Cooling System Questions

One of the commonly encountered problems with galvanized coatings of all kinds is “white rust” or “white storage stain”. It is manifested as a bulky, white, powdery deposit that forms rapidly on the surface of the galvanized coating under certain specific conditions.

White rust can cause considerable damage to the coating and is always detrimental to the galvanized coating’s appearance. The surface of galvanized coatings is almost 100% zinc. It is the durability of the zinc that provides the outstanding anti-corrosion performance for steel, yet zinc is a relatively Îreactiveâ metal. It is the stable oxides that form on the zinc’s surface that determine its durability, and these oxides are formed progressively as the zinc is exposed to the atmosphere. Carbon dioxide in particular is a contributor to the formation of these stable oxides.

With newly galvanized steelwork, the zinc’s surface has been subjected to little oxidation and is at its most vulnerable. For this reason, Industrial Galvanizers uses a chromate passivation in conjunction with its galvanizing operations to provide protection to the galvanized coating during the Îyouthâ period of the coating. This passivation coating provides short term protection to the zinc to give the stable oxides time to form on the surface.

White Rust Formation

Pure water (H2O) contains no dissolved salts or minerals and zinc will react quickly with pure water to form zinc hydroxide, a bulky white and relatively unstable oxide of zinc. Where freshly galvanized steel is exposed to pure water (rain, dew or condensation), in an oxygen deficient environment, the water will continue to react with the zinc and progressively consume the coating. The most common condition in which white rust occurs is with galvanized products that are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods.

Avoiding White Rust Formation

There are a number of simple steps that can greatly reduce or eliminate the formation of white rust. These are:

1. Keep the packed work dry
2. Pack the items to permit air circulation between the surfaces
3. Stack the packed items to allow water to drain out
4. Treat the surface with proprietary water repellent or barrier coatings to prevent moisture contact with galvanized surface

Treating Galvanized Surfaces Affected by White Rust

Once the galvanized surface has been attacked and the zinc hydroxide compounds have formed, it is desirable to remove the oxide products from the surface because:

• their presence inhibits the formation of stable carbonate based oxides and
• they are unsightly

The effect on the galvanized coating can range from very minor to extremely severe and various levels of remedial treatment are available to deal with white rust problems at the various levels at which they are likely to occur.

The following treatments are recommended to deal with white rust on galvanized products:

Light white rusting such as this may appear on newly galvanized work after periods of heavy rain. Because these trailer parts are well ventilated and well drained, the white rusting will be limited and is easily removed.

1. Light white rusting
   This is characterized by the formation of a light film of white powdery residue and frequently occurs on galvanized products during periods of heavy rain. It is particularly evident on areas that have been buffed or filed during quality assurance operations. These treatments remove the passivated surface from the galvanizing and expose unoxidized zinc to attack from rainwater. Provided the items are well ventilated and well drained, white rust rarely progresses past this superficial stage. It can be brushed off if required but will generally wash off in service with normal weather. No remedial treatment is generally required for this level.
2. Moderate white rusting
   This is characterized by a noticeable darkening and apparent etching of the galvanized coating under the affected area, with the white rust formation appearing bulky. The galvanized coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, less than 5% of the galvanized coating will have been removed and thus no remedial work should be required as long as the appearance of the affected area is not detrimental to the use of the product and the zinc hydroxide residues are removed by wire brushing. If appearance is unacceptable, the white rust affectedarea can be treated as follows:
   • Wire brush the affected area to remove all white corrosion products
   • Using a cloth pad wet with aluminum paint, rub the surface with the pad to apply a thin film of aluminum paint to the affected area to blend it with the adjacent unaffected galvanized surfaces
3. Severe white rusting
   This is characterized by very heavy oxide deposits. Items may be stuck together. Areas under the oxidized area may be almost black of show signs of red rust. A coating thickness check will determine the extent to which the galvanized coating has been damaged. Remedial treatment to reinstate the coating should be undertaken as follows:
   • Wire brush or buff the affected area to remove all oxidation products and rust if any
   • Apply one or two coats of approved epoxy zinc-rich paint to achieve required dry film thickness of 100 microns minimum
4. Re-Passivating the Galvanized Surface
   Where white rusting has occurred and the item may be subject to continuing exposure that may propagate similar corrosion, re-passivating of the surface can be done by treating the surface with a solution of 5% sodium dichromate 0.1% sulfuric acid, brushing with a stiff wire brush for 30 seconds before thorough rinsing of the surface.

Conclusion

White rust is a post-galvanizing phenomenon. Responsibility for its prevention lies in the manner it is packed, handled and stored prior to the galvanized productâs installation and use. The presence of white rust is not a reflection on the galvanized coating’s performance, but rather the responsibility of all those involved in the supply chain to ensure that the causes of white rust are recognized and the risks of its occurrence minimized on newly galvanized steel.

