BAC PCT Series, Operation & Maintenance Manual

Page: 27 / 38

21
W W W . B A L T I M O R E A I R C O I L . C O M . A U
M A R 1 0 8 - 2
PCT COOLING TOWER
Water Treatment
BLEED RATE
BLEED LINE CALCULATIONS
Water Treatment
Bleed Rate
A proper water treatment program, administered under the supervision
of a competent water treatment specialist, is an essential part of
routine maintenance to ensure the safe operation and longevity of
evaporative cooling equipment, as well as other system components.
Bleed Rate
1. In evaporative cooling, evaporation of a small portion of the re-
circulating spray water as it flows through the equipment causes
the cooling effect. As this water evaporates, the impurities
originally present remain in the re-circulating water. The
concentration of the dissolved solids increases over time and can
reach unacceptable levels. In addition, airborne impurities are
often introduced into the re-circulating water. If these impurities
and contaminants are not effectively controlled, they can cause
scaling, corrosion and sludge accumulations that reduce heat
transfer efficiency and increase system operating costs, potentially
shortening the useful life of the equipment.
2. The degree to which dissolved solids and other impurities build
up in the re-circulating water may be defined as the cycles of
concentration. Specifically, cycles of concentration is the ratio
of the concentration of dissolved solids (for example - chlorides,
sulphates, etc.) in the re-circulating water to the concentration of
the same material in the make-up water.
3. In order to optimise heat transfer efficiency and maximise
equipment life, “bleed” or “blow-down” a small amount of re-
circulating water from the system. This controls the cycles of
concentration to maintain the quality of the re-circulating water
within the guidelines given on Page 28.
4. Replenish the “bleed” water with fresh make-up water, thereby
limiting the build-up of impurities.
5. Bleed/blowdown: Accomplish the bleed automatically through a
solenoid valve controlled by a conductivity meter. The conductivity
meter set point is the water conductivity at the desired cycles of
concentration and should be determined by a competent water
treatment expert. (Note: The solenoid valve and conductivity meter
may be supplied by BAC or others).
6. Alternatively, use a bleed line with a valve to continuously bleed
from the system. In this arrangement, adjust the rate of bleed using
the valve in the bleed line. Measure the rate of bleed by filling a
container of known volume while noting the duration. Check the
bleed rate and water quality periodically to ensure that adequate
control of the water quality is being maintained.