Continuing its proud heritage, this Tenth Edition provides thorough coverage of the latest in the theory and practice of environmental control system design. This bestselling book encompasses mechanical and electrical systems for buildings of all sizes, featuring design guidelines and detailed design procedures for every topic covered and supported by more than 2,200 illustrations—over 225 new to this edition!
Note: The Figures and/or Tables mentioned in this sample chapter do not apprear on the Web.
INTERIOR AIR QUALITY
Although ventilation is the '' V'' in HVAC, this textbook has not previously had a separate chapter on either ventilation or interior air quality (IAQ). Ventilation has been a subissue within heating and cooling system design. However, in centuries past, there were special systems to deal with providing outside air to buildings, even on a residential scale. Banham (1969) describes both large and small buildings where outside air was deliberately introduced. In the home that Dr. John Hayward built for his family in 1867, the Octagon in Liverpool, England (Fig. 6.1), outdoor air was brought into the basement, slowed to precipitate some particulates, then heated to help it rise throughout the four-story building. Ceiling vents just above gas lights drew '' vitiated'' air from each room, which was then vented to a '' foul air chamber'' in the attic. From there, a large shaft utilized a combination of a siphon and the stack effect. Powered at its low point by ever-present heat from the kitchen cooking range, it drew the foul air down, then up a very high chimney to discharge.
Several trends have combined to bring IAQ back into prominence. First, an increasingly large percentage of people's time is now spent indoors, and in more tightly controlled environments, as a service-based economy overtakes a manufacturing- based one. Second, the oil embargo of 1973 raised the world consciousness of finite energy sources, producing a sudden and powerful rush toward energy-conserving design. This in turn encouraged designers to limit the introduction of outdoor air that demanded cooling in summer and heating in winter. Third, a proliferation of chemicals has produced a vast array of potential air pollutants, from synthetic products permanently installed within buildings, to equipment used indoors, to cleaning fluids used in maintenance. With more time spent in less fresh air and surrounded by more pollution sources, we experienced increasing numbers of buildings with sick building syndrome (SBS). One definition of SBS is that more than 20% of the occupants complain of symptoms associated with SBS, such as headaches, upper respiratory irritation, and irritations of the eyes, among others. If these symptoms disappear after leaving the workplace (weekends are especially good periods of contrast), SBS is even more strongly indicated. The building designer has an elusive task when air quality is at issue, because so little can be predicted. Heat flow rates, occupancy schedules, and typical weather patterns can be combined to predict with some confidence how much energy will be consumed by a building; construction types can then be altered in the design stage to yield predictably different results. As yet, designers have few tables that yield rates of outgassing for various materials at given temperatures, nor have we the averages and the design conditions for the quality of outdoor ('' fresh'') air, even though we do have such information for its temperature and humidity. Yet controlling the quality of indoor air may be as important as controlling its temperature and humidity. Buildings that are thermally comfortable can still suffer SBS when pollutants are sufficiently numerous. Designers know that saving energy for heating and cooling lowers the cost of maintaining a building; employers know that lost productivity from either on-the-job illness or sick leave can result in much greater costs. One estimate for a large office building compared the cost of increasing ventilation and improving air filtration to the value of projected health and productivity benefits. Initial improvements yielded estimated benefit-to-cost ratios of 50 to 1 (increased ventilation) and 20 to 1 (improved filtration).
IAQ depends on four major considerations, three of which depend largely on the designer:
1. Limit pollution at the source (choose materials and equipment carefully).
2. Isolate unavoidable sources of pollution.
3. Provide for adequate supply and filtering of fresh air (and return air).
4. Maintain the building and its equipment in clean condition.
The designer can thus provide for improved IAQ by careful location on site; choosing of materials and (where possible) equipment; zoning to isolate pollutant sources; and providing clean, adequate, and well-distributed outdoor air, air-cleaning devices, and building commissioning (often including a '' flush mode'' following completion of construction). It is up to the building managers to maintain the building's IAQ health by a regular equipment maintenance program, regular interior cleaning, and a careful selection of cleaning agents. Furthermore, a flush of the building after every unoccupied weekend or holiday period will be helpful in removing accumulated pollutants from finishes and furnishings.
