11
Operation and Maintenance Instructions
Water Treatment and Water Chemistry
Proper water treatment is an essential part of the maintenance
required for evaporative cooling equipment. A well designed
and consistently implemented water treatment program will
help to ensure efficient system operation while maximizing the
equipment’s service life. A qualified water treatment company
should design a site specific water treatment protocol based
on equipment (including all metallurgies in the cooling system),
location, makeup water quality, and usage.
Bleed or Blowdown
Evaporative cooling equipment rejects heat by evaporating a
portion of the recirculated water into the atmosphere as warm,
saturated discharge air. As the pure water evaporates it leaves
behind the impurities found in the system’s makeup water and
any accumulated airborne contaminants. These impurities
and contaminants, which continue to recirculate in the system,
must be controlled to avoid excessive concentration which
can lead to corrosion, scale, or biological fouling.
Evaporative cooling equipment requires a bleed or blowdown
line, located on the discharge side of the recirculating pump,
to remove concentrated (cycled up) water from the system.
EVAPCO recommends an automated conductivity controller
to maximize the water efficiency of your system. Based on
recommendations from the water treatment company, the
conductivity controller should open and close a motorized
ball or solenoid valve to maintain the conductivity of the
recirculating water. If a manual valve is used to control the
bleed rate, it should be set to maintain the conductivity of the
recirculating water during periods of peak load at the maximum
level recommended by the water treatment company.
Galvanized Steel – Passivation
‘White Rust’ is a premature failure of the protective zinc layer
on hot dip or mill galvanized steel which can occur as a result
of improper water treatment control during the start-up of
new galvanized equipment. The initial commissioning and
passivation period is a critical time for maximizing the service
life of galvanized equipment. EVAPCO recommends that the
site specific water treatment protocol includes a passivation
procedure which details water chemistry, any necessary
chemical addition, and visual inspections during the first six
(6) to twelve (12) weeks of operation. During this passivation
period, recirculating water pH should be maintained above
7.0 and below 8.0 at all times. Since elevated temperatures
have a harmful effect on the passivation process, the new
galvanized equipment should be run without load for as much
of the passivation period as is practical.
The following water chemistry promotes the formation of white
rust and should be avoided during the passivation period:
1. pH values in the recirculating water greater than 8.3.
2. Calcium hardness (as CaCO
3
) less than 50 ppm in the
recirculating water.
3. Anions of chlorides or sulfates greater than 250 ppm
in the recirculating water.
4. Alkalinity greater than 300 ppm in the recirculating
water regardless of pH value.
Changes in water chemistry control may be considered after
the passivation process is complete as evidenced by the
galvanized surfaces taking on a dull gray color. Any changes
to the treatment program or control limits should be made
slowly, in stages while documenting the impact of the changes
on the passivated zinc surfaces.
• Operating galvanized evaporative cooling equipment
with a water pH below 6.0 for any period may cause
removal of the protective zinc coating.
• Operating galvanized evaporative cooling equipment
with a water pH above 9.0 for any period may destabilize
the passivated surface and create white rust.
• Re-passivation may be required at any time in the
service life of the equipment if an upset condition occurs
which destabilizes the passivated zinc surface.
Water Chemistry Parameters
The water treatment program designed for evaporative cooling
equipment must be compatible with the unit’s materials of
construction, as well as other equipment and piping used
in the system. Control of corrosion and scale will be very
difficult if the recirculating water chemistry is not consistently
maintained within the ranges noted in
Table 3
.
Property
Z-725
Type 304
Type 316
Galvanized Steel
Stainless Steel
Stainless Steel
pH
7.0 – 8.8
6.0 – 9.5
6.0 – 9.5
pH During Passivation
7.0 – 8.0
N/A
N/A
Total Suspended Solids (ppm)*
<25
<25
<25
Conductivity (Micro-Siemens/cm) ** <2,400
<4,000
<5,000
Alkalinity as CaCO
3
(ppm)
75 - 400
<600
<600
Calcium Hardness CaCO
3
(ppm)
50 - 500
<600
<600
Chlorides as Cl-(ppm) ***
<300
<500
<2,000
Silica (ppm)
<150
<150
<150
Total Bacteria (cfu/ml)
<10,000
<10,000
<10,000
* Based on standard EVAPAK
®
fill
** Based on clean metal surfaces. Accumulations of dirt, deposits, or sludge will increase corrosion potential
*** Based on maximum coil fluid temperatures below 49°C
Table 3
- Recommended Water Chemistry Guidelines
