MEASUREMENT OF VENTILATION RATE BY CONCENTRATION-DECAY METHOD WITH LAW-ENVIRONMENTAL-LOADING TRACER GAS

Transcription

1 MEASUREMENT OF VENTILATION RATE BY CONCENTRATION-DECAY METHOD WITH LAW-ENVIRONMENTAL-LOADING TRACER GAS M Hori 1,*, M Soma, 2 and T Mizoguchi 3 1 Yokohama National University; Yokohama, Japan 2 Urban Renaissance Agency, Tokyo, Japan 3 Urban Renaissance Linkage Co. Ltd., Tokyo, Japan ABSTRACT Tracer gases and its monitoring method were studied in measurement of ventilation rate by use of concentration-decay method. Five kinds of gases of sulfur hexafluoride, nitrous oxide, carbon monoxide, carbon dioxide and isobutene were compared from viewpoints of global warming potential, toxicity, performance and cost of the gas and the monitor. The system of isobutene and a photo ionization monitor for the measurement was proposed and the procedure of the system and performance conditions in field was studied. INDEX TERMS Ventilation rate, Tracer gas, Low-environmental-loading, Photo ionization monitor, Isobutene INTRODUCTION Information of ventilation rate by measurement of concentration decreasing of a tracer gas give us air life in indoor space. Carbon dioxide and sulfur hexafluoride are usually used as the tracer gas(jis A.146, ASTM 1993). Measurement in concentrations of order of 2 to 5 ppm with carbon dioxide consumes large amounts of liquefied carbonate for one time. Sulfur hexafluoride used in concentrations of a hundredth of carbon dioxide several tens of ppm has a very high global warming potential. In recent years N 2 O of 31 in global warming potential(gwp) was proposed in ISO(ISO CD 16-8 Draft 2 October 24) and is used at concentration of 5ppm. The product of GWP and the concentration for use is appreciably smaller than that of sulfur hexafluoride of 237 in GWP, while it is appreciably larger than that of carbon dioxide used approximately at 2 ppm. On the other hand, carbon monoxide of less than 1ppm except for nearby loads and smoking rooms can be regarded as one of the tracer gas. For example, Carbon monoxide once was applied to the measurement of air change rate in ice skating arenas (Demokritou p et al. 2). However, it nowadays is hardly used to measure a ventilation rate. It is seemed to be acutely harmful. In this study isobutene was used as law-environmental-loading tracer gas instead of these gases. Its concentration was monitored with a monitor on PID: photo-ionizing detector. Performance characteristics of this system were examined to fix the procedures and conditions for measurement of ventilation rate. MATERIAL AND METHODS Isobutene, 1-butene, of a liquidized gas in cylinder of 3 Kg on market was used. The PID monitor is a VOC monitor of Mini RAE 2 type and 11.2eV lamp made by RAE Systems Inc. Sulfur hexafluoride was monitored by photo-acoustic PGM- 76 of B & K Inc.(PAS). They have function of a data logger and the outputs were recorded every minute. VOCs in an object room were collected to a charcoal tube and analyzed with CS 2 abstraction and GC-MS. Ventilation rate was measured as following procedures: The tracer gases were discharged from the respective cylinder in places in an object room and air of the room was stirred with two blowers. Output of the monitor was monitored during the measurement. Ventilation rate was calculated by the following formula. * Corresponding author horim@ynu.ac.jp 3338

