


The actual hourglass is hollow and is designed to generate a draft,
exploiting the venturi effect.  Around the base of the hourglass is a
ring of water towers.  Warm river water, coming from the steam condenser
in the plant, is sprayed over louvres.  The draft being pulled through
the tower cools the water by both evaporation and convection.  The
sensible heat extracted from the cooling water is the driving force for
draft generation.

It should be noted that the hourglass-shaped cooling towers are used on
both fossile and nuclear plants.  It should also be noted that at 
locations where water is plentiful, the cooling towers are only used part
time, when the discharge temperature would exceed some release limit.
It was once thought that the warm discharge water was damaging to fish.
Fishermen know that is thoroughly incorrect.  Nontheless, stringent,
usually state, regulations remain in some instances.  Since it typically
takes 60,000 hp worth of pumping to move the volume of water needed 
to cool a 1000 MWe plant, the cost of using the towers is not insignificant.