Pull your socks up
March 1st 2007

Fabric ducts, or 'socks' are ideal for ventilating large, high spaces such as factories and warehouses, but are often ignored in favour of more familiar systems. Paul Russon, managing director of Euro Air UK, explains why they should be given more consideration

In delivering heated or cooled air to a large space, the conventional approach in the UK involves a combination of fans, steel ductwork and grilles. The result is generally a system where high air velocities are required to get the air to the shop floor, creating draughts which move dust around and make people uncomfortable.

An increasingly popular alternative is the use of fabric ducting, which has been used for many years in the food processing and pharmaceutical sectors and is now making major inroads into other applications. Capable of providing cooling, heating or both; fabric ducting offers low air velocities, thorough mixing of conditioned air, effective filtration, acoustic control and ease of maintenance. It is also visually striking and can be used as an aesthetic feature within the space.

And on top of all that, fabric ducting is a lower capital cost with faster installation, and, because the ducting is in the space, less void space is required and coordination of services at design stage is easier.

Often referred to as a 'sock', fabric ducting consists of a non-shrink, heavy gauge polyester (standard or flame retardant) sewn into a specific shape – usually round, half round or quarter round.

It is designed to inflate under pressure from the air handling unit, at which point conditioned air passes through the weave of the fabric at low velocity into the conditioned space.

Different weave options allow for the perfect 'supply ratio' between volume and permeability and the size of the duct is determined by the required air volume and the static pressure, balanced to ensure minimal movement of the inflated sock. This arrangement provides a much large surface area for delivery of supply air, compared to a conventional grille or diffuser system. For example, 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.

And these figures can be varied to suit the exact needs of the project by choosing one of the seven weaves available, providing a high level of flexibility.

These factors become increasingly important in areas where there are high heat gains and where a conventional system would need to deliver high velocity, low temperature air to maintain the set temperature – resulting in uncomfortable draughts for people in the space.

Higher velocities also increase the energy consumption of fans and the system noise, whereas fabric ducting has a proven sound absorbing effect.

Slots and nozzles In addition to the seven weave options, further flexibility in air distribution can be achieved through the use of slots and nozzles strategically located in the sock.

These are designed to supplement the general air flow with directional flows of conditioned air as and when needed.

When designing these systems it is also possible to vary the ratios of air passing through the weave and through the slots or nozzles to create different air patterns.

The slots or nozzles can also be positioned strategically at different points on the circumference of the sock to create a bespoke system that meets the precise needs of the space.

Cleaning and hygiene Fabric ducts are designed with ziptogether sections so when unzipped the sections will fit into a standard washing machine. Then it's simply a matter of washing with normal washing powder at 40°C, followed by several cold rinses.

Much simpler than cleaning rigid ductwork! Furthermore, use of polyester means there is no organic content available for bacterial growth and they are nonhygroscopic so they will not absorb moisture from the air. They also have a long life, as they are manufactured by fusing multiple polyester fibres into a tough, non-tear fabric with a high material density of 400g/m2 and a large internal surface area. This creates a system with a higher dust capacity than would be the case with a flat weave.

As with any HVAC system, the successful application of a fabric ducting system depends on good design and specification. Getting it right requires a thorough understanding of the design criteria, a high quality product and the technical support to match the product to the needs of the project.