The potential of fabric ducts, or socks, is often ignored in favour of the more familiar, comfortable rigid ductwork approach, suggests Paul Russon, managing director of Euro Air UK
FOR most engineers, there is a significant comfort factor in using something that is tried and tested. In delivering conditioned air to a space, the conventional approach in the UK involves the usual combination of fans, steel ductwork and grilles - even when it is not the best solution.
It may provide comfort to the specifier but it may not be the best way to provide comfort to the occupants of the space.
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 leisure, fitness, retail, electronics and clean room 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. Less important from an engineering perspective, 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 co-ordination of services at design stage is easier.
Often referred to as a sock, fabric ducting consists of a material sewn into a specific shape - usually round, half round or quarter round - which is designed to inflate to shape under pressure from the air handling unit.
The size of the duct is determined by the required air volume and the static pressure is balanced to ensure minimal movement of the inflated sock.
The material used is a non-shrink, heavy gauge polyester (standard or flame retardant) with different weave options to allow for the perfect supply ratio between volume and permeability. As the duct is normally mounted within the space, there is a large surface area for delivery of the supply air, compared with 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.
Avoiding draughts was a key consideration for leading digital printer Real Digital. Inevitably, in a printing environment, a considerable amount of paper dust is generated and any draughts could cause the dust to enter the sensitive electronic printing equipment. To avoid this problem, a dedicated extract system to remove paper offcuts is combined with fabric ventilation socks to provide heating, cooling and general ventilation without draughts.
'We have been very pleased with the ventilation system, which maintains a comfortable environment without the draughts and background noise associated with conventional ventilation,' commented Real Digital chief executive Peter Rivett. 'We were also impressed with the way Euro Air was willing to work closely with us to ensure the system was precisely matched to our needs,' he added.
In this respect, fabric ducting offers high levels of flexibility by using different combinations of diffusers. In addition to introducing the air through the weave of the fabric, slots and nozzles can be laser-cut into the fabric to provide 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 zip-together 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 non-hygroscopic 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.
Euro Air UK T: 0116 272 1231