Sources and Impacts
Indoor air pollution can be described both in terms of the types of contaminants (gaseous, organic, or particulate) and the types of effects (odors, irritants, toxic substances). We not only inhale contaminants, we also absorb some and ingest some; not only the nose is involved in sensing IAQ. For some contaminants, the only method of avoidance is to design for their exclusion; equipment will not remove them, although increased ventilation can reduce their impact. Examples are asbestos, radon, and pesticides. Table 6.1 summarizes some common indoor air pollutants, their effects, and simple strategies to ameliorate them.
(a) Odors. One of the most immediate indicators of IAQ problems is odor detection. People are sensitive to odors over an extraordinary range, while equipment to detect and classify odors is woefully lacking. Odors are perceived most strongly on initial encounter, then '' fatigue'' occurs and perception fades; thus, visitors are more likely to detect odors than are the long-term inhabitants of a space. Odors may be simply unpleasant, or may be indicators of a more serious IAQ problem that has physiological consequences. When an unfamiliar odor is detected, our reactions are either positive, neutral, or negative, depending on whether we perceive a threat associated with the odor.
Sometimes odors are directly traceable to a source, but in office environments the odor is usually more complex. A typical blend for an office environment's odor may be body odor, grooming products (perfumes, colognes), copy machines, cleaning fluids, and outgassing from materials. More rarely now, tobacco smoke may also be present. This complexity produces an interesting reaction; people tend to become less sensitive to each of the component odors, with the result of overall masking. However, an architectural masking approach-- the deliberate introduction of a '' perfume'' to cover offending odors-- is rarely successful. (Sweet-and-sour may work nicely for the palate, but olfactorily, it can be uniquely nauseating.) Conversely, as the indoor environment is freed of multiple odors, people become more sensitive to the one or two odors that remain.
Sometimes a simple measurement of carbon dioxide concentration is used as a first indicator of potential body odor and/ or inadequate ventilation problems, because the CO2 concentration indoors is generally proportional to human concentration.
Filtering out the odors from indoor air is usually achieved with electronic or with activated charcoal filters, described in Section 6.5.
(b) Irritants. Unlike odors, which are immediately perceived and fade with prolonged exposure, irritants are often imperceptible at first but cause increasing distress over time. Symptoms of irritants include itching or burning eyes, sneezing, coughing, dry nose and throat, sore throat, and tightness of the chest. Most irritants are present in the form of particles and gas dispersoids (Table 6.2).
Sources of irritants include the building itself and the equipment and occupants within. New and newly renovated buildings are particularly prone to problems from outgassing of paints, adhesives, sealants, office furniture, carpeting, and vinyl wall coverings. Volatile organic compounds (VOCs) are chemicals containing carbon molecules that are volatile; that is, they off-gas or evaporate from the surface of materials at room temperatures. The list is long: methane, ethane, methylene chloride, trichloroethane, CFCs, HCFCs, HFCs, formaldehyde, and hydrocarbons such as styrene, benzene, and alcohols. All are now found more frequently in these new spaces.
Long-term occupancy brings other irritants. Ozone, a friend in the upper atmosphere but a smog component below, is produced from copy machines, high-voltage electrical equipment, and-- ironically-- from electrostatic air cleaners. Mineral fibrous particles can result from the breakdown of interior duct insulation and fire-proofing. Hydrocarbon compounds come from copy machines and their papers. Tobacco smoke is a mixture of gases and fine particles especially irritating to sensitive individuals. Low humidity can exacerbate problems with irritants, producing symptoms similar to those from chemicals. Carpet shampooing yields organic solvents and ammonia; nighttime cleaning coinciding with reduced or nonexistent ventilation is especially problematic. In contrast, night maintenance with increased ventilation rates-- as with cooling by night ventilation of thermal mass-- can reduce this threat.
As with odor control, the impacts of irritants can be reduced with increased outdoor air. Filtering for the removal of irritants usually consists of particulate filters; less common are gaseous-removal filters, air washers, and electronic air cleaners. These are also discussed later in Section 6.5.
(c) Toxic Particulate Substances. At the top of this list is asbestos, widely used in buildings until its toxicity was realized in the 1970s. We encounter asbestos in tightly bound form as in asbestos-cement and in vinyl-asbestos floor tiles, and in loosely bound form as in sprayed-on asbestos insulation. The latter form is particularly dangerous, readily releasing toxic asbestos fibers over the life of the material. With asbestos, neither increased ventilation nor filtering is acceptable; it must be either removed under stringent controls to isolate the air within the affected space, or sealed and left in place.
