to the wet bulb temp, the larger the tower.3. What is the flowrate and the inlet temp of the water? This will determine how much heat you need to remove.The amount of heat to be removed will determine the tower height (contact time with air while water falls) and air velocity through the tower cell (how quickly heat is removed while the water falls). Typical design L/G ratios range from 0.75 to 1.50. Once the previous questions are answered, the theoretical KaV/L should be calculated via the numerical integration equation shown in "Cooling Towers: Design and Operation Considerations" (see below).Then it is necessary to design the cooling tower fill and air distribution to meet the KaV/L value. Companies that manufacture cooling towers usually have standard towers that they manufacture (as opposed to custom towers). You can see some standards at the second link below.Each one of these towers has a maximum L/G ratio associated with it. I suspect that you're interested in bridging the gap between the L/G ratio and the geometric construction. Since the L/G ratio is always supplied by the cooling tower manufacturer, I don't know how they arrive at it. I suspect it a long history of experience and trial-and-error, but I guess they could have correlations to determine this.
Links: Cooling Towers: Design and Operation Considerations

Cooling Tower Standards

Cooling Tower Experts


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