by James Piper
Well-designed - and maintained water treatment programs offer multiple benefits to facility executives
In their search for ways to improve the operating efficiency of their building's HVAC systems, facility executives are focusing attention on an important, unglamorous and often overlooked maintenance task: water treatment. To take full advantage of the many recent improvements in the design of energy-using systems, the heat transfer surfaces within the systems must be kept clean and free of scale. And the best method of accomplishing this is the comprehensive water treatment program. But helping to keep systems operating at peak efficiency is only one of the benefits of water treatment programs. Effective water treatment can enhance operating safety, protecting both personnel and equipment. Systems that have been operated with a well-designed water treatment program also have longer service lives than those where the quality of water being circulated has been ignored.
But when it comes to water treatment, one size does not fit all. Programs must be tailored to a specific application, including the type of system, the types of metals found in that system, the quality of the makeup water being introduced into the system and the rate at which makeup water is being added.
Water treatment programs also are not one-time efforts, which once initiated can be forgotten. Water treatment programs require continuous monitoring. Samples must be taken on a regular basis and analyzed to determine the specific contaminants that are present in the water system and their concentrations. For chemical-based systems, the application of chemicals to the system must be monitored to ensure that only the needed level of chemicals is added to the system. Too high a concentration level can be as hard on the system as one that is too low.
There are three separate building systems that can benefit from water treatment programs: boilers, HVAC closed loop water systems and cooling towers.
Boiler Water Treatment Programs
There are four basic steps in boiler water treatment programs: clarification, demineralization and softening, deaeration, and the addition of amines.
All circulating steam systems contain suspended solids. The solids can be introduced into the system by boiler makeup water, or they can come from piping and other components within the system. If these solids are allowed to remain in the system, they can interfere with the operation of steam traps and valves, or they can accumulate sufficiently to block portions of the system. Clarification, the first step in boiler water treatment, is designed to remove the larger suspended solids in the system. In most cases, removal can be achieved through the use of properly placed filters. To assist in removal of smaller suspended solids, chemical coagulants are added to the system to help clump these particles together so they can be captured by the filters.
The second step is demineralization and softening. All water contains certain levels of dissolved minerals or salts, such as silica, iron, calcium and magnesium, which tend to form scale on heat transfer surfaces. Scale decreases the energy efficiency of the system. Scale on boiler tubes can also result in the formation of localized hot spots that can lead to tube failure.
The two most common processes for demineralization and softening are lime-soda softening and ion-exchange. The water treatment system based on the lime-soda process uses a form of hydrated lime to react with the calcium and magnesium bicarbonates in the boiler feedwater to form precipitates. These precipitates then can be removed by settlement or by filtration. The ionexchange process removes a number of ionized impurities, including calcium, magnesium, iron and manganese. The third step in boiler water treatment, deaeration, involves removal of dissolved gases from the boiler feedwater, including oxygen, carbon dioxide and hydrogen. These dissolved gases are highly corrosive to most components, including boiler tubes, steam piping, steam traps and especially condensate return systems. The most common deaeration systems use steam to heat the boiler's feedwater, causing dissolved gases to be carried off with vented steam.
The addition of amines to the boiler water is a treatment method designed to reduce corrosion within the condensate return system. All steam systems, including those with good water treatment programs, have oxygen and carbon dioxide in the condensate return system. As a result, corrosion will continue to take place. To help protect condensate return system components from this corrosion, afilming amine is added to the boiler feedwater. Carried throughout the system by the steam from the boiler, these amines form athin, protective layer on metal surfaces primarily in the condensate system, helping to reduce contact between the corrosive condensate and condensate system components.
Closed Loop Water Systems
Corrosion is an electro chemical process that erodes metal surfaces in the closed loop system, including piping and chiller tubes. The rate at which corrosion takes place is primarily a result of the level of oxygen in the system, the degree of alkalinity or acidity of the circulating water, the temperature and velocity of the circulating water, and the concentration of dissolved and suspended solids in the water.
There are two types of corrosion: generalized and localized corrosion. The former attacks all metal surfaces in the system exposed to the water. Although generalized corrosion releases oxides that contribute to scaling, it is a relatively slow process and less of a concern than localized corrosion.
Localized corrosion attacks small areas on metal surfaces, resulting in rapid pitting and perforation. With the average chiller tube being less than 3/32 of an inch thick, localized corrosion can quickly eat through the material, resulting in the mixing of refrigerant and water. Controlling corrosion requires knowing what types of metals are used in the system, how susceptible they are to corrosion and what the operating conditions are in the system. Chemicals introduced to control corrosion must be carefully matched to the system requirements in order to avoid doing even more damage to the system. Once in place, the system must be continuously monitored through water sampling to ensure the proper mix is being used.
