EFFECTIVE AND EFFICIENT VENTILATION Ventilation Efficiency Indices

Effective air distribution in ventilated rooms and proper quantity of condi­tioned air are essential for creating comfortable conditions, removing contam­inants, and reducing initial and operating costs of air conditioning and ventilation systems. The degree to which a ventilation system fulfills ventila­tion requirements is described in the literature in terms of “ventilation effec­tiveness,” “ventilation efficiency,” “ventilation performance,” etc. Liddament in his review of technical information related to ventilation effectiveness stated that “the subject of ventilation effectiveness is made unnecessarily complex by the lack of uniformity in terminology. Frequently, terms are interchanged or different terms are used to describe the same concepts.”1

Ventilation efficiency has traditionally been defined as the ratio between contaminant concentration in the occupied spaces and the concentration in the exhaust air.2 Sandberg and Skaret differentiate between the terms “air change efficiency” and “contaminant removal effectiveness.” Air change efficiency is “a measure of how effectively the air present in a room is replaced by fresh air from the ventilation system,” whereas “contaminant removal effectiveness” is “a measure of how quickly an air-borne contaminant is removed from the room.” A third similar criterion that is used is “contaminant removal efficiency.” 3

In the current review, the term “effectiveness” of air distribution will be used to describe the ratio of the occupied zone area (where thermal comfort and contaminant concentration are within ranges required by standards and codes) to the total occupied zone area. This hygienic criterion allows one to judge how well the HVAC system fulfills its main task—creating thermal com­fort conditions and controlling contaminants in the occupied zone.

Industrial halls are typically large enclosures—indoor spaces that typically comprise one or more zones of occupancy. A large height combined with heat sources often results in room airflow patterns controlled by buoyancy flows.

A characteristic of many industrial halls is that zones of occupancy take up only a small portion of the room volume and height. In addition, the flows are normally buoyancy dominated. This results in a vertical temperature strat­ification that can be utilized for room air conditioning design in order to achieve effective climatization along with low energy consumption.

A process is found effective when “it produces a desired effect”; e. g., ventilation in a room is effective when it produces sufficiently good air
quality in the breathing zone. However, that air quality may be achieved without the ventilation being efficient. A ventilation process is considered efficient only when it is “effective with little waste of effort,”4 i. e., being cost-effective.

Being cost-effective is normally equivalent to achieving a ventilation de­sign where the indoor climate meets required specifications (target values), ap­plying a low rate of supply air, along with avoidance of heating or cooling. That way, climatization would be effective as well as efficient.

A review of ventilation efficiency indices is given below. Complete mixing ventilation is the comparative basis for all efficiencies, with the value 1.0. That may be useful, as complete mixing ventilation often is assumed in early design phases.

Which ventilation efficiency index should be chosen for assessment de­pends on the actual scenario in the enclosure.

The contaminant removal effectiveness can be used when emission data for contaminant sources are available.

Air exchange efficiency indices can be used for cases where no or little in­formation on sources is available, whereas ventilation efficiency, which con­cerns workers, can be used where very detailed information is available on sources and activities.

Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK