Water can be the real culprit in cooling system problems
By Steve Chase and Robert Schneider
TES Engineering
Cooling tower problems can present major threats to retail mall operations. When an engineering firm's site review reveals leaking, collapsing, or under-performing units, owners often ask about switching to air-cooled systems, but water-cooled systems have many comparative advantages.
Water-cooled systems on average use 25% less electrical energy. For a typical enclosed mall with an annual electric budget of $1.5 million, that saves $375,000 per year. Towers require just one-third of the rooftop heat rejection surface as air-cooled systems. Existing roof structures may not have adequate support for the heavier weight of air-cooled equipment and the electrical power requirements may need to be increased. And, depending on regional temperatures, water-cooled systems can provide the opportunity for free cooling in winter.
When these advantages are presented, the decision is usually to replace the mall's cooling towers, but to protect the owner's investment, it's important to find the real culprit to prevent another premature tower failure. It's also the time to build in operational efficiencies and reduced energy usage.
Water problems
Water, for all its benefits, also can cause problems such as corrosion and sediment or mineral build-up that damage equipment and prevent proper performance. System designs must consider water and sewer costs, maintenance costs, pumping and piping needs, the natural pH or hardness of the city water and possible changes in the water supply—for example, city water systems served by river water can experience shortages and changes in composition during droughts. Most cooling towers are open, exposing the water to contaminants and pollutants from outside air brought into the system. Effects can be chemical, such as increased alkalinity from exposure to lime from a nearby concrete plant, or microbial, since water supports growth of bacteria at 95˚F.
Because variations occur in pH, corrosion rates, evaporation that causes mineral build-up, and bacterial counts, active water treatment is needed. The answer has always been chemical treatment, but other methods are available as alternatives or adjuncts.
Chemical treatment
Traditionally, chemical water treatment systems have taken a broad-spectrum approach using either chlorine or bromine. The chemicals are an expensive maintenance item, so monitoring is important to insure they are being used properly and effectively. Biocides need to be rotated to keep bacteria from developing resistance. In addition, determining specific issues can allow the use of targeted chemicals, cutting overall cost—and can resolve major cooling system problems.
For example, a mall in El Paso, TX, sought help for old roof-mounted chilled water air handling units that were inefficient and leaked air and water. The engineering inspection also revealed that the cooling towers were white—instead of black—when viewed from a distance. Further investigation showed that the fill was mostly blocked with calcium deposits, which greatly reduced the amount of available air for evaporation. The mall was operating tower fans on full speed to cool the water, negating the value of the variable frequency drives (VFD) that had been installed to help reduce energy consumption. There were no back-up towers, so tower failure would have resulted in a loss of air conditioning for the property.
It turned out that the mall's water treatment system was not functioning and was hard to service; the previous vendor had been doing "drive-by" treatment and just sold chemicals instead of monitoring performance and troubleshooting. A new chemical water treatment system was designed to include proper injection of scale and corrosion inhibitor chemicals selected for the city water's calcium and pH. (The polymer chain of chemicals should be adjusted for each region's water.) The design for the replacement towers also included "sweeper" piping and filters to help keep the towers clean, along with maintenance service platforms to make access easier for cleaning crews.
Non-chemical treatment options
Some states and cities are banning the discharge of chemically treated tower water into sewer systems, which means that make-up water for cooling towers can no longer bleed off into the sanitary system. In addition, the push toward sustainability has the U.S. Environmental Protection Agency, the Department of Energy, and the American Society of Heating, Refrigeration and Air-conditioning Engineers looking at non-chemical alternatives for water treatment. Several technologies are beginning to move to the U.S. from Europe, with adoption highest on the coasts. These include:
- Filtration systems – Green sand, diatomaceous earth, and charcoal filters are used to remove impurities and minerals;
- Filters – Bag filters, side-stream filters, and sweeper piping are added at critical points in the cooling water system to remove debris, prevent mineral deposits, and eliminate or reduce blowdown;
- Magnets – Have performed well in closed-loop systems such as steam heating to remove minerals, but are not applicable to open systems like cooling towers;
- Electric pulse systems – While relatively expensive, they are being used effectively in small systems up to 150 tons by changing minerals into non-sticking powders (which also incorporate bacteria) that are easily filtered and removed during blowdown or that settle to the cooling tower basin for annual removal; and
- Water softening – Sodium chloride removes bacteria as effectively as chlorine or bromine with no odor or health and safety concerns (it is also used in swimming pool disinfectant systems). Care must be taken when softening, since it will create unstable ions that can cause corrosion.
