Mean ventilation effectiveness - a sensitive parameter: an experimental study.

In order to improve the indoor air quality a change in the ventilation system is often one of the actions. The mean ventilation effectiveness or contaminant removal effectiveness is then often presented as a measure. Great care should however be taken when evaluating a system using these parameters. Measurements have shown that in displacement ventilation the mean ventilation effectiveness can fluctuate very much for the same air flow, the same cooling load, the same inlet - outlet devices and the same height of the tracer gas supply. The vertical position of the tracer gas supply as well as the distance from heal sources in form of people, lamps, PCs etc. is of major influence. The density of the tracer gas used in the measurements is in some cases also an influencing factor. The paper presents some results concerning these facts obtained under laboratory conditions which give an idea of the spread that can be obtained when measuring the mean ventilation effectiveness in office buildings. The paper also shows U1at an increase¢ ventilation air flow rate does not always improve the mean ventilation effectiveness in displacement ventilation; in fact the effect can be the opposite. The ventilation effectiveness was found to have a maximum at a certain ventilation flow rate.