Document NeBobN6vK3MV510zvQER9e0w8

548 CHAPTER 29 1965 Guide; And.Data Book to-the two jet centerlines increases until it equals.the jet centerline velocity. From this point on, a maximum velocity of the .combined jet stream is on the midway line, the profile is almost as though emanating from-asingle outlet of twice the area of one of the two outlets. Koestel and Austin1* have determined the sparing between outlets for noninterference between the jets. For a K value of 6J> the outlets should be placed' three .to eight;diameters apart, for V, values from 500 to 1500 fpm.. ." Effect of Resistance on Return Path of Jet Air Laboratory experiments on jets usually involve recirculated air with negligible resistance to Sow on'the return path of the jet air. Experience .and experiments in tunnels of amall cross? sectional area in mines, where considerable resistance is offered to the return flow of jet air to outlets, sbow that the expansion of the jet terminates abruptly at a distance that is independent of velocity of discharge and but slightly affected by' size of outlet. Such distances are determined primarily by ain and length of return path. In a long 5 ft X`6 ft tunnel, a jet may not travel more than 25 ft whereas in a relatively short open ing, 25 by 60 ft, it may travel more than 250 ft. Few engineer ing data are available on this phase of jet expansion but it can be'of great practical importance.* ISOTHERMAL RADIAL FLOW JETS / In the radial jet the cross-sectional area at any HirfAmy from the outlet varies as the square of this distance, the game as for ah axial jet. Experiments have shown that the centerline velocity gradients and the cross-sectional velocity profiles are similar to those of Zone 3 of axial jets and that the angles of divergence are about the same. In using-Fig 8, X/H should be used as abscissa instead of X/yX. Jets from ceiling plaques have the same form as one-half of a free radial jet. The jet is wider and longer than a free jet, with the maximum velocity close to the wall For a basic equation for radial flow outlets resulting from the air distribution research at Case Institute of Technology . see Reference 17. ' NONISOTHERMAL AXIAL-FLOW JETS The paths assumed by horizontally projected heated and chilled jets under the influence of buoyant forces are of great practical importance for heating and cooling with outlets located in walls. An equation describing the behavior of jets of this type has been presented by KoesteL1* VERTICALLY PROJECTED HEATED AND CHILLED JETS Equations for outlet characteristics that affect the down throw of heated air have been developed by investigators at Kansas State College (see Bibliography, ASHAE Coopera tive Research at Kanra* State College). Equations for temperatures and velocities in heated and chilled jets based upon the research at Case Institute of Technology are given in Reference 19. DUCT ARRANGEMENTS Duct Approaches to Outlets The importance of the manner in which the air stream is introduced into the outlet cannot be overestimated. To ob tain correct air distribution, the velocity of the air stream must be as uniform as possible over the entire to the duct, and perpendicular to the'outlet face. No air outlet can compensatefor improper duct approach:. ' :> A wall grille installed at the end of a'horizontal duct and a ceiling outlet at the end of a vertical duct receive the air perpendicularly and at (for' practical purposes) uniform velocity over the entire duct cross-section, provided the system is carefully designed. However, .very,few outlets are installed in this manner. Most ode waJToutlets sire ingtntfaj either at the end of vertical ducts' or in the side of horizontal ducts, and most oeiling outlets are attached, eithez; directly to the' bottom of horizontal ducts or] to special vertical take off ducts which connect'the outlet with the liorizbntal 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. (See Chapter 30.) ` "' The influence of the duct approach on outlet' performance has been investigated for...vertical, stack, heads -.with -plui^ openings*? or equipped with grilles0 and for;side,outlets on horizontal ducts.*1 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. 10 shows the direction of flow, distribution and velocity (measured'12.in? from opening) .of the air for various types of stack h^d? tested; expandingfrom a 14 X.'6 'ih: 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 direction* of flow and the distribution pattern are generally indicative of per formance obtainable with -nonexp&nding elbows of gimikr shapes for a range of velocities 200 to .1400 fpm. In teste conducted with 3:X 10 in!.,: 4'X'9 ini,'and 6X6 in. ride 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 the outlet were not so effective as curved deflectors. A ring!* scoop type deflector at the outlet did not' improve the flow pattern obtained from a plain outlet, and was not desirable. Dbcharge from a Long Slot When a long slot receives its air supply from one end only, the important design factor is the ratio of the area of the slot to the area of the supply duct, and both the air stream profiles and the duct pressure requirements are determined by this ratio.*1 Fig. 11 showB the changing profile, as the.slot area is increased for a rounded entrance slot (C<j.=. 