UFC 3-240-13FN
25 May 2005
4-1.5
Common Cooling Water System Problems. Water-related problems can
cause system downtime, loss of equipment efficiency, the need for capital replacement
of equipment, and can increase the risk of disease from pathogenic microorganisms. An
open recirculating cooling tower system has a greater potential for these problems than
does a once-through cooling water system, due to the air- and water-mixing design of
the open recirculating system. These problems are associated with water-caused
deposits, corrosion, or microbiological organisms, and occur for various reasons:
The cooling tower is essentially a huge air scrubber that can introduce
materials such as microorganisms, gases, dust, and dirt into the
circulating water, which provides an excellent growth environment for
pathogenic microorganisms. These materials can contribute to the
formation of deposits and cause corrosion.
If the water is not properly treated and its quality maintained, corrosion
and scale and solids deposition can occur. The potential for these
problems results from the nature of the cooling system design and the
operating conditions, including water evaporation, mineral concentration,
and water temperatures of up to 54 C (130 F).
The constant addition of makeup water results in increased quantities of
mineral constituents that can form scale, deposits, and corrosion.
Blowdown control and proper water treatment can minimize these
problems.
The film fill contains small water and air passages that can become
plugged, thereby causing a reduction in cooling tower operational
efficiency due to reduced water evaporation (see paragraph 4-1.4.3).
Current designs for heat exchangers and cooling towers provide for more
efficient operation than in the past, but unexpected water problems may
occur. Some of the more prevalent potential problems are described in
paragraphs 4-1.5.1 through 4-1.5.4.
4-1.5.1
Enhanced and Super-Enhanced Chiller Condenser Tubing. Recent air
conditioning chiller equipment designs incorporate enhanced and super-enhanced
chiller condenser tubes. Previous designs have used smooth-bored waterside
condenser tubing. The enhanced tube is machined with rifled grooves that provide an
increased surface area and a resultant increase in heat transfer; however, the rifled
grooves and ridges tend to entrap SS (i.e., dirt, silt, sand, and old corrosion products),
which are deposited from the cooling water as it passes through the tube. This
deposition of material on metal surfaces can create a type of localized corrosion called
"under-deposit corrosion." This situation has resulted in numerous cases of tube failure.
The super-enhanced chiller tubes have even finer grooves and ridges, making this type
of tubing even more susceptible to under-deposit corrosion. (See Figures 4-12 and 4-
13, which show photos of super-enhanced copper tubes.)
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