Some of the respirable particles (see Table 6.2) that result from incomplete combustion are toxic. Incomplete combustion can occur from smoking, in woodstoves, fireplaces, gas ranges, and unvented gas or kerosene space heaters. Lacking control of combustion at its source, the remedies are to isolate the source insofar as possible, exhaust air from the immediate vicinity, increase outdoor air to the area, and utilize particle filtering.
(d) Biological Contaminants. Because living things inhabit both buildings and outdoor air, there will be biological contaminants such as bacteria, fungi, viruses, algae, insect parts, and dust within buildings. Moisture encourages both the retention and growth of these contaminants; standing water (in HVAC system components) and moist interior surfaces are likely trouble sites. Allergic reactions and infectious and noninfectious diseases can result. Outbreaks of Legionnaire's disease occurred when improperly maintained HVAC systems within large buildings incubated then distributed the diseasecausing microorganisms. Now, residential humidifiers, dehumidifiers, and air conditioners' drain pans are suspect.
Remedies for biological contaminants begin with good design and end with vigilant maintenance. Although exposure to ultraviolet light is sometimes employed, equipment such as filters are rarely an effective solution in this case.
(e) Radon and Soil Gases. Radon is a radioactive gas that decays rapidly, releasing radiation at each stage. It is colorless and odorless, thus undetectable by humans. If we inhale radon, radiation release in the lungs can cause lung cancer. Other soil gases include methane (usually odiferous), and some pesticides that can volatize and enter buildings with soil gases. Effects on humans are not likely to be beneficial. In many of the buildings with high levels of radon, the problem has been traced to exposed earth. Radon penetrates through cracks and openings around plumbing, and below-grade spaces are particularly at risk. Penetrations of below-grade walls and floors should be both minimized and well sealed; under-slab ventilation (Section 6.4c) may be appropriate, especially in areas of high radon risk.
( f ) Multiple Chemical Sensitivity. This is an unusual and sometimes controversial condition, also known as environmental illness, in which IAQ becomes the prime indoor environmental issue. People with this affliction avoid environments with any of the foregoing risk factors. The controversy arises because the causes of this condition are poorly understood; if causes are mysterious, so might be the remedies.
In San Rafael, California, '' Ecology House'' is an apartment complex for low-income 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 OSB; the heating system is radiant hot water, not forced air. Barbecues and fireplaces are absent; painted surfaces minimized; any window coverings are alternatives to curtains. There is even an '' airing room'' where, for example, newspapers can be hung to evaporate ink odors before they are read.
6.2 Predicting IAQ
Designers want to know how much outdoor air and how much filtering will produce an acceptable indoor environment. These questions are elusive.
(a) Ventilation Rate. The most common remedy for SBS (after controlling the pollution source) is to increase the rate of outdoor air ventilation. Recommended rates of ventilation are found in Table 4.22. Although very small amounts of outdoor air will provide sufficient oxygen, and human body odor control is usually achievable at a rate of from 6 to 9 cfm (3 to 4.5 L/ s) of outdoor air per occupant, outdoor air has more work to do than provide oxygen and control odors. The current minimum rate for offices (Table 4.22) is 20 cfm (10 L/ s) of outdoor air per occupant. Because this rate is based on a density of 7 persons per 1000 ft 2 (per 100 m 2 ), it corresponds to an outdoor air rate of 0.14 cfm/ ft 2 (0.1 L/ s m 2 )of floor area. In a typical office with an 8-ft (2.4-m) ceiling, this corresponds to slightly more than 1 ACH (air change per hour).
The less volume of space per occupant (i. e., lower ceilings or greater density), the higher the rate of ventilation is appropriate. An analogy: One fish will pollute a small pond more rapidly than it will a larger pond.
Two units have been proposed to integrate the various indoor air pollutants in the same way as they are perceived by human beings. The olf is a unit of pollution (1 olf . the bioeffluents produced by the average person); the decipol is a unit of perceived air quality. These are related in this proposed comfort formula:
Q . 10 G over Ci -- Co
where Q . ventilation rate, L/ s G . total pollution sources, olf Ci . perceived indoor air quality, decipol Co . perceived outdoor air quality, decipol
At present, Ciis recommended to be set at 1.4 decipol, which represents an expectation of 80% of occupants satisfied with indoor air quality. Co and G may be roughly estimated from Table 6.3. The greatly increased ventilation rates for existing buildings in Part C [compared to ASHRAE's 0.14 cfm/ ft 2 (0.1 L/ s m 2 ) of floor area] suggest dealing with indoor pollution at its source rather than increasing outdoor air flushing-- and related energy consumption-- to such high levels. These proposals are discussed in more detail in Fanger (1989).
