AMAZON multi-meters discounts AMAZON oscilloscope discounts
(cont. from part 1)
5. PASSIVE AND LOW-ENERGY APPROACHES FOR CONTROL OF IAQ
This section deals with passive ventilation for control of indoor air quality. Ventilation is an approach that assumes that "the solution to pollution is dilution." Another IAQ strategy, air cleaning, almost always involves forcing air through various filtering devices using active systems. Filters and other equipment associated with active approaches to both ventilation and air cleaning are discussed in a following section.
(a) Windows
Operable windows are one of the oldest and most common "switches" of all. Passive ventilation through windows and skylights is influenced by the position of the open window; if wind strikes the glass surface in its open position, it will be deflected.
The direction of the wind approaching the window is generally unpredictable. Also, whereas for simple ventilation (without cooling) it is usually desirable to keep wind away from people, for cooling at temperatures above the standard comfort zone, wind across the body is helpful . For these reasons, a window that can be opened in a variety of positions can be useful; some examples are shown in Fig. 4.
Perhaps the best aspect of operable windows is that they give the building occupants some control over the source of outdoor air. Perhaps the worst aspect is that they rarely offer any means of filtering this incoming air. They also can confound attempts by a central HVAC system to regulate airflow and the resulting pressure. Sometimes they admit air (windward side); at other times, they exhaust it (leeward side). Note that the EPA Campus (Fig. 2) elected to install fixed, not operable, windows.
Some windows offer more free area of opening than others of the same size. Figure 5 com pares some common window types. The pattern of incoming flow is also highly influenced by the way in which these windows open. The outward flow is somewhat affected as well. Insect screens will reduce the flow of air. Estimating wind-driven airflow through window openings is discussed in Section 8.
FIG. 3 Zoning for IAQ: the office copier. This notorious source of VOCs
can be located at the perimeter (a), where access to fresh air and direct exhaust
is simple; however, perimeter space is prime real estate for daylight and views.
If the copier is located in less-desirable interior space (b), exhaust air
can create negative air pressure, drawing air from adjacent offices and containing
the VOCs; longer runs of exhaust ducts are required.
FIG. 4 A window that can open in more than one position can enhance ventilation
performance. (a) The window tilts from the top, directing the incoming air
toward the ceiling; fresh air does not directly encounter workers within the
space. (b) The window swings inward, allowing incoming air to move across people,
enhancing warm-weather cooling.
FIG. 5 The percentage of actual openable area varies with the window type.
FIG. 6 The Commerzbank is a recent addition to Frankfurt, Germany's, skyline.
Operable windows in this 56-story tower are tilted in from the top by occupants,
while a fixed outer pane with continuous venting slots at the top and bottom
keeps air cur rents under control. Shown here in exhaust mode, the window can
also supply fresh air to the office. Under adverse outdoor conditions, the
building management system (BMS) locks the inner window in the closed position.
Windows work best in the presence of wind. In calm conditions, they may still admit-or exhaust- air due to the stack effect. The taller the building, the more pronounced this effect. Operable windows in very tall buildings have been shunned by designers until recently. The Commerzbank in Frankfurt, Germany, is a 56-story tower. Each office's exterior window (Fig. 6) is operable in temperate weather, and the occupant decides the degree of openness; a lock-out is controlled by a building management system (BMS). The full-height office window's outer skin consists of fixed single-glazed safety glass, with 5-in. (125-mm) ventilation slots all across the top and bottom. These serve the 8-in. (200-mm) wide cavity between the outer and inner window skins.
The inner window is double-glazed, hinged at floor level, and has a motor-operated tilt-in mechanism at the top. Motorized blinds for solar shading are located within the cavity between the window skins.
A central atrium provides a stack effect so that an open window is usually a source of incoming rather than exhaust air, although the top and bottom slots allow for a slight stack effect at each window.
FIG. 7 Two wind-gravity (turbine) ventilators accent the skyline of the
addition to Barton Hall at the University of New Hampshire. These draw hot
exhaust air from the auditorium; cool outdoor air is admitted through low windows
and shutters in the auditorium's north wall.
FIG. 8 Performance of some passive ventilators compared to a simple open
stack. The popular turbine ventilator is identified as #1 but is outperformed
by many other devices.
Table 7 Turbine Ventilator Performance
(b) Stack Effect
Several applications of the principle that hot air rises are applicable to IAQ. Estimating airflow due to the stack effect is discussed in Section 8, along with more detailed calculations. Devices can be used to enhance the stack effect by creating suction when wind blows across the top of a stack. Probably the most common (available in chain-store catalogs) are wind gravity or turbine ventilators (Fig. 7); typical performance characteristics are listed in Table 7. This is probably not the most effective topping device, however.
Figure 8 compares volumetric airflow results for a turbine and several other ventilators with those for a simple open stack. The tests were done in a wind tunnel at the Virginia Polytechnic Institute.
Because the stack effect works more force fully with increased height, intakes should be as low as possible. When these openings are near the ground, pre-cooling of summer intake air is possible.
