Document ENqKMm6XwmD8rEo8va32y0RL

740 CHAPTER 40 1948 Guide outlet) of the air, based on tests 6, are shown in Fig. 6 for various types of stack heads expanding from a 14 in. x 6 in. stack to 14 in. x 9 in. outlets, without grilles. The air velocity for each was 500 fpm in the stack below the elbow, but the direction of flow and the distribution, patterns are generally indicative of' performance obtainable with non-expanding elbows of similar shapes for a range of velocities 200 to 1400 fpm. Some of the conclusions drawn from the tests were: 1. Experiments with various elbow outlets on the 14 in. x 6 in. vertical stack5 with stack air velocities of 200 to 14Q0 fpm indicated that enlargement of the outlet area, whether used in connection with square or rounded elbows, would not reduce either the angle of discharge (which was 20 to 30 deg above the horizontal) or the outlet velocity. The effect of the enlargement of the outlet was mainly to increase the reverse flow area -in the lower part of the outlet, but in each case enlargement of the outlet reduced the static pressure in the duct below the elbow. 2. .Splitters'in the elbows had the effect of dividing the air stream into a number of streams flowing^ through rounded elbows and therefore lowered the angle of discharge, reduced or eliminated the reverse flow area, and made the outlet velocity quite uniform. 3- Turning vanes'having 2 in. inner and 1 in. outer radii located in the center of the elbow were found most effective in improving performance in regard to angle of dis charge, outlet velocity, and elimination of reverse flow area. 4. Pressure loss through stack heads may be reduced by use of splitters or turning vanes or by increasing the inner radius of an elbow. Considering the sum of the velocity and static pressure as a measure of the energy required to change the direction of the air stream and to deliver.the air into the.atmosphere, and considering the energy required for a plain fitting as 100 per cent, it was found that turning vanes dropped the energy requirement of square type stack heads to 45 per cent. Splitters reduced the energy requirement to 90 per cent in long radius elbows and to 74 per cent in short radius turns. In expanding heads splitters reduced the energy requirement to 58 per cent. Side Outlets in Horizontal Air Ducts When air is supplied to a room from side outlets in horizontal ducts it is necessary to use directive devices within the duct at each outlet in order to obtain a uniform velocity of delivered air and to obtain a direction of flow perpendicular to the face of the outlet. In tests6 conducted with 3 in. x 10 in., 4 in. x 9 in., and 6 in. x 6 in. outlets in a 6 in. x 20 in. hori zontal duct at duct velocities of 200 to 1400 fpm (in the 6 in. x 20 in. section) it was found that multiple curved deflectors produced the best flow characteristics. Vertical guide strips in the outlet were not so effective,as curved deflectors. A single scoop type deflector at the outlet did not improve the flow pattern obtained from a plain outlet and was therefore not found to be desirable. .Ceiling Outlets, on Horizontal Ducts Ceiling outlets are usually installed below horizontal supply ducts so that the supply air has to make a 90 deg turn before entering the outlet itself. The shorter the connection between bottom of duct and outlet, the greater is the need for directive devices to obtain uniformity of flow. Generally speaking, conditions and remedy in such cases strongly re semble those for side outlets in horizontal air ducts. Ceiling ducts often have a rectangular cross section while the connections to the ceiling out lets are circular. It will then be quite difficult to install turning vanes successfully, particularly 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 area 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. Air Distribution 741 Volume Control Various methods are used to regulate volume of supply and return (exhaust) 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, need and feasibility of directional control; (2) 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 be located near any air outlet and is practical only where no intermediate setting between full open and closed is required. 2. Hit-and-miss Damper. Two slotted plates or discs, closely adjacent; by moving one of the two plates the respective slots may be opened or closed. This type of. volume '/ Hinges y Either two high velocity 3777777; or air spills down ward Fig. 7. Effect of Various Damper Arrangements Designed for Straight Blow control may be installed close to an air outlet and it is easy to operate, but its main dis advantage is that even in the open position the air passage area is blocked by at. least 50 per cent. This requires oversizing of the air outlet in order to avoid excessive increase of noise level. 3: Splitter Damper'. A single blade sheet metal plate hinged at one edge,; usually located at the branch connection of a duct or outlet. It is easy to operate but often causes irregular air flow in the duct. When used in connection with, and near an outlet additional directional control is required. "' " 4. Butterfly Damper. A single blade sheet metal hinged in the middle, usually located in a straight duct run. It is easier to handle than a splitter damper, since only halFthe motion is necessary to change its setting. However, if located too close to an air outlet, it is objectionable because its operation frequently results in a condition whereby too high velocity jets are created along the sides of the duct or the air spills immediately downward into the occupied zone (see C Fig. 7). 5. Louver Dampers. Numerous designs have been developed incorporating a series of splitter or butterfly dampers across the duct or air outlet. Their main advantage consists in retaining greater uniformity of air flow and in requiring less depth for instal lation. Some designs provide for louver blades moving in.opposite directions, and while decreasing free air passage area, retain a constant air flow direction along the axis of the duct air outlet connection (see A and B in Fig, 7). RETURN AND EXHAUST INTAKES The selection of return and exhaust intakes depends on: (1) velocity in occupied zone near intake, (2) permissible pressure drop through in take, and (3) noise. 1. Velocity. The. effect of air flow through return intakes upon air movement in the room is slight. Air handled by the intake is drawn from