ageing

Air age is an important index to evaluate indoor air quality in ventilated rooms. The traditional definition of air age is limited to the indoor part (i.e., the air age at the inlets of air supply diffusers is assumed to be zero). Total air age extends the traditional air age concept by considering the influence of air delivery process on air age distribution. In this paper, we report the development of an algorithm to calculate total air age in a room ventilated by multiple air-handling units (AHUs).

Recently the air quality has caused people's great attention, and the appraisal of the air quality has become a hot issue in the worldwide. This paper assets that the age-of-air should be an important index to evaluate the air quality. Since the architecture place becomes more and more complicate and diversity, the traditional searching method has a lot of limitations. With the development of computer technique, the superiority of computational fluid dynamics (CFD) method becomes more and more obvious, thus the CFD begins to be applied to the HVAC.

This study has the objective of applying the "age of air" concept and tracer gas techniques in the evaluation of ventilation characteristics of workplaces equipped with a general ventilation system. A Class I OK cleanroom was selected for this study and C02 was used as a tracer gas. After the tracer gas was released in the form of a pulse into the cleanroom, its concentration was measured as a function of time at various locations by non-dispersive infrared photometers. The average age of air at each location was then calculated from the C02 concentration data.

The work presented in this paper is aimed at the definition of tracer gas experimental procedures for measuring the air change rate, the age of air and the air change efficiency in real buildings under mechanical ventilation conditions. The measurement procedures, based on the decay method, were validated in a special experimental chamber and implemented in two rooms of a building under real operating conditions. Measurements of volumetric flow rate through the air ducts of two buildings, performed by means of the constant emission rate method, will be shown and commented.

A condition often assumed when designing a naturally ventilated building is where air enters at low level and leaves at high level due to the stack effect. It then follows that, at upper levels, the air may be relatively 'stale' since it has previously passed through the lower storeys. An analogous situation may arise when wind is blowing, in which the air entering through the windward face becomes stale as it passes through the building to the downwind sections. It is not well understood how ventilation may, in reality, be affected by this.

After a short introduction of the "age of air" concept in ventilation, a theorem of conservation for the age of air at the outlet is presented. Restrictions to validity and two applications (to measurements and numerical simulation) are described. A rigorous analytic proof is shown based on field theory.