NATURAL DRAFT COOLING TOWER

Typical power-plant cycles which incorporate the natural-draft hyperbolic cooling tower are shown in the schematics.

Note that only 40% of heat-energy input is converted to power in the fossil-fuel plant, while 45% is discharged to condenser cooling water; the remaining 15% is lost up the stack & in the ash.

A thermal plant is even less efficient, converting only about 33% of its heat energy input into power, with 62% discharged to cooling water & 5% lost.

Remember, nonetheless, that heat discharges from electric power plants are among the lowest of all energy-conversion processes.

 A natural draft hyperbolic cooling tower are evaporative in design, they contain no fans.

WORKING PRINCIPLE

Flow of air through shell is created by the density difference between atmospheric air and that inside the tower which has been warmed by the hot water from plant condensers.

As with the mechanical-draft-type, two basic airflow schemes in relation to water flow are cross flow & counter flow.

• In counter flow design provides the more efficient means for heat transfer because coolest water contacts coolest air initially, although uniformity of air and water distribution achieved in cross flow type may offset this advantage.
• In the cross flow design, air flow is normal to water movement, and usually more fill is required to transfer a given amount of heat; however, the cross flow tower has a lower air-pressure drop. Selection of either arrangement depends primarily upon operating conditions required.
• Heat transfer, of course, takes place within the tower’s fill area: with cross flow towers, this section is outside the shell.

Typically, the area consists of wood, asbestos-cement or plastic fill, glass-reinforced polyester grids for supporting the fill; poured or precast support beams and support columns; and asbestos-cement or polyvinyl chloride (PVC) drift eliminators, placed behind the fill to separate water droplets that may become entrained in the air stream.

Basic fill arrangements are splash packing and film packing (sketches above).

Function of both is to generate as much air/water interface as possible with minimum air-pressure loss.

In the splash type, usually selected for cross flow towers, hot water falls over wave shaped fill in such a way that the droplets are constantly reforming, and thus presenting a fresh surface to the cooling air.

In the film type, most often found in counter flow towers. The sheet-like fill consists of multiple vertical surfaces down which hot water flows in extremely thin continuous films; cooling air passes over these films. Film-type fill occupies less volume, generally requires less shell height, but is more subject to clogging; splash packing is somewhat easier to repair or replace, if necessary.

A distribution system dispenses inlet hot water evenly over the fill. Typically system for a cross flow tower consists of vertical risers and an open water-distribution basin (left sketch above). Condenser hot water is pumped through risers to the basin” from which water flows through holes set in a precast-concrete slab. A nozzle is installed in each hole, with an integral splash plate that distributes hot water evenly over the fill surfaces below” For a counter flow tower, risers feed a closed-pipe system; cross piping contains nozzles for even distribution of water to the fill.

The basin at the bottom of the tower collects the cooled water for return to plant condensers.

It is sized to hold enough water to parried tower operation for several hours without having to add makeup water.

A drain system is normally integral with the basin, for removal of silt deposits; this system also acts to control basin water level in case of flow surges.

Cold-water return is most often a sloped canal at bottom of the collecting basin, which feeds into circulation pumps. Screens are placed ahead of the pumps to prevent debris and other foreign matter from entering return system.

A hyperbolic tower, then, is an hour, glass-shaped structure sitting on “stilts” in a shallow basin of water. The stilts, or diagonal support columns, provide an open area through which air enters the shell. In a counter flow tower, the fill is located above the columns inside the shell; in a cross flow tower, the fill is external to the shell and forms an annular ring around the base. A canopy seals the ring to the shell.

In either type of tower, hot water from plant condensers is pumped to the

Top of the fill, and flows or splashes down through the fill to the basin, while air sweeps either up or across the fill area. Above the fill the 300 feet or more of the tower is empty, functioning merely as a chimney.

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