VENTILATION STRATEGIES TO IMPROVE INDOOR AIR QUALITY

Transcription

1 VENTILATION STRATEGIES TO IMPROVE INDOOR AIR QUALITY Q Luo *, G Tang and W Huang College of Civil Engineering, Hunan University, Changsha 4182, China ABSTRACT IAQ may be not always improved by only increasing ventilation rates, and favorable ventilation strategies are the keys to improve IAQ. The impacts of ventilation on IAQ are discussed, measured or experimented on four aspects. (1) Some regeneration compositions owing to potential chemical reactions among indoor contaminants on some indoor conditions may be more harmful to habitants; (2) Some outdoor contaminants may flow into indoor space with fresh air, and return air may be another indoor contaminants source; (3) A favorable air distribution and high ventilation efficiency is the key to improve IAQ in breathing zone; (4) Cleaning duct system may play double roles, namely to improve IAQ and energy efficiency. Some ventilation strategies to improve IAQ are proposed and discussed, they are: (1) Low-temperature air supply can increase acceptability of IAQ and decrease the rate of VOCs emission; (2) Demand-controlled ventilation systems can automatically control the rate of ventilation in response to variations of IAQ; (3) A task supply ventilation system can focalize fresh air to breathing zone. INDEX TERMS IAQ; Potential chemical reaction; Air distribution; Ventilation strategy INTRODUCTIONS Recognition of the critical relationship between ventilation and indoor air quality (IAQ) may seem to have occurred relatively recently. Not until 1989 did ASHRAE modify their ventilation standard to address the growing concerns regarding degraded IAQ and sick buildings. Indoor contaminations ranging from volatile organics to microbial organisms did not begin to substantially influence recommended ventilation rates until after the World Health Organization provided a working definition of sick building syndrome in 1983 in response to widespread reports of work related health complaints. A review of the history of ventilation reveals that IAQ concerns played a significant role in the development of technologies and strategies for building ventilation and heating during the nineteenth century. IAQ, however, may be not always improved by only increasing ventilation rates, and proper ventilation strategies are the keys to improve IAQ. This paper discusses the impacts of ventilation on IAQ and proposes some favorable ventilation strategies. IMPACTS OF VENTILATION ON IAQ Indoor Contaminants And Potential Chemical Reactions A building space can be regarded as a chemical vessel for contaminants emitting from furnishings, building materials and habitants, and contaminants also flow into with supply air and out with exhaust air simultaneously. Potential chemical reactions may occur among ozone and indoor pollutants on some environment conditions and some regeneration compositions may be more harmful to habitants (Weschler et al 1997). The potential chemical reactions among indoor contaminants mainly include as follows: (1) Chemical reactions between O 3 and NO X. The reaction velocity basically depends on the NO X concentration and the indoor temperature. O 3 +NO NO 2 +O 2 O 3 +NO 2 NO 3 +O 2 (2) Chemical reactions between O 3 and unsaturated hydrocarbon. R 1 R 2 C=CR 3 R 4 +O 3 R 1 R 2 C=O+R 3 R 4 COO R 3 R 4 COO presents strong oxidability. Some regeneration compositions, such as CO 2 NO 2 SO 3 or O 3, may be * Corresponding author hunanluoqinghai@163.com 3313

2 produced respectively because of chemical reactions between R 3 R 4 COO and CO NO SO 2 or O 2. R 3 R 4 COO may be auto-decomposed into aldehyde, ketone, acid and analogous low-carbon substances. (3) Photochemical reactions of NO X. Such reactions are the basic conditions of photochemical smog. NO 2 +hυ (29~43 nm) NO+O( 3 P) O( 3 P)+O 2 +M O 3 +M O 3 +NO NO 2 +O 2 (4) Photochemical reactions of aldehyde or ketone: The photolysis of formaldehyde may present in two approaches: HCHO+ hυ (λ<37 nm) H+HCO; H 2 +CO; H+O 2 HO 2 ; HCO+O 2 HO 2 +CO; The photolysis of high-carbon aldehydes may present as follows: CH 3 CHO+ hυ CH 3 +HCO; CH 4 +CO; H+CH 3 CO; H 2 +CH 2 CO; The photolysis of ketones may be following: