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Figure 4. kelaticnshiNs betweer the roorn acceptaGility factor, V2 anc the room contamindtion factor, Q2, as a function of the air cleaner efficiency, ~L, when the passive active air excnange ratic, M, is negligible (i.e., r, = 0). z N O sissLszzOz

Figure 5. kelationsr.iFs uetween the rooM acceptability factor, F:., and the rocm contamination factor, Q2, as a function of the air cleaner efficiency, E2, when Che passive to active air exct:ange ratio, M, is low (i.e., M = 0.1). 4Z6B4SIZOZY

Figure 6. keiaticnsnips between the room acceptability factor, K2, and the roon, contamination factor, Q2, as a function of the air cleaner efficiency, E` when the passive to active air excnange ratio, M, is moderate (i.e., M = 0.5). ezssLsizOZ

., , ! Q=•N/I Txo 17 0 0 \ ~~ . . t •o\ . ~ ~ ! G S 1 1.3 7 O= • N/ITTxe ,-~ t•., Figure 7. Itelationships between the roon acceptability factor, K2, and the room contamination factor, Q2, as a function of the air cleaner efficiency, E2, when the passive to active air exchange ratio, M, is high (i.e., M= 1.0). szssLsizOz

WOODS AND KRAFTHEFER ON OCCUPIED SPACES FILTRATION 207 which holds for all values of H. The graphs in Fig. 4 are presented for M= 0, which represents interior zones where infiltration and na- tural ventilation are negligible. Fig. 5 is presented for M= 0.1, which represents exterior zones with relatively tight envelopes. For example, if the supply air rate for thermal control is 6 air changes per hour, the infiltration rate would be 0.6 air changes per hour. In Fig. 6, M= 0.5, which is representative of exterior rooms that may have windows partially opened. Fig. 7 represents conditions in which the rates of passive and active air exchanges are equal, M= 1.0, This condition may exist in some occupied spaces with large window areas. Thermal control in this ca"se would be difficult and probably energy intensive. The graphs in Figures 4 to 7 identify several! performance charac- teristics that are useful for evaluation of alternative room venti- lation control strategies: o For all values of Mi the Roan Acceptability Ratio, K2, will ex- ceed 1.0 (i.e., the indoor concentration will exceed the outdoor concentration) for all values of Q2 greater than zero when re- moval control is not employed ( i. e. , E2 = 0) . o When removal control is employed (i.e., e2 > 0), a critical value of Q2 exists for all values of M at which a "cross-over condition" exi sts: i o For values of Q2 smaller than the critical value, lower Room Acceptability Ratios; K2, can be achieved by minimizing the outdoor air ratio, H. That is, when the generation rate of a contaminant within the mini-environment and tNAC system is less than the transport rate of the contaminant from outdoors, the ratio of indoor to outdoor concentrations will be reduced by reducinQ the percentage of outdoor air used for ventil ation. o For values of Q2 greater than the critical value, lower Room Acceptability Ratios, K2, can be achieved by maximizing the outdoor air ratio, H. That is, when the generation rate of the contaminant within the mini-environment and HVAC system is gre- ater than the transport rate of contaminant from outdoors, the ratio of indoor to outdoor concentrations will be reduced by increasing the percentage of outdoor air used for ventilation. o The critical value of Q2 is increased by increasing the effi- ciency of' the air cleaner. For example, the cross-over condi- tion is increased from Q2 - 0.2 to 1.0 when the air cleaner efficiency is increased from e2 = 0.2 to 0.9. o The rel ative importance of the outdoor air ratio, H; is greater at lower val!ues of M and s2. These relationships may be observed by ccmparing Figures 4 and 7. o When M= 0 and e2 = 0 (Fig. 4), the only method of ventila- tion control is dilution through the fN AC system. Thus, when H i's changed and Q2 remains constant, a substantial change in K2 will result. Conversely, when M- 1.0 and E2 = 0(Fig. 7), the amount of dilution air fran natural ventilation and infil-

