Water Treatment And Water Chemistry; Bleed Or Blowdown; Galvanized Steel - Passivation - EVAPCO ESW Series Operation And Maintenance Instruction

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 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.
Bleedrate (GPM) =
*Cycles of Concentration is the ratio of the concentration of dissolved icons in the recirculating water, divided by the concentration of dissolved ions in the makeup water.
Galvanized Steel – Passivation
'White Rust' is a form of premature corrosion 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 CaCO3) 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 download a copy of EVAPCO's Engineering Bulletin 36A at evapco.com.
Evaporation Rate (GPM)
(Cycles of Concentration - 1)
21