Verify the system has proper flow across the conductivity probe. Next, inspect the probe for scale buildup and clean if necessary.  Then recalibrate the controller after confirming the current conductivity reading using a handheld meter.

In light commercial to industrial applications may rely on cooling towers to remove waste heat. While there are many different types of cooling towers and evaporative condensers used with hvac systems the basic methods of operation and maintenance are fairly common to all designs.

Because of the inherently rugged construction of modern cooling towers, proper maintenance is often overlooked until major  cooling system problems develop. Such problems are avoidable if you understand the basic principles of tower operation, and perform periodic inspection and maintenance of the cooling tower system.

It also helps to understand that a cooling tower is a collection of systems that work together. Here’s an overview of how these systems operate:

Hot water from the condenser is delivered to the top of the cooling tower by the condenser pump through distribution piping. The hot water is sprayed through nozzles onto the heat transfer media (fill) inside the cooling tower. Some towers feed the nozzles through pressurized piping, others use a water distribution basin and feed the nozzles through gravity.

A cold water collection basin at the base of the tower gathers cool water after it has passed through the heat transfer media. The cool water is pumped back to the condenser to complete the cooling water loop. Cooling towers use evaporation to release waste heat from an hvac system. Hot water flowing from the condenser is slowed down and spread out in the heat transfer media (fill). A portion of the hot water is evaporated in the fill area, which cools the bulk water. Cooling tower fill is typically arranged in packs of thin corrugated plastic sheets or, alternately, as splash bars supported in a grid pattern.

Large volumes of air flowing through the heat transfer media help increase the rate of evaporation and cooling capacity of the tower. This airflow is generated by fans powered by electric motors. The cooling tower fan size and airflow rate are selected for the desired cooling at the design conditions of hot water, cold water, water flow rate and wet bulb air temperature.

Hvac cooling tower fans may be propeller type or squirrel cage blowers depending on the tower design. Small fans may be connected directly to the driving motor, but most designs require an intermediate speed reduction provided by a power belt or reduction gears. The fan and drive system operates in conjunction with a starter and control unit that provides start/stop and speed control.

As cooling air moves through the fill, small droplets of cooling water become entrained and can exit the cooling tower as carry-over or drift. Devices called drift eliminators are used to remove carry-over water droplets. Cooling tower drift becomes an annoyance when the droplets fall on people and surfaces downwind from the cooling tower. Efficient drift eliminators remove virtually all of the entrained cooling water droplets from the air stream.

Water quality counts

The water quality system insures the cooling water remains free of contamination and buildup of dissolved solids. The most basic system includes a makeup water line to replace the water evaporated during cooling. A water bleed-off or blowdown line, which removes excess dissolved solids from the bulk cooling water, is also required.

Warm water in the cooling system is a natural habitat for microorganisms. Chemical treatment is required to eliminate this biological growth. Several acceptable biocides are available from water treatment companies for this purpose. It may also be advisable to periodically add additional water treatment chemicals to the cooling tower to control corrosion, pH (acidity) and hardness. Water treatment companies will recommend the best treatment program for your cooling system.

Cooling towers naturally wash particulate matter from the air that flows through the tower. This solid material can accumulate and deposit in the fill and distribution systems. Removal of suspended and entrained solids can be accomplished through filtration and periodic manual cleaning. Various types of filtering systems are effective for keeping the cooling water clean and reducing the amount of manual cleaning.

Although it may seem obvious, the cooling tower must include a means to support and contain the cooling water and all the systems described above. The cooling tower structure and casing may be manufactured from various materials like steel, wood, fiberglass or concrete. The material of choice must resist the effects of water and chemical corrosion, and environmental influences from sun, wind, snow and ice. The design and material selection of the cooling tower structure significantly affects the service life and cost of the equipment.

Successful cooling tower operation requires periodic attention and preventive maintenance to each of the major components. All operation and maintenance activities must provide for worker safety. Some safety aspects to consider include:

• Safe access around the cooling tower or evaporative condenser, including all points where inspection and maintenance activities occur
• Fall protection around inspection and maintenance surfaces, such as the top of the cooling tower
• Lockout of fan motor and circulating pumps during inspection and maintenance
• Protection of workers from exposure to biological and chemical hazards within the cooling water system
• Cooling tower location must prevent cooling tower discharge air from entering the fresh air intake ducts of
  any building

The following general operating tips apply to all types of cooling towers. Each manufacturer will have procedures that apply to a particular tower design:

1. When starting a new tower, inspect and remove any accumulated debris. Fill the cooling system with water. Set the makeup water valve during this filling process to the recommended operating water level in the collection basin. Continue filling until the static water depth in the cold water collection basin is at the overflow level. Vent trapped air pockets where possible. Start the circulating pump, adding water as needed to maintain positive pump suction.
2. Balance waterflow following the tower manufacturer’s procedure to insure even distribution of hot water to all areas of the fill. Poorly distributed water can lead to air bypass through the fill and loss of tower performance.
3. Follow your water treating company’s recommendations regarding chemical addition during startup and continued operation of the cooling system. Galvanized steel cooling towers require special passivation procedures during the first weeks of operation to prevent “white rust.”
4. Before starting the fan motor check the tightness and alignment of drive belts, tightness of mechanical hold-down bolts, oil level in gear reducer drive systems and alignment of couplings. Rotate the fan by hand, insure that blades clear all points of the fan shroud.
5. Bump the fan, checking for proper forward rotation. If the fan runs in reverse, switch the connection point of two of the three motor leads. Check the function of the motor control system at all forward speeds and reverse operation, if so equipped. Check for stable operation with no leakage of lubricants from mechanical seals and no excessive vibration in the tower structure.
6. When the cooling tower operates without heat from the hvac system, the fan motor may run with amperage above motor nameplate level. Motor amperage will reduce when hot water is circulated over the tower.
7. The motor control system is designed to start and stop the fan to maintain return cold water temperature.
8. The fan motor must start and stop no more frequently than four to five times per hour to prevent motor
   overheating.
9. Bleed water rate from the cooling tower should be adjusted to maintain between four to eight concentrations of dissolved solids. The table on the previous page shows the minimum bleed flow rate (as a percentage of cooling water flow rate) to maintain desired levels of concentration.

To keep equipment running trouble free, the following maintenance procedures should be conducted in the time frame indicated:

Monthly –

• Closely observe the operation of the cooling tower and look for changes in sound or vibration level. Any unusual sound  related to the rotating equipment is cause to shut down the fan until the problem is isolated and corrected.
• Check fan drive belts for tightness and wear. Check gear reducer and associated lube lines for leakage.
• Inspect air inlet louvers and clean accumulated debris. Check for proper operation of the makeup valve, full open to full close.
• Remove any debris that may have accumulated at the pump suction screen.
• Monitor the rate of silt buildup in the cold water collection basin. Make a record of the deposition rate.

Semiannually –

• Lubricate motor bearings and fan shaft bearings in accordance with the tower manufacturer’s recommendation.
• Check the gear reducer oil level with the fan motor off. Add oil, as required, to the full mark. Check the tightness of all bolts in the fan assembly and mechanical equipment support. Tighten fan shroud and fan guard bolts.
• Inspect drift eliminators inside the tower. Remove any debris or scale buildup. Inspect the cold water collection basin for silt accumulation.
• Drain and clean the basin if significant amounts of silt have accumulated.

Annually –

• Conduct a thorough inspection of the complete cooling tower. Clean debris from collection points in the tower.
• Check hot water distribution system for algae and silt buildup. Clean and flush with water and disinfect.
• Lubricate motor and fan shaft bearings. Change drive belt if excessive wear or cracking of belt is observed.
• Replace gear reducer lubricant. Follow manufacturer’s recommendations. Some synthetic hydrocarbon lubricants allow extended oil change intervals for up to five years.

As for general housekeeping tips, cooling towers must be thoroughly cleaned on a periodic basis to minimize bacterial growth. Unclean cooling towers promote growth of potentially infectious bacteria, including Legionella Pneumophila.

Health service officials recommend that you regularly inspect the cooling tower for dirt, scale and algae; conduct periodic flushing and cleaning of the cooling system; and maintain a complete water treatment program, including biological treatment. Flushing and cleaning is recommended twice per year. Louvers, drift eliminators and accessible fill surfaces should be washed by flushing with a moderate pressure water noble.

After refilling the cooling system with water and prior to operating the cooling tower fan, it is very important to perform biological treatment of the water. First, circulate water throughout the system with the condensing water pumps. Then, execute one of the following biocide treatments:

Treat the system with sodium hypochlorite to a level of 4 to 5 mg/l (ppm) free chlorine residual at a pH of 7.0 to 7.6 and hold the residual level for six hours. Resume the biocide treatment that had been in use prior to system shutdown for sufficient time to bring
the system under good biological control. Follow the recommendation of your water treatment company.

If the tower has been shut down for a period of time and not drained, perform one of the biological pre-treatments above within the water storage vessel (cooling tower sump, drain down tank, etc.) before operating the condenser-pumps. Then, circulate the treated water over the cooling tower with fans off. When the biological treatment has been satisfactorily maintained for six hours, the fans may be turned on and the system returned to service.

Cooling towers perform an important function within hvac systems. Evaporative cooling efficiently reduces operating expense when compared to air-cooled systems. A basic understanding of cooling tower operation and maintenance procedures will keep a cooling water system running trouble free, year after year.