MIL-HDBK-1003/7
exceeded as a result of higher vacuum or higher water temperature, the cooling water at
this point will partially flash into vapor (steam) and the siphon will collapse.
Where the maximum allowable siphon is exceeded because of difference in
elevation between the water level of the source and top of the condenser water box, a
seal well with an adjustable discharge weir must be provided at the end of the discharge
conduit to limit the siphon head. The adjustable weir is made up of stop logs that are
installed by the operators as necessary to obtain the maximum possible head recovery,
which can be afforded by the maximum practical siphon.
Cooling Towers. Cooling towers consist of a structure, some type of fill to
12.7
break the warm water into droplets, warm water distribution system, air circulation
system, and basin for collection of cooled water. For a detailed description of
construction and theory of operation of cooling towers, see American Society of Heating,
Refrigerating, and Air Conditioning Engineers, ASHRAE Handbook, HE Equipment Volume.
12.7.1
Mechanical Draft Towers
12.7.1.1 Induced Draft Wet Type. This type utilizes propeller fans with exhaust
velocity recovery fan cylinders, which are mounted at the top of the cooling tower at
the fan deck. Typical fan size for power plant use would be 28-foot diameter and
powered with a 200 hp motor (single or two-speed) through a right angle gear reducer.
Atmospheric air is drawn in through louvers or grating, through the falling water and
fill, through drift eliminators, and out through the fans and fan cylinders.
12.7.1.2 Induced Draft Wet/Dry Type. This type combines a wet type as described above
with a finned tube heat exchanger. The warm water to the tower usually flows first
through the heat exchanger and then is distributed over the fill of the wet section.
Atmospheric air is drawn in through the heat exchangers in parallel with air through the
wet section. The addition of the heat exchanger reduces water consumption and water
vapor discharge.
12.7.1.3 Induced Draft Dry Type. This combines induced draft fans and finned tube heat
exchanger surface. Water flows through the coils while air is drawn across the outside
of the finned tubes.
Natural Draft Towers. Natural draft cooling towers are generally economical
12.7.2
for use only with very large scale power generating plants.
12.8
Cooling Water System Chemical Treatment
12.8.1
Chemical Analysis. The chemical analysis, the source, and the physical
analysis of the raw water should be examined to determine what treatments are necessary.
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