The Cottage Restaurant in Oregon took some advantage of this principle. The Olivier Theatre at the Bedales School in rural Hampshire, England (Fig. 9), uses a gently sloping site to similar advantage. The tightly packed audience of a theater generates considerable heat, as do the lights. For this theater, the maximum acceptable temperature around the audience is about 77ºF (25ºC); the heat produced by people and lights provides a temperature increase of about 12.6ºF (7ºC). Therefore, whenever the outdoor tempera ture is about 64.4ºF (18ºC), cooling of the incoming air is needed.
In this theater, air is introduced to an "under croft" (crawl space) with a concrete floor, on which are built many concrete block walls, forming an indirect path for the incoming air. This undercroft is cooled by night ventilation, and thus is made ready for the next event's heat gains. The inlet openings are 5% of the theater floor area; the surface area of the undercroft is 32 ft 2 (3 m2) per person. The audience of 270 people is ventilated and cooled by the air rising from this undercroft, through openings that total 3.5% of the floor area. Gaining heat, the air rises toward the central cupola, aided when necessary by a "punkah" fan, and exits through four louvered sides of the cupola, with a total outlet area of 6% of the floor area. The overall height of this stack is 51 ft (15.5 m); a maximum of 15 ACH is expected at around 5ºF (3ºC) difference (warmer inside than outside). This ventilation-cooling system is silent and utilizes no refrigerant. In the heating season at partial occupancy, the stack outlets are closed, and the fan can be run in reverse to send collected warm air downward to the floor.
FIG. 9 Stack ventilation caps the Olivier Theatre at the Bedales School
in rural Hampshire, England. (a) View from the east. (Photo by VIEW/Dennis
Gilbert.) (b) Section shows the openings that admit outdoor air to the concrete "undercroft," where
it is cooled, then admit ted below seats to the auditorium; gaining heat, it
rises (assisted when necessary by a punkah fan) and exits out the four-sided
dampered openings in the cupola. (Designed by Fielden Clegg Architects, Bath,
Avon; engineering by Max Fordham and Partners, London.)
FIG. 10 Relative risks of high radon levels in the soil by county in the
United States. The darker the zone, the higher the risk (zone 1, for example,
presents more risk than zone 2). (Courtesy U.S. Environmental Protection Agency.
From A Guide to Radon. Environmental Health Center, National Safety Council.
Washington, DC. 1996.) (c) Under-slab Ventilation
Although the theater uses the ground's coolth to advantage, there are some places where the ground contains radon (or other soil gases). A map of the United States (Fig. 10) shows the relative risk from radon, a long-term harmful gas, on a county-by-county basis. Buildings on former industrial sites or landfills could be threatened by other dangerous soil gases. Even ordinary sites can be threatened by methane gas from a leaking sewer line. A precaution against soil gas is to design for a passive sub-slab depressurization system. This involves at least one 4-in. (100-mm) pipe open at both ends.
The lower end is set into a layer of clean, crushed rock at least 4 in. (100 mm) thick that lies immediately below the floor slab. The object is to allow air within this rock layer to enter the open end of the pipe. The slab is poured and carefully sealed around the pipe, and the pipe is extended (through interior walls) through the roof, where it can vent radon and other soil gases to a safer place. Heat from the building drives the stack effect within this pipe.
(d) Preheating Ventilation Air
Fresh air brought directly into a space during the winter will improve IAQ, but at the expense of thermal comfort. Several passive or low-energy strategies to mitigate this problem are available. The office building in Fig. 11 is surrounded by a 4-ft wide (1.2-m) cavity between the inner and outer glass surfaces. Air within this cavity is heated, both by the sun and by indoor heat sources, and rises out of a damper-controlled opening. Although this building does not utilize such heated air for ventilation, it demonstrates the possibility of such an approach. See the Comstock Center as another example.
The use of a south-facing wall as a winter preheating device with an (unglazed) transpired collector (available as Solarwall) is illustrated in Fig. 12. Aluminum sheeting, specially finished for solar absorption and penetrated by thousands of tiny holes, is the exterior surface. Behind this is a cavity kept under negative pressure by a fan.
Outdoor air is drawn through the holes, heated by the dark outer surface, and drawn up the cavity to the fan and then on to the space. Insulation and the interior surface complete the south wall.
Thus, heat loss from the building through the south wall is recaptured by the inflowing outside air. In summer, a fan draws air directly from the outside, bypassing the solar cavity. The holes at the top of the wall serve as outlets for the stack effect produced by the solar gains through the outer surface. There are numerous installations around the world-one of the largest, at 108,000 ft2 (10,034 m2), is for an aircraft manufacturer in Quebec, Canada. For the design procedure, see NREL (1988).
FIG. 11 The Occidental Chemical Corporate Office Building, Niagara Falls,
New York. (Architects: Cannon Design, Inc.) (a) Fully exposed to sun and wind
despite its downtown site, the building appears as a conventional curtain-wall
box. (b) An ordinary-looking plan with a central core and suspended ceilings.
