Socks for all climates

In the UK, the conventional way of introducing fresh or conditioned air to a space involves a combination of fans, steel ductwork and grilles – an approach that many engineers feel familiar, and therefore comfortable, with. However, this is not always the most appropriate solution; in fact there are many situations where ducting made from a polyester fabric is a more sensible alternative.

This has certainly been recognised in continental Europe, where fabric ducting is the first choice for many food processing, pharmaceutical, leisure, fitness, electronics and cleanroom applications. Fabric ducting is able to provide cooling, heating or both, with the benefits of low air velocities, thorough mixing of conditioned air, effective filtration, acoustic control and ease of maintenance.

Furthermore, fabric ducting costs considerably less to install than conventional metal ductwork and grilles, and has significant operational benefits that impact directly on the bottom line.

The principles of fabric ducting are very simple. 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 strict guidelines are applied in balancing the static pressure in the sock and the duct diameter 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. The duct is normally mounted within the space, which provides a large surface area for delivery of the supply air, compared to a conventional grille or diffuser arrangement.

With a metal ductwork system the only way to introduce the conditioned air is through grilles, which may each have a surface area of less than 1m2. Therefore, the air has to enter the space at high velocity (anywhere from 0.5m/s upwards) to ensure enough warm or cool air is delivered and that it is distributed throughout the space.

In contrast, with fabric ducting air diffuses through the weave of the fabric across the entire surface area of the sock. To put this into perspective, a 10m long, 400mm diameter round fabric duct has a surface area of 12.5m2.

The result is high air volumes equally distributed at low velocities, avoiding the draughts that would normally result from introducing high volumes of air through a series of grilles. The 10m x Ø400mm sock described above, for example, will 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, perhaps because of machinery, thus requiring high volumes of cold air through the ventilation system. With grilles the only ways to offset higher heat gains are to increase the velocity of supply or use colder air, both of which cause uncomfortable draughts.

Higher velocities also increase the energy consumption of the fans and the noise from the system. With fabric ducting, the flexibility of the material has a proven sound absorbing effect, avoiding the problems of fan noise and reflected noise from the space that are often encountered with metal ductwork systems.

Dust reduction

Avoiding draughts was a key consideration for leading UK digital printer Real Digital, based in south London. Using leading-edge print processes for fast turnaround of printed items, Real Digital boasts the first ever truly scalable digital print process, as well as the world’s largest finishing line, and has the capability to produce a million personalised communications in a day.

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.

Food for thought

At Arla Foods in Denmark, for example, traditional low impulse socks proved ideal for the palleting room but one of the storage rooms required a more precisely engineered solution. Here, there was a requirement for high velocity and good distribution in shelving areas to cool the products as quickly as possible. However, the free-blowing unit coolers used previously had also subjected staff working at one end of the space to uncomfortable draughts.

To overcome this problem, Arla installed a nozzle sock system, with nozzles placed 100mm apart on the side of the sock facing the shelving. This spacing ensured a long throw of air with optimum entrainment of room air to provide fast cooling of stored products. At the same time, staff were protected from draughts, commenting they were unaware of the ventilation and that working conditions were considerably more comfortable than before.

Cleaning and hygiene

All ductwork, whether it is rigid or fabric, needs to be cleaned if acceptable indoor air quality levels are to be maintained. Clearly, this is particularly important in clean room, pharmaceutical and food processing applications.

However, rigid ducts are often quite difficult to access for cleaning, so the process is time-consuming and expensive, not to mention disruptive to the processes in the space. In the worst cases, the cost and complexity of the operation leads to it being deferred for longer intervals than is satisfactory for hygiene requirements.

Fabric ducts, on the other hand, are designed to be easy to clean. They are designed with zip together sections so when the component sections are unzipped and removed from the rail they will fit into a standard washing machine. Cleaning is then a matter of washing at 40°C with normal washing powder, followed by several cold rinses. They can even be spin-dried or drip-dried (but not tumble-dried) or re-installed while damp and left to dry on the rails.

Installation

There are some significant differences between the installation of metal ducting and fabric ducting, relating to aesthetics, capital cost and maintenance..

As noted above, fabric ducting is nearly always installed in the space – making it highly visible. For this reason, it can be supplied in a range of colours to suit the aesthetics of the space and is often used as a design feature. At Real Digital, for example, the socks were supplied in blue to complement the company’s corporate colours.

A further benefit of siting the fabric ducting in the space is that less void space is required for services, which can save money on both new build and refurbishment projects.

Installation costs are also important, of course, and fabric ducting can be installed very quickly with minimum disruption, using a simple two-piece rail design. In a typical installation, the fabric ducts are practically the last thing to be installed, sliding simply onto the pre-installed rails and zipping together.

In contrast, metal ductwork requires specialist fabrication skills, usually with additional insulation of the ductwork, and considerable disruption during installation. As a result, the combination of time savings and lower initial cost associated with fabric ducting result in significant savings on the cost of the project, compared to using a rigid duct system.

Fabric ducts 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, so that high levels of dust can be tolerated without affecting performance.

The fact that they are polyester also means there is no organic content available for bacterial growth and they are also non-hygroscopic so they will not absorb moisture from the air.

At the same time, the fact that fabric ducting is “low cost” does not mean it is “low performance”. On the contrary, fabric ducting has been proven in many different applications over many years to offer high performance with low cost of ownership.

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.