Page 98 - Membranes for Industrial Wastewater Recovery and Re-Use

78 Membranesfor lridirstrial Wastewater Recovery and Re-use

tower to the atmosphere as hot moist vapour. Evaporative cooling systems
provide air flow either through natural draft or mechanical draft systems.
Typically, natural draft systems are hyperbolic towers (> 50 m high) with air
flow rates of about 2000 m3 min-l (Kemmer, 1988; Puckorius and Hess, 1991;
State of California, 1980). Density gradients between the less dense heated air
and the more dense cool ambient air result in positive air flow.
Mechanical draft systems employ fans to move air through the towers either
through forced draft (fans at the bottom of the tower) or induced draft (fans at the
top of the tower to pull air up through the tower). Rotating fans are used to pull
cool, dry air into the tower at a rate of about 600-750 m3 air per m3
recirculating water. Warmed heat exchanger water is pumped to the top of the
tower where it descends through the rising air stream. Packing material inside
the tower breaks up the water flow to improve the contact efficiency between the
air and water. A fraction of the water evaporates and leaves the tower as vapour.
Mechanical draft cooling towers (Fig. 3.2) tend to have vapour plumes that are
about 30% shorter than those produced by natural draft towers, thus reducing
the need for plume abatement.
A variety of types of cooling systems are used, depending on the application
and water availability (Table 3.1). Cooling water can be used once and returned
to its original source (once-through cooling) or reused in an open recirculating

Figure 3.2 Vpour plume from cooling system at McKag Bay waste-to-energHfacility, Tampa, Florida