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Laboratorieundersøgelser af luftrenseres effekt over for tetrachlorethylen

Summary and conclusions

• Efficiency of the air cleaner
• Efficiency of the air cleaner in practice
• Adsorption capacity
• Outcomes of field investigations

The purpose of the project was to investigate the effect of an air cleaner on the concentration of tetrachloroethylene in room air. Investigations were conducted on a mechanical air cleaner consisting primarily of a dust filter, an electrical fan and an activated charcoal filter mounted in a sound-absorbing cabinet. The cleaner is intended for use as a provisional, acute preventive measure in dwellings where the indoor climate is unacceptably influenced from a nearby soil and groundwater contamination.

The investigations were performed in a 32 m² test-room in the laboratory at the Danish Building Research Institute.

The cleaner investigated was developed by NIRAS - Consulting Engineers and Planners A/S in co-operation with Østergaard Filter ApS and Fyns Amt. The outer dimensions of the unit are approximately 60 x 40 x 30 cm and it weighs approx. 29 kg. The activated charcoal filter weighs approx. 4 kg.

The investigations included measurements of the mechanically induced airflow through the cleaner and analyses of the efficiency of the cleaner at high as well as low room air concentrations of tetrachloroethylene (initial concentration approx. 20 mg/m³ and approx. 200 µg/m³). In addition, the charcoal filter was exposed to very high concentrations in order to investigate the liberation of tetrachloroethylene from the highly contaminated filter.

The concentration of tetrachloroethylene in the room air was measured using a photo-acoustic gas analyser (make Innova type 1312) as well as membrane inlet mass spectrometry, MIMS. The detection limit for tetrachloroethylene is about 200 µg/m³ for the Innova and about 0,5 µg/m³ for MIMS.

Efficiency of the air cleaner

The investigations showed that the performance of the air cleaner, with respect to reducing the concentration of tetrachloroethylene in the room air, matches what can be deduced theoretically. For practical purposes the figure below can be used for assessing the reduction in the room air concentration of tetrachloroethylene.

The figure shows the ratio of the equilibrium room air concentration with the cleaner in operation (c_{room, 0}) and without the cleaner in operation (c_room) as a function of the ratio of the airflow through the cleaner (q_cleaner) and the outdoor air supply to the room (q_room). The red line in the figure illustrates as an example, that if the cleaner is placed in a room where the outdoor air supply to the room (q_room) is half the airflow through the cleaner (q_cleaner), i.e.q_room = 2, the concentration in the room air (c_{room, 0}) can be expected to be reduced to about 35 percent of the initial concentration (c_room). It is assumed that the concentration of tetrachloroethylene at the exhaust of the air cleaner is zero.

Efficiency of the air cleaner. The curve shows the relation between the equilibrium concentration in the room air with the cleaner in operation (c_{room, 0}) and without the cleaner in operation (c_room) as a function of the ratio of the air flow through the cleaner (q_cleaner) and the outdoor air supply to the room (q_room).

Efficiency of the air cleaner in practice

With regard to ventilation in dwellings, the current building regulations dictate a ventilation rate of 0.5 air changes per hour. In multi-storey buildings the ventilation must be provided mechanically whereas in detached single family houses the ventilation may be either mechanical or natural.

However, a large number of dwellings have been built according to earlier building regulations where for example the requirement for mechanical ventilation in multi-storey buildings did not exist. Field investigations have shown that on average the air change rate in existing dwellings is somewhat lower than 0,5 air changes per hour. Measurements in flats in renovated and non-renovated multi-storey buildings built between 1930 and 1960 showed an average ventilation rate of about 0.47 air changes per hour. In naturally ventilated detached single family houses built during the eighties the average ventilation rate was measured at around 0.35 air changes per hour.

Assuming a ventilation rate of about 0.4 air changes per hour in a dwelling of 100 m², the outdoor air supply to the dwelling is about 90 m³/h. Assuming three air cleaners are placed in the dwelling and the fans in the cleaners are adjusted to 75 m³/h equalling about 2/3 of maximum performance, the ratio of the air flows mentioned will be 2.5 (q_room = 90 m³/h, q_cleaner = 225 m³/h, q_cleaner/q_room = 225/90 = 2.5). From the green dashed line in the above figure it appears that the resulting equilibrium concentration can be expected to be about 30 percent of the initial concentration.

The investigations of the air cleaner were performed in the laboratory in a room of 11 m² and a volume of 32 m³ and a mixing fan was in operation in the room. The air cleaner was not tested in large rooms and it must be concluded that with respect to reducing the room air concentration of tetrachloroethylene the cleaner is performing according to expectations. In practice, the efficiency of the air cleaner will depend on the individual ventilation conditions.

Adsorption capacity

Determination of the adsorption capacity of this particular charcoal filter and specifically regarding tetrachloroethylene as a function of the concentration in the room air requires certain specific investigations, which have not been performed within this project.

For activated charcoal filters in general and at room air concentrations lower than 1 ppm the adsorption capacity can typically be estimated to be about 10 percent of the weight of the filter. The weight of the filter investigated is 4 kg. Assuming the filter will in practice be exposed to concentrations of about 1 mg/m³ (0,15 ppm) and conservatively assuming an adsorption capacity of 5 percent of the weight of the filter, the lifetime of the filter can be estimated at about 2700 hours (~4 months) of continuous operation with an airflow through the cleaner of 75 m³/h ((4000 g × 5%)/(0,001 g/m³ × 75 m³/h)).

Outcomes of field investigations

Appendix A details the outcomes and findings of field investigations in 6 dwellings. The appendix is a reproduction of a document prepared by Jacob Jensen, Skude & Jacobsen Consulting Engineers A/S who were responsible for the investigations. The measurements were performed prior to and independently of the investigations documented in the main report, and the measurements have been conducted without the participation of the Danish Building Research Institute.

The results of the measurements establish an effect from placing air cleaners in dwellings exposed to an unacceptable influence of tetrachloroethylene. However, it was not possible to reduce the concentration in any of the dwellings below the emission criteria of 6 µg/m³. Assessment of the correlation between the effect of the cleaner and external conditions has not been possible under existing circumstances.

That kind of assessment would include among other things:

• Knowledge of source strength and emission rate of the contaminants
• Knowledge of pathways in buildings
• Air change rate measurements before and after installing air cleaners
• Measurements of contribution from possible sink effects.

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Version 1.0 September 2010, © Miljøstyrelsen.