SIMPLE AND CHEAP AIR CHANGE RATE MEASUREMENT USING CO 2 CONCENTRATION DECAYS

Transcription

1 SIMPLE AND CHEAP AIR CHANGE RATE MEASUREMENT USING CO 2 CONCENTRATION DECAYS Roulet, C.-A. 1 ; Foradini, F. 2 1 LESO-PB, EPFL, Lausanne, Switzerland. (claude.roulet@epfl.ch) 2 E4Tech, Rue Louis Ruchonnet 57, CH 13 Lausanne. (foradini@e4tech.com) ABSTRACT Buildings are aired to evacuate indoor pollutants, in particular those produced by the occupants. CO 2 is a good indicator of occupancy, as long as there is no other significant source. This is the case in most buildings. Cheap, portable analysers and loggers that allow easy recording of CO 2 concentration in a room or in the exhaust duct of a ventilation system are available in the market. Peak value of the CO 2 concentration during occupancy is an indicator of the minimum airflow rate per person. Analysis of the decays observed when the occupants leave the building provides the nominal time constant of the ventilated space, which is directly dependent on the outdoor airflow rate from the ventilation system and infiltration. Depending on the state of the ventilation system during the decay, this method provides either the total outdoor airflow rate provided by the system, or the infiltration rate. When combined with a simple pressure differential measurement, this method can also be used to check air tightness of building envelopes. A user-friendly computer program helps with the interpretation of the records. The paper describes the method, its application to several rooms and buildings, and its validation by comparison with SF6 tracer gas measurements. RÉSUMÉ Les bâtiments sont aérés essentiellement pour évacuer les polluants, notamment ceux produits par les occupants. Le gaz carbonique (CO 2 ) est un bon indicateur du débit d'air neuf par personne, dans la mesure ou aucune autre source n'interfère, comme c'est le cas dans la plupart des bâtiments. On trouve sur le marché des analyseurs légers et relativement bon marché, qui permettent d'enregistrer la concentration de CO 2 présent dans une pièce ou dans la canalisation d'extraction d'air au cours du temps. Les valeurs maximales de cette concentration pendant la période d'occupation donnent une idée du débit d'air minimum par personne. L'analyse de la décroissance de la concentration que l'on observe après le départ des occupants permet d'obtenir la constante de temps nominale et le débit d'air spécifique (ou taux de renouvellement d'air) de la pièce ou de la zone ventilée par l'installation de ventilation. Suivant que l'installation de ventilation fonctionne ou non, on obtient soit la constante de temps nominale globale, soit celle relative à l'infiltration seulement. Cette méthode, combinée avec une simple mesure de pression différentielle, permet de contrôler l'étanchéité des enveloppes de bâtiments. Un logiciel convivial a été développé, qui facilite l'interprétation des enregistrements. Cette communication présente la méthode, son application à plusieurs locaux ou bâtiments, ainsi que sa validation par comparaison avec une mesure avec un gaz traceur inerte(sf 6 ).