(c) On all four sides, a 4-ft (1.2-m) cavity allows for maintenance of movable
daylight louvers and window washing, as well as for ample natural ventilation
by the stack effect. Although cavity airflow is released at the roof, it could
be utilized during winter as tempered fresh air to the interior.
Another approach to both residential winter ventilation and heat exchange is the breathable wall combined with an exhaust air heat pump. This sys tem depends upon a house being under negative pressure, which is ensured by forced exhaust air. A heat pump then takes heat from the exhaust air and delivers that heat either for space heating or domes tic water heating. The fresh air to replace that being expelled is drawn in through the outside walls by a unique combination of fiberglass lap siding board, fiberglass insulation batts, breathable sheathing, and no vapor barrier. This allows a slow, steady stream of cold air to enter, be warmed by the insulation, then enter the house. More information is avail able from the National Research Council Canada.
FIG. 12 Winter preheating of fresh air on a south wall using an unglazed
transpired collector (a). Tiny holes in the aluminum skin admit air to a cavity
outside the wall's insulation, where it is preheated by sun on the dark aluminum,
as well as by heat escaping through the insulation. A fan draws this air to
a supply plenum, after which it is distributed to the space. In summer (b),
the fan draws directly from the outside, and the collector is self-ventilated
by the stack effect. (Courtesy of the National Renewable Energy Laboratory.)
6. ACTIVE APPROACHES FOR CONTROL OF IAQ
This section concerns equipment that moves, heats or cools, humidifies or dehumidifies, and cleans air. A large range of capacities is involved, from room-sized to central whole-building air handling. A general note about heating and cooling system choice: Acceptable IAQ will be easier to achieve if the heating and cooling systems utilize forced-air motion, because some filtering is built into the air-handling equipment. However, separate air-cleaning systems are becoming increasingly common, so radiant heating systems with separate forced-air cleaning can yield high IAQ (given adequate outdoor air). For cooling, an economizer cycle provides up to 100% outdoor air at times, and cooling by night ventilation of thermal mass provides many complete air changes during the nightly building maintenance activities that are so fume-producing. Evaporative cooling provides a continuous flow of outdoor air.
(a) Exhaust Fans
Exhaust fans remove air that is odorous and/or excessively humid before it can spread beyond bathrooms, kitchens, or process areas, creating a negatively pressured area that further limits the spread of undesirable air. In buildings with heating systems without air motion (radiant heating), exhaust fans are often the only built-in devices for moving air. They are often very noisy, which is sometimes useful for covering noises associated with bathrooms, but also noisy enough to discourage their use over long periods of time.
In its most simple application, the exhaust fan is a stand-alone device, with no thought about where the replacement air will be drawn from, and rarely much concern about where this unwanted air will be discharged to. (Discharge into attics, basements, or crawl spaces is prohibited by code.)
ANSI/ASHRAE Standard 62.2-2007, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, requires intermittent (user controlled) exhaust fans of at least 50-cfm (25-L/s) capacity for bathrooms and 100 cfm (50 L/s) for kitchens. The intakes for these fans should be as close to the source of polluted air as possible so as not to drag such air across other locations before it leaves the space. In kitchens, this is directly above the range (grease, odors, and water vapor); in bath rooms, in the ceiling above the toilet (and shower) in order to remove the most moist air. Some options are shown in Fig. 13.
A more comprehensive approach in a residence includes the addition of a principal exhaust fan, which should be centrally located (drawing from the greatest space), quiet, and suitable for continuous use. Its exhaust capacity should be at least 50% of the entire system's capacity. In turn, this entire capacity is typically no less than 0.3 ACH. When this principal exhaust fan is operating, outdoor air must be brought in, tempered, and circulated throughout the residence. The tempering can be done by mixing with indoor air or by heating. Figure 14 shows two approaches to whole-house exhaust, one for forced-air systems and one without forced-air motion.
This additional continuous exhaust capacity may cause problems of inadequate air for, or spill age of fumes from, combustion equipment. Such equipment (furnaces, stove-top barbecues, etc.) with a net exhaust greater than 150 cfm (75 L/s) must be provided with a makeup air fan that turns on/off with the equipment.
FIG. 13 Options for exhaust air from toilet rooms: (a) Daylight enters from
the sides; incoming air from the lower window picks up heat and moisture near
the ceiling and exits through a high clerestory. (b) Two skylights admit daylight;
one exhausts hot, moist air at the ceiling. Makeup air will be needed from
the adjacent rooms. (c) Adding an exhaust fan will ensure negative pressure
in the bathroom, isolating moisture and odors from the adjacent rooms. (d)
In public toilet rooms, exhaust directly above toilets is desirable. In cold
weather, small radiant heat lamps with timer switches could add task heat and
light.
FIG. 14 Ensured ventilation for residences. (a) Ventilation system with
forced-air heat. (b) Partially distributed exhaust system with a vent. (Based
upon Canadian Research Council studies.)