Cooling Towers
As with both boiler and closed loop systems, the water that is circulated in the system contains dissolved minerals that if left untreated will form scale on heat transfer surfaces, decreasing efficiency. What's more, cooling towers are open systems with a significant makeup water requirement. Evaporation, drift and the need to bleed off a portion of the circulating water result in an ongoing need for makeup water. For example, a typical cooling tower serving a 350-ton building chiller will lose three to five gallons of water per minute from evaporation and drift. This makeup water becomes an ongoing source for new dissolved minerals in the system.
Compounding the problems is the fact that few of the minerals present in the system are carried off by the evaporating water, resulting in a steady increase in the concentration of dissolved minerals in the circulating water.
While bleeding off a portion of the circulating water will help to limit their concentration, a water treatment program is required to keep their levels within acceptable limits.
There are three primary goals to cooling tower water treatment programs: controlling scale formation, reducing corrosion and minimizing the growth of microscopic organisms. In areas having hard water, scale control is the driving concern, while corrosion protection is the major concern in areas with soft water. Water treatment programs must evaluate the conditions that exist at the site to determine the type and quantity of chemicals that are required to bring water quality within the acceptable range. Systems typically continuously monitor water conditions in the condenser water system, adding the proper quality of chemicals required to control both scale and corrosion.
The most common method of controlling the growth of microscopic organisms is to add a biocide to the circulating water. Biocides can be either oxidizing or nonoxidizing depending on the particular needs of the system. They are added to the system typically through a small pump on a timer that introduces a set amount of the biocide on a regular schedule. Monitoring of the levels of growth in the system is required to ensure that the proper dose and schedule are being used.
Non-chemical Treatment
There are also alternatives to traditional chemical biocides. One is the use of ultraviolet light to kill organisms.
The systems operate continuously, eliminating under dosing and over dosing problems found in biocide-based systems. With no moving parts, the systems are practically maintenance-free. With no chemical requirements, ultraviolet-based systems have very low operating costs. Ultraviolet-light-based systems, when combined with a filtration system at 40 to 50 microns, can provide performance that matches or exceeds that of biocide-based systems. However, UV systems leave no residual biocides in the water.
Another option is the use of copper-silver ionization. These systems use electrodes to generate parts per billion levels of copper and silver in the cooling tower water to kill micro biological life. They operate continuously and leave a residual in the water. At least one company says that it offers a non-chemical program for all aspects of cooling tower water treatment. That program combines copper-silver ionization, filtration and electronic scale control technology.
Implementation
Facilities have the option of implementing the program in-house or contracting all orpart of the program out to firms
that specialize in water treatment. If in-house personnel are used,they must be fully trained in all of the procedures
involved in the water treatment program, including how to safely handle the chemicals involved. If the program is
contracted out, it is important that a qualified contractor be selected, one that is experienced in working with
systems similar to the ones in the facility. Regardless of the method of implementation, the result will be better
performing and longer lasting systems.
Above article appeared previously in the December 2001 issue of Building Operations Management.
The benefits of water treatment in boiler systems extend beyond the boilers themselves to include both the steam distribution and condensate return systems. Throughout the entire steam system, water treatment works to maintain operating efficiency and extend the service life of practically all components.
There are two major concerns with closed loop heating and cooling systems: scale and corrosion. Scale, like scale in boiler systems, is the result of precipitation of salts found in the water. These salts tend to adhere to heat transfer surfaces, are difficult to remove, and reduce the efficiency of the heat exchanger. Fouling factor is a measure of the resistance to heat transfer often used in evaluating chiller performance. In new chillers, the fouling factor is typically 0.0002 or lower. Even a thin layer of scale on the tubes can raise a chiller's fouling factor to 0.0025, resulting in an increase in chiller condensing temperatures by five degrees and an increase in overall compressor energy use by 20 to 25 percent. In most systems, scale is controlled by adding chemicals to reduce the calcium hardness of the water.
Cooling towers are natural dirt collectors. Leaves, dust, dirt and other contaminants easily enter condenser water systems through the cooling tower. Untreated, these contaminants accumulate, blocking piping and reducing the effectiveness of heat transfer surfaces. Equally important, the warm waters found in cooling towers and condensing water systems are perfect breeding grounds for micro-organisms such as legionellosis that, in addition to potentially blocking the chiller's heat exchangers, can pose a serious health risk to those who work close to or are exposed to wind-borne water droplets from the cooling tower. A comprehensive water treatment program is the most effective means of controlling both particulate and biological contaminants.
Although most water treatment programs are chemical based, there are alternatives. For example, electronic technology can be used for scale control and can play a role in controlling corrosion.
Water treatment programs will require installation of specialized equipment, generally including chemical feeders, monitoring sensors and sampling ports. Once installed, equipment operation must be monitored. Water samples must be taken and analyzed, typically weekly. And adjustments will have to be made in the program to match changing water conditions.
First published January 2002
