4 replies to this topic

[Sridhar P]

Dear Sir,

How to decide on the Cooling Tower Recirculation allowance for deciding the design air inlet WBT. Can anyone provide the relevant extracts/reference.

Thanks and Regards,
P.Sridhar

[kkala]

If I understand the query, following points can make an answer.
1. Wet bulb temperature of air depends on climatic conditions (it can be up to, say, 28 oC). Water recirculation does not seem to affect it.
2. Cooling water recirculation depends on its tendency to form scales, which should be avoided. You may want to look at "http://www.gewater.c...#COOLING TOWERS. Recirculation is the sum of all cooling water streams entering equipment for cooling and going out of equipment to the cooling tower.

[Sridhar P]

Dear Sir,

It is not the water recirculation, it is the short circuting of cooling tower exhaust air to cooling tower fresh air inlet. This increases the cooling tower cold air inlet temperature by some fraction. I need some reference/extracts saying the actual requirment of recirculation allowance.

Thanks and Regards,
P.Sridhar

[kkala]

It is not the water recirculation, it is the short circuting of cooling tower exhaust air to cooling tower fresh air inlet. This increases the cooling tower cold air inlet temperature by some fraction. I need some reference/extracts saying the actual requirment of recirculation allowance.

In the past I found qualitative information on this topic in the book "A working guide to Process equipment" by Norman & Elizabeth Lieberman, McGraw-Hill, 2008 (3rd edition). According to that book, forced draft towers can suffer from air recirculation, but not induced draft towers. Probably there are exceptions to this statement, expressing a statistical trend.
Searching the web can give results, e.g. http://www.energyboo.../pdf/310314.pdf. Configuration of towers and prevailing wind seems to play a role.
Probably quantitative parameters to offset the phenomenon are known only by manufacturers of specific cooling towers (proprietary information), or you can measure performance of a specific tower.

Note on 25 Jul 11: Having gone to the country, I could look at Lieberman's book, Chapter 17, to see that statement for forced / induced draft fan concerns actually air coolers. Scale formation upstream forced draft fan causes some air flow downwards, escaping out through the void between fan blades and wall. Induced draft fan does not create this parasitic flow, even though scale formation will decrease flow (constant in the previous case). One may suppose that case for cooling towers has similarities to air coolers , but this is not written in the book. Sorry for my misunderstanding.

Edited by kkala, 25 July 2011 - 07:41 AM.

[tarafdar]

Dear Mr. Sridhar P,
95% summer hours maximum WBT for the area is the starting point for C.T. design.To this an allowance of 1-3 oF is added for recirculation of hot air.
I take this information from "Rules of thumb for Chemical Engineers" by CARL BRANAN.