Figure 5: Example of a swimming pool using a high impulse non-permeable circular ventilation system with 4.5 mm holes


weaving-method is a more simple cotton bond, with a much smaller surface than that of the HDC fabric. HDC materials will hold more dust and increase the period between washing intervals, along with helping to minimise any increase in pressure loss.


Air distribution with textile-based ventilation systems The air distribution principle used for a simple textile-based ventilation system is fundamentally different from that of a conventional ventilation system equipped with metal ducting and discreet diffusers. Uncoated textile-based systems use permeable fabrics to distribute the air into the room, diffusing through the surface area of the material, enabling air to enter the room at a low velocity (as in Figure 2) – sometimes termed ‘low impulse’ systems. Directed systems use non-permeable fabric materials (coated in a self-extinguishing and flame-retardant plastic coating), with the addition of nozzles and/or laser cut holes to project and distribute the air supply to the room at higher velocities, sometimes termed ‘high impulse’ systems (Figure 3). There are also hybrid systems that use a mix of the permeable material for diffusion and additional openings for projection and further distribution of the air (Figure 4).


‘Low impulse’ supply systems The air distribution principle for these systems is often based on displacement ventilation principles, where the air is supplied at a slightly lower temperature than the room air. Because of the difference in density, the cooled supply air being heavier than the warmer room air, the supply air continues moving towards the floor. As


48 CIBSE Journal May 2013


with other displacement systems, heat from sources – such as people, equipment and solar gain – generates convection currents, resulting in the air being displaced, so moving heat and pollutants away from the occupied zone to be extracted at high level. Supplying air at a low velocity is suitable for cooling or for distributing large volumes of air at a temperature similar to that of the room air. High air-change rates (and associated cooling) can be achieved with low terminal velocities in the occupied area, owing to the air being discharged over the entire surface of the ducting. For example, each metre run can provide about 700 watts-cooling using a room/supply ∆T of 3K in an office-type application (approximately 200 W cooling per m2


floor area) – the diameter of the duct


being determined by its capacity to convey the total flowrate for that particular branch (using manufacturers’ data sheets).


Figure 7: Nozzles added to the textile ducting during the manufacturing process


In permeable low impulse supply systems,


the textile surface is effectively a fine mesh, allowing the supply air to pass through the surface at a low, uniform discharge velocity (normally less than 0.1 m·s-1


). The


discharge velocity should be kept below (approximately) 0.40-0.50 m·s-1


, as beyond


this the room air will start to entrain and mix with the low temperature supply airflow. The noise generated by this low-velocity supply is commensurately low and, when appropriately designed and installed, can be used where noise levels down to 20 dBA are required. Since the surface of the material is continuously ‘washed’ through by the diffusing supply air, the surface of the material does not exhibit condensation, so allowing supply air at a temperature below the dew point of the room air. Such low-velocity systems are unsuitable


for heating applications, as warmer supply air will settle at high level and is likely to ‘short circuit’ back into the extract system.


‘High impulse’ supply systems These rely on purpose-made openings in the non-permeable duct material to provide jets of supply air. These may be in the form of small holes or slots (factory cut and finished), arranged in specific locations and groupings in the textile duct, or by using nozzles fixed into the fabric that allow both area and direction of the air jet to be more closely controlled. Ducts with many small holes (jets) in the


Figure 6: Olympic temporary portal at Heathrow Airport, using a high impulse non-permeable circular ventilation system with nozzle diffusers


textile result in large induction of room air, so the velocity is reduced more quickly (and the throw reduced) compared to nozzle systems (Figure 5). Adding nozzles to the duct increases the supply options, so that systems may be manufactured to meet very


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