March 1, 2012
By Robert F. Goodfellow, IAQCP
There are many advantages to having an effective indoor air quality (IAQ) strategy, which may not always include the obvious––breathing clean air. After all, most people now spend an average of 90 per cent of time indoors. (See the U.S. Environmental Protection Agency’s (EPA’s) “An Introduction to Indoor Air Quality (IAQ).” Visit www.epa.gov/iaq/voc.html).
One needs to look at the technologies that improve IAQ and address specific problems. From there, one can give some insight into the many less obvious potential advantages of a successful IAQ strategy.
Today’s predominant indoor air quality strategies are largely a holdover from 20 or 30 years ago when smoking was still prevalent indoors. Building construction was getting tighter following the oil embargos of the 1970s––which served to lock in pollutants––and increasing ventilation air was still seen as the best way to purge indoor contaminants. Today, inside air is cleaner without the smoke, but there is still a vast array of other pollutants present indoors, including those from:
Throughout the last century, HVAC filters were used primarily to protect HVAC equipment, not the building occupants. Larger dust particles created the biggest problems, so filters were developed to trap them. Submicron particles, because they did not adversely affect equipment performance, went largely ignored. Today, there are many IAQ products and solutions that allow designers the opportunity to customize an IAQ strategy based on occupant needs.
Outdoor air is the most common source of IAQ problems in a building. Proximity to industries, busy highways, or other sources of polluted air can allow the entrainment of particles and odours into air intake vents. With the banning of smoking and today’s modern sustainable building materials, outdoor air is typically more polluted than indoor air in any urban environment. Figure 1 gives several examples where outdoor air quality can affect IAQ.
Identifying contaminants of concern
There are three types of airborne contaminants: particles, biological, and gas phase.
The ambient air outdoors in a typical urban environment contains about 35 million particles (in the size range of 0.5 µ [about 0.02 mils] or larger in diameter) per cubic metre. By weight, the volume over 0.5 µ represents about 99 per cent. However, by particle count, the same particles represent only about eight per cent. This means the overwhelming majority of airborne particles are invisible to the naked eye.
Airborne contaminants that measure below 0.5 µ in size are potentially detrimental to health because they are absorbed through lungs into bodies. Low-efficiency filters cannot remove these contaminants from the air.
Conventional filtration technologies and mechanisms
Passive, mechanical filters are common, accounting for more than 90 per cent of all HVAC filters because of their low price and large number of users.
Usually consisting of a loose pad of coarse fibres, and sometimes coated with a light adhesive to help large particles stick, these filters screen out only large particles, and have virtually no effect on airborne materials that cause allergies or sickness. Passive filters work like a sieve. The sizes of particles collected depend on the density of the filter media. The trade-off for a denser, more efficient filter is pressure drop. Passive filters also include pleated filters, which use a finer mesh and catch smaller particles. Since the finer mesh also creates more resistance to airflow, the surface is ‘extended’ with pleats to offer more surface area for air to pass through. Pleated filters are usually 25 to 152 mm (1 to 6 in.) thick.
High-efficiency particulate air (HEPA) and ultra-low penetration air (ULPA) filters are also passive filters. (High-efficiency particulate air (HEPA) filters are rated 99.99 per cent efficient with particles 0.3 µ and larger in diameter, while ultra-low penetration air (ULPA) filters are rated 99.99 per cent efficient with particles 0.12 µ in diameter). They contain hundreds of square feet of filter paper folded into a couple of square feet of space. They remove very small particles, including some viruses. Since they are several inches thick, they require duct modifications. Bypass HEPA filters only clean a small percentage of air as it bypasses the primary HVAC system to avoid excessive pressure drops.
Passive electrostatic filters have polypropylene fibres that pick up a static charge as the air passes over them. The charge helps hold fine particles and gives it greater efficiency than comparable passive media. Designed primarily for residential use, these filters are washable and can be reused. Since the charge is passive, it can wear off over time as fibres become coated. Depending on the density of the fibres, passive electrostatic filters can also have a higher static pressure drop. The higher the pressure drop, the more energy it takes to push air through the system, and the greater the wear on the air-moving equipment.