FIG. 15 Makeup air heaters/coolers. (a) Distributed on a factory roof, these
units prevent negative indoor air pressures by providing replacement (makeup)
air for the air that is exhausted from processes indoors. Often, a separate
heating/cooling system will be used for gains or losses through the building
skin. (b) Heat recovery ventilator, utilizing a cross-flow core, serves from
200 to 900 cfm (100 to 450 L/s), with supply air entering at a temperature
0.6 to 0.8 that of exhaust air temperature. (© Conservation Energy Systems,
Minneapolis, MN.)
(b) Heating/Cooling of Makeup Air
Where climates are mild and/or energy is inexpensive, special equipment (Fig. 15) other than heat exchangers can be used to heat and/ or cool a particularly large quantity of makeup air. Especially common in factories or laboratory buildings with high exhaust air requirements, these simple devices often supplement the building's main heating/cooling system, which deals primarily with heat gains/losses through the building skin. In warm, dry climates, evaporative coolers are often used for makeup air because they are already designed to utilize 100% out door air. Even in hot and more humid climates, indirect evaporative cooling can help lower the temperature of makeup air. In Fig. 16, out door air is at 104ºF (40ºC) and 10% RH (point A). Two streams of outdoor air are involved. An evaporative cooler cools one air stream along a constant wet-bulb temperature line to point B, where it is now 70% RH but considerably cooler at 73ºF (23ºC): in the comfort zone, but warm and humid. At this point, it enters an air-to-air heat exchanger. The other out door airstream enters the other side of this heat exchanger, again at point A conditions. As the two streams exchange heat, they move toward the same temperature: the evaporatively cooled stream moves from point B to point C, about 86ºF (30ºC) and 42% RH and is then exhausted. On the other side, outdoor air moves from point A to point D, about 91ºF (33ºC) and 16% RH. This second airstream is indirectly evaporatively cooled.
Although it is still well above the comfort zone, it can then be either cooled by typical refrigerant systems or evaporatively cooled until it reaches the comfort zone.
FIG. 16 Indirect evaporative cooling can pre-cool fresh air. The psychrometrics
of the process.
FIG. 17 Air-to-air heat exchangers are particularly helpful in cold weather.
(a) The basic principle of operation. (b) A superinsulated home is heated by
coils fed from the domestic water heater, plus a heat exchanger to preheat
incoming cold air. (From: Alberta Agriculture, Home and
Community Design Branch, Low Energy Home Designs; © 1983 by Brick House Publishing
Co.) (c) Fan-powered heat exchanger utilizing a heat/moisture exchange wheel
(see also Fig. 20). This model is adjustable from 70 to 200 cfm (35 to 100
L/s), can be ceiling-, wall-, or floor-mounted, and operates at 75% to 80%
thermal efficiency. (Courtesy of Air Xchange, Inc., Rockland, MA.)
Table 8 Representative Heat Exchanger Data for Smaller Buildings
(c) Heat Exchangers
As the tightness of construction increases and fewer air changes per hour (ACH) occur from infiltration (unintended air leaks), forced ventilation becomes more attractive as a means of reducing indoor air pollution. When a heat exchanger is used, it is possible to maintain an adequate supply of fresh air without severe energy consumption consequences. Figure 17 illustrates the basic principle of a simple air-to-air heat exchanger that is becoming increasingly common for tightly built small buildings. Note that the outgoing and incoming airstreams must be adjacent.
Some commercially available heat exchangers are capable of extracting 70% or more of the heat from exhaust air. The lower the volume of airflow, the higher the efficiency. Table 8, Part A, shows representative sizes, airflows, and efficiencies for these devices. For the best diffusion of incoming fresh air through a building, the heat exchanger should be incorporated at the central forced-air fan (Fig. 17b). When a central forced-air system is not available, heat exchangers can be placed at various points in a building; typically, each heat exchanger is then equipped with its own fan. These may serve as makeup air units that are more energy efficient than the devices discussed in Section 5.6(b).
Some cautions about air-to-air heat exchangers:
1. Avoid using them on exhaust airstreams that are contaminated with grease, lint, or excessive moisture (through cooking and clothes drying in particular) because clogging, frosting, and fire hazard problems can develop.
2. In colder winter conditions, a built-in defroster, which will consume energy, will be needed.
3. Carefully locate the outdoor fresh air intake.
Keep this intake as far as possible from the exhaust air outlet to avoid drawing contaminated exhaust air back into the building. Keep the intake away from pollution sources such as vehicle exhaust, furnace flues, dryer and exhaust fan vents, and plumbing vents.
A student housing complex in Greensboro, North Carolina, utilizes heat exchangers on the exhaust air from bathrooms (Fig. 18). These exhaust devices are called energy recovery ventilators (ERVs). Each floor of the three-story complex has four apartments, each with 1078-ft 2 (100-m2) floor area and 8-ft (2-m) ceilings. Each apartment has its own air-to-air ERV that accepts air from the two small bathrooms in the apartment. Control is by individual switches in the bathrooms.
Prefiltered outdoor air is drawn into the ERV, exchanging heat with the outgoing exhaust air.
