Document XOK69mobx0V5zq9vRVDg072mB

554 Chapter 30. 1945 Guide from the supply opening area and average face velocity. This fact should l?e remembered in considering the use of register dampers, particularly in those cases where there must be considerable throttling with the damper ' to balance a poorly designed system. Where reduction of noise is im portant, it is recommended that balancing dampers be placed in the duct ahead of the acoustic duct lining. Similar unequal face velocities, aggravated by a deflection of the air stream, are obtained with the arrangement shown in Fig. 3. The latter may be corrected by inserting a turning member in the elbow back of the outlet face as shown in Fig. 4. The importance of straightening the air stream and effecting uniform distribution over the-entire face of the. supply opening cannot be over-emphasized*. Dampers of special construction, as illustrated in Fig. 5, may be used to maintain a constant direction of blow, approximate distance of blow, and constant outlet velocity regardless of the damper's position. The' capacity of dampers diagrammed as A and B will be roughly in proportion to the position of the operating lever. They are particularly effective for Hinges, //Vz Either two J high velocity / jets / I'777777, or air spills down ward Fig. 5.- Effect of Various Damper Arrangements Designed for Straight Blow cooling work with oversized grilles. The single leaf damper shown as C in Fig. 5 is objectionable in that it frequently results in a condition whereby two high velocity jets are created along the sides of the duct, or the air spills immediately downward on the occupants below the outlet. Operation of Ceiling Outlets .The relationships presented for sidewall outlets will' apply with the proper modification to the operation of ceiling outlets. With the ceiling 4 outlets, the . angle of distribution may be. a full 360 deg/ This extreme angle of distribution results in a high rate'of induction and short blow, and also permits air to be introduced into a space satisfactorily at a relatively high temperature differential and high velocity. Ceiling outlets may handle greater quantities of air than a comparable sidewall outlet without the creation of excessive air motion. In contrast to wall outlets, the ceiling outlet does not necessarily have to be equipped with adjustable features, other than means to regulate air quantity. Frequently sections of ceiling outlets are blanked off to avoid having the air stream strike wall or column obstructions. TYPES OF SUPPLY AND RETURN OPENINGS Perforated Outlets. Due to the non-adjustability and small vane ratio of perforated sheet metal outlets, they have not met with favor as supply *A.S.H.V.E. Research Report No. 1155--The Performance of Stack Heads, by D. W. Nelson.-D. H. KransandA. F.TuthiU (A.S.H.V.E. Transactions. Vol.46.1940,p.205). A.S.H.V.E. Research Report. No. 1226--Performance of Side Outlets on Horizontal Ducts, by D. W. Nelson and G. E. Smedberg (A.S H.V.E. Transactions, Vol. 49, 1943, p. 58). , Air Distribution _____________________________________ . - ___________________________________________ 555 openings. ' They, are useful primarily where directional air control is unnecessary, and for return air openings. Vaned Outlets. Outlets equipped with both vertical and horizontal adjustable varies are particularly suited to sidewall distribution. For . proper control over the air flow, the vane ratio should be from 1 to 2. Outlets with non-adjustable vanes may be employed; however, they should only be used where the performance is not critical or can be adequately predicted. Registers. Fixed vanes or perforated grilles equipped with a single blade damper are termed registers. They are primarily used for residential heating systems, where the outlet distribution is not critical and low cost , is of importance. Slotted Outlets. Slotted outlets essentially consist of either flat steel plates containing a number of long narrow slots or a single long narrow slot, with a free area of approximately 10 per cent. In order to give a. good conversion from static pressure to velocity pressure, the sides of the . slots are rounded to give a venturi effect. Due to their high aspect ratio, the slotted outlets have a greater induction effect than the comparable vaned outlets of equal area; thus, the throw of the slot is slightly less. They are primarily useful where an unobtrusive means of distribution is desired, and where it'is desirable to submerge the outlets into the room decoration. Since these outlets offer greater induction at a given noise level, they are useful in obtaining proper air motion when otherwise limited by design. Ejector Nozzles refer to outlets which operate, at high static pressures,. and which are constructed to give a high conversion from static in the duct to velocity pressure in the outlet, and have a high induction effect due to their high outlet velocity. _ The ejector is chiefly used for long throws and industrial process installations, such as drying, freezing, . cooking-, etc. Another type of ejector is sometimes referred to as a louver . nozzle having a 45 to 90 deg elbow, which can be rotated similarly to a universal joint about an axis perpendicular to the surface to which it is fastened. These outlets give a considerable degree of adjustability and are, therefore, desirable for use in confined spaces where spot cooling is employed. Ceiling Outlets. There are two general classifications of ceiling outlets, one the simple ceiling plaque, usually with limited induction effect, and the other a concentrically vaned ceiling outlet with high induction effect. Plaque outlets, although cheap and simple in construction, are difficult to control and are not generally satisfactory for comfort cooling where the entering air is more than 12 F below the room temperature. The vaned ceiling outlet with marked induction will distribute air uniformly over an entire half sphere. The induction effect is greatest in the direction of the axis of the outlet, and least in the plane perpendicular to the axis and located at the ceiling level. Thus the induction is greatest in the vertical direction where the least blow can be tolerated, and least in the horizontal plane at the ceiling where the greatest blow is both desired and permissible. Perforated Ceilings. Such outlets consist of metal or composition board ceiling with small perforations through -which air may be supplied to a room. The free' area of the ceiling is about 10 to 15 per cent. The per forated ceiling can be designed to give a lower rate of room air motion for a given air supply than any other type of outlet. For this reason, the perforated ceiling is particularly applicable in. installations requiring a