Cooling Tower Design

To select a cooling tower you need gpm, inlet temp, desired outlet temp and wet bulb temp. Most domestic towers are CTI certified which means you can expect to get the capacity certified by the manufacturer. If you plan to oversize for future growth, less than ideal installation or whatever, increase the design wet bulb temperature. If you screw with the flow rate, you may get spray nozzles that are too big. Google "Cooling Tower ch_17" and click on first entry for a check list to review before purchase.

Thank u vry much for ur reply. i would like to know if delta T requirement for Process cooling is 5 deg C.And if we select some margin for temperature range , say suppose we select CT with delta T of 6 deg C. What major change would it cause in working of CT, when process water will come to CT at delta T of 5 only.

Everything depends upon the wet bulb temperature, which is a function of the weather locally.

If a specific process temperature is required that is some or all of the time below the local wet bulb temperature range, then a refrigeration plant is needed between the process and the cooling tower circuit.

ya , thats ok.

Say if local WBT is 27.5 deg C.And I have designed CT for 38 deg C to 32 deg C(delta T =6deg C).

And for most of the time the return water temperature to CT is less than 38 deg C ,say 37 or 36 deg C.

What effects would it have on working of cooling tower...?

The lowest temperature to which water can be cooled in a cooling tower is the wet bulb temperature of air surrounding the cooling tower.The difference between the temperature of water leaving the tower and the wet bulb temperature is called the wet bulb approach of the C.T. the temperature difference between the temperatures of water entering and leaving the tower is called the range of the C.T.Load on the C.T is the load on the Condenser ( Heat rejection on the condenser or heat exchanger). What type of cooling tower ? Is it induced /forced type or atmospheric type?.

It is a induced draft cooling tower.

mrswamy, thanks , I'm going to check my CT's "approach" today--

aaborkar:

Think of it this way... The difference between the inlet temp and outlet temp of a cooling tower multiplied by the flow rate will give you the rate of heat rejection.

A tower will always reject the heat it is given (except if it's so high as to melt the wet deck, etc.). The tower cannot reject more heat than it is given.

Since the flow rate through the item being cooled is the same as the flow rate through the tower, the delta T at each will be the same.

The 'approach' is defined as the difference between the tower's leaving water temperature and the wet bulb temperature.

A 'small' tower will operate with a 'large' approach. As the tower is made bigger and bigger the approach will become less and less. An infinitely sized tower would operate at zero degree approach and your leaving water temperature would be the wet bulb temperature. While all along the product of your 'range' and 'flow rate' at the tower will continue to match the product of the flow rate and delta T of the item being cooled.

Go to a cooling tower manufacturer's web site (Baltimore Aircoil for example) and play with their selection software. Notice how increasing the design wet bulb quickly expands the size of the required tower.

Be sure to use a design wet bulb that is correct for your location during the hottest season of the year. Then, if you want to add a degree or two you'll have a lot of safety. Naturally, if you want to account for future expansion, select a tower for the future conditions but be sure to tell the manufacturer the true flow rate so that the tower is fitted with the correct spray nozzles. If you want to boost the flow later, you may have to get new nozzles.