25 May 2005
deposit inhibitors. This requirement refers to operational parameters such as maximum
temperature, minimum velocities, usage patterns, and whether or not layup periods
occur. The best chemical treatment programs cannot overcome the stresses of poorly
operated systems. A good example of system stress is intermittent chiller operation that
allows for SS accumulation (deposits) in chiller tube surfaces due to low flow or lack of
flow. This situation creates a strong potential for under-deposit corrosion and for MIC
that the chemical treatment may not be able to overcome completely. Stagnant water
can also cause deposits and corrosion. High water temperatures can cause scale. The
maximum water temperature should be known so that the proper scale-control method
can be used.
Water Quality. Water quality within the cooling water system is a critical
consideration. The source may be potable or reuse or recycled water. The various water
constituents must be known so appropriate water treatment chemicals can be selected
for controlling specific water quality parameters.
Cooling Water Treatment Programs
Categories. For the purpose of developing a water treatment program,
cooling tower recirculating water systems can be divided into three basic categories:
Small-sized towers capacity less than 88 kilowatts (25 tons)
Medium-sized towers 88 to 352 kilowatts (25 to 100 tons)
Large-sized towers - greater than 352 kilowatts (100 tons)
The water treatment program selected for cooling tower systems will vary
depending on the size of the tower (among other factors). Treatment program
considerations are described in paragraphs 4-6.5 and 4-6.6 for small-, medium-, and
large-sized towers, respectively.
Cycles of Concentration (COC). The COC of small, medium, and large
cooling towers may be controlled by blowdown, either automatic or manual, to avoid the
high levels of silica and calcium salts that can lead to scale. If the cooling tower is not
chemically treated, the COC should be adjusted to be slightly less than the lowest
allowable COC value based on all scale-forming water ingredients (see paragraph 4-
3.3). The use of chemical treatment permits a higher COC, which is usually limited by
either the chemical effectiveness or the water treatment control.
EXAMPLE: An untreated cooling tower system can operate at 10 COC based on the
level of silica without scale forming, but can operate at only 2 COC based on the level of
calcium carbonate. Accordingly, the tower system should be operated at a maximum of
2 COC. The system can likely be operated at 5 to 6 COC when an effective chemical
treatment is applied and maintained to control calcium carbonate.
Treatment Program for Small Cooling Towers. Typically a small cooling
tower will have a rating of less than 88 kilowatts (25 tons). The treatment program used
for small towers should be designed to be simple to control and easy to apply, with very