This fresh air is then fed directly into the air handler for each apartment's heat pump, adjacent to the ERV. Thus, the bathrooms are under negative pressure relative to the rest of the apartment.
Outside, the fresh air intake is located high on the wall, the exhaust outlet lower. Intake and outlet locations on the walls are separated by a mini mum of 8 ft (2 m).
FIG. 18 Energy recovery ventilator (ERV) serves two small bathrooms (a)
in a student housing complex in Greensboro, North Carolina. The ERV is activated
by switches in either bathroom. Air is drawn out beside the water closet and
exchanges heat (at about 85% efficiency) with incoming fresh air. This tempered
fresh air is then mixed with some return air and fed directly to the indoor
unit of a heat pump (b) located above the ERV. The supply air is then fed to
other rooms, ensuring a negative pressure in the bathroom while the ERV operates.
Each of 16 apartments has its own ERV and heat pump. (Design by Harry John
Boody, Jamestown, NC.)
A heat pipe (Fig. 19) also transfers sensible heat between adjacent airstreams. Within the heat pipe, a charge of refrigerant spends its life alternately evaporating, condensing, and migrating by capillary action through a porous wick. Because the only thing that moves is the refrigerant and it is self-contained, no maintenance and long life are likely. Efficiency ranges from 50% to 70%; modular sizes are available to 54 in. × 138 in. × 8 rows deep (1.4 × 3.5 m).
Heat pipes can assist in the dehumidification and cooling of incoming air. The typical cooling process using cooling coils is shown in Section 8. This potentially energy-intensive process often overcools the air to "wring out" (condense) water, then reheats the air. The heat pipe pre cools the air (subtracting heat) before the cooling coil, then warms it (adding heat) after the cooling coil. No energy input is required. A configuration of the heat pipe for this task is shown in Fig. 19b.
FIG. 19 The heat pipe is a self-contained device with no moving parts. (a)
It silently transfers sensible heat between adjacent fresh air intake and stale-air
exhaust airstreams. (From AIA: Ramsey and Sleeper, Architectural Graphic Standards,
9th ed.; © 1994 by John Wiley & Sons, Inc. Reprinted with permission of
John Wiley & Sons, Inc.) (b) The heat pipe can increase moisture removal
by first pre-cooling incoming hot air before it reaches the cooling coils.
Then the heat is returned to the cooled air, bringing it to a temperature humidity
combination acceptable for supply air to a space. (Courtesy of Heat Pipe Technologies,
Inc., and Environmental Building News.)
Energy transfer wheels (Fig. 20) go further than the two preceding devices in that they transfer latent as well as sensible heat. In winter, they recover both sensible and latent heat from exhaust air; in summer, they both cool and dehumidify the incoming fresh air. Seals and laminar flow of air through the wheels prevent mixing of exhaust air and incoming air. A further precaution in the process purges each sector of the wheel briefly, using fresh air to blow away any unpleasant residual effects of the exhaust air on the wheel surfaces.
Carryover of exhaust air qualities, except those of heat and moisture, is between 4% and 8% without purging and less than 1% with purging. Efficiency ranges from 70% to 80%, and available sizes range up to 144 in. (3.6 m) in diameter. Table 8, Part B, shows some representative sizes, airflows, and efficiencies. A smaller example of this device is shown in Fig. 17c.
(d) Desiccant Cooling
Another rotating-wheel process involves desiccant cooling. Desiccant cooling systems (Fig. 21) are attractive because they use no refrigerants (that may contain CFCs), and they lower humidity without having to overcool the air. The desiccants (such as silica gel, activated alumina, or synthetic polymers) in an active system must be heated to drive out the moisture they remove from the incoming air; at present, natural gas is typically used, but solar energy is a promising substitute due to its plentiful summer availability. Waste heat from other mechanical systems may also be used. In a passive desiccant system, heat from a building's exhaust air is enough to release and vent the moisture removed from incoming air.
Research on materials suitable for desiccant cooling and solar-driven regeneration should produce improvements in the types available as well as their performance. For an extended discussion of both solid- and liquid-based desiccant systems, see Grondzik (2007).
(e) Task Dehumidification and Humidification
Humidity affects comfort; for a sedentary person, a 30% RH change will produce about the same com fort sensation as a 2ºF (1ºC) change in tempera ture. Higher humidity adds to hot air discomfort, making evaporation more difficult, increasing skin wettedness, and increasing the friction between skin and clothing or furniture surfaces. As RH exceeds 60%, problems with IAQ increase due to mold and mildew growth. Lower humidity produces cooler and drier sensations, but too-low humidity can irritate the skin.
FIG. 20 Energy transfer wheels. The wheel surface is impregnated with lithium
chloride (or similar material), which absorbs moisture and transfers it to
the other airstream. The wheel delivers moist air in winter (a) and dry air
in summer (b). Cross section (c) through the wheel and the two airstreams it
serves. Exhaust air may be filtered to help keep the wheel clean. (d) Multiple-unit
installation.