Electronic air cleaners
There are many kinds of electronic air cleaners, the most common of which are electrostatic precipitators, popularized in the 1980s. They consist of ionizing wires that charge airborne matter, and charged collector plates. The particles are reflected by oppositely charged plates, and collected on grounded plates. Electrostatic precipitators are generally the most effective at removing submicron particles (e.g. mist and smoke), rather than larger ones (e.g. dust and grains). Electrostatic precipitators have a very low static resistance to airflow but require more frequent maintenance. Once ionizing wires and/or collector plates become coated, performance drops off. Most electronic air cleaners draw between 20 and 40 W. (See Frost & Sullivan’s “North American HVAC Air Filters Markets” from March 2005. Visit www.frost.com/prod/servlet/report-brochure.pag?id=A994-01-00-00-00#report-technologies).
Another type of electronic air cleaner is the active electrostatic or polarized-media air cleaner. This air cleaner does not create charged particles or ozone and draws between 2 and 6 W. Polarized-media air cleaners use mechanisms of both passive filters and electronic air cleaners in addition to agglomeration:
Carbon filters, usually configured in cells or trays, are used primarily for removing odours, vapours, and gases through adsorption. The material attracts and holds gases and vapours in its pores, thereby eliminating odours. Carbon filtration systems are more expensive than traditional filters.
Filter performance tests: what do they mean?
There are various tests used to measure filter performance. Unfortunately, some are designed for passive filters and some for air cleaners, and others only measure single-pass efficiency.
The most common benchmark for filter efficiency is the minimum efficiency reporting value (MERV), a testing protocol designed in 1987 as American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 52.2, Method of Testing General Ventilation Air-cleaning Devices for Removal Efficiency by Particle Size. The standard rates the effectiveness of clean, passive HVAC filters. MERV uses conductive dust in a single-pass test to measure passive filter performance and is not applicable to electronic air cleaners or air cleaners using multiple air exchanges.
Many building practices rely on MERV. Hospitals, for example, often require pre-filters of MERV 8, and MERV 13 or 14 final filtration. The Leadership in Energy and Environmental Design (LEED) system encourages sustainable green building and development practices through various performance criteria. One point is available for indoor environmental quality if the filter or air cleaner achieves a minimum of MERV 13 under Indoor Environmental Quality (EQ) Credit 5, Indoor Chemical and Pollutant Source Control. However, the program does not distinguish between filters or electronic air cleaners.
Clean air delivery rate (CADR) is a measurement resulting from testing procedures developed as American National Standards Institute/Association of Home Appliance Manufacturers (ANSI/AHAM) AC-1-1988, Portable Household Electric Cord-connected Room Air Cleaners. The test is performed in a 28.5-m3 (1006.5-cf) chamber with a circulating fan. It uses a logarithmic regression to compare particles’ removal rate by an air cleaner to the natural decay rate within the chamber over 10 or 20 minutes.
Thermal DOP efficiency
Thermal dioctyl phthalate (DOP) efficiency refers to another test specific to the very highest efficiency passive filters, including HEPA filters, using a heated aerosol mist of dioctyl phthalate.
The efficacy of different filters varies depending on particle size. Higher efficiency is usually better, but it can get complicated. Some filters may catch all the lint, but few catch the smaller particles. The filters that perform best across the entire range of particle sizes are HEPA filters and electronic air cleaners.
Other key performance comparisons
For high-efficiency filtration, there are other key indicators that are as important as a filter’s rating or its ability to remove particles and other airborne contaminants from the air. There are two that most impact an IAQ strategy.
In addition to the pressure impact this produces, dust-holding capacity is the best indicator of how long a filter will last. For instance, a 51-mm (2-in.), MERV 13, 610 x 610-mm (24 x 24-in.) filter holds approximately 40 g (1.3 oz.) of dust before reaching its dirty design static pressure of 36 mm (1.4 in.) water. Therefore, if the average building produces 130 g of dust per cubic metre each month, or 48 g per 610 x 610-mm filter, one would have to replace a 51-mm deep MERV 13 filter every two weeks and a 102-mm (4-in.) deep MERV 13 filter every month to keep the static below 36 mm (1.4 in.) water and maintain a mid-life static pressure of around 23 mm (0.9 in.) water, not including the impact from a pre-filter.
By comparison, some polarized-media electronic air cleaners can hold up to 1600 g (51.4 oz.) of dust at the recommended change state of 15 mm (0.6 in.) water, or twice its initial static pressure. An air cleaner that holds 10 times the dust of cartridge and bag filters and up to 100 times the dust of shallow-bed passive filters will greatly impact maintenance intervals and other ongoing costs of filtration.