One reason for greatly increased ventilation rates above the guidelines given in Table 4.22 is the concept of replacance. Table 6.4 shows that at the rate of 1 ACH of outdoor air, an indoor space would have only 63% '' new'' air after 1 hour; about 8 hours at this rate is required for all the '' old'' air to be exhausted. There is, then, a difference between the fresh air input rate (ACH) and the replacance-- the fraction of air molecules at one specified time that was not in the indoor space at an earlier, reference time. This relationship, along with details of air pollutants (and of heat exchanger design, for energy conservation), is thoroughly discussed in Shurcliff (1981).
The new campus for the Environmental Protection Agency (EPA) in Research Triangle Park, North Carolina, was designed with special emphasis on IAQ (Fig. 6.2). The designers considered several alternatives for fresh air provisions, deciding that a simple variable air volume (VAV) system, set at a minimum of 3 ACH (of combined fresh and recycled air) would be acceptable. If the system were designed with a typical minimum, 1 ACH would have resulted in periods when neither heating nor cooling were required (spring and fall typical conditions). However, a 6 ACH alternative would have had dramatically increased energy consumption.
(b) Material Safety Data Sheets. Manufacturers of building materials furnish these reports, which list all chemical constituents that make up at least 1% of the material (and are not deemed '' proprietary''). Unfortunately, this does not predict emission rates. The designer is left with the suspicion that the higher the percentage content of a chemical, the more likely its out-gassing.
(c) Testing. When a client is especially interested in IAQ, full-scale time tests can be used. At the new EPA campus (Fig. 6.2), the contractor was given a target for allowable air concentrations (Table 6.5) for IAQ. Any material assemblies deemed likely to contribute more than one-third of these allowable air concentrations, and used in large quantities, were to be tested as assemblies before acceptance. One desirable outcome of such testing is the possible avoidance (or shortening) of an anticipated flush-out of the completed building with outdoor air for 90 days before occupancy.
6.3 Zoning for IAQ
After implementing pollution control at the source (cleaner equipment, prohibiting smoking, careful material choices, etc.), remaining unavoidable pollutant sources should be identified. Then isolate the more sensitive areas from the contaminators. This is sometimes difficult, as in '' open offices'' where walls are unwelcome but copying machines are essential. In such cases, erect as much of a barrier as is possible around the offender, then '' task ventilate'' to remove the contaminated air immediately. Sometimes air pollution sources also produce unwanted sound, in which case the argument for a more complete barrier may become more compelling.
Many health care and laboratory buildings have '' clean'' and '' dirty'' zones, even separate circulation pathways. Often, differential air pressures are maintained to discourage air flow from dirty to clean; higher pressure in clean areas, lower in dirty areas. Lower-pressure areas can be created simply by installing exhaust fans from such spaces, as well as limiting the volume of supply air. Higher-pressure areas can be created by installing make-up air equipment, as well as increasing the volume of supply air from the HVAC system.
On a site-planning scale, try to locate air intakes upwind from pollution sources. Because winds frequently change direction, this may be more a matter of adequate separation distance than direction. The most obvious example is a major air intake for a central HVAC system, which should be as far as possible from parking areas, delivery docks, and streets-- and from the exhaust outlets from that same HVAC system or outlets from other buildings' systems. Even exhaust outlets should be located carefully, because there is a possibility that at times outdoor air can be drawn into these '' exhaust'' grilles. The mechanical equipment room is the typical location for both intake and exhaust; energy conservation devices such as heat exchangers benefit from close proximity of intake and exhaust. Most animals use the same '' ducts'' to breathe in and exhale, obviously inviting such air re-entrainment. However, for a building, separation of these openings is prudent design.
