8
Water Treatment
BLOWDOWN
–Blowdown, or bleed-off, is the continuous removal of a portion
of the water from the circulating system. Blowdown is used to prevent the
dissolved solids from concentrating to the point where they will form scale.
The amount of blowdown required depends upon the cooling range (the dif-
ference between the hot and cold water temperatures) and the composition
of the make-up water (water added to the system to compensate for losses
by blowdown, evaporation and drift). The following table shows the amount
of blowdown required to maintain different concentrations with various cool-
ing ranges:
BLOWDOWN–% OF CIRCULATING RATE
e
g
n
a
R
g
n
il
o
o
C
s
n
o
it
a
rt
n
e
c
n
o
C
f
o
r
e
b
m
u
N
X
5
.
1
X
0
.
2
X
5
.
2
X
0
.
3
X
0
.
4
X
0
.
5
X
0
.
6
)
C
°
8
7
.
2
(
F
°
5
8
7
.
8
3
.
5
2
.
8
1
.
1
1
.
8
0
.
6
0
.
)
C
°
6
5
.
5
(
F
°
0
1
8
5
.
1
8
7
.
1
5
.
8
3
.
5
2
.
8
1
.
4
1
.
)
C
°
3
3
.
8
(
F
°
5
1
8
3
.
2
8
1
.
1
8
7
.
8
5
.
8
3
.
8
2
.
2
2
.
)
C
°
1
1
.
1
1
(
F
°
0
2
8
1
.
3
8
5
.
1
5
0
.
1
8
7
.
1
5
.
8
3
.
0
3
.
)
C
°
9
8
.
3
1
(
F
°
5
2
8
9
.
3
8
9
.
1
2
3
.
1
8
9
.
4
6
.
8
4
.
8
3
.
.
e
t
a
r
r
e
t
a
w
g
n
it
a
l
u
c
ri
c
e
h
t
f
o
%
2
0
.
0
f
o
tf
ir
d
n
o
d
e
s
a
b
e
r
a
s
r
e
il
p
it
l
u
M
EXAMPLE:
7000 GPM circulating rate, 15° cooling range. To maintain 4
concentrations, the required blowdown is .38% or .0038 times 7000 GPM
which is 26.6 GPM.
If tower is operated at 4 concentrations, circulating water will contain four
times as much dissolved solid as the make-up water, providing none of the
solids form scale or are otherwise removed from the system.
CHEMICAL TREATMENT
–In some cases chemical treatment of the cir-
culating water is not required if adequate blowdown is maintained. In most
cases, however, chemical treatment is required to prevent scale formation and
corrosion. Sulfuric acid or one of the polyphosphates is most generally used
to control calcium carbonate scale. Various proprietary materials containing
chromates, phosphates or other compounds are available for corrosion control.
When water treatment chemicals are required, the services of reliable water
treating companies should be obtained.
Slime, a gelatinous organic growth, and algae, a green moss, may grow in the
cooling tower or heat exchangers. Their presence can interfere with cooling
efficiencies. Proprietary compounds are available from water treating compa-
nies for the control of slime and/or algae; however, compounds which contain
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