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Ventilation effectiveness - How well do ventilation systems work?December 2011

Effective ventilation, whether provided by mechanical or natural means, is crucial to provide a comfortable, healthy and ultimately productive working environment.

The effective ventilation of buildings has always been a primary design requirement. What is specified and what is procured will almost wholly determine what the client will get. But also ease of commissioning and maintenance is vital to the performance of any ventilation system. 
Leakage testing of display cabinet at Victoria & Albert Museum in London

This article describes the basics and the main methods for checking indoor ventilation rates.

Ventilation effectiveness or as often called Contaminant Removal Effectiveness

The effectiveness of a ventilation system can be determined by how effective it is at removing internally produced contaminants.

There are mathematical calculations described as multipoint age-of-air analysis that can be used to determine the ventilation effectiveness of a system. The term "age-of air" refers to how long a particular molecule of "air" has been in the room since it was supplied into the space.

Calculation of ventilation effectiveness (åv) takes into account pollution concentration in exhaust air, supply air and breathing zone indoor air. perfect dilution ventilation system.

Short-circuiting

Zero ventilation effectiveness åv means the ventilation system is short-circuiting which happens if a supply grille and an extract grille are located close together.

Dilution ventilation system

If the ventilation effectiveness is 1, this is a perfect dilution ventilation system (in reality, most ventilation systems have åv < 1). This system is common in many buildings. If installed correctly, it will produce effective ventilation. However, it is commonly installed incorrectly and so does not produce effective removal of contaminants throughout the building.
Ventilation short circuiting - A supply grille (nearest) and an extract grille (furthest) can short circuit if located close together (click image to zoom)

Displacement ventilation system 

If the ventilation effectiveness tends towards infinity, this is a perfect displacement ventilation system. This is when cold air is supplied at low level and at low volume into the space. The cold air travels slowly at low level until it reaches a warm object and it is entrained into the warm plume from the object and so cools it. For this system to work effectively the cold air must be able to reach all areas where there are heat loads and the high level extracts must be adequate to extract all the warm air from the plumes in the space.

Tracer gas - methods and strategies

A reliable method of measuring effectiveness of ventilation is using a tracer gas. There are two types of tracer gas that BSRIA have used, carbon dioxide and also nitrous oxide that needs specialist measuring equipment.

Single-point measurement methods - Release in the supply or in the occupied zone, and with measure in the extract
This simple calculation only works in a well-mixed zone as it only gives one ventilation rate for the whole space. In reality, a space is not well mixed in all areas so the levels of tracer gas must be measured at various strategic locations in the zone.

Multi-point measurement strategy - single point release in the supply or in the occupied zone, and with multi-point measurement in the space
This will show how well the levels of gas are reduced at various locations and how contaminants present in the space would be removed by the ventilation system.

From BSRIA's experience, tracer gas testing can effectively be used to determine the ventilation rates (air change rate) for many types of structures.

  • Museum display cases - checking air change rate that needs to be very low
  • Houses - checking the ventilation rate
  • Schools - checking the ventilation rate
  • Laboratory test rigs - checking the ventilation rate is correct for the mock-up
  • Hospital isolation room mock-ups - checking contaminant removal
  • Laboratory fume cupboards - checking the external plume dispersion

BSRIA Test & MicroClimate
Phone: +44 (0)1344 465600
Email: test@bsria.co.uk  

 

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