Steely alternative that won’t blow your money

With the rapid rise in worldwide steel prices the cost of galvanised steel ductwork has also risen, resulting in dramatically increased costs.

As a consequence, there has been a trend towards increased use of fabric ductwork distribution systems.

Traditional galvanised steel ductwork, usually circular spiral wound, is the norm for virtually all ductwork installations for heating, cooling and ventilation systems, but for many applications there is a viable alternative. There are numerous installations in modern buildings where the distribution ductwork is exposed within the heated or air conditioned space. For these installations, the fabric distribution duct is ideally suited and provides very even air distribution and aesthetics in addition to cost savings.

The fabric distribution ductwork plenum may be manufactured from a variety of materials to suit different applications and is inflated by the air pressure inside the duct. Modern fabric ductwork systems are far more sophisticated than simple air socks, which diffuse air through the entire surface of the duct, or polythene tube type ducts which have a simple row of holes down the length of the duct. These simple systems are useful for specific applications but lack the versatility and air distribution features of the modern fabric duct.

Computer design technology now allows fabric duct systems to be individually created to match the proposed criteria of the individual application. They may be used for heating, air conditioning or ventilation with duct mounting heights between 2m and 15m.

Air discharge may be selected from a number of specially designed perforations to promote high levels of air induction and subsequent air mixing. The distribution and location of the outlets is unique to each application and depends on the air leaving temperature, mounting height and terminal velocity criteria to be achieved. For open plan areas, air distribution would generally take place along the full length of the duct, but for racked areas the ducts can be manufactured specifically for air discharge between the racks.

The distribution outlets are designed to induce large volumes of secondary air from around the duct into the discharge air stream. The combination of uniform air distribution down the full length of the duct, plus the large volume of induced secondary air, promotes substantial low velocity air movement within the building. This results in very even temperature distribution with little or no discernable air movement at occupancy level.

For ducts installed at high level, the outlets can be designed to increase the amount of air induction to provide auto de-stratification and thereby eliminating excess roof space temperatures and reclaiming heat from lighting for example.

Whilst increased steel costs have made the fabric ductwork system more cost effective, the savings are not purely related to material. Site installation costs are also reduced. The lightweight systems can be installed three or four times quicker than traditional steel ductwork, and  since most ductwork installations are at high level and covered by the stringent “Working at Heights” legislation, this can lead to significant reductions.

What’s more, the induced air movement around the duct removes the risk of condensation forming on the ducts used for air conditioning applications. Therefore there is no requirement for insulation of the duct in the air conditioned space. As the system is self balancing and does not require any grilles or terminals, balancing and commissioning is simplified providing yet further savings. The fabric distribution duct is available in a wide range of colours so that it may be colour coded into the building fabric without having to pay for painting and subsequent maintenance.

Joe Kiernan of Reznor comments “Over the last 12 months we have seen a growth in applications of our Airmixing fabric ductwork distribution systems used in conjunction with our gas fired heating and ventilation units. The Reznor AirMix distribution duct has become increasingly attractive and many more installers are now realising the benefits of lower installation costs and improved air distribution as a result of installing Reznor AirMix.

The system also allows the installer to purchase the distribution duct and the heater from a single supplier”