The new campus for the EPA (Fig. 6.2) is an example of predesign planning for IAQ. This is a 1,000,000 ft 2 (92,900 m 2 ) building complex, serving a population of more than 2000 on 133 acres (54 hectares) of farm land that has reverted to second-growth hardwoods. Table 6.6 is a summary of design decisions and their impact on IAQ. Some of the more visible design consequences are the separation of parking and building, the concentration of parking in a structure (less impact on the existing landscape, more control of vehicle fumes), and the height of the exhaust stacks from the laboratories. Many other design decisions are hidden within the building's materials and HVAC system, as detailed in Table 6.6.
The topic of zoning includes decisions about local versus central equipment. Should individual exhaust fans be installed (creating selective lower-pressure areas) or a central exhaust fan (that can discharge up a very tall stack)? Should air cleaners be installed locally, where they may be selected according to the degree of pollution, or central cleaning, where it can be more easily and regularly maintained? What about heat exchangers for tempering incoming air: lots of smaller ones, or one large one? The larger and more complex the building, the more likely that a mix will result of local, specialized zones and a large, more general zone centrally served. Figure 6.3 explores the issue of the location of the office copier; at the edge, where task ventilation is easy, but plentiful daylight and view may be '' wasted'' on this function; or away from the edge, where a central exhaust system is more likely to be utilized.
6.4 Passive and Low-Energy Approaches to Ventilation
This section deals with ventilation, the approach that assumes '' the solution to pollution is dilution. '' The other major approach, air cleaning, almost always involves forcing air through various filtering devices. Equipment that controls both ventilation and air cleaning is discussed in the section following.
(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 (Fig. 3.27). For these reasons, a window that can be opened in a variety of positions can be useful; some examples are shown in Fig. 6.4. A multiple-position window in the Wallasey school was shown in Fig. 1.10.
Perhaps the best aspect of operable windows is that they give the building occupant 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 air flow and resulting pressure. Sometimes they admit air (windward side); at other times, they exhaust it (leeward side). Note that the EPA Campus (Fig. 6.2) elected fixed, not operable, windows.
Some windows offer more '' free area'' of opening than others of the same size. Figure 6.5 compares 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. Details of windows and screens are discussed in Chandra et al. (1986). Estimates of air flow driven by wind through window openings are found in Section 5.6a, detailed calculations in Section 5.14a.
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. 6.6) is operable in temperate weather; the lock is controlled by a building management system (BMS), although the occupant decides the degree of openness. The full-height office window's outer skin is 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 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.
(b) Stack Effect. Several applications of the principle that hot air rises are applicable to IAQ. For the simple stack effect, estimates of air flow are found in Section 5.6b, detailed calculations in Section 5.14b. Devices can be used to enhance the stack effect by creating suction when wind blows across the top of the stack. Probably the most common (available in chain-store catalogs) are wind gravity or turbine ventilators (Fig. 6.7); their performance characteristics are listed in Table 6.7. This is probably not the most effective topping device, however. Figure 6.8 compares the volumetric airflow results for the turbine and several other ventilators with those for a simple open stack. The tests were done in a wind tunnel at Virginia Polytechnic Institute.
Because the stack effect works more forcefully with increased height, intakes should be as low as possible. When these openings are near the earth surface, precooling of summer intake air is possible. The Cottage Restaurant in Oregon (Fig. 1.14) took some advantage of this. The Olivier Theatre at the Bedales School in rural Hampshire, England (Fig. 6.9) 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 25.C (77.F); the heat produced by people and lights provides an uplift of about 7C. (12.6F.). Therefore, whenever the outdoor temperature is about 18.C (64.4.F), cooling of the incoming air is needed.
At this theater, air is introduced to an '' undercroft'' (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, thus 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 3 m 2 (32 ft 2 ) 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 (see later Fig. 7.4), and exits through four louvered sides of the cupola, with a total outlet area at 6% of the floor area. The overall height of this stack is 15.5 m( 51 ft); a maximum of 15 ACH is expected at 3C. difference, inside warmer 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.
(c) Underslab Ventilation. Although the theater used the ground's coolth to advantage, there are some places where the ground contains radon (or other soil gases). A county map of the United States (Fig. 6.10) shows relative risk from radon, a long-term harmful gas. Buildings on former industrial sites or landfills could be threatened by other dangerous soil gases. Even ordinary sites can be threatened with methane gas from a leaking sewer line. A precaution against soil gas is to design for a passive subslab 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 at a safer place. The heat from the building drives the stack effect within this pipe.