Room exhaust air passes through the upper chambers, and incoming fresh air passes through the lower chambers. Wheels rotate at 8 to 10 rpm. (e) Rooftop unit supplies up to 1000 cfm (500 L/s). (Courtesy of Air Xchange, Inc., Rockland, MA.)
FIG. 21 Desiccant cooling utilizes a rotating wheel impregnated with either
silica gel or another desiccant material. The wheel removes moisture from incoming
hot outdoor air and delivers some of it to outgoing exhaust air. The remaining
moisture must be driven from the wheel by a heat source: natural gas or, ideally,
solar energy. (National Renewable Energy Laboratory.)
FIG. 22 Refrigerant dehumidifiers typically are installed as free-standing
units in smaller buildings.
FIG. 23 Air filter efficiencies. Methods of determining efficiency vary;
the Arrestance Method can be used across all groups, while the Atmospheric
(Dust-Spot) Method should be used for medium- and high-performance filters.
The Detection Operational Program (DOP) Method is a military standard.
(From D. Rousseau and J. Wasley. 1997. Healthy by Design. Hartley & Marks. Point Roberts, WA.)
For spaces that need only dehumidification rather than mechanical cooling, refrigerant dehumidifiers are commercially available (Fig. 22). Their advantage over desiccant dehumidifiers is that the air temperature remains essentially unchanged during dehumidification (desiccant dehumidifiers raise the temperature of the dried air). These devices consume energy to run the refrigeration cycle, however, and this energy is added, as heat, to the space. Accumulated water must periodically be removed from these units and, if untended, could become a source of disease. Most refrigerant dehumidifiers encounter operating difficulties at air temperatures below 65ºF (18ºC), at which point frost forms on their cooling coils.
This could cause problems in a tightly enclosed residence in winter.
Task humidifiers are widely available and often used to relieve symptoms of respiratory illnesses.
Again, the problem of bacterial and mold growth in water reservoirs arises; some products add an ultraviolet (UV) lamp to counteract this threat.
(f) Filters
There is a wide variety of particle air pollutants, as described in Table 2. The larger particulates are the easiest to remove, but much smaller respirable particles pose a greater threat to health. Not all pollutants can be removed by filters. Figure 23 compares filter groups and testing methods with associated filter efficiencies. Groups I, II, and III are widely used and are generally illustrated in Fig. 24. The highest-efficiency filter, the high efficiency particulate arrestance (HEPA), is most often found in special air cleaners for unusually polluted or IAQ-demanding environments. Air filter characteristics are summarized in Table 9. See ASHRAE Standards 52.1-1992 and 52.2-2007 for further details on air filter ratings. Specialists recommend using dust collectors rather than filters when the dust loading equals or exceeds 10 mg/m3. Again, it is wisest to remove pollutants at the source.
Particulate Filters. Particulate filters are very common and come in several guises. Panel filters are furnished with HVAC equipment and function mainly to protect the fans from large particles of lint or dust. Because they are relatively crude, they are not really considered to be air-cleaning equipment. Media filters are much finer, using highly efficient pleated filter paper within a frame.
They function both by straining and impaction.
The larger particles are strained out by the closely spaced filter fibers, while some of the smaller particles that would otherwise pass through are pushed into the fibers due to air turbulence. Particulate filters need regular maintenance, especially media filters, which can become blocked and cause dam age to HVAC equipment and increased energy consumption if not replaced frequently enough. Media filters of high quality are expected to perform at an efficiency of 90% (minimum) and are typically at least 6 in. (150 mm) deep; this is for a six-month minimum life cycle.
Table 9 Air Filter Characteristics
FIG. 24 Several air filter types (a, b, c); not shown are pleated and roll
types.
Adsorption Filters. Adsorption filters are for gaseous contaminant removal and vary according to the pollutant in question. Activated charcoal filters are the most common of these types, absorbing materials with high molecular weights but allowing those of lower weights to pass. Other adsorption filters use porous pellets impregnated with active chemicals such as potassium permanganate; the chemicals react with contaminants, reducing their harmful effects.
Adsorption filters must be regularly regenerated or replaced.
High-quality adsorption filters contain gas adsorbers and/or oxidizers with sufficient capacity to remain active over a full service cycle of 6 months at 24 hours per day, 7 days per week.
Air velocity should be such as to allow the air to remain in the filter for about 0.06 second.
Air Washers. Air washers are sometimes used to control humidity and bacterial growth. The moisture involved can pose a threat if these devices are not well maintained.
Electronic Air Cleaners. Electronic air cleaners can pose a different threat due to ozone production, but have the advantage of demanding less maintenance. Static electricity is produced in the self charging mechanical filter by air rushing through it; larger particles thus cling to the filter. The more humid and/or higher the air velocity, the lower the filtering efficiency. In a charged media filter, an electrostatic field is created by applying a high dc voltage to the dielectric material of the filter.
Many particles are not polarized, however, due to an insufficiently strong field. A two-stage electronic air cleaner first passes dirty air between ionizing wires of a high-voltage power supply. Electrons are stripped from the particulate contaminants, leaving them positively charged. Then these ionized particles pass between collector plates that are closely spaced and oppositely charged. The particles are simultaneously repelled by the positive plates and attracted to the negative plates, where they are collected.
