Page 193 - Boiler_Operators_Handbook,_Second_Edition

178 Boiler Operator’s Handbook

The fill is designed to convert the falling water to a film air velocities in the tower that are higher than design.
along the surface of the fill and thin sheets or droplets of Sometimes it only occurs at high loads but it can be a
water between parts of the fill. The air is drawn through problem at lower loads if the fill is distorted, damaged,
the fill to contact the air over that extended surface to covered with organic growth or otherwise altered to re-
achieve the heat transfer and sweep vaporized water out duce or block air flow in some parts of the tower so the
of the tower. The fill can consist of redwood slats in sev- velocities have to be higher in other parts to compensate
eral alternating layers (Figure 5-38) but so many of them for it. Towers can contain baffles to redirect water splash-
burned up when idled during the winter that it’s an un- ing off the fill back into the tower and they can be bent
common material in towers today, used primarily where or otherwise altered by high winds, falling tree limbs,
the tower is always in service. There are many variations and accidents to increase liquid water loss. Drift is sim-
and designs of plastic fill used today. ply a waste of water and should be addressed when it
Part of the plume can be drift. Drift identifies drop- is detected. Consistent pools of water on adjacent roofs
lets of water that are swept off the fill by the air and (evident in Figure 5-37) are indications of drift.
leave the tower without being vaporized. Those drop- Cooling towers in electric power plants and large
lets are usually much larger than the fog and drop out industrial plants can operate without a fan forcing the
on adjacent structures. Excessive drift is an indication of flow of the air. You’re probably familiar with photo-
graphs of nuclear power plants and the
tall hyperbolic cooling towers (Figure
5-39) which use the differential pres-
sure produced by the difference in den-
sity of the atmospheric air and the air
heated in the towers and containing
a larger volume of lighter steam. The
plume that we associate with cooling
towers is actually droplets of water that
condense as the exhaust of the cooling
tower is cooled by the slightly cooler
atmospheric air. A better name for that
plume would be fog.
In an electric power plant colder
condenser water, cooled by the cool-
Figure 5-38. Cross section of redwood cooling tower
ing tower, increases the energy the tur-
bine can produce. The water is cooled
as much as possible, stopping in the
winter slightly above 32°F to prevent
freezing of the water. Cooling tower
water used in industry for cooling of
production equipment or product can
also be allowed to vary in temperature
but typically the lower limit is set high-
er to minimize thermal shock and en-
sure the temperature control valves on
the cooling equipment maintain con-
trol. For chilled water systems a cooling
tower is normally operated to maintain
a leaving temperature of 85°F to match
the design condition for most chillers.
Maintaining the temperature maintains
pressure to ensure adequate liquid flow
through the throttling device. In cir-
cumstances of low loads, and where the
Figure 5-39. Hyperbolic CT in power plant