17
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
ensureefcientsystemoperationwhilemaximizingtheequipment’sservicelife.Aqualiedwater
treatmentcompanyshoulddesignasitespecicwatertreatmentprotocolbasedonequipment
(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.
EVAPCOrecommendsanautomatedconductivitycontrollertomaximizethewaterefciencyof
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 rate of bleed 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
servicelifeofgalvanizedequipment.EVAPCOrecommendsthatthesitespecicwatertreatment
protocol includes a passivation procedure which details water chemistry, any necessary chemical
addition,andvisualinspectionsduringtherstsix(6)totwelve(12)weeksofoperation.Duringthis
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.
For more information on passivation and white rust, please request a copy of EVAPCO’s
Engineering Bulletin 36.
To Next Page
Loading...
Need help?
General