(d) Preheating Ventilation Air. Winter fresh air brought directly into a space will improve IAQ, but at the expense of thermal comfort. Several passive or low-energy strategies are available. The office building in Fig. 6.11 is surrounded by a 4-ft (1.2-m) wide cavity between inner and outer glass surfaces. Air within this cavity is heated, both by the sun and by indoor heat sources, and rises out 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 Wallasey school (Fig. 1.10) and the Comstock Building (Fig. 8.72) for examples.
The south-facing wall is used as a winter preheat opportunity in the (unglazed) transpired collector, available as Solarwall (Fig. 6.12). 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, the fan draws directly from the outside, bypassing the solar cavity. The holes at the top of the wall serve as outlets for a 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 ft 2 (10,034 m 2 ), for an aircraft manufacturer in Quebec, Canada. For a design procedure, see Federal Technology Alert (April 1988).
Another approach to both residential winter ventilation and heat exchange is the '' breathable wall'' combined with an exhaust air heat pump. This system depends on the house being under negative pressure, assured by installing a heat pump that takes heat from forced exhaust air and invests that heat either in space heating or domestic water heating. The fresh air to replace that being expelled is drawn in through the outside walls, in a unique combination of fiberglass lap siding board, fiberglass insulation batts, a breathable sheathing, and no vapor barrier. This allows a slow, steady stream of cold air to enter and be warmed by the insulation, then enter the house. More information is available from the Canadian National Research Council.
6.5 Equipment for IAQ
This section is about 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: IAQ will be easier to achieve if the heating and cooling systems utilize forced air motion, because some filtering is built in to the air-handling equipment. However, separate air-cleaning systems are increasingly common, so radiant-only heating systems with added forced-air cleaning can yield high indoor air quality. For cooling, the economizer cycle (see Fig. 8.35) 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 (sections 5.6 d, e and 5.14 d, e) provides a continuous flow of outdoor air, more humid than outdoor conditions.
(a) Exhaust fans. This is one device we all have used. Exhaust fans remove air that is odorous and/ or excessively humid before it can spread beyond bathrooms or kitchens, creating a negatively pressured area in the process that further limits the spread of undesirable air. In buildings that have non- air-motion heating systems (radiant heat), exhaust fans are often the only built-in devices for moving air. They are often very noisy, sometimes useful for covering other sounds associated with toilets, 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 from where the replacement air will be drawn, and rarely much concern about where this unwanted air will be discharged outdoors. (Discharge into attics, basements, or crawl spaces is prohibited.)
ANSI/ ASHRAE 90.2-1993, Energy-Efficient Design of New 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 intake for these fans should be as near 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 bathrooms, in the ceiling above the toilet (and shower) in order to remove the warmest (therefore most moist) air. Some options are shown in Fig. 6.13.
A more comprehensive approach in a residence includes the addition of a '' principal exhaust fan, '' which should be centrally located (draw from the greatest space), quiet, and suitable for continuous use. Its exhaust capacity should be at least 50% of the entire system 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 by mixing with indoor air, or by heating. Figure 6.14 shows two approaches to whole-house exhaust, one for forced-air systems, one without forced air motion.
This additional, continuous exhaust capacity might cause problems of inadequate air for or spillage of fumes from combustion equipment. This equipment (furnaces, stove-top barbecues, etc.) with a net exhaust greater than 150 cfm (75 L/ s) must be provided with a make-up air fan that turns on/ off at the same time.
(b) Heating/ Cooling of Make-Up Air. Where climates are mild and/ or energy inexpensive, special equipment (Fig. 6.15) other than heat exchangers can be used to heat and/ or cool a particularly large quantity of make-up 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's skin. In warm dry climates, evaporative coolers are often used for make-up air because they are already designed to utilize 100% outdoor air. Even in hot and more humid climates, indirect evaporative cooling can help lower the temperature of make up air. In Fig. 6.16, outdoor air is at 104.F (40.C) and 10% rh (Point A). Two streams of this outdoor air are involved. An evaporative cooler cools one air stream along a constant WB temperature line to Point B, where it is now 70% rh but considerably cooler at 73.F (23.C): in the comfort zone, but rather hot and humid. At this point, it enters an air-to-air heat exchanger (see Section 6.5c). The other outdoor air stream enters the other side of this heat exchanger, again at Point A. As the two streams exchange heat, they move toward the same temperature: the evaporatively cooled stream moves from Point B to Point C, about 86.F (30.C) and 42% rh, and is then exhausted. On the other side, outdoor air moves from Point A to Point D, about 91.F (33.C) and 16% rh. This second air stream 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.