2 INTRODUCTION Carbon gas is naturally present in the air at a ground concentration of about 36 ppm in clean air. In cities and areas with strong human activity, this concentration may be higher, up to 7 ppm. Occupants, pets, and flue-less heating or cooking appliances are CO 2 sources in buildings. Therefore, the concentration within buildings is larger than outdoors. In properly ventilated buildings however, this concentration should not exceed 15 to 2 ppm. The indoor concentration of any contaminant depends on: - the contaminant source strength - the airflow rates (from outdoors and adjacent rooms) - the concentration of that contaminant in outdoor air. The general relationship between these variables may be rather complex, but in many cases, simple algorithms can be used to obtain useful information. METHOD CO 2 generated by occupants can be used as a tracer gas, since it is easy and cheap to measure. There are compact and light CO 2 analysers in the market for a few thousand Euros, which includes a data logger. If the source strength is known, airflow rates can be calculated from a record of CO 2 concentration using the usual techniques. Equivalent outdoor airflow rate Air may enter into a measured zone not only directly from outdoors, but also from neighbouring zones, whose CO 2 concentration may differ from outdoor air. These inter-zone airflows influence the CO 2 concentration in the measured zone, but can be measured only with complex and expensive techniques [1]. The concept of "equivalent outdoor airflow rate" is introduced to offset this inconvenience. It corresponds to the outdoor airflow rate that would result in the same CO 2 concentration in the measured room without inter-zone airflows. In the following, this quantity will simply be mentioned as "outdoor airflow rate". Outdoor airflow rate per person An adult person produces on the average and for most of the time (i.e. quiet or doing light work, about 1 W metabolic rate) about 2 l/h carbon dioxide. At steady state, and assuming that occupants are the only CO 2 sources, the equivalent outdoor airflow rate per person, Q e is related to CO 2 concentration C (C i indoors and C e outdoors) by: S Qe = Ci Co where S is the CO 2 source strength, i.e. about 2 l/h. The equivalent outdoor airflow rate per person can then be assessed during the periods of time when steady state can reasonably be assumed, that is when the CO 2 concentration is constant. Nominal time constant Another way is to use the CO 2 concentration records when there is no CO 2 source in the building. During these periods, the concentration decays down to background concentration, by dilution with outdoor air. If there is good mixing and if the outdoor airflow rate is constant, the decay is exponential and the decay factor corresponds to the air change rate. If the outdoor airflow rate is not constant, the decrement calculated from two measurements of concentration taken at time t 1 and t 2 provides an unbiased estimate of the average equivalent outdoor airflow rate [2].

3 EXAMPLES OF APPLICATION Decay experiments Carbon gas concentration was recorded every 5 minutes during 19 winter days in a room at the second floor of the LESO building. This room is usually occupied by one person during office hours. The evolution of temperature, relative humidity and carbon gas concentration is shown in Figure 1. A base outdoor concentration, C o, was determined from the minimum values at the end of long decay periods (week ends). It was about 6 ppm. This outdoor concentration was first deduced to the CO 2 concentration to get the increase resulting from indoor sources. On December 9 th, a stabilisation of the CO 2 concentration at 15 ppm is observed. This corresponds to 22 m³/(h person). Temp [ C] and RH [%] Temp. C RH % CO2 ppm CO2 [ppm] Figure 1: Records of temperature, relative humidity and carbon gas concentration. Several decay periods can be observed in Figure 1, especially during the nights. These decay periods are selected. For each period, initial time and final time are determined and a normalised concentration, C N, is calculated for each measurement time: ln[(c-co)/(ci-co)] C N C( t) C() = C() C Date Time [h] Figure 2: Logarithm of normalised concentration versus elapsed time since decay start. o

4 The air change rate is the slope of the line that represents ln(c N ) versus time. The lines are drawn in Figure 3, and results are summarised in Table 1. In this table, the confidence intervals (last column) are calculated from the dispersion of the concentration measurements around the regression lines, using a,1% probability (99.9% confidence). Table 1: Specific airflow rates ("air change rates") calculated from the various decays. From at To at Air change rate : :32.11 ± : : ± : :32.15 ± : :37.86 ± : :32.89 ± : :7.83 ± : :47.92 ± : : ± : :17.11 ±.3 The average specific airflow rate is.1 ±.5 h -1. Another experiment in a neighbouring room is shown in Figure 4, which was produced by the TGD computer program developed to interpret decay experiments. The rectangles, selected by hand are drawn on the decay periods. Figure 4: Records of carbon gas concentration in another office room in the LESO. The TGD software calculates the resulting air change rates and their confidence intervals. Table 2 shows the results as reported by the software. Table 2: Specific airflow rates ("air change rates") calculated from the various decays, as reported by the TGD software. from to n L δn L :1: :53:17.12 ± :7: :3:26.67 ± :26: :58:4.122 ± :45: :2: ± :39: :41:13.88 ±.1 The average specific airflow rate is.1 ±.2 h -1, but the standard deviation of the results from the five decays is.2. The significant differences between the successive decays can be attributed to variations of indoor-outdoor pressure differences resulting from stack effect and from wind.