Pressure drop (resistance to airflow)
The pressure drop is another important performance measurement. Any filter in the air stream increases resistance and reduces flow. This is an important consideration for designing a forced air distribution system. Electronic air cleaners offer relatively low resistance. With passive filters, the static pressure increases with efficiency.
Additionally, lower static pressure directly corresponds to lower brake horsepower. Since brake horsepower drives fan energy and so on, lower static pressure corresponds directly to energy savings.
In a recent amendment within ASHRAE 90.1-2007, Energy Standard for Buildings Except Low-rise Residential Buildings, the allowable brake horsepower for each type of system and space use has been clarified. Section 6.5, “HVAC Air System Design and Control,” sets allowances for brake horsepower based on system type and application. Often, these levels can be difficult to meet with traditional passive filtration.
The role of ventilation
Ventilation has traditionally been the most widely used means of addressing airborne contaminants. In commercial buildings, most codes rely on ventilation rates specified by ANSI/ASHRAE 62.1-2010, Ventilation for Acceptable Indoor Air Quality. Today’s standard has two methods for determining the necessary outdoor air levels: the ventilation rate procedure and the more recent IAQ procedure.
The ventilation rate procedure is the easier of the two to apply and most buildings are designed and operated under its guidelines. The ventilation standard specifies a certain number of cubic feet per minute per person for various building types. For example, in office spaces and schools, the HVAC system must bring in 22 to 27 m3/hr (13 to 16 cfm) per person of outdoor air, depending on occupant density. When the ventilation rates were adopted in 1989, two things were assumed—the only means of dealing with contaminants in the space is by dilution with outdoor air (i.e. there is no air cleaning), and the outdoor air level is sufficient to accommodate ‘moderate smoking’ (based on 30 per cent of the occupants smoking one cigarette per hour). Later, standards were amended to allow 25 m3/hr (15 cfm) per person in non-smoking offices. Today, the ventilation standard also requires outdoor air quality meet certain criteria. In practice, however, this is something not often determined or monitored.
The IAQ procedure allows for greater fine-tuning and more efficient operation of the HVAC system and the outdoor air levels. However, because it uses a series of complex calculations, it is less straightforward to use than the ventilation rate procedure. The IAQ procedure essentially gives designers and operators outdoor air credit for things such as no smoking, good airflow patterns, and air cleaning. ASHRAE has developed formulas for calculating contaminant levels in a space and guidelines as to what levels are of concern. Simulation programs can put the data into easily interpretable formats that make meaningful predictions for carbon dioxide (CO2) and contaminant levels.
Operational savings from reduced ventilation
In a typical building with no smoking and no unusual contaminant sources, outdoor air levels can often be reduced from 22 to 27 m3/hr per person to between 13 and 17 m3/hr (7.5 and 10 cfm) per person. Such a reduction can yield significant operational savings. For example, in a small office building with a 60-tonne rooftop unit, annual savings on utility costs alone can be expected to be in the range of $3000 to $12,000, depending on the building’s geographic location (hot, humid climates have the greatest costs/savings), the utility rates, and the hours of operation.
The other benefit to this approach is outdoor ventilation air does not inadvertently create IAQ issues. As discussed, in many urban environments, outdoor air is usually more polluted than indoor air. For example, in the monitoring of black carbon and ultra-fine particle levels (< 0.1 µ [about 0.004 mils] in size) inside and outside an office building in a 2010 Washington, D.C., study, the indoor levels were 92 to 99 per cent lower than those outdoors. Bringing dirty outside air inside does not improve IAQ, whereas cleaning and re-using the indoor air does.
Reduced outside air can also favourably impact equipment selection, sometimes translating into smaller equipment. This can have the net effect of essentially paying for the filtration system on the project.
Designing for the right IAQ strategy
The first step is to decide which airborne contaminants need controlling. The most effective filters tend to cost the most, so the decision should balance the price against the effectiveness needed. Vehicle emissions or combustion gases require a filter that removes much smaller particles, such as an electronic air cleaner, polarized-media air cleaner, or HEPA or ULPA filter. The electronic air cleaner can be a good choice given effectiveness on a wide range of particle sizes and the low pressure drop. HEPA and ULPA filters are the most effective, but create significant pressure drop, and replacement costs can be high.