CH 3 C=OCH 3 + hυ CH 3 +CH 3 CO; While the photolysis of anisomerous ketones may present following two approaches: CH 3 C=OC 2 H 5 + hυ CH 3 CO+C 2 H 5 ; CH 3 +C 2 H 5 CO O 3 is mainly produced by furniture, or follows into the space with supply air. It is a strongly oxidizing agent. Outdoor luminaries or indoor illuminants provide the basic conditions of photochemical reactions. Aerosols play the role of catalyzers of chemical reactions among indoor contaminants. Outside Supply Air And Contaminants. Adequate outdoor ventilation air to dilute the concentration of indoor air contaminants has been proved an essential, practical, and cost-effective means to improve IAQ in practice. However, some outdoor contaminants, such as SO 2 NO X CO, bacteria and dust, may flow into indoor space, and return air may be another indoor contaminants source. Figure 1 and figure 2 illustrate the surveying data of air input, indoor CO 2 concentrations, indoor and outdoor concentrations of respirable dusts of a complex building, which was surveyed at seven times in a day in July 24 in Changsha. This is a building complex integrating a hotel, office rooms, council-chambers and other recreation rooms. The fresh air systems could not work efficiently due to wanting management. The fresh air fans were equipped dispersedly, and not controlled by the auto-control system. The majority of fresh air louvers were thoroughly opened while the IAQ was investigated at each time. Only impurity with diameter more than 2 mm can be captured by filters of air-conditioning system, so the concentrations of outdoor gaseous contaminations affected IAQ directly. Fig.1 shows that the indoor CO 2 concentrations decreased with increasing air input. As can be seen from Fig.2, the indoor concentration of respirable dusts increased with that of supply air. The concentration of respirable dusts of the 2nd surveying time was the largest, namely 82 μg/m 3, because the corresponding air input was the largest, namely 157.9m 3 /h.p, which was respectively times CO2 Concen. (ppm) CO2 Concen. Supply Air Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ Ⅶ Measuring Sequences Figure 1. Indoor CO 2 concentrations varying with supply air at different measuring times PM1 (μg/m 3 ) Outdoor Air Indoor Air Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ Ⅶ Measuring Sequences Figure 2. Indoor and outdoor concentrations of respirable dusts at different measuring times Air intake (m 3 /h.p) 3314

3 of that of the other six surveying times. The air supply should be ensured according to healthy standards and the impact of return air. Improving filters performances is significant to improve IAQ. Indoor Air Distribution Menzies (1997) inquired inhabitants of two healthy buildings. He discovered that the rate of inhabitant with sick building symptom was 7.12%~14.9% for a building with fresh air input of 34 m 3 /(h p), however, 11%~14.5% for another building with fresh air input of 85 m 3 /(h p). Which illustrated that IAQ may be not always improved by only improving ventilation rates, and proper ventilation strategies are the keys to improve IAQ. Adequate out door ventilation air is necessary to dilute the concentration of indoor air contaminants, however, the IAQ of breathing zones may be effected by heating sources, contaminant sources, airflow encumbrances and air distributions simultaneously. Improving air distribution may play double roles, namely improving IAQ and decreasing ventilation rate. Three ventilation patterns were simulated by means of CFD method (Li et al 23). The Reynolds averaged Navier-Stokes equations and the RNG k-εturbulence model were used to predict 3D airflow pattern in a room, where barriers, pollutant sources and a centralized heating source were laid out. The indoor air velocity and temperature fields and CO 2 distribution were studied numerically. It was illustrated by computational results that the displacement ventilation might bring the best IAQ and thermal comfort in the breathing zone and the higher ventilation efficiency. Maintenances Of Ventilation System Dusts deposited in the duct system not only pollute fresh and indoor air, but also increase duct resistance and further decrease the air rate. On