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By Shankar Sapaliga
Deputy General Manager
AC&R BG, Voltas Ltd., Mumbai
Shankar Sapaliga is a mechanical engineer with 36 years experience in the air conditioning industry. He has been working with Voltas Ltd. for the past 27 years and is a member of ISHRAE.
More technological changes have occurred in the last 100 years than in the previous 1000 years. Modern automobiles, aircraft, communications, industry, power plants and air conditioning have changed the way most of mankind lives today. But all this progress has also polluted our natural resources, most notably the air around us. While the outdoor air in our major cities is now being cleaned by government intervention in the quality of fuel consumed by automobiles and their engine exhaust, as well as by industry being compelled to move out of city limits, indoor air within our offices, hotels, hospitals, theatres and shopping malls can only be cleaned by the building maintenance staff.
IAQ became one of the leading environmental issues in the USA during the early 90s. In India the awareness came many years later and today in our major cities it is fairly common to come across persons suffering from allergic bronchitis (asthma), allergic rhinitis (stuffy nose) and other breathing problems caused both by the polluted outdoor air as well as the air inside our carpeted homes and sealed offices with central air conditioning, over which the individual has no control. Newspaper articles on the subject and reports by doctors in government as well as private hospitals about the increasing incidences of pollution-caused allergies now appear frequently.
Studies abroad have shown that 70% or more of all IAQ problems involve the HVAC system and that most are due to inadequate care and maintenance of the system. The air ducts, which are the "lungs" of a building can accumulate deposits of construction dirt, dust, cigarette tar, smoke, insects and other air borne pollutants. Dirty air ducts can also become an ideal breeding ground for mold spores, mildew, pollen, bacterial colonies and other health - threatening microorganisms. The air that occupants of such buildings constantly breathe is exposed to all this dirt inside the duct as well as growths of fungus and mold inside the coils and drain pans of the air handling units.
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Most ductwork is concealed above false ceilings and the term "out of sight, out of mind" applies very well to these ducts. A few years ago, a well respected hospital in Mumbai decided to revamp their air conditioning system for a suite of operation theatres. False ceilings were torn down, ductwork dismantled but before they were disposed of, the consulting engineer for the new AC system decided to look inside these ducts and was horrified to see all the dirt stuck to the walls of the ducts. He had all the dirt removed and weighed and found that there was 17kgs of it, including construction debris and greencoloured lint, which could only have come from the green cotton garments worn by the team of surgeons and their assistants inside the operation theatres.
Foreign hotel chains such as Marriott and Hyatt and fast food restaurants such as McDonald have introduced their standard maintenance routines followed in the USA for years, into their Indian operations with duct cleaning as a normal procedure.
Such news travels fast among the engineer's fraternity who handle maintenance in large centrally air conditioned buildings and today there is a greater realisation among maintenance engineers that cleaning ducts is serious work and must be done.
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About five to six years ago a few companies in India imported duct cleaning equipment from the USA and added this facility to their Service Department's activities as a diversification measure. The American and Canadian embassies in New Delhi also imported such machines and pretty soon we have today about half a dozen companies offering their cleaning expertise to clients in all the major cities of India. Still a very small number by American standards where there are hundreds of companies doing such work. Of course the market there is vast since most homes install all-air ducted systems for heating and cooling in addition to the commercial, office and institutional establishments.
The only acceptable method today is "source removal" which involves the physical removal of contaminants from inside the ducts. To achieve proper cleaning, the duct work must be put under a vacuum, after which the contaminants are dislodged from the inner walls of the duct work using high pressure compressed air. See Figure 1.
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The portable power vacuum unit, see Figure 2, is the single most important piece of equipment required. It should be highly portable, lightweight and have a small "footprint" so that it can be easily loaded and unloaded from transport vehicles, moved up or down narrow stairways and fit into small places. The blower inside the unit should be capable of delivering 1800 to 2500 cfm for light commercial work and 6000 cfm or more for large commercial projects.
The static head, which is a measure of the vacuum unit's ability to overcome the internal static resistance from filters and external static resistance of the ductwork should be a minimum of 100 to 150 mm. This should enable the unit to maintain adequate airflow even with dirty filters. The best blower type for this application is a backward-curved aerofoil followed by backward - inclined or radial fans. Forward - curved blowers are not well suited for the high level of static resistance encountered in duct cleaning.
Vacuum units should have three progressive filtration stages to ensure that the air is almost perfectly clean before discharge into the air conditioned space. The firststage coarse prefilter should be cleanable and reusable. The second-stage intermediate filter should be 65 to 85% efficiency to capture most of the remaining visible contaminants. The final-stage HEPA filter will have a minimum efficiency of 99.97 percent at 0.3 microns.
Let's walk through the steps of typical duct cleaning from set up to clean up. Always remember, safety first. Turn the thermostat to the off position, and shut off the electrical power to the air handling unit and any other electrical installations such as duct heaters, humidifiers etc. This is your protection while cleaning the blower wheel and housing in the air handler.
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During the duct cleaning procedure, the most important concern should be the safety and well being of the facility's occupants, the facility itself and the workers performing the cleaning service. Considering that one of the most commonly cited reasons for having air conditioning systems cleaned is to safeguard the health and safety of building occupants, both the cleaning contractor and the client have the responsibility to ensure that the cleaning process does not expose occupants to new hazards.
The kitchen is the main source of fire in a restaurant, hotel or canteen and the exhaust hood with connected exhaust ducts is where the fire commonly starts because both the hood and duct are covered with oil from the cooking process. Grease filters provided at the intake of the hood do not capture all the oil and a substantial part of it gets past and must be cleaned very regularly from inside the duct hood and fans. A different cleaning process is required for such ducts.
Laundry exhaust ducts are also prone to fire because of the cotton fluff and dust which accumulate inside the ducts over a period of time. Regular duct cleaning in such areas is a necessity to prevent fires and damage to clothes as well as equipment.
In February 1989, a group of air duct cleaning professionals in the USA joined together to help educate the public about the need for properly performed HVAC system cleaning services. They formed the National Air Duct Cleaners Association (NADCA) and made it their goal to promote Source Removal methods of duct cleaning. They have since developed duct cleaning "standards" as well as training material for new entrants in this growing service field.
In India, duct cleaning is in its infancy right now but is slowly being recognised as an essential maintenance tool wherever air distribution ducts are installed. The cleaning equipment itself is fairly simple to manufacture and it's merely a matter of time before some entrepreneur sets up shop to produce this locally thus cutting down the high cost of imported equipment.
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