Issues with odours and/or volatile organic compounds (VOCs) will also impact the filtration type. Pet stores, hair salons, medical buildings, and assisted living facilities are just a few examples where odour and VOC issues can be expected. The most popular technologies that address these gas phase contaminants include polarized-media air cleaners, carbon filters, and photocatalytic oxidation (PCO). Technologies that address biologicals (i.e. living airborne organisms) include polarized-media air cleaner HEPA and ULPA filters, and ultraviolet (UV) lamps. The most widely used germicidal lamps operate in the C band (UV-C).
The second step is to determine if any sustainability initiatives will impact equipment selection. LEED certification, for example, requires a minimum of MERV 13 for New Construction (NC) v2.2, EQ Credit 5. Additional LEED points may be available in other areas. Reducing HVAC energy, for example, qualifies for a point under Energy and Atmosphere (EA) Credit 1, Optimize Energy Performance. The availability of carbon credits, which are popular in the European community, is also impacted by energy savings.
The third step is to compare the initial price to the trade-offs in lifecycle and operating costs. This becomes increasingly important as the need for high-efficiency filtration rises. With this also comes providing for adequate space for the mechanical systems. Static pressure and ventilation air greatly impact energy consumption. The service replacement expenses and length of maintenance intervals influence ongoing operational costs. These should be reviewed to determine the system return on investment (ROI). In some cases, a high-efficiency filtration system, where there is a need for high-efficiency filters, can pay for themselves in just a few years.
Case in point
Victoria, B.C.’s 947 Fort Street building is a 4088-m2 (44,000-sf), six-storey, class ‘A’ office tower. The development is a landmark building on Fort Street, which already features a vibrant and eclectic mix of retailers, restaurants, and street front professional offices. The structure is ideally located along one of Victoria’s main arterial bus, car, and bicycle routes.
Designed by Genivar (formerly HWT Consultants), the facility was constructed as a LEED Gold-certified office building. Polarized-media air cleaning systems were included as one of the building’s design elements. Developers saw the value of incorporating indoor atmospheric quality management plans and programs into the project design to increase IAQ during occupancy. In addition to adding value offered by a superior indoor environment, developers also recognized the filtration system would add to the building’s sustainability and reduce operating costs. The system features a low air pressure drop relative to 80 per cent or greater passive mechanical filters. This reduces required fan energy and lowers energy consumption, which helps earn LEED points. The system also features a very high dust-holding capacity that extends filter replacement schedules from months to years.
The polarized-media electronic air cleaners installed in a custom air handling unit (AHU) also feature a heat wheel which provides clean, tempered air to hybrid heat pumps. These pumps use the compressors for cooling only, and use hot water coils for heating. The project also includes an air to water heat pump on the roof. The pump plant operates much the same as geothermal, but in Victoria’s moderate climate, it uses air as the heat transfer source to provide heating and cooling for the building.
Quantifiable benefits of an IAQ strategy
The construction industry has made great progress in implementing energy-saving strategies that improve building efficiency and carbon footprint. Nearly all of them, however, are geared to reduce heating and cooling system consumption. The absence of an effective IAQ strategy in the design stage can lead to problems including health risks, annoying odours, high energy consumption, and excessive operational costs.
The advantages to having an effective IAQ strategy are many. Studies have shown better indoor air quality improves health, well-being, and productivity. (For more, see “Health and Productivity Gains from Better Indoor Environments and their Relationship with Building Energy Efficiency,” by William J. Fisk in the Annual Review of Energy and the Environment (vol. 25, 2000). Also see “Estimates of Improved Productivity and Health from Better Indoor Environments” by William J. Fisk and Arthur H. Rosenfeld in Indoor Air: International Journal of Indoor Environment and Health (vol. 7, issue 3, 2007), and the Occupational Health & Safety’s (OH&S’s) “IAQ and Worker Productivity.” Visit ohsonline.com/articles/2010/09/01/iaq-and-worker-productivity.aspx). Careful planning can eliminate nuisance issues down the road. Today, there are air cleaners available that extend maintenance intervals from months to years. There are even systems can actually pay for themselves through lower operating costs.
Robert F. Goodfellow, IAQCP, is vice-president of marketing with Dynamic Air Quality Solutions. He has more than 20 years of experience in the heating, ventilation, air-conditioning, and refrigeration industry. Goodfellow can be reached via e-mail at email@example.com.
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