0.93) with con stant duct cross-section. . . ... . Hie air discharge from a slot in a.tapered duct will.be uni form (Fig. 12) for a relationship between discharge angle- and slot-duct dimensions.14 , .. ` ' 'A.Ct cot 0 = ---- ... (15) 9 discharge angle, degrees. A. " slot area, square feet. , Ad = duct crass-sectional area at upstream end, square feet. C4 coefficient of discharge. RETURN AND EXHAUST INTAKES The selection of return and exhaust intakes depends on: (I) velocity in occupied zone near intake; (2) permissible pressure drop through intake; and (3) noise,. Space Airi Distribution- L < 549 1 Velocity. The control of the room .air motion for the maintenance of effort'conditions, depends'on' the , proper' selection of the supply outlets. The effect of air flow through return intakes upon air movement in the room is slight. Air handled by the intake approaches the opening from all direc tions and its velocity decreases rapidly as:the distance from the opening increases. Therefore, drafty conditions will rarely be encountered near return intakes. Recommended return intake face velocities are given in Table 3. 2 permissible Pressure Drop.' The permissible pressure drop will depend on the choice of the designer..Table 4 gives pressure drop through plain lattice intakes as a function of free area and face velocity.;... Table 3 .... Recommended Return Intake Face Velocities . .hsSoko Location ( Vdocffy Over Gratt - Ana Fpm . Above occupied zone.................................... Within occupied zone, not near seats... Within occupied zone, near seats............. . 800 up 600-800 .400-600. Door or wall louvers....;...................... . .200-300 Undercutting of doors (through under- ' cut-area). .`..V.- -'200-300 fig. 11 .... Shape of Air-Stream Envelopes as'Slot ! Area is Increased - x\\\\\\\\\\\\\\ -fig. 12.\ ;.' Uniform Air How from' a,Slot Supplied ,, by a Tapered. Duct - height, amount of: outside wall area, and number of air changes required. In. some cases,, stratification of warm air may cause,short-circuiting. ; Floor locations of returns are-used in heating installations, for idling or side-wall supply. When located so that air isdrawn across exposed walls, the performance of the system may be somewhat improved. In general, floor locations tend to.collect dirt and refuse. Wall and door;.locations of exhaust outlets depending on- their elevation,, have the characteristics, of either -floor or ceiling- returns. In large buildings with many small rooms,; the return air may- be brought through door grilles or door, undercuts into the corridors, and then to a common return or exhaust. The pressure drop through door>retums should not be excessive; otherwise the air distribution to the room1 may be seriously unbalanced with the opening or closing-' of the doors.''Outward leakage through ` doors'or windows cannot be counted upon for dependable results. (See Chapter 30.) - - 1 ;' ' BALANCING'THE SYSTEM . . . -Proper pressure drop allowance should be made for con trol or directive devices; . \] ' 3. Noise. The problem of noise generated by return in-* takes is the same as that for supply outlets. In computing resultant room noise levels from the operation' of an air-con-, ditioaing system, the return intake must be included as. a part of the total grille area. The major difference between: the supply outlets and return intakes is the frequent installs-,1 tion of the'latter at ear level. When'so located, it is recom7> mended that the return intake velocity:!* not in.excess of 75 percent of the maximum permissible outlet velocity. The location of return and exhaust intakes does not criti cally affect air motion; unless room air' velocities in the. occu pied zone near the intake exceed comfort' limits. The.locations>of return or exhaust intakes are, however, important for obtaining the desired room temperature equalization.-' - Ceiling locations for exhaust outlets are recommended for bars, kitchens, lavatories,; dining rooms, .dub' rooms, etc. where warm air will rise to th'A ceiling leveL In heating installations, location of the return grilles in the-ceiling or high on the wall is not recommended, as it may result in stratification of the conditioned' air, and--depending on the relative location ` of 6upply *nd return outlets--in shortcircuiting. Some ceiling outlets combine the supply and return-open-; mgB in a single unit- This method is'used for heating as well ns for cooling applications:.' However, the application for. heating is more critical and requires consideration of ceiling.' In designing a system, the engineer aims to size ducts and outlets in such a'manner that the supply of air is property distributed. He may, however, feel it necessary to oversee certain trunks, branches, or outlets to allow flexibility or to permit future redistribution, so that tbe system as designed may not be self-balancing. Even if ducts are designed for self-balancing and outlets are properly selected,-all air sys tems must be balanced, that is, the amount of air-through supply ducts, supply outlets,-outdoor air intakes/ exhausts, return outlets, and ducts must be property adjusted so that the air quantities correspond closely with the design quan tities. Ralanwingj therefore, is part of the field test procedure to which each air-conditioning system should be subjected in order to determine-whether'the capacity, and performance, of the equipment correspond with the design. Table 4 .... Approximate Pressure Drops for Lattice Return Intakes Indw* Wofar Gaga---Standard Air Parcaa* Fra# Atm 50 60. 70 -80 Foe# Vttodfy, Fpm 400 500 600 700 soo - 900 1000.: 0.06 0.04 0.03 0.02 0.09 o.oel 0.05 0.03 0.13 0.09 0.07 0.05 0.17 0.12 0.09 0.07 0.22 0.16 0.12 0.09 0.2S ,0.35 0.20 0.24 n 0.16- 0.18 0.11 0.14-*