Technical Guide TG2010.25
2Pages

Invisible.AC Limited St John's Innovation Centre Cowley Road, Cambridge CB4 0WS United Kingdom T +44(0)870 9619661 E info@invisible.ac W www.invisible.ac Technical Guide TG2010.25 Plenum Duct Design and Installation Instructions Introduction Many Invisible.AC installers are using both Fibreboard and metal duct successfully for the main trunk line (plenum). This Technical Guide describes the best methods for using both rectangular and round types of duct. Duct Size Round metal duct is used for some systems and is effective because its stiffness make it an effective attenuator of low-frequency breakout noise. Equally, stiff fibreboard rectangular duct is effective against breakout noise. If metal rectangular duct is used, it is recommended that it be of the heaviest practical gauge in order to avoid 'drumming' resonances. Invisible.AC provides plenum adapters for both fibreboard and metal ducts. The recommended cross-section of the plenum duct depends on its length and of course on the airflow requirement. In most cases, where the plenum duct length is less than 40m, the plenum will be between 160 mm and 200mm diameter, or, if rectangular, at least 160mm x 160mm. Rectangular fibreboard of internal dimensions 250mm x 100mm is a popular choice. In certain instances a larger duct crosssectional area will be needed. Specifically, a larger duct is necessary for air handlers rated at 14kW or over before any splitting of the flow or if the plenum length is more than 40m. Table 1 lists the maximum recommended airflow for various round and rectangular duct sizes. For rectangular duct sizes, the duct is sized for equivalent flow resistance. Where flow is split, it is acceptable to reduce the duct size immediately after the tee. Invisible.AC has available appropriate reducer tees. Note: In terms of pressure drop, an elbowis equivalent to 5m and a tee equivalent to 9m of straight duct. Installation The duct must be designed for 1000Pa internal pressure. It can be fibreboard, spiral metal, seamless, or snap lock metal. Fibreboard duct is normally of 25mm wall thickness. In the case of sheet metal a thickness of at least 0.5mm is commended in order to retain sufficient strength after the branch mini-duct holes are cut. Table 1. Recommended Plenum Duct Sizes Round Rectangular Maximun Diameter Equivalent Airflow l/s

Invisible.AC Limited St John's Innovation Centre Cowley Road, Cambridge CB4 0WS United Kingdom T +44(0)870 9619661 E info@invisible.ac W www.invisible.ac Sealing It is extremely important to seal all duct seams and joints, including any longitudinal snap-lock seams. The method of sealing should follow the ASHRAE Handbook-HVAC Systems and Equipment. Insulation The Invisible.AC systems are designed to produce a greater temperature drop across the heat exchanger than a conventional system. To prevent condensation or excessive duct losses to the surrounding air, wrap the duct with at least 40mm...