Page 114 - Plant design and economics for chemical engineers

9 0 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

extent from the loss of sensible heat to the air. In dry cooling towers, the
temperature reduction of the condenser water depends upon conduction and
convection for the transfer of heat from the water to the air.
Mechanical draft cooling towers either force or induce the air which serves
as the heat-transfer medium through the tower. For their driving force, natural
draft cooling towers depend upon the density difference between the air leaving
the tower and the air entering the tower.
Cooling ponds are generally only considered for heat removal when
suitable land is available at a reasonable price, since such systems are simple,
cheap, and frequently less water-intensive. It is normally assumed that all heat
discharged to a cooling pond is lost through the air-water interface. With low
heat-transfer rates, large surface areas are required.
When land costs are too high, spray ponds often provide a viable alterna-
tive to cooling ponds. It is estimated that a spray pond requires only about 5 to
10 percent of the area of a cooling pond due to the more intimate air-water
contact. In addition, drift losses and corrosion problems are less severe than in
cooling towers.

NOISE CONTROL. The design engineer should include noise studies in the
design stage of any industrial facility. Generally, acoustical problems left for
field resolution cost roughly twice as much. Unnecessary costs incurred in
post-construction noise work may include the replacement of insulation, re-

TABLE 11
Equipment noise sources, levels, and potential control solutions

Sound level in dBAf
Equipment at 3 ft Possible noise control treatments

Air coolers 81-94 Aerodynamic fan blades, decrease
rev./min and increase pitch, tip and
hub seals, decrease pressure drop.
Compressors go-120 Install mufflers on intake and exhaust,
enclosure of machine casing, vibra-
tion isolation and lagging of piping
systems.
Electric motors go-110 Acoustically lined fan covers,
enclosures, and motor mutes.
Heaters and furnaces 95-110 Acoustic plenums, intake mufflers,
ducts lined and damped.
Valves < 80 to 108 Avoid sonic velocities, limit pressure
drop, and mass flow, replace with
special low noise valves, vibration
isolation and lagging.
Piping go-105 Inline silencers, vibration isolation
and lagging.
i Defined as the sound intensity measured in units equal to ten times the logarithm of the
square of the relative pressure associated with the sound wave.