CPD PROGRAMME
zone is taken to be the local zone around the duct where the air velocity is greater than that required for occupant comfort (Figure 9).
Figure 10: Example of a duct system layout
Figure 8: The near zone for a horizontal low impulse supply system – the volume that is not suitable for occupied use
the jet draws in more room air, just as with ‘traditional’ air diffusion devices, the velocity of the air will reduce and, if appropriately sized, the supply air will be completely mixed with the room air before it reaches the occupied zone and the air velocity has dropped to a reasonable value. A textile-based high impulse supply
Figure 9: The near zone for a vertical low impulse supply system – the volume that is not suitable for occupied use
specific air distribution needs (Figure 6). So, for example, systems may be produced for industrial areas that have nozzles to ‘air wash’ the soffit to remove condensation, while other nozzles are oriented to satisfy the comfort needs of the occupants below. The air may be directed along adjacent surfaces, employing the ‘coanda effect’ to increase the effective throw. These systems will provide mixing ventilation, with the air being delivered outside the occupied zone at high velocity. So, for example, using plain holes of approximately 4.5 mm diameter will deliver air at 6 m·s-1
pressure of 50 Pa, and 16 m·s-1
with an internal duct static at 200 Pa
duct pressure. The distribution pattern of the holes will determine the throw and spread. To provide more control of the design of the supply air pattern, nozzles (typically 12-60 mm diameter) may be selectively factory fitted (Figure 7) according to the application, giving velocities of around 8 m·s-1
(at 50 Pa) to 16 m·s-1 (at 200
Pa) (nozzle size being determined by duct mounting height and the throws required for the room). Throws would typically be
from a few metres, using 12 mm nozzles, to 25 m-plus with larger 60 mm nozzles, for both heating and
cooling. The velocity of the air moving through the
room will reduce the local air static pressure and entrain air from the room volume (where there is a higher static pressure). As
www.cibsejournal.com Figure 11: Example of single and double suspension, and how the duct will look with and without air flow May 2013 CIBSE Journal 49
system may be used for cooling systems, tempered ventilation and heating applications, since the supply air is delivered with sufficient velocity to allow the air to mix with the room air independent of the temperature difference between the supply and the room air. Just as with ‘traditional’ diffusers, these systems may be characterised by ‘throw’ and ‘spread’. Unlike low impulse supply systems, the location of the exhaust has only a minor influence on room air distribution. Air travelling through the coated, non- permeable flexible ductwork must be above the dew point of adjacent room air, otherwise condensation will form on the duct’s surface.
Occupied zone The occupied zone is the target volume where the system aims to maintain the required conditions. It is by no means fixed but will vary from one project to another, depending on use of the space. Typically, the occupied zone is defined as the zone from the floor up to a height of 1.8 m where occupants are in a standing position, and up to 1.1 m for people who are seated. The term ‘near zone’ may be used in the case of horizontal low velocity supply systems (Figure 8), to indicate the zone under the textile ducting where there is an appreciable risk of a ‘cold downdraught’. For vertically-mounted systems, the near
Installation The duct (Figure 10) is typically suspended using aluminium track or point systems, designed to allow speedy installation and removal (for cleaning), as well as ensuring that the system maintains its shape and appearance (Figure 11). Since textile ductwork relies on the pressure of the air inside the duct to maintain its consistent shape, specific care should be taken to ensure that there are appropriate pressures – not only to move the air from the duct into the conditioned space but also to minimise the effect of pressure perturbations (or ‘fluttering’). The sections of duct are connected
with ‘zip’-style fasteners, allowing easily manageable sections for installation and ready access. Abrupt changes in direction and interference from framework can cause mis-shaping, vibration and noise in the fabric. However, radial bends, angled sections, dog-legs, and complicated shapes can be made in textile ducting – it is not restricted to straight runs. As the air travels through the constant diameter of the duct, its velocity will drop as the air is diffused into the space. By virtue of static regain, the static pressure will tend to rise (in spite of the frictional pressure drop), and this should be considered when designing systems. The noise produced by textile ducting is
very low in comparison to metal systems, and so higher duct velocities of 6-7 m/s are possible while still achieving low noise ratings. It is important to note that there is little sound absorption in the material, so attenuation will still be needed to remove noise from the central distribution system and air handling, as with a metal-ducted system. © Tim Dwyer, 2013.
● This article was produced with the input of Mark Bailey, with reference material and all images supplied by KE Fibertec UK.
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