208 FLUID FILTRATION: GAS tration may be equal to ( i. e. , H= 1.0) or exceed ( i. e. , H < 1.0) that supplied by the HVAC system, thus K2 is less sen- sitive to changes in H. o As the air cleaner efficiency increases for a specific _contam- inant, the relative importance of H decreases (i.e., see graphs for• E2 - 0.9) for alI values of M. o The value of Q2 will vary in a "constant air volune" system only as N and xo vary. Hawever in a "variable air volume" system, the value of Q2 wi11 also vary with the thermally controlled values of z and rtm. The relationship between thermal and indoor air quality control of the mini-environment is apparent form Equations 10 and 14 and i'rom Figures 4 to 7. To achieve acceptable thermal and air quality, energy consu,nption, and life-cycle costs, simultaneous control of roan tem- perature, relative hu,niditg, supply and return air rates, and parti- culate and gaseous concentrations is required. To evaluate the cap- ability of a system to control these factors simultaneously, the fol- lowing procedure is recommended: 1. Determine the minimun value of supply air flow rate, zk, re- quired to maintain thermal acceptability while the air distri- bution performance index (ADPI) is maintained at a value of at least 80% (151. 2. For the value of zriyn, determined in Step 1, the values of xo expected during the times of minimum supply air flow rates, and the expected generation rates of the contaminants in the roem and in the HVAC system during these periods, calculate the val- ues of Q2 for these contaminants. 3. Specify the acceptable values of K2 for each of the contami- nants of concern. 4. Specify the air cleaner efficiency, E2, for each of the con- taminants of concern. 5. Determine the outdoor air ratios, H, required for dilution con- trol to ccmpliment removal control specified in Step 4. 6. Compare the minimua value of H, determined in Step 5, to the value of H that will minimize energy consurption during the periods considered in Step 1. 7. Reiterate Steps 4-6 until the environmental acceptability, energy consumption, and life-cycle criteria are optimized. Personal Exposure Control Room ventilation control can be effective in maintaining accept- able indoor concentrations of contaminants. However, this control method is usually base.l on the assumptions that the contaminant is uniformly mixed throughout the controlled space. This assumption im- plies that the occupants will be exposed to the same contaminant con-

WOODS AND KRAFTHEFER ON OCCUPIED SPACES FILTRATION 209 centrations throughout the controlled space. Uniform mixing seldom occurs and, in sane cases, stratification within rooms can be signi- ficant. Thus, roan ventilation control, alone, is not always suffi- cient to provide the required air quality for personal exposure con- trol. This phenomenon has long been recognized in hospitals. Examples include: - o Laminar flow clean benches 1'n laboratories and pharmacies. o Oxygen tents in patients' rooms. o Biological cabinets in laboratories. o Portable laminar flow equipment in operating rooms. o Portable "sterile air" equipment to protect bed-ridden and am- bulatory patients that are immunosupressed. The air quality control interaction between the mini- and mi'cro- environments can be expressed in terms of the following steady-state equations: o The mass balance of the contaminant in the micro- env ironment: 2x2(1 - 1) + Nx1 - m1x1 [15] where Nxl is the generation rate of the contaminant in the micro-environment. i . o The fraction of micro- to mini'-environmental mass air flow rate: I a = m1 / rip [' 167 where a, the room-coupl ing coefficient, may be passive ( i. e. , related to ventilation efficiency) or active ( i. e. , related to the forced air into the micro-environmental by blowers). And the mass balance of air flow in the mini-environment: rtg = mg + mi 1171 Pq ua ti'ons 15 through 17 may be combi ned w ith Bq ua tion 5 to prov ide an expression for the "Micro-environmental Acceptability Ratio," K1 = xl/x2: xl~ Kl = _ _ (1 - e1) + Q1 X2 where: [18] Q1 = Nx1/(a(zmm + mi)x2) (19]' Functional relationships from Eq [18] are shown in Fig. 8 between the micro-envirormental acceptability factor, K1, and the micro- environmental contamination factor, Q1, with the air cleaner of efficiency, c1, as the parameteric function. Because of the ex- pression of Q', these relationships are valid for all values of M ( i. e. , mi/zmnl) . Fran Figure 8, performance characteristics are