the other hand, the temperature and humidity in the air-conditioning duct system are adaptive conditions for bacteria to grow and breed. An air-conditioning system may be a contaminant source, and cleaning duct system may play double roles, namely to improve IAQ and operating efficiency. Fan (24) introduced special cleaning equipments and cleaning process for HAVC system. Table 1 presents the comparison between data of supply air, return air and bacteria in two guest rooms of a hotel before and after cleaning ducts. It indicated that bacteria decreased by 43.2%, while return air increased by 16.1%, and supply air increased by 36.2% simultaneously. The sampling and testing method was according to concerned criterion (Health Ministry of PRC, 22) Table 1. the comparison between ventilation data before and after cleaning ducts Supply air (m 3 / h) Return air ( m / ) Bacteria ( / m Before cleaning After cleaning In addition, after the air-conditioning system of an office building was cleaned, the indoor bacteria decreased by 85.3% of maximum, and supply air increased by 55% of maximum (Liu et al 23). VENTILATION STRATEGIES TO IMPROVE IAQ Low-Temperature Air Supply In the past few years, some views based on experiments were pointed out that the temperature and humidity play a remarkable impact on experimenters comfort in climatic chambers (Fanger 22). Figure 3 illustrates the results of experimenter rate dissatisfying to the IAQ with varying enthalpy in a climatic chamber (Fang et al 1999). Figure 3 indicated that the indoor temperature and humidity not only affect on habitants thermal comfort, but also remarkably impact on sensing IAQ. The 3315 Dissatisfied Rate (%) ,3% 3 h 23,5% Enthalpy (kj/kg) 3 ) 28,7% Figure 3. Impact of temperature and humidity on occupants sensing IAQ

4 acceptability of IAQ decreased with the increasing temperature and relative humidity. The latest experiment results revealed that the sensation of IAQ was better in the environment with temperature of 2, relative humidity of 4% and supply air of 3.5 l/s p than that in the environment with corresponding parameters of 23, 5% and 1 l/s p. On the other hand, the rate of VOCs emitted into the air by building materials increased with the increasing indoor temperature. Experiment results also indicated that the impacts of temperature and humidity on habitants sensation of IAQ were more remarkable than the impact on the rate of VOCs emission. In an environment with lower relative humidity, habitants may feel air fresher, and substances go moldy more difficultly (Yin 24). Energy efficiency is one of major advantages of low-temperature air supply. Experiment investigations and theory analysis indicated that dry-bulb temperature may rise reasonably with wet-bulb temperature decreasing while the comfort index was kept at the same levels. If indoor dry-bulb temperature was raised 1, air delivery can decrease 5 %~6 % regardless of sensible heat load owing to indoor temperature increasing, and refrigerating capacity can decrease about 1% correspondingly(elovitz 1999). Demand-Controlled Ventilation (DCV) Investigation on DCV is being in the ascendant (Chao 24, Pavlovas 24, Zhang 25). DCV systems provide a means of automatically controlling the rate of ventilation in response to variations in IAQ. Essentially, a sensor is used to track IAQ and to modulate the rate of ventilation to ensure that air quality does not deteriorate. DCV systems are particularly beneficial in locations of transient occupancy or where pollutant loads, specific to an environment, vary over time. DCV systems are effective when: 1) Outdoor air supply can be controlled (i.e., minimum infiltration or other losses). 2) The occupancy pattern or dominant pollutant is variable. 3) Space heating or cooling energy loads can be minimized. 4) The controlled pollutant (or pollutants) is dominant. The advantage of DCV is ventilation rate can be optimized to meet prevailing need. The disadvantages of DCV are following: 1) Sensors and control systems can be expensive. 2) Currently methods are essentially restricted to carbon dioxide and humidity control. 3) Although mixed-gas sensors can provide general control, the individual mixture of gas is uncertain. 