Book - School of Science and Technology

Ductwork components and auxiliaries 445Parallel rotatingContra - rotatingMulti - blade Iris ButterflyFigure 16.10 Typical balancing dampers for insertion in doctingAccess openingsAccess into ductwork should be provided at every component, at both sides of plantitems, and at regular intervals along the run to allow for inspection and cleaning.Balancing dampersPermanently set dampers are used in low pressure systems to regulate the air flowquantities at the commissioning stage. These are installed downstream of the fan in themain duct, in every branch duct from the main duct, in every sub-branch serving three ormore terminals and at every terminal. The butterfly type is normally used in circular ductsand a multi-bladed type in rectangular sections, with some form of locking devices. Thecontra-rotating type, commonly called opposed blade dampers, is superior from theregulating point of view, to the type where all vanes rotate in the same direction. Aselection of the more popular types of damper is shown in Figure 16.10. Where tight shutoffis essential, it is necessary for the dampers to be felt-tipped and to close on to a feltedframe, and with edge seals.Dampers should only be installed in medium or high (pressure) velocity systems whereabsolutely necessary. Where these must be provided, it will probably be necessary toprovide attenuators upstream and downstream together with protection to reduce noisebreakout from the duct.Controllable dampers, adjusted either manually or automatically, are dealt with inChapter 22.Fire and smoke dampersIn principle, fire dampers must be installed where ducts pass through fire compartmentwalls, floors or other elements, with the possible exception of low level penetrations lessthan 0:013 m 2 in section. Ideally, dampers should be built in to the compartment element.Where this is not practical, the damper should be located close to the element andconnected to it by 6 mm thick (minimum) steel plate with flanged joints. Fire/smokedampers may also be required where ducts penetrate smoke barriers in ceiling voids. Therequirements may be relaxed in toilet blocks, where a `shunt' duct arrangement, as shownin Figure 13.11, may be an acceptable alternative.Figure 16.11 illustrates some typical fire damper constructions. The swing blade type,Figure 16.11(a), consists of a heavy steel casing with the steel damper blade kept open by a