210 FLUID FILTRATION: GAS identified that are useful for evaluation of alternative personal exposure control strategies: o A simple linear relationship exists between the micro-environ- mental contamination factor, Q1, and the micro-envirormental acceptability factor, KI , with El as a parameter. A critical val'ue of Q1 does not exist. o The micro-envi~rormental contamination factor, Q1, is affected by: o The supply air flow rate, zriyn, and the natural ventilation and infiltration rates, mi. For variable air volume sys- tems, or for rooms with operable windows, these changes can be significant (e.g., the room air exchange rate can vary by factors of two or more). Thus, if (zirm + mi ) is decreased due to the thermai load in the roan controlled' by a variable air volume system, the value of Ql would increase, and, if u, x2, andE1 remained constant, the value of K1 woul~d increase, resulting in a degradation of acceptability. o The roan coupling coefficient, u, which may be a passive function of room stratification or ventilation efficiency ( i. e. , a« 1). Conversely, a may be an active function of a portable or local forced air device ( i. e., fan). In the lat- ter case, the value of a may be much greater than one (e, g. , laminar flow clean bench). Note that the air flow rate into the micro- env i ronment, represented by the factor ~Y(zrifi + (ni), may consist entirely of air recirculated within the mini-environment and need not have been directly supplied by the ENAC system. o The contaminant concentration in the mini-envirorment, x2. 'Ihus, when x2 is large, the mi'cro-environmental accept•- ability ratio, K1, will be more sensitive to the trans- ported contamination than to that generated within the micro- environment. Of particular note, if the value of the factor (zmm + mi) i's large and the ai'r cleaner efficiency, el, is high, the value of x2 may be decreased before steady-state is achieved. i Although the interaction between thermal and indoor air quality control in the micro-environment is not as evident as it is in the mini-envirorment, a relationship still exists. For example, the heat from a computer terminal is first dissipated into the micro-environ- ment before it is dissipated in the mini'-envirorment and the 1wAC system. As a result, both the dry-bulb and mean radiant temperatures i'n the micro-envirornent can be significantly higher than in the mini- envi'rorment. Thus, simultaneous control of temperature, humidity, particulate and gaseous contaminants is also required within the micro-environment of sane functional areas. In others, reductions in energy consunption and operational costs may be achieved by employing micro-envirormental control strategies [13, 141. To evaluate the capability of a system to control these factors simultaneously, the following procedure is recommended:

WOODS AND KRAFTHEFER ON OCCUPIED SPACES FILTRATION 211 t 2 3 ~ S 41 . Nxl/cllZmT+ .njlx2 Figure 8. Relationships between the micro-environmental accep- tability factor, KI and micro- env i ronmental contamination fac- tor, Q1 as a: function of the ai r cl eaner eff iciency, E 1, for all values of the passive to active air exchange ratio, M. 1. Determine the minimum value of zri>m + mi from Step 1 for evalu- ation of Room Ventilation Control. 2. Specify the acceptable values of Kl for each of the contam- inants of concern. 3. If passive room coupling is expected (i.e., stratification exists), assume a is the ventilation efficiency (11]'. 4. If active room coupling is to be used, select desired value of a. 5. For the mini- env ironmental air flou rate, determined in Step 1!, the values of x2 expected for the contaminants of concern in the mini-environment and the expected generation rates of the contaminants in the micro-environment during these periods, calculate the values of Q1 for these contaminants. 6. Specify the air cleaner efficiency, e1 , for each of the con- taminants of concern, 7. Compare the resultant K1 values from Equations 18 and 19 or from Figure 8 to those specified in Step 2.

212 FLUID FILTRATION: GAS i I 8. Re-iterate Steps 3 through 7 until the environmental accept- ability, energy consumption, and life-cycle criteria are opti- mized. CONCLUSIONS We conclude that removal control is vital to the development of effective control strategies for acceptable indoor air quality. Removal control is the only method in use today that can provide assurance that indoor cncentrations of contaminants can be controlled to values less than those outdoors. Two factors are the primary de- terminants of the effectiveness of removal control: remo%al effi- ciency and air circulation. Yet, standards for evaluating either of these factors for air quality acceptability are essentially non- existent. New standards are needed that will allow evaluation of air cleaners and system performances in terms of the contaminants to be removed. For example, air cleaner efficiencies could be defined in terms of characteristic gases, vapors, and particulates; air cl eaner capacities could be evaluated in terms of products of the efficiencies and air flow rates through them; and system performance could be evaluated in terms of air cleaner capacities, percent of air recir- culation, and effectiveness of roao air distribution. We also conclude that air quality control cannot be isolated from thermal control of occupied spaces. Stratification within the oc- cupied spaces is highly influenced by convection and system air dis- tribution patterns; psychrometric processes within the system can result in interactions with the aiYborne contaminants; and the !N AC systems, themselves, may act as secondary sources of contamiantion if they, are not properly maintained. These relationships are parti- cularly important to the coupling between zones within an occupied space, such as "smoking"'and "non-smoking" zones in office areas, or patient and staff zones in chemotherapy treatment areas in hospitals. Finally, we conclude that control strategies can be developed for occupied spaces to optimize occupant acceptability with owning and operating costs. By minimizing contaminant generation rates whilie providing the appropriate combination of removal and dilution rates as functions of thennal and contaminant 1'oad characteristics, occupant acceptabil ty can be achieved and~ costs of operation, can be reducedy thereby increasing productivity within the occupied spaces. REFERENCES [1] ASHRAE Standard 62-1981. Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society of Heating, Refrigerating, and'Air Conditioning Engineers, 1981. (2) Krafthefer, B.C., and Bonne, U. Energy Use Implications of Metta- ods to Maintain Heat Exchanger Cleanliness. ASfIRAE Trans. 92 (Part 1), 1986 (In Press).