It is important to correctly diagnose the problem and its location to choose the appropriate water treatment method or combined solutions. A mall in Boston, MA, had cooling towers that needed to be replaced due to massive corrosion and major water leaks after just nine years—less than half of their normal life. Investigation showed that the make-up water was from a city well for non-potable uses, which also fed the mall's toilets/urinals and irrigation system. Flush valves were a regular maintenance item, there were rust stains in the WCs, sidewalks were stained brown, and grass did not grow.
A third-party water consultant tested the well water and found it had a pH of 5.7, was full of manganese, and had a very high bacteria count. The mall's existing water softening system was inappropriate and had over-corrected the pH, so the resulting alkaline water degraded the towers by attacking the protective galvanizing.
After evaluating several water treatment options, a comprehensive water filtration system was developed that included green sand filters—that attract manganese—and potassium permanganate injections to control the pH. The old system added chemicals before the blowdown solenoid, which wasted around a third of the chemicals and rendered the system ineffective. The two 5,000–gallon storage tanks that held the non-potable water were cleaned and the softener system was eliminated for the make-up water. Piping changes included sweeper piping in the tower basins to keep debris from outside air from collecting inside the towers, and a non-loss side stream water cyclone filter and bag filter were added to collect solids from the tower water. This greatly reduced the chiller tube fouling, increasing chiller efficiency. The filtered water was returned to the tower. A water treatment company performed weekly testing with quarterly reviews by the water consultant, and on-site maintenance staff was trained.
The new cooling towers were designed as all stainless steel for extra corrosion insurance and used variable frequency fan drives (VFD) for improved efficiency. The integrated solutions produced annualized savings as follows:
| Reduced water & sewer charges |
$32,935 |
| Chemical savings |
1,000 |
| VFD energy savings |
2,000 |
| Chiller operation (side stream filtration) |
30,300 |
| Elimination: mechanical seals replacement |
1,000 |
|
|
|
| Total Annual Savings |
$67,235 |
In addition, solving the water issues allowed the mall to use the free cooling system from the building's original construction. Sediment, which had collected in the plate and frame, was flushed and the heat exchanger was put back into service.
Value of maintenance
Maintenance of water-cooled systems is an important management operational responsibility. Water systems should be designed to be maintainable, with all elements easily accessible.
Often, inoperable free cooling systems can be restored to efficient use by cleaning. Another Boston mall was planning to replace 25-year-old cooling towers, and noted that tenants on the upper floors always complained that it was too warm. The tower pump had just been rebuilt, and the mall was repairing one compressor per year on tenant area air-conditioning, with costs ranging from $9,000 to $19,000 depending on the tonnage. Inspection of the system showed that compressors in the water-cooled air conditioning units were operating, even though the tower water was cold enough for free cooling. None of the units could be maintained because there were no access doors to clean the free cooling coils or the D/X coils.
his partially cleaned CACU coil shows the degree of clogging that had been present,
rendering the free cooling system inoperable.
The main solution was to disassemble the air conditioning units, remove the coils, and clean them. Sheet metal spacers with access doors on both sides were added for future cleaning of all coils. The design changed the free cooling control valve on the air conditioning units to electronic two-way and added a refrigeration water regulating valve, which also enabled the addition of a VFD to the building pump for horsepower savings. It also added a tower bypass line to help regulate the water temperature during the winter, as well as ladder access to the towers for easier cleaning. The amount of air delivered to the tenants increased dramatically, which reduced the compressor run times and the fan horsepower. The free cooling coil increased comfort during the winter—at a fraction of the cost of running 60 HP compressors. Energy usage has dropped, and compressors will potentially have longer life.
Conclusion
Digging deeper to correctly identify problems and integrating water treatment, cleaning, and HVAC design can resolve underlying issues in water-cooled systems to insure optimum long-term performance. This approach also makes for happier tenants and saves energy.