Document 0J3j77zoVgB4r1DmgO36o53NV

782 CHAPTER 30 1958 Guide outlets are attached either directly to the bottom of horizontal ducts or to special vertical take-off ducts which connect the outlet with the horizontal duct. In all these cases, special devices for directing and equalizing the air flow are necessary to obtain proper direction and distribution of the air. The influence of the duct approach on outlet performance has been in vestigated for vertical stack heads with plain openings12 or equipped with grilles12 and for side outlets on horizontal ducts.14 In the tests conducted with the stack heads it was found that it is necessary to provide splitters or guide vanes in the elbows at the top of the vertical stacks regardless of the shape of the elbows, whether of rounded, square or expanding types. Cushion chambers at the top of the stack heads have no beneficial effect. Fig. 13 shows the direction of flow, distribution and velocity (measured 12 in. from opening) of the air for various types of stack heads tested, expand ing from a 14 x 6 in. stack to a 14 x 9 in. opening, without grille. The air velocity for each was 500 fpm in the stack below the elbow, but the direc tion of flow and the distribution pattern are generally indicative of per- Fig. 13. Outlet Velocity and Air Direction Diagrams fob Stack Heads with Expanding Outlets Stack 14 in. x 6 in. Outlets 14 in. x 9 in. Stack Velocity 500 fpm A. Rounded Throat and Rounded Back. C. Square Throat and Back and 6 Guide Vanes. B. Rounded Throat and Back and 2 Splitters. formance obtainable with nonexpanding elbows of similar shapes for a range of velocities 200 to 1400 fpm. In tests conducted with 3 x 10 in., 4x9 in., and 6 x 6 in. side outlets in a 6 x 20 in. horizontal duct at duct velocities of 200 to 1400 fpm (in the 6 x 20 in. section) it was found that multiple curved deflectors produced the best flow characteristics. Vertical guide strips in tVe outlet were not so effective as curved deflectors. A single scoop type deflector at the outlet aid not improve the flow pattern obtained from a plain outlet, and was there fore not found to be desirable. Directional Control Many devices for directing and equalizing air flow in side wall and ceil ing outlets are now commercially available, and should be used whenever necessary. They are indispensable for all side outlets in horizontal ducts an for most ceiling outlets. Ceiling outlets are usually installed below horizontal supply ducts so t a the supply air has to make a 90-deg turn before entering the outlet its _ The shorter the connection between bottom of duct and outlet, the grea is the need for directive devices to obtain uniformity of flow. Generany speaking, conditions and remedy in such cases strongly resemble those i side outlets in horizontal air ducts. Ceiling ducts often have a rectangu cross-section, while the connections to the ceiling outlets are circular- ^ will then be quite difficult to install turning vanes successfully, particularly! Air Distribution 783 if the ducts are. shallow and the connection areas are comparatively large. This will be the case when more than one outlet is installed on one duct run, and restrictions of duct areas must be avoided. In such cases good results have been obtained by using a series of vertical guide strips, installed at right angles to the direction of air approach in the outlet connection where it leaves the horizontal air duct. Volume Control Various methods are used to regulate volume of supply and return (ex haust) air. Some of these accomplish only minor changes in volume; most of them however permit a range of adjustment from maximum air supply to complete shut-off. \' . When selecting type and location of such dampers, the following points must be considered, especially when the volume control , feature is to be located near the air outlet itself .'.(1) deflection of air stream by the damper; Hinge /A///A / Either two ////// high velocity or air spills down ward Fig. 14. Effect of Various Dampeb Arbangements Designed fob Straight Blow (2) need and feasibility of directional control; (3) increase of noise level due to irregular and localized high air velocities caused by damper operation. The following types of volume control are most frequently encountered: 1. Slide Damper. A single plate which can be pushed across the duct. Since its operation changes the free area of air passage in a one-sided manner, it should not atVs'f*- "?ear ay air outlet, and its use is practicable only where no intermedi ate setting between full open and closed is required. onp ^\L.and-Mi^ Damper. Two slotted plates or discs, closely adjacent; by moving voli.m tfte tw<? Plates the respective slots may be opened or closed. This type of its mn'6 c?-n^ j may he installed close to an air outlet and it is easy to operate, but bv At i , mVan*ae is that even in the open position the air passage area is blocked ** Percent. This requires oversizing of the air outlet in order to avoid cessive increase of noise level. locafJd^i^iT hamper. A single blade sheet metal plate hinged at one edge, usually causes' at i branch connection of a duct or outlet. It is easy to operate, but often let , r.?lr Aw >n the duct. When used in connection with, and near an out- , additional directional control is required. locate^1^1^ Damper. A single blade sheet metal plate hinged in the middle, usually onlv haUiL strai?ht duct run. It is easier to handle than a splitter damper, since to an ai ttle motion is necessary to change its setting. However, if located too close condition i i *t is objectionable because its operation frequently results in a the air two high velocity jets are created, along the sides of the duct, or g j SP11,S immediately downward into the occupied zone (see C Fig. 14). of splitteVer Dampers. Numerous designs have been developed incorporating a series consists i 01 butterfly dampers across the duct or air outlet. Their main advantage installati11 reiamin? greater uniformity of air flow, and in requiring less depth for end While'S "Ome designs provide for louver blades moving in opposite directions, the axin deceasing free air passage area, retain a constant air flow direction along 8 01 the duct air outlet connection (see A and B in Fig. 14).