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Carryover and Indoor Air Quality Part II

In the last issue, we discussed the causes of carryover of water from the air handler and other ‘wet’ system components into the air stream. The problems (especially increased microbial growth) that are caused by excess moisture in the air stream were discussed. As we see more proof of the major role that microbial growth plays in creating indoor air problems, it becomes essential that we do everything we can to minimize growth. This especially includes the elimination of excess moisture wherever it exists. This issue, we will look in depth at some of the causes of carryover and what we can do to minimize it.

As we learned last time, coil design is important in minimizing carryover. This is a good reason to select equipment manufactured by quality companies. Often, the realities of some system needs do not permit design that will eliminate carryover. Very large coils, air washers, and humidifying systems all represent such vast sources of moisture that some carryover is likely. For this reason, metal baffles or mist eliminators are often incorporated into their design to catch the water droplets and help them drop out of the air stream. They also create turbulence that helps the water droplets flash into water vapor, which is more likely to be carried out of the system and not deposit on the walls of the plenum or ducts. Never remove mist eliminators or make any change that would interfere with their proper operation.

Unfortunately, the turbulence generated by mist eliminators can, under some conditions, cause water vapor to condense into droplets and actually lead to a wetting problem. It is extremely important to keep the mist eliminators and other like obstructions in the air path clean and well maintained. This way, water droplets that do form on these surfaces will have less of a chance of supporting microbial growth, as food sources will not be abundant.

Air washers and humidifiers carry their own set of risks. Because they utilize water they have the potential for significant moisture contamination. It is important that they be well maintained, the temperature and airflow be maintained at the design levels, and their operation carefully monitored. Again, cleanliness is vital here.

Designers are well aware of the risks from humidifiers. In recent years there has been a trend toward the use of ultrasonic vaporizers or dry steam as a source of the water vapor needed to achieve an acceptable level of humidity in dry climates. Even with these devices, it is common for water droplets to be deposited on duct surfaces as far as fifteen feet from the humidification source. This is true even when the device is properly sized and in good working condition. The service technician should always carefully inspect the plenum or duct downstream from such moisture sources. Any indication of excessive moisture and especially evidence of microbial growth should lead to immediate action to prevent further contamination.

The more common challenge will be preventing moisture contamination from cooling coils. This is a basic task faced by service technicians every day. A good way to start is to learn from the manufacturer. In building the unit, extreme measures are taken to maintain the cleanliness of the unit. This is because dirt or contamination (especially on cooling coils) will lead to excessive carryover. Obviously then, system cleanliness is a key. The faster water runs off the coil surfaces, the less water is available to be re-released into the air stream. This is because anything that blocks the path of the water will serve as a mini dam and slow its progress. In addition, even a slight amount of contamination; will change the surface tension of the interface between the water film and the metallic surface. Increased surface tension will lead to greater retention of water on the surface.

When contamination buildup is great enough, The contamination will soak up water and serve as a ‘wick’, actually feeding water into the air stream. It is no accident that indoor air quality experts stress the importance of frequent cleaning of coils and other system components (especially fans and their housings). The cleaner the system, the less chance for carryover and its related problems.

Sometimes, however, cleaning, if done improperly, will result in carryover problems. When a coil is very dirty, a technician will often resort to using a high acid ‘condenser’ cleaning agent to clean an evaporator or chilled water coil. This carries a real possibility of causing excessive carryover. Acid cleaning agents rely on a chemical reaction between the acid and the metal surface for much of their cleaning ability. This reaction actually removes a portion of the surface, taking the dirt with it. This ‘etching’ action is not even so the surface of the coil is left with many microscopic pits. These pits trap water droplets and greatly slow the flow of water off the coil and fins. Much of this retained water will be carried into the air stream as it moves through the coil.

Unfortunately, many of the so-called ‘alkaline’ coil cleaners also have an etching action. The only difference is that the chemical activity is caustic rather than corrosive. Despite the difference in names, the result is still the same, a damaged surface and increased carryover. So, cleaning agent selection is important. Technicians often reject the more neutral compounds because they are not satisfied with their cleaning ability. Fortunately, recent advances in engineered surfactant technology have allowed the development of cleaners that are more pH balanced yet still highly effective in cleaning ability.

Another requirement for correct cleaning, is the complete removal of the debris that is loosened from surfaces. High water pressure can help here but must be used carefully to avoid damaging the coil and other system components. Of equal importance is a strong foaming action in the selected cleaning agent. Many users avoid foaming cleaners because the foam can be difficult to deal with in the limited space of an air handler. Here too, recent developments in cleaning technology are helpful. New techniques allow the formulation of a product that foams well to provide a mechanical action to remove and suspend debris yet that foam quickly recedes and can be easily washed away.

In the next issue we will review how to properly clean the air handler in a HVAC system. This is a subject that is considered so common sense that it is not often taught or written about. Yet, improper cleaning may be on of the major causes of both IAQ problems and equipment failure.

Bob Baker is a member of ASHRAE and Chairman and CEO of BBJ Environmental Solutions, Inc., a manufacturing company specializing in providing clean air through environmentally responsible products, such as BBJ MicroBiocide, BBJ Micro Coil Clean, BioSoft Spray Disinfectant/Cleaner, and Fresh Duct, as well as The Indoor Air First Aid Kit.