Consider the following statements in respect of natural-draft cooling towers:

1) Theoretically the water can be cooled to even below the dry-bulb temperature of the induced air flow.

2) Nature-draft cooling towers are 100 m or more in height.

3) The inner and outer surfaces are surfaces of revolution of a segment of a hyperbola about the vertical axis-affording improved strength rather than any thermodynamic augmentation.

Which of the above statements are correct?

In natural draught cooling tower the flow of air occurs due to the natural pressure head caused by the difference in density between the cold outside air and the hot humid air inside.

The pressure head developed is

ΔP_d = (ρ_o - ρ_i) gH

Where H = height of the tower above the fill ρ_o = density of outside air ρ_i is the density of inside air. Because of the relatively small difference between ρo and ρ_i the H must be large so as to result in the desired ΔP_d which should be able to balance the air pressure losses in the tower. 

The tower body has hyperbolic profile which offers superior strength and great resistance to the outside wind loading compared to other forms.

As the water comes into contact with the air, the water evaporates by taking away the energy from the remaining water, which reduces the temperature of water below the dry bulb temperature of the air if the air is relatively dry. 

Nature-draft cooling towers are 100 m or more in height.