Cooling Tower Design from Scratch

Where does one start when designing a mechanical draft cooling tower from scratch? This can be quite a daunting task as there are many variables and opposing objectives when optimizing the design. The steps below should give an appropriate starting point for the design process with my cooling tower software. The software is then employed to optimize the design. The process typically involves some trial-and-error to obtain the desired water outlet temperature.

For a start the following design conditions must be specified:
  • Wet Bulb Temperature
  • Design water flow rate per cell (mwi)
  • Dry Bulb Temperature
  • Hot Water Temperature
  • Cold Water Temperature.

It is also advisable at this stage to know what type of fill will be employed. The decision of the type of fill will be a function of the water quality and the fouling propensity of the fill.

The first important design decision one needs to make is what the frontal area of the fill (Afr) should be. One parameter that can guide you in this decision is the water mass velocity, Gw, where Gw = mwi/Afr. Your fill supplier should be able to tell you what the typical recommended range for Gw is.

If the recommended Gw for the fill is known Afr can now be calculated, where Afr = mwi/Gw

It is further assumed that the tower is square. The actual area of the cooling tower is larger than the area of the fill due to the area that the fill supports etc. are taking up. Lets assume for argument sake that the actual plan area is 1.05 times the area of the fill. Assuming a square tower the actual inside dimensions of the tower can now be determined.

Cooling tower fills are generally packed in layers where the layers have a fixed height. Pick any number of layers that will give a practical height, e.g. 2m.

The next decision is the required air flow rate. A first guess for the air flow rate is based on the following. According to one publication the ratio of the water and air flow mass velocities (Gw/Ga) should be between 0.75 and 1.5 for mechanical draft cooling towers. A Gw/Ga ratio of one is therefore a good first assumption, i.e. the air flow rate is equal to the water mass flow rate.

The rest of the inputs in the software should be typical for mechanical draft cooling towers. A trail and error process will now be conducted to achieve the design water outlet temperature. With the first run of the software the water outlet temperature will probably be too hot or too cold. A decision can now be made to l alter the air flow rate, the fill height or both.

When the tower design is complete a fan must be selected. Refer to the second point in the blog entry entitled “Fan Specification” on how to do this.
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