How clean is your air?

Published:  05 January, 2012

BSRIA, IAQ, tracer gas, ventilation
Fig. 1 This common approach to ventilation is effective if installed correctly, but can easily lead to short circuiting so that contaminants in air are not removed effectively.

Contaminants in air are generally invisible, and the occupants of a building many not be aware of them — so how do you find out how well a ventilation system removes them? Colin Judd gives an insight into the tracer-gas method.

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 you specify and what you procure will almost wholly determine what you’ll get.

But also ease of commissioning and maintenance is vital to the performance of any ventilation system. This article describes the basics and the main methods for checking indoor ventilation rates.

Ventilation effectiveness is sometimes called contaminant-removal effectiveness. The effectiveness of a ventilation system can be determined by how effective it is at removing internally produced contaminants. Calculations can be used to determine the ventilation effectiveness of a system. This is described as multipoint age-of-air analysis. 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. The equation below can be used to determine the age of air in the breathing zone.

Ev = (Ce-Cs)/(Ci-Cs)

Where the C terms are the concentration of pollutants in exhaust air (e), supply air (s) and the breathing zone (i).

If we assume the supply air is unpolluted, the equation simplifies to

Ev = Ce/Ci

There are three special cases for ventilation efficiency.

• Short circuiting, Ev = 0.

• Perfect dilution system, Ev = 1. In reality Ev is less than one for most ventilation systems.

• Perfect displacement system, Ev tending to infinity.

Positioning a supply and extract grille close to each other can cause short circuiting (Fig. 1). Such an arrangement is common in many buildings and will deliver effective ventilation if installed correctly. However, incorrect installation is common, so that contaminants are not removed effectively.

Displacement ventilation involves cold air (cooler than room air) being supplied at low level and low volume into the space. The cold air travels slowly through the space at low level until it reaches a warm object, when it is entrained into the warm plume from the object and cools it. For displacement ventilation 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.

The effectiveness of ventilation can be revealed using tracer gases. There are two types of tracer gas that we have used, carbon dioxide and nitrous oxide. Each gas has its pros and cons.

Carbon dioxide is readily available, easily measurable and mixes well in air. However, there is a need to measure background/external levels.

In contrast, although nitrous oxide requires specialist measuring equipment, it is readily available and mixes well in air. There is also no need to measure background/external levels

Using a tracer gas to measure the effectiveness of a ventilation system requires the gas to be released into the space at a known flow rate until steady-state conditions have been reached; the supply of gas is then turned off.

The decay rate of the gas is then logged over time until the level of the gas reaches zero (nitrous oxide) or the background level (carbon dioxide). During the steady-state decay period, the decay rate gives the air-change rate of the space.

Fig. 2: Tracer gases provide an effective way of measuring the effectiveness of a ventilation system at removing contaminants. The tracer gas can be introduced into the supply system or into the space itself and measured in a number of ways, as explained in the text.

This simple calculation only works in a well mixed zone. In reality, a space is not well mixed in all areas. Therefore, to accurately measure the effectiveness of a ventilation system, the levels of tracer gas must be measured at various strategic locations in the zone. This will show how well the levels of gas are reduced at various locations throughout the space.

Fig. 2 shows four ways of releasing tracer gas into the space and measuring it.

The tracer gas can be released into the supply air and measured in the extract (Fig. 2a).

A variation on that approach is to release the tracer gas into the occupied zone and measure it in the extract (Fig. 2b).

The above two methods give only one ventilation rate for the whole space and do not indicate what is happening in the corners of the room.

To obtain a better picture of what is happening at various points in the space requires measurements to be taken at a number of points (Figs 2c and 2d). Multi-point measurement shows how a contaminant in the space would be removed from it by the ventilation system.

Fig. 2c shows single-point release of tracer gas in the supply air and multi-point measurement in the space. Fig 2d shows tracer gas released into the space and multi-point measurement in the space.

From BSRIA’s experience, tracer-gas testing can be used to determine the ventilation rates (air-change rate) for the following types of building.

• Museum display cases with very low air-change rates of 0.1 to 0.2 a day, using nitrous oxide.

• Laboratory test rigs, using nitrous oxide to check the ventilation rate is correct for the mock-up.

• Houses, using carbon dioxide to check the ventilation rate.

• Schools, using carbon dioxide to check the ventilation rate.

• Mock-ups of hospital isolation rooms, using nitrous oxide to check the removal of contaminants.

• Laboratory fume cupboards, using nitrous oxide to check dispersal of the external plume.

Colin Judd is micro-climate team leader at BSRIA.



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