(c) Heat Exchangers. As the tightness of construction increases and fewer air changes per hour (ACH) occur from natural infiltration, 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 6.17 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 air streams 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 6.8, Part A shows representative sizes, airflows, and efficiencies. For the best diffusion of incoming fresh air through a building, the heat exchanger should be incorporated at the central forced-air fan (Fig. 6.17b). When a central forced-air system is not available, heat exchangers can be placed at various points in the building; typically, each heat exchanger is then equipped with its own fan. These may serve as make-up air units that are more energy-efficient than the devices discussed in Section 6.5( 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 require energy, will be needed.
3. Carefully locate the outdoor fresh air intake. Keep this air intake as far as possible from the exhaust air outlet to avoid drawing indoor-contaminated air back into the building. Keep it 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. 6.18). These exhaust devices are called energy recovery ventilators (ERV). Each floor of the three-story complex has four apartments, each with 1078 ft 2 (100 m 2 ) 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. All intakes and outlets are separated by a minimum of 8 ft (2 m) on the exterior walls.
The heat pipe (Fig. 6.19) 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 the porous wick. Because the only thing that moves is refrigerant and it is self-contained, the ideal of no maintenance and long life is obtained. Efficiency is from 50% to 70%; modular sizes are available to 54 in. . 138 in. . 8 rows deep.
Heat pipes can assist in the dehumidification and cooling of incoming air. The typical cooling process using cooling coils was shown in Fig. 5.43. This potentially energy-intensive process first overcools the air to '' wring out'' (condense) water, then reheats the air. The heat pipe precools 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 in this task is shown in Fig. 6.19b.
Energy transfer wheels (Fig. 6.20) 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 can 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. Afurther 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 is from 70% to 80%, and available sizes range up to 144 in. in diameter. Table 6.8, Part B shows some representative sizes, airflows, and efficiencies. An smaller example of this device was shown in Fig. 6.17c
(d) Dessicant Cooling. Another rotating-wheel process involves dessicant cooling. Dessicant cooling systems (Fig. 6.21) are attractive because they use no refrigerants that contain CFCs and they lower humidity without having to resort to overcooling air. The dessicants (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 source due to its plentiful summer availability. Waste heat from other mechanical systems may also be used. In a '' passive'' dessicant system, the heat from the building's exhaust air is enough to release and vent the moisture removed from incoming air.
Research into materials suitable for dessicant 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 dessicant systems, see Lorsch (Ed.) (1993).
(e) Task Dehumidification and Humidification. Humidity affects comfort; for a sedentary person, a 30% rh change will produce about the same comfort sensation as a 2F. (1C.) change in temperature. Higher humidity adds to hot air discomfort, making evaporation more difficult, increasing skin wettedness, and increasing the fric-tion between skin and clothing or furniture surfaces. As rh exceeds 60%, problems with IAQ increase due to mold and mildew growth. Lower humidity produces sensations of cooler and drier; but too-low conditions can irritate the skin.
For spaces that need only dehumidification rather than mechanical cooling, refrigerant dehumidifiers are commercially available (Fig. 6.22). Their advantage over dessicant dehumidifiers is that the air temperature remains essentially unchanged during dehumidification (dessicant dehu-midifiers raise the temperature of the dried air). However, these devices consume energy to run the refrigeration cycle, and this is added, as heat, to the space. Accumulated water must periodically be removed from the unit, and if untended could become a source of disease. Most refrigerant dehumidifiers encounter operating difficulties at air temperatures below 65.F (18.C), 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, often used to relieve symptoms of respiratory illnesses. Again, the problem of bacterial and mold growth in water reservoirs arises; some products add a ultraviolet light (UV) bulb to counteract this threat.
( f ) Filters. There is a wide variety of particle air pollutants, described in Table 6.2. The particulates of larger size 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 6.23 compares filter groups and testing methods with associated filter efficiencies. (A new ASHRAE Standard method is under development, and will be issued under 52.2P.) Groups I, II, and III are widely used and are generally illustrated in Fig. 6.24. 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 6.9.