(g) Locating Air-Cleaning Equipment
Before the advent of IAQ concerns, buildings were often designed with rather crude panel filters located only at the HVAC equipment; they were primarily intended to intercept materials that might adversely affect combustion or heat exchange. In addition to these equipment-protecting devices, a building requiring high IAQ will now have a combination of high-efficiency particle filters and adsorption filters.
The panel filters provided with HVAC equipment are usually located upstream from the unit fan. High-efficiency particle and adsorption filtering systems should be located downstream from the HVAC cooling coils and drain pans to ensure that any microbiological contaminants from those wet surfaces are removed rather than being distributed throughout the building.
In buildings where a filtering system must be integrated with a central HVAC system (typical of small buildings), the HVAC system should be capable of continuously circulating air at the rate of 6 to 10 times per hour and of operating against the considerable static pressure that results from high-efficiency filters.
(h) Ultraviolet (UV) Radiation
Since early in the twentieth century, UV radiation in the C band (200-280 nm wavelength) has been used to kill harmful microorganisms, but under tightly controlled conditions. Now there are UV lamp units that work within HVAC systems, promising to control fungi, prevent the development and spread of bacteria, and reduce the spread of viruses. As an additional benefit, cooling coils and drain pans stay cleaner. The germicidal output of these lamps is somewhat higher at room temperatures (and above) than at lower temperatures. These devices take up very little space (they are placed within ductwork) and generate no ozone or other chemicals. They are even more effective when installed in a UV-reflective duct interior: aluminum seems to be the best UV reflector commonly available. UV lamp life is 5000 to 7500 hours.
UV radiation also looks promising as a treatment for VOCs. The National Renewable Energy Laboratory is helping to develop a process that bombards polluted air with UV radiation in the presence of special catalysts. Pollutants including cigarette smoke, formaldehyde, and toluene are quickly broken down into molecules of water and carbon dioxide.
FIG. 25 A stand-alone electrostatic air filter operates independently of
the central HVAC system; no ductwork is required. Airflow rates vary; the slower
the flow, the more efficient the filtering. Flush-mount (recessed in suspended
ceiling), ceiling surface-mount, wall-mount, and portable models are available.
(Courtesy of Tectronic Products Co. Inc., East Syracuse, NY.)
(i) Individual Space Air Cleansing
Energy conservation considerations have reduced the air circulation rate in many central air handling systems; moving less air reduces the energy used by fans. One result can be low distribution efficiency, which causes poorly mixed air within the occupied spaces. With local (individual space) air-filtering equipment, both a high circulation rate and proper air mixing are achievable.
Each such unit has its own fan that can operate either with or without the central HVAC fan. An example of an independent electrostatic air filter is shown in Fig. 25.
FIG. 26 This air-cleaning system uses a combination of high-voltage dc and
high-frequency ac. The supply air emerges with greatly reduced submicron particles,
which have coagulated into larger particles more easily carried by air currents.
As this air moves through the occupied space, it picks up submicron particles
and is either exhausted or returned to the equipment, where it is filtered
to remove the larger particles.
Air circulation through these filters should occur at rates between 6 and 10 times per hour.
The air is then ducted to diffusers, hence circulating in a sweeping pattern across a space to return air intakes on the opposite side.
A variation on electrostatic air cleaning is shown in Fig. 26. In this equipment, a mixture of outdoor and indoor air is passed through a complex electrical field produced by both high voltage dc and high-frequency ac. This supply air emerges with greatly reduced submicron particles that have coagulated into larger particles more easily carried by air currents. As this air moves through the occupied space, it picks up submicron particles and is returned to the equipment (some is exhausted), where it is filtered to remove the larger particles.
Portable air cleaners abound. One more elaborate model combines UV germicidal radiation with carbon/oxidizing media and HEPA. It is a counter-height rolling device about 15 in. (380 mm) square that emits cleansed air from the top.
It claims to purify air in a space of about 12,000 ft 3 (340 m3).
(j) Controls for IAQ
A large number of air quality-monitoring devices are now available, some of which can control the operation of IAQ-related equipment. One of the oldest and simplest devices measures the concentration of CO2 in parts per million (ppm). Wherever there are indoor concentrations of people, elevated levels of CO2 can be expected. Thus, CO2 becomes a kind of "canary in the coal mine," an early indicator of pollutant buildups due to occupancy. In the Bedales School theater (Fig. 9), the operation of the exhaust air dampers in the cupola is controlled by a building management system. Under ordinary conditions, damper opening is regulated by information from the CO2 monitor. This can be overridden by indoor and outdoor temperatures (summer night ventilation is encouraged), air velocity monitors below the seats (too much draft could produce discomfort), the presence of wind and rain (openings limited), or a fire alarm (damper fully open to aid in smoke extraction).
