air change efficiency

The main task of every ventilation system is to dilute and extract pollutants from indoor air, most importantly in occupied space. This is usually achieved by exchanging polluted indoor air with less polluted outdoor air. In the case of a mechanical ventilation system, this process requires a fan power to be provided which is approximately proportional to the power of three to the resulting airflow. Because of this, reducing the necessary airflow to be provided by the ventilation unit e.g., by 10% would lead to a reduced power supply of about 27%.

Outdoor air change qualifies the air that enters into the buildings. The outdoor air moves freely along the urban mesh favoured by the wind forces and stresses. Buildings, trees and other constructions alter the natural air flow pattern inside the cities, creating stagnated air masses in those wind-protected regions. Some outdoor spaces such as light shafts and confined light shafts inhibit the correct exchange of the stagnated air with fresh air coming from the outskirts and suburban areas. 

The paper investigates the possibility for using a traditional ventilation system with ceiling mounted diffusers to provide heating under winter time conditions in relatively cold climates – in buildings with low transmition losses such as “passive houses”. The analysis is done through a number of CFD simulations of a simplified office. It is shown that even small over-temperatures reduce the Air Change Efficiency substantially. On the other hand even very small internal heat sources increase the efficiency.

Nowadays, indoor air quality has become a major concern. Regarding the fact that people spend most of their time indoors, it is necessary to study the performance of the ventilation system in order to limit the risks on occupants’ health. This study evaluates the ventilation effectiveness of supply-only ventilation (SOV) and extract-only ventilation (EOV) in terms of air exchange efficiency and contaminants removal effectiveness. These indicators are measured as function of air change rate and inlet/outlet devices positions using the gas tracer technic.

The "step-down" tracer gas technique was used to evaluate experimentally in a mechanically ventilated test room the effect of varying thermal boundary conditions, inlet flow rates, and inlet - exhaust grids position on the Air Change Efficiency (ACE) values. The paper shows that the measured global ACE values are strongly correlated to the Archimedes number (Ar).

The purpose of this study is to identify the ventilation effectiveness of a displacement ventilation system in a concert hall with 501 seats, where a large amount of outside air is required for ventilation. Displacement ventilation was considered appropriate to reduce the amount of outside air. Light bulbs were placed on all the seats to simulate the heat source from the audience. From the measured concentrations, the local mean age of air at the breathing point with the displacement ventilation system was found around one third of that of the fully mixed condition.

The influence of a thermal heterogeneity boundary conditions on the air change efficiency (ACE) of a mechanical ventilation system in a test room was experimentally evaluated by means of the "step-down" tracer gas technique in 24 different experimental conditions. The experiments were performed under isothermal condition, varying the air supply temperature with respect to the walls and varying the surface temperature of a wall with respect to the other walls and the supply air, simulating both heating and cooling situations.

The age of the air in a room is normally determined either from a pulse response or from a step change response (up or down). There are a certain number of problems involved in applying these two theoretical models, especially those associated with the duration of the injection, which must either be infinitely short or infinitely long. A hybrid method that consists of injecting a known quantity of tracer for a given time offers the advantages of both methods.