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Thinking outside the duct

Choosing the best whole-life ventilation solution necessitates a working knowledge of the options available. Paul Russon of Euro Air UK reveals some interesting facts about fabric ventilation used for heating and cooling
Thinking outside the duct
THERE are clear signs that many more building operators are now starting to think in terms of whole life costs rather than just capital cost. So this is the ideal time for specifiers and contractors to be taking a good look around at what is available on the market, rather than just sticking with the familiar.

The supply of conditioned air to a space is a case in point. The traditional approach of steel ductwork and grilles and/or diffusers may still be fine for some projects but it is not always the best option in terms of whole life costs. There are a many applications where fabric or textile ducting is a more sensible and cost-effective choice but it is not selected because the specifier isn't familiar with the technology, or harbours misconceptions.

For example, most people who have heard of fabric ducting will tend to think of it as a simple woven fabric sock that allows air to pass through its weave slowly. And that is the basis of the simplest form of fabric ventilation - but there are many options, including the use of nozzles or slots to provide directional air flows to supplement the ventilation flow.

There is also a misconception that fabric ducting is suitable only for cooling. However, there are a number of variations on the standard theme that enable effective heating or even the use of the same sock for heating and cooling.

Furthermore, fabric ducting provides a lower capital cost with faster installation, and, because the ducting is in the space, less void space is required and co-ordination of services at design stage is easier. Maintaining system hygiene is also easier as the zip-together sections of a fabric duct can be removed and washed in a washing machine.
And because a fabric duct system only requires a pressure of around 100Pa to operate, less energy is consumed by fan motors, compared with steel systems.

The basis of a standard sock fabric duct is that its large surface area allows delivery of high volumes of air at low velocity through the weave, thus avoiding draughts. A 10m long, 400mm diameter round fabric duct has a surface area of 12.5m2 and can provide an airflow of 3000 m3/h at a velocity of just 0.07m/s. These figures can be varied to suit the project by choosing one of the seven weaves.

Slots and nozzles

Where higher velocities of more precise air distribution are required, for heating or cooling, a sock with slots or nozzles or perforations will combine the distribution through the weave with directional air streams. The width of the slots or the numbers of nozzles will determine air volume and velocity.

Nozzle diffusers are 18mm nozzles set out in rows along the duct to provide a high induction, turbulent air pattern that is for entraining room air at high level. For long throws such as high bay warehouses a 50mm nozzle diffuser has been introduced that provides higher velocities and higher induction.

Slot diffusers are 2-30mm linear slots running the length of the duct, providing a predictable laminar air pattern for delivery of air to very specific locations - such as demisting windows in a swimming pool hall. Perforations are 4mm diffusers set out in straight rows of 50 per linear metre, providing high induction capability with broad air delivery patterns.

Injection

Taking this principle further is the Inject sock system, a specialised non-permeable duct made using a PVC coating both internally and external to the duct. The duct is penetrated by multiple rows of 4mm inject holes to deliver 100% of the supply air and produce a stirring effect that entrains and mixes with the room air around the duct. It is ideal for swimming pools and applications requiring fresh air supplied above dew point.

Membrane systems

When using a high delta T, a single duct used for both heating and cooling can create unacceptable velocities during cooling. The answer is to use a non-permeable membrane within the duct, connected to a two-position motor. In cooling mode the membrane blocks off the bottom half of the duct allowing normal permeation through the weave on the top of the duct. In heating mode the membrane blocks off the top half of the duct allowing warm air to flow through slots or nozzles located in the lower half of the duct. Thus, acceptably low velocities are achieved in either mode.

When all these options are considered it, hopefully, becomes clear fabric ventilation is far more versatile than many realise. When compared with steel ductwork and grilles it offers lower capital and installation costs, is cheaper to run and easier to maintain. A no-brainer as they say.


Euro Air UK T: 0116 272 1231
E-mail: info@euro-air.co.uk
1 September 2007

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