2 VR (N)= Ln(C ) Ln(Cτ 1 ) / τ 1 While C : Concentration at starting time of time when the concentration-decay rate have become constant approximately; Cτ 1 : Concentration at time when τ 1 elapse; τ 1 : measurement time (h). C and Cτ 1 is readings of monitors. Reactivity of isobutene with ozone was examined by a chamber test as the followings. The air glass chamber of 2L in volume has a fan for stirring and an ultra violet(uv) lamp for ozone generation and the concentrations in the chamber was monitored with the PID monitor and an ozone monitor of UV absorption. The lamp was stopped when ozone attained at 1ppm, then 2% isobutene of 1 ml was injected with a glass syringe to the chamber. After the gases have mixed, VOCs of 1ml in the chamber were determined by a gas chromatograph. RESULTS AND DISCUSSION Comparison of isobutene and sulfur hexafluoride Both of isobutene and sulfur hexafluoride was scattered in such a way that the concentrations were approximately 1ppm in a residence of 37 m 2 in floor area and full-time ventilation and the concentrations at one point were simultaneously monitored. Time courses of outputs of monitors were shown in Figure Ln [ppm] Isobutene SF Time [min] Figure 1. Examples of time course of monitor outputs for isobutene and sulfur hexafluoride in a parallel measurement. The ordinate is plotted with logarithmic data. PID is not interfered by sulfur hexafluoride and the influence of isobutene to PAS is small. This results shows that VR of both of them was 1./h, and VR of isobutene and PID agrees with that of hexafluoride and PAS. PID outputs in new buildings and volume of isobutene scattered PID outputs in indoor air of some new buildings were surveyed in both of summer and winter, and in parallel VOCs in the air were determined. One of them had mechanical ventilation and the others were in passive ventilation. Outputs for outdoor air except for air that VOCs from automobiles and the other sources were not negligibly small is usually regarded as zero air, compared to that of indoor air. Zero of PID is adjusted with outdoor air. Figure 2 shows an example of outputs of PID for outdoor air, those for indoor air before and after scattering of isobutene. While the zero was adjusted by clean air through a column packed with active carbon. PID monitors are calibrated with a standard gas of isobutene of 1 ppm in the manual. The outputs were normalized by isobutene and shown by ppm. The calibration, however, was not need usually because of use of relative concentration course in this measurement. The level of output, background for the VR measurement, in new buildings in summer and winter in Japan were, respectively, 3.8 to 9.9 ppm and 6.8 ppm in an average (n=5) and.1 to 3. ppm and 1.4 ppm in an average (n=1). On the other hand outputs for outdoor in residential area were.4 to.5 ppm and 3339

3 negligibly small Indoor.1 Indoor Isobuten[ppm] 1 5 Outdoor Tracer gas was scattered Output[V] Outdoor Tracer gas was scattered 1:3 11: 11:3 12: 12:3 13: 13:3 Time 1:3 11: 11:3 12: 12:3 13: 13:3 Time Figure 2. Example of time courses of output of PID monitor and semiconductor TVOC monitor in a field. The ordinate is plotted with law outputs, but not with logarithmic data. Left: PID, Right: Semiconductor. The relationship between the outputs and concentrations of selected VOCs measured in parallel was analyzed. PID is sensitive to unsaturated and aromatic hydrocarbon. It was cleared that the outs put is governed not by toluene, but by a-pinene in Japanese new building. A TVOC monitor of semiconductor gas sensor was applied in parallel to compare with PID. The result shown in Figure 2 (right) shows that change rate of the relative concentration is less than that of PID because of the low selectivity. On the other hand PID outputs had a high relative coefficient to TVOC of more than 5mg/m 3, but not to that of less than.5 mg/m 3. The concentration level of isobutene needed in the measurement is estimated five to ten times that of the background. The concentrations in summer and winter were, respectively, 5 to 1 ppm and 1 to 5 ppm. The background has been measured before scattering, the volume of isobutene scattered is able to be selected. The reaction of isobutene in environmental air Isobutene is one of the most sensitive gases in PID monitor, while it is unstable in indoor and outdoor air. The reactivity with, especially, with ozone is.2ppm -1 min -1 in rate constant at 25 C. Time courses of PID output and ozone concentration after mixing ozone of 1 ppm with isobutene of 1 ppm in the chamber experiment is show in Figure 3. After elapse of 1 min ozone was not detected, and the output increased for 1 min. The result shows that isobutene reacts rapidly with ozone for a few min to change to isobutene oxide and till 1 min at the latest. Concentration level of ozone of less than.1 ppm except for neighborhood of the sources such as a PPC machine and discharger of air cleaner. On the other hand the time taken to disperse tracer gas approximately uniformly is 2 min and more than in the door air under conditions in the following survey. Therefore it is regarded that influence of the reaction to the measurement is negligibly small Isobutene(ppm) Iso but yle ne[ pp m] 1 5 Isobutene Isobutylene Instruction value 1.5 Ozone(ppm) Ozone Ozone Time (min) 334

