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540 CHAPTER 29 1965 Guide'And Data Boole 3. Natural convection currents form a stagnant zone from the .mary-and total air far above the occupied sone. Therefore, the ceiling down during cooling and from the floor up during `V'outlets are capable of handling relatively large quantities of air heating. It should be noted thatthis zone ia formed above the '..j, at large temperature differentials. terminal'point'of the total'airduriiS'heating'ahd below'the terminal point during cooling.-Since^tnia sone.is a result of the natural convection currents, the air'Velocities, within the cone are usually low (15-20 fpm) and the air.stratifiea in layers of in |*j .' :' is During beating, the warm total air tends to rise,'.and then occupies the region near the ceiling. This results in a large stag- nant sone between the floor and some distance above the floor, Temperature variations will be small in the region between the creasing temperatures from a low to a high leveL The concept of a stagnant layer and the total air, that is to say, the air in this stagnant'sone is an important tool in'the proper application and selection'of outlets as it permits due consideration of the natural convection currents from.warm and cold surfaces and internal -loads. . 4. The return intake affects the room air motion only within its immediate vicinity. The intake should, in general, be located in the stagnant sone so as to return the warmest room air during cooling and the coolest room air during heating. The importance of the location depends on the relative of the stagnant sone which results with the various types of outlets. 5. The general room air motion (shown by dear areas in Figs. | region is uniformly mixed. 1 There ia a large temperature gradient between the floor and the --.stagnant layer, primarily as a result of the natural convection -; currents, due to the heat load and the construction of the space. This condition indicates that the outlets should have additional - ' radiation to take care of the-perimeter exposures. On the other hand, in an interior sone. where the loading is not severe, the stagnant layer is practically nonexistent, and conditions through out the whole room are uniform with a minimum temperature gradient! ' 2 to 6) ia a gentle drift toward the total air."Room conditions are O.-