Again, it is wise to remove pollutants at the source. ASHRAE Standard 62-1989, Ventilation for Acceptable IAQ, recommends dust collectors rather than filters when the dust loading equals or exceeds 10 mg/ m 3 . This reference offers considerable useful information about filters.
Particulate filters. Particulate filters are very common; 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 properly considered to be air cleaning equipment. Media filters are much finer, using highly efficient filter paper in pleats within a frame. They work 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 the air turbulence. Particulate filters need regular maintenance, especially the media filters, which can become blocked and cause damage to HVAC equipment 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 6-month minimum life-cycle.
Adsorption filters. Adsorption filters are for gaseous removal and vary according to the pollutant in question. Activated-charcoal filters are the more 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 regularly be regenerated or replaced.
High-quality adsorption filters contain gas adsorbers and/ or oxidizers and are of sufficient capacity to remain active over a full service cycle of 6 months at 24 hours per day, 7 days per week. The air velocity should allow the air to remain in the filter about 0.06 second.
Air washers. Air washers are sometimes used to control humidity and bacteria 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 velocity the air, the less the filtering efficiency. In the 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 concern about IAQ, buildings were often designed with rather crude panel filters only at the HVAC equipment itself; they were primarily intended to intercept materials that might adversely affect combustion or heat exchange. In addition to these equipment-protecting devices, a building deserving a high IAQ will now have a combination of high-efficiency particle filters and adsorption filters.
The panel filters provided with the HVAC equipment are usually located upstream from the unit's fan. Subsequently, the high-efficiency particle and adsorption filtering system should be located downstream from the HVAC cooling coils and their drain pan, to ensure that any micro-biological contaminants living on those wet surfaces are removed rather than being distributed throughout the building.
In buildings where the filtering system must be integrated with the central HVACsystem (typical of small buildings), that system should be capable of continuously circulating at the rate of 6 to 10 times per hour, and of operating against the rather considerable static pressure that results from high-efficiency filters.
(h) Ultraviolet Light (UV). Since early in the 20th century, ultraviolet light 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 devices 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 higher) than at colder temperatures. These devices take up very little space (they work within ductwork) and generate no ozone or other chemicals. They are even more effective when installed in a UV-reflective duct interior: aluminum seems the best UVreflector commonly available. Tube life is from 5000 to 7500 hours. UV light also looks promising as treatment for VOCs. The National Renewable Energy Laboratory is helping develop a process that bombards polluted air with UV in the presence of special catalysts. Pollutants including cigarette smoke, formaldehyde, and toluene are quickly broken down into molecules of water and carbon dioxide.
The EPICenter at Montana State University was introduced in Fig. 1.5. This building contains both chemistry laboratories and classrooms, and is considering photocatalytic oxidation as a process for both laboratory fume hoods and general indoor air quality. UVlight is focused on a catalyst in the presence of water vapor, to destroy airborne microorganisms and VOCs. Hydroxy radicals are generated in this process, more powerful than chlorine in their ability to oxidize VOCs and neutralize microbes. The combination of IAQ strategies in this project is shown in Fig. 6.25.
(i) Individual Space Air Cleansing. Energy conservation considerations have reduced the air circulation rate in many central air handling systems; moving less air saves energy used by fans. One result can be a poor distribution efficiency, which means 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. 6.26.
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. 6.27. In this equipment, a mixture of outdoor and indoor air is passed through a complex electrical field produced by both a high-voltage dc and a 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 exhaust air), where it is filtered to remove the larger particles.
Portable air cleaners abound. One more elaborate model combines ultraviolet germicidal light 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 m 3 ).
( j) Controls for IAQ. There are now a large number of air quality monitoring devices, some of which control the operation of related equipment. One of the oldest and most simple devices measures the concentration of carbon dioxide (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 build-ups due to dense occupancy. In the Bedales school theater (Fig. 6.9), the operation of the exhaust air dampers in the cupola is controlled by a BMS. Under ordinary conditions, the damper openings are 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 velocity could produce discomfort), the presence of wind and rain (openings limited), or by a fire alarm (fully open to aid in smoke extraction).
Where air pollution is expected from sources in addition to humans, monitors may be installed for carbon monoxide, or for combinations of VOCs, or fuels such as propane, butane, or natural gas; even for a 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; mounting height depends on which gas is to be monitored.
Such devices can regulate ventilating heat exchangers, such as the ERV (shown in Fig. 6.18). This could be especially useful in 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.
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