4) High concentrations of harmful pollutants could go undetected. Task Supply Ventilation (Tsv). Conventional modes of supply air always dilute pollutants for entire building space; however, supply air diffusing to the breathing zone is no longer fresh after it mixed pollutants in above space. TSV can focalize fresh air to breathing zone. The supply air is ducted directly to occupants where flow rate and comfort conditions can be individually adjusted by locally heating or cooling. TSV decrease fresh air age and improve air quality in the breathing zones. It might provide the farthest satisfactory microenvironment for occupants according to individual demand (Martin 2). Figure 4 is comparison of CO 2 concentrations in the breathing zone between conventional ventilation and TSV in a climatic chamber. The rate of fresh air was 8.66 l/s p for both ventilation manners. 5 people were simulating to work, among which 4 people sit quietly, and 1 people walk about. As can be seen from figure 4, TSV brought a higher IAQ for occupants in shorter time than conventional ventilation with the same rate of supply air. The further experiment results revealed that TSV demanded less fresh air than conventional ventilation modes to obtain the same CO 2 concentration in the breathing zone. TSV is also more superior to conventional modes in the energy efficiency. CO2 Concentration (ppm) conventional individual Measuring Time (Min.) Figure 4. comparison of CO 2 concentrations in the breathing zone between conventional ventilation and TSV 3316

5 CONCLUSIONS IAQ may be not always improved by only improving ventilation rates. The impacts of ventilation on IAQ mainly include as follows: (1) indoor contaminants and potential chemical reactions; (2) outside supply air and contaminants; (3) indoor air distribution; (4) maintenances of ventilation system. Some ventilation strategies to improve IAQ are proposed. (1) The low-temperature air supply system can improve indoor environment and IAQ. (2) DCV systems are particularly beneficial in locations of transient occupancy or where pollutant loads, specific to an environment, vary over time. (3) TSV is more superior to conventional ventilation modes to improve IAQ in the breathing zone and to increase energy efficiency of ventilation systems. REFERENCES: Chao CYH., Hu JS., 24. Development of a dual-mode demand control ventilation strategy for indoor air quality control and energy saving. Building and Environment 39 (4) : Elovitz KM Understanding what humidity does and why. ASHRAE J. 41(4): Lin F. 24. Improve IAQ by Cleaning HAVC System, Cleaning World, 2(4): (In Chinese) Fang L., Clausen G. and Fanger PO Impact of temperature and humidity on chemical and sensory emissions from building materials. Indoor Air, 9(4): Fanger PO. 22. Human requirements in future air-conditioned environments. In: Proceedings of Advances in Building technology. HongKong, P Health Ministry of PRC, 22. Sterilizing Criterion. Edition 4. Liu D., Chen PL., Ji L., et al. 23. Air duct cleaning an effective way to improve indoor air quality. Heating Ventilating & Air Conditioning, 33(4): (In Chinese) Liddament M. 2. Ventilation Strategies. In: John D. Spengler, Jonathan M. Samet, John F. McCarthy. Indoor Air Quality Handbook, New York: McGraw-Hill Companies, Menzies D., Pasztor J. and Nunes F Effect of a new ventilation system on health and well-being of office workers. ARCH.ENVIRON.& HEALTH. 52 (5): Pavlovas, Vitalijus, 24. Demand controlled ventilation: A case study for existing Swedish multifamily buildings. Energy and Buildings, 36(1): Weschler CJ. and Shields C Potential reactions among indoor pollutants. Atmospheric Environment, 31(21): Xu L., Weng PF. and Sun WM. 23. Numerical analysis of indoor air distribution and indoor air on three ventilation patterns. Acta Aerodynamica Sinica, 21(3): (In Chinese) Ping Y. 24. Design method for ice-storage low-temperature air supply systems (1): indoor design conditions, comfort and IAQ. Heating Ventilating & Air Conditioning, 34(5): (In Chinese). Zhang L., Chow TT., Fong KF., et al, 25. Comparison of performances of displacement and mixing ventilations. Part II: indoor air quality. International Journal of Refrigeration, 28(2):