Where contaminants are expected from sources in addition to human beings (typically the case), monitors may be installed to detect carbon monoxide, or combinations of VOCs, or fuels such as propane, butane, or natural gas- or even for depletion of oxygen. These monitors can be installed as stand-alone alarms or with additional relays that activate equipment. They are about the size of a programmable thermostat; the mounting height depends upon which gas is to be monitored.
Such devices can regulate ventilating heat exchangers, such as the ERV (shown in Fig. 18). This could be especially useful during unoccupied periods, holidays, and weekends. Many HVAC systems are shut off during such extended periods; VOCs from finishes and furnishings continue to accumulate, however. Periodic flushing from ERVs, controlled by VOC monitors, will help maintain acceptable air quality and could eliminate (or greatly reduce) the need for a Monday morning pre-flush.
7. IAQ, MATERIALS, AND HEALTH
The effect of IAQ on occupant health and productivity seems to be an emerging building design issue. This concern stems partly from increasing interest in green design (with its focus upon environmental quality and the resulting impact on materials choices) and partly from continuing stories about the adverse health impacts of poor IAQ (especially cases involving mold and mildew).
(a) Multiple Chemical Sensitivity
This is an unusual (and sometimes controversial) condition, also known as environmental illness.
The controversy arises because the causes of this condition are poorly understood and likely involve numerous factors. When causes are mysterious, establishing targeted remedies becomes difficult.
People with multiple chemical sensitivity are likely to see the provision of outstanding IAQ as a primary intent of the design process. People with this condition avoid environments with any known environmental risk factors.
Ecology House (in San Rafael, California) is an apartment complex for people with multiple chemical sensitivity. Its construction avoided plywood, using Douglas fir sheathing instead; the floors are tile instead of wall-to-wall carpet; cabinets are metal, not plywood or oriented strand board; the heating system is radiant hot water, not forced air. Barbecues and fireplaces are absent, painted surfaces are minimized, and any window coverings are alternatives to curtains. There is even an "airing room" where, for example, newspapers can be hung before they are read in order to evaporate ink odors.
(b) Materials and IAQ
There is a rapidly growing availability of environ mentally responsible building materials. Many such products address the issue of outgassing via low-VOC formulations. Others provide alternative materials to replace less-benign products in common use. Even so, quantitative data are hard to find. Manufacturers of building materials are required to provide Material Safety Data Sheets (MSDS). These reports list all chemical constituents that make up at least 1% of a material (and are not deemed proprietary). Unfortunately, this information does not predict pollutant emission rates. A designer is left with the suspicion that the higher the percentage content of a chemical, the more likely its outgassing.
(c) Green Design and IAQ
The U.S. Green Building Council's LEED rating system has helped to improve the visibility of acceptable IAQ as a design objective. LEED for new construction and major renovations, for example, establishes compliance with ASHRAE Standard 62 and control of tobacco smoke as prerequisites for LEED certification. Beyond these minimums, designers may choose to address CO2 monitoring, low-emitting materials, or pollutant source control strategies (among others) to achieve rating points.
References and Resources
AIA. 1994. Ramsey, C. G. and H. R. Sleeper. Architectural Graphic Standards, 9th ed. American Institute of Architects/John Wiley & Sons. New York.
ASHRAE. 1992. ANSI/ASHRAE Standard 52.1 1992: Gravimetric and Dust-Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. Atlanta, GA.
ASHRAE. 2007. ANSI/ASHRAE Standard 52.2 2007: Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.
ASHRAE. 2007. ANSI/ASHRAE Standard 62.1 2007: Ventilation for Acceptable Indoor Air Quality. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.
ASHRAE. 2007. ANSI/ASHRAE Standard 62.2 2007: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.
Banham, R. 1969. The Architecture of the Well-Tempered Environment. University of Chicago Press, Chicago, and the Architectural Press, London.
Chandra, S., P. W. Fairey, and M. Houston. 1986. Cooling with Ventilation, Solar Energy Research Institute. Golden, CO.
Fanger, P. O. 1989. "The New Comfort Equation for Indoor Air Quality," in ASHRAE Journal, October.
Grondzik, W. (ed.). 2007. Air-Conditioning System Design Manual, 2nd ed. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA.
Hellmuth, Obata and Kassabaum. 2001. The Greening Curve: Lessons Learned in the Design of the New EPA Campus. U.S. Environmental Protection Agency, EPA 220/K-02-001.
The New Buildings Institute (scroll to link for Commercial Building Indoor Air Quality)
NREL. April 1988. Federal Technology Alert: Transpired Collectors (Solar Preheaters for Outdoor Ventilation Air). National Renewable Energy Laboratory, U.S. Department of Energy. Boulder, CO.
Rousseau, D. and J. Wasley. 1997. Healthy by Design. Hartley and Marks. Point Roberts, WA.
Shurcliff, W. A. 1981. Air-to-Air Heat Exchangers for Houses. Brick House. Andover, MA.
U.S. Environmental Protection Agency, Research Triangle Park Campus
U.S. Environmental Protection Agency, Indoor Environments Division (IAQ)
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