SPX Marley NCWD, User Manual, Page: 20 / 32

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In order to achieve water quality which is acceptable to the cooling tower (as
well as the remainder of your circulating water system), the selected water
treatment company must work from a relatively constant level of concentra-
tions. This stabilization of contaminant concentrations is usually accomplished
by blowdown, which is the constant discharge of a portion of the circulating
water to waste. As a rule, acceptable levels on which to base a treatment
schedule will be in the range of 2-4 concentrations. The following table shows
the minimum amount of blowdown (percent of flow) required to maintain dif-
ferent concentrations with various cooling ranges*:
* Range = Difference between hot water temperature coming to tower and cold water tem-
perature leaving tower.
EXAMPLE: 700 gpm circulating rate, 18°F cooling range. To maintain 4
concentrations, the required blowdown is 0.458% or .00458 times 700 gpm,
which is 3.2 gpm.
If tower is operated at 4 concentrations, circulating water will contain four
times as much dissolved solid as the makeup water, assuming none of the
solids form scale or are otherwise removed from the system.
When water treatment chemicals are added, they should not be in-
troduced into the circulating water system via the cold water basin
of the cooling tower. Water velocities are lowest at that point, which
results in inadequate mixing.
Cooling Tower Inspection and Maintenance
Microorganisms including Legionella bacteria can exist in premise
plumbing including cooling towers. The development of an effective
water management plan (WMP) and implementation of maintenance
procedures are essential to prevent the presence, dissemination and
amplification of Legionella bacteria and other waterborne contami-
nants throughout premise plumbing. Before operating the cooling
Note
Warning
maintenance
Cooling Range
Number of Concentrations
1.5X 2.0X 2.5X 3.0X 4.0X 5.0X 6.0X
5º F .78 .38 .25 .18 .11 .08 .06
10º F 1.58 .78 .51 .38 .25 .18 .14
15º F 2.38 1.18 .78 .58 .38 .28 .22
20º F 3.18 1.58 1.05 .78 .51 .38 .30
25º F 3.98 1.98 1.32 .98 .64 .48 .38
Multipliers are based on drift of 0.02% of the circulating water rate.