5 Air tightness test The two office rooms mentioned above have a 45 m³ volume and a 9 m² envelope area. The equivalent outdoor airflow rate is,1 45 = 4,5 m³/h. Assuming that this air flows through the envelope leakage (in absence of occupants, office doors were closed), and that the pressure difference is about 4 Pa, the specific leakage rate can be obtained by dividing this airflow rate by the envelope area. We get,5 m³/(h m²), which fulfils the requirements of the Swiss standard SIA 18 for new buildings [3]. Such a result cannot be taken as granted, since the pressure differential was not measured, but it shows that a reasonable estimate of the leakage characteristic of the envelope can be obtained with a simple measurement. Occupancy CO 2 concentration was recorded during a day in the main exhaust duct of an office building where about 1 person are employed. The outdoor airflow rate was also measured in the ventilation unit using the constant injection tracer gas dilution method [4]. Outdoor concentration measured in the air inlet duct was 376±4 ppm. CO2 [ppm] : 11: 12: 13: 14: Figure 5: Record of carbon gas concentration in the exhaust duct of an office building. Nb. of ccupants : 11: 12: 13: 14: Figure 6: Number of occupants versus time in the building, deduced fromco 2. Figure 5 shows a quasi-steady state period between 1:3 and 11: at about 415 ppm, that is 4 ppm above outdoors. Assuming that the CO2 source strength corresponds to 1 persons, we get 5' m³/h for the equivalent outdoor airflow rate. An independent measurement of the outdoor air flow rate pulsed by the air handling unit gave 5'±1' m³/h. Using this airflow rate, the number of occupants in the building can be calculated for the next hours, resulting in Figure 6. This curve is of course a bit delayed in time, because of the building nominal time constant (12 minutes). VALIDATION D. Bloomfiel (BRE, UK) said in 199 that validation "is impossible, has no meaning, but is essential". The purpose of the experiment described below is therefore not to fully validate the method, but to check that it provides results of the same order of magnitude as a well accepted method like tracer gas decay with sulphur hexafluoride (SF 6 ). At the end of the lectures in a university auditorium air-conditioned by an independent unit, a decay measurement was performed using SF 6, while CO 2 concentrations was also recorded. The records shown in Figure 7 indicate three distinct periods: a strong decay during the first hour, when the air conditioning system is still on; then it is switched off for the night, and the decay results from infiltration only. In the next morning, ventilation is on again (SF 6 stronger decay) while students attend another lecture (CO 2 concentration increase).

6 Normalised concentration CO2 SF6 2 3 Elapsed time (hours) Figure 7: Records of SF 6 and CO 2 concentration in an auditorium after a lecture. Rectangles mark the records used for comparisons. Results shown in Table 3 show a fair agreement, except for the time period 3, when the concentration is close to zero. Table 3: Comparison of air change rates from CO 2 and SF 6 decays CONCLUSIONS Air change rate from ID SF 6 CO ± ± ±.8.29 ± ± ±.6 The feasibility of determining the specific airflow rate from the decay of CO 2 concentration is proven. This method has the advantages of decay measurements: facility, rapidity, cheap and light measurement material, but also their inconveniences: impossibility to follow changes in airflow rates, necessity of perfect mixing at the beginning of the test, and "global" assessment including air coming from neighbouring rooms. This method, combined with a simple pressure differential measurement, can be used to check the air tightness of building envelopes. ACKNOWLEDGEMENTS This work, including tests and the development of the TGD software, was financed by the Swiss Federal Office of Energy (OFEN) under contract Nr The authors cheerfully thank Mr.Zuraimi Bin Mohd Sultan from the National University of Singapore, for having performed the measurements in the auditorium. REFERENCES 1. Roulet, C.-A. and L.Vandaele, Airflow patterns within buildings - measurement techniques. AIVC technical note , Bracknell, Berkshire RG124AH, GB: AIVC ASTM E : Standard test method for determination of air leakage rate by tracer dilution., in Annual Book of Standards, ASTM, Editor. 1988, ASTM: Philadelphia, PA. 3. SIA 18: Isolation thermique et protection contre l'humidité dans les bâtiments. 1999, SIA: Zurich. 4. Roulet, C.-A., et al., DAHU: Diagnosis of Air Handling Units, in Air Distribution in Rooms - Ventilation for Health and Sustainable Development, H.B. Awbi, Editor. 2, Elsevier: Reading, UK. p I