Measuring Effectiveness: Targeting the Breathing Zone

CIBSE Journal, Vol. 65, no. 8, August 2023

Author:

Summary of Dr. Kishor Khankari’s Presentation Prepared by Mr. Tim Dwyer

Abstract:

Traditional methods of measuring ventilation effectiveness depend on well­ mixed spaces that don’t often occur in reality, says Tim Dwyer, who introduces a new metric developed by Dr Kishor Khankari that targets the critically important breathing zone and enables engineers to accurately model ventilation designs.

Ventilation effectiveness is a description of an air distribution system’s ability to dilute and remove internally generated pollutants from a building, zone or space compared with a perfect air­ mixing condition. In his presentation, A computational fluid dynamics (CFD) approach for evaluation of ventilation effectiveness, Dr Kishor Khankari explained that it provides a measure of how a clean air supply meets the ventilation goals of the space, with the aim of keeping occupants healthy, comfortable and productive in occupied space by concentrating on the breathing zone.

He emphasized that the supply air should move through the space with minimum recirculation and stagnation, which otherwise can promote zones with high contaminant concentrations. He also noted that the precious resource of clean air should not short circuit without doing its work ñ it should reach the breathing zone, sweep up the contaminant, and not leave the space without removing the contaminants. However, the air always takes the path of least resistance, but that path is not intuitive and is difficult to imagine ñ it is important to understand that path.

Dr. Khankari introduced a new approach to ventilation effectiveness – the Spread Index, SITC. This defines the extent of room volume occupied by a certain concentration, where SI = Volume of the room above TC/Volume of room, where TC is target concentration. The goal is to minimize SI and to maximize ventilation. The another metric introduced by Dr. Khankari is the Purge Time (PT) – it is the time taken to reduce SI from its maximum value to zero
Dr. Khankari demonstrated how CFD modelling can be employed to readily estimate the Spread Index. The Spread Index provides qualitative and quantitative aspects of ventilation effectiveness. Better results come from properly distributing the clean supply air without the need for so many air changes in achieving the same result at lower air changes. So, it begs the question, why are ventilation effectiveness theories evolving around dilution all the time when it ís not necessarily the dilution that is the solution for all pollution?
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