Using Negative and Positive Pressure

21st March 2018

Using Negative and Positive Pressure, RVT Group

Negative pressure
Dust and fumes can be extracted from a contained space if negative pressure is created. The basic principle is the same behind the humble bathroom extractor fan, which draws out moisture and odours because the forced air ventilation creates a negative pressure within the room (as long as any doors and windows are closed).

On construction sites, the basic principle is the same – just on a larger scale. The spaces that need to be cleared of dust and fumes are generally much bigger than the average bathroom, and the elements that need extracting are large amounts of dust and fumes generated by building (or demolition) activities. The basic requirement is therefore for much more powerful fans, but the final choice of machine will depend on several other factors, considered later.

Creating negative pressure is particularly vital in sensitive environments, such as hospitals, where activities generating dust or fumes may be in close proximity to live working environments. Sealing the area where the work is taking place and then using a fan both to extract dust and create negative pressure will ensure harmful particles cannot escape into the surrounding areas. Dust within the sealed area can be captured using a dust extractor and dust filter (or a unit that fulfils both options, such as the Raptor).

Positive pressure
Generating positive pressure is useful in a variety of situations. For example, if powerful fans are used to drive fresh air through ducting to provide tunnel ventilation, the positive pressure delivering the air will displace stale and contaminated air. It is vital that the ducting should run to the furthest extremity, so that the bad air is continually forced back along and out of the tunnel. Using positive pressure to force bad air out is preferable to using extraction under negative pressure in these circumstances; relying on extraction alone is very risky as undirected air flow will always follow the easiest route and it is likely that 'dead spots' will arise where the build-up of dangerous gases, such as carbon dioxide, takes place. To ensure gas levels are kept within safe limits, the fans used to drive air under positive pressure can be augmented with additional fans set up in the outflow to extract harmful gases under negative pressure.

Ventilation is better provided by forcing fresh air under positive pressure

Positive pressure also has a very practical value in helping to dry out damp interiors. If heat from a dry source (such as our mobile oil-fired heaters) is driven through ducting under pressure, moisture within a building is driven out and, at the same time, external moisture is prevented from entering. This double benefit of dry heat under positive pressure results in rapid drying, to the extent that separate dehumidifiers are often unnecessary. Apart from countering the safety hazard implicit in mixing damp and electricity, drying out plastering quickly can often be the difference between a job finishing on time or running late.

Selecting the right equipment
Where negative pressure is used to extract air containing flammable, abrasive or even explosive particles, such as dust generated by drilling or sanding activities, it is vital that these particles are kept away from the fan's motor, both to prevent damage to the fan itself and to avoid any potential for fire or explosion. In such situations, a centrifugal fan should be used because its motor sits outside the flow of air created. The contaminated air can then be ducted well away from the where the dust-generating activity is happening, ideally through a filter to capture and remove harmful particles from the air entirely.

Centrifugal fan, showing the motor sitting outside the airflow

If the intention is to create negative pressure systems to remove fumes (if non-flammable) and odours from the air, then either centrifugal or axial fans can be used.

Either type of fan can also be used to drive a positive pressure system. In general, while axial fans tend to be noisier than centrifugal ones – which might rule out their use in certain sensitive environments – they require less power to move the same volume of air.

Axial fans require less power than equivalent centifugal fans

A further consideration is where to site the fans. Thanks to their power, sufficient to drive or extract air through long duct runs, they can often be sited well away from the area to be ventilated or cleared of dust. Even so, the available working and access space may put limits on the size of the equipment that can be deployed (which, of course, is also true of heaters).

There is a wide range of situations in which fans and heaters can be deployed to create negative or positive pressure, which is why RVT's range runs from self-contained dust extraction and filtration units designed for small spaces through to large fans capable of maintaining negative or positive pressure over extended duct runs. We also have products for creating sealed containment areas.

We back up our products with many years' experience of designing and setting up extraction, ventilation and heating systems to provide our customers with a complete hazard control service.

If you would like us to recommend a dust extraction, ventilation or drying solution for your site, take advantage of our free site assessment service, operating across the UK.



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