June 11, 2007
This is the next one as I promised earlier in my first one "Cooling Towers: Not the Coolest One". This time I was motivated by the search of someone on cooling towers capacity assessment. I have already mentoned many factors & procedure to find out the capacity of cooling towers under the title Performance Monitoring - Item 9 to find out NTU. However, this search from some student led me to re-write it in simple way.
Let us understand how CT (cooling towers) capacity is fixed for design. (I am including this to give you an overview).
1. Assessment of all process loads. (e.g. Coolers, Condensers, etc.)
2. These process loads are then added with safety margins of 20-30% depedning on the confidence of design engineer either in terms of process temperature or in terms of flow approximation and then finally in terms of circulation pump capacity & head.
3. Thus, before hand we have ~20-30% margin in the CT flow rate compared to actual requirement.
4. Then CT is deisgned for worst case of WBT (Wet Bulb Temperature) which prevails only for 2-3 months in a year anywhere in the world in general. This in general gives you ~10% extra capacity in terms of heat load on CT.
Let us assume that these are the major prevailing factors which are contributing to the load on CT and no design or operating issue which we have discussed in the previous article is not there.
Now we can divide the loading of CT in two parts - One is flow loading & Second is heat Loading.
In General it is not advisable to increase flow loading beyond 10% or so of design value due to mechanical reasons for tower stablity & safety.
Thus the recommended way is to increase the heat load on CT utilizing the design margins available due to several reasons as discussed above & in my previous post.
Now the Mathematics is easier to find out the capacity of CT.
Recently I need to increase the CW flow rate from a tower of 1050 M3/hr whcih was having a 3 cells of 350 M3/hr each. My requirement was 80-100 M3/hr.
When I estimated the total load on tower it was running with 1050 M3/hr flow rate but with 8°C range against design of 10°C range.
By simple maths, you can estimate that the
Heat load capacity was 1050 x 10 = 10.500 Gcal/Hr.
Actual load 1050 x 8 = 8.400 Gcal/Hr.
Additional Load 100 x 10 = 1.0 Gcal/Hr
Obviously there is margin in the CT & is possible to load it even the flow rate is running at design value of 1050 M3/Hr.
So total New load = 8.4 + 1.0 = 9.4 Gcal/Hr
With Design Flow = 1050 M3/hr
Delta T (Range required) = 8.95 ~ 9°C.
So I need to reduce the CW flow rate in other consumers such that the range of colling becomes 9°C from 8°C this will reduce the flow rate by ~100 M3/hr in other users giving me the flexibility or cushion to accomodate new load.
The purpose is to explain that CT should not be viewed as a stand alone flow machine it should always be treated as a combined entity for flow & temperature which is its heat removal capacity.
I have written it to help the users /students who are not clear in the concepts of CT. If you have any questio kindly post them in comments section & I will be happy to answer them as soon as possible for me.