4 Figure 3.Time courses of PID output and ozone concentration after mixing ozone of 1 ppm with isobutene of 1 ppm in a chamber. Procedures of measurement The ventilation rate was measured in residences with and without mechanical ventilation system to determine the procedures. One of them was 2LDKand 83 m 2 in floor area, and did not have a mechanical ventilation system. The other was 1DK and 37 m 2 in floor area, and a ventilation system was operated. Both of them had been stirred with two pieces of electric fan. Sampling at multi points to inform an average air life in a space is desirable. One point sampling of simple method, however, was performed in this study. The procedures were as the follows: 1. Exterior doors and windows have been shut. Closets, shelves and interior doors had been opened, and the fans have also been started before measurement. If the residence has a mechanical ventilation system, it has been started before a few hours. 2. Output of PID for background in the residence was measured. 3. An operator scattered isobutene gas little by little from a mini cylinder around the rooms while he read outputs till the output showed five to ten times the background. 4. The operator went out of the room. The output was recorded with a data logger. The PID was available with a lead pipe also outside of the residence. 5. The output had been monitored for one hour or more than. The time course of outputs in a residence without a mechanical ventilation system is shown in Figure4. The result shows that a stationary state was reached in the decrease of output when 2 min or more than had elapsed from the start. On the other hand the time course outputs in a residence with a mechanical ventilation system is shown in Figure 1. It is estimated that a stationary state was reached in the decrease in a residence with a mechanical ventilation system when approximately 1 min had elapsed, compared with the decrease without a mechanical ventilation system. 4.7 Ln (ppm) Time Passage Time(min) Figure 4. Example of time course of PID output in a residential room without a mechanical ventilation system. The ordinate is plotted with logarithmic data. Comparison of measurement systems FID and semiconductor gas sensor are available for the sensitivity as the monitor of isobutene, but they are insufficient in the selectivity of isobutene because of many kinds of VOCs and the level of TVOC in indoor air. On the other hand PID sensitive for unsaturated compounds satisfies both of the sensitivity and the selectivity. Carbon monoxide is monitored with an electrochemical sensor on potential control electrolysis. Carbon dioxide is monitored on IR absorption or detector tube method. The cost of PID monitor is more expensive than that of them, and one fifth of that of PAS. An electrochemical sensor for carbon monoxide is lower in price than the other monitor, and IR for carbon dioxide is the most stable monitor. Property of isobutene is as follows: 57 in mol weight; 1.96 in gas space gravity; 1.6 to 1 % in explosion range. Isobutene scattered from a cylinder is diluted in indoor air a ten thousandth times to concentrations of 5 to 1 ppm by stirring. The ratio of lower limit of explosion and 1ppm is 1. Therefore the average concentration of room is estimated not to exceed the explosion limit. Isobutene is one of odorous gases and its toxicity is estimated to be less than that of CO and more than that of SF 6 5) though its maximum allowance concentration is yet provided. 3341

5 On the other hand the toxicity of production from isobutene and ozone 6) is negligibly small because of the low concentration in indoor air, environmental loading for photochemical pollution in summer seems not to be negligible in the local area through it is temporary. GWP of carbon dioxide, Sulfur hexafluolide and nitrous oxide are, respectively, 1, 23,9 and 31. on the other hand that of carbon monoxide and isobutene is unknown, but estimated to be equivalent to the level of carbon dioxide. The ventilation rate measurement system of a gas and a monitor is to be evaluated comprehensively, and the evaluation is relative. Comparison of five kinds of system as a ventilation rate measurement system comparison is shown in Table 1. The low evaluation of carbon dioxide comes from consume of large amounts of it. CONCLUSION The system of isobutene and PID for ventilation rate measurement was proposed and the procedure for the measurement system was established. No system for measurement of ventilation rate by concentration-decay method is perfect. There is a great difference between the four systems of a gas and a monitor. Ranking of the systems changes when the evaluation point is changed. The system of isobutene and PID was rated high from viewpoints of global warming potential and risk of acute toxicity. Table 1. Comparison of 4 kinds of system as a ventilation rate measurement system Gas Monitor Comprehensiv Gas Env. Loading Toxicity Performance Cost Performance Cost e evaluation I**** ** *** * SF 6 - *** *** * *** * 11* CO 2 * *** * ** *** *** 13* CO **** - * *** ** **** 14* N 2 O ** * *** ** ** *** 13* Evaluation:- Blank Extremely low * Low ** Middle *** Good **** Superior REFERENCES ASTM (1993) Standard Test Methods Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution, Philadelphia, PA, American Society for Testing and Materials (Standard Designation E ). Demokritou p, Chen Q., Yang C. and Spengler J: The impact of ventilation on air quality in indoor ice skating arenas, Healthy building 2 Vol.2, 47. ISO CD 16-8 Draft 2 October 24, Indoor air-determination of local mean ages of air in buildings for characterizing ventilation conditions, 6. Irving Sax N. (1984), Dangerous Properties of Industrial Materials Sixth Edition, VAN NOSTRAND REINHOLD COMPANY. JIS A.146, Measurement method for indoor ventilation (Carbon dioxide method). Wilkins CK., Clausen PA., Wolkoff P., et al. 21. Formation of strong airway irritants in mixtures of isoprene/ozone and isoprene/ozone/nitrogen dioxide. Environ. Health Perspect. 19(9):