ventilation effectiveness

The transient accessibility of supply air (TASA) and transient accessibility of contaminant source (TACS) in ventilated rooms are important indices to evaluate the effect of ventilation and the indoor air quality (IAQ). These indices can be measured by experimental method or calculated with computational fluid dynamics (CFD) tools. Compared to the measurement method, the numerical method has a lot of advantages such as fast, flexible and with detailed data. In this paper, the calculation and validation of the TASA and TACS are introduced.

Air quality in offices depends on the ventilation system ability to remove contaminants from the occupied zone. In a low polluted building air quality mainly depends on the human presence and carbon dioxide is normally used as indicator of human bioeffluents.

Today an acceptable indoor air quality is mainly defined by specifying the required level of ventilation in air changes per hour or the outside air supply rate. This would be equivalent to defining the requirements for thermal comfort by specifying the level of heating or cooling in Watts. The increasing societal need for energy efficiency will often result in very tight buildings. This means that the amount of outside air supplied by infiltration is not enough to provide the required ventilation.

Exhaust effectiveness indicates how effectively contaminated air can be removed from a space, whereas air change effectiveness indicates how effectively distribute fresh air into the space. It is intended to describe the exhaust effectiveness based on the residual-life-time of contaminant in the context of logical extension of supply effectiveness based on LMA.

These Guidelines recommend the ventilation required to obtain a desired indoor air quality in a space. The first step is to decide the air quality aimed for in the ventilated space. A certain air quality is prescribed to avoid adverse health effects while a decision is required on the level of perceived air quality aimed for in the ventilated space. Three different comfort levels are suggested. The next step is to determine the pollution load on the air caused by pollution sources in the space.

Mixing and displacement ventilation are common systems in commercial buildings, while mixing ventilation is used in residential buildings. Displacement ventilation provides fresh air to the occupied zone in a more efficient way than mixing ventilation but it is important to know how well it works with a floor system for heating or cooling. Can, for example, a floor heating system warm up the supply air too fast and destroy the displacement effect?

The aim of that guidebook (74 pp), published by REHVA is to provide a clear analysis of the definitions of ventilation effectiveness and to present case study examples

The ordinary displacement ventilation system (DV) needs large installation spaces for large supplydiffusers with lower supply air velocity to avoid the draft discomfort at the foot level, and large supply airvolume to control allowable vertical temperature gradient within the occupied zone. Furthermore, theDV is difficult to heat without other warming means in the winter. The swirling induction type HVACsystem (SWIT) was developed as utilizing a compact air diffuser increasing the induction function bymeans of swirling air jets.

In this paper, the indoor environmental quality for the commuter train space was analyzed for assumingthe load factor of seat capacity to be a parameter of analysis. Especially flow fields, temperature and airquality distributions in vehicle space were analyzed by using CFD technique. Furthermore, it reports onthe result of examining the controllability of indoor climate in the vehicle when the displacementventilation system was applied in it.

For that study, the constant concentration dosing method and the decay method were used to test air changes performances in an office equiped with a VAV mechanical ventilation system and in a domestic apartment with natural ventilation . The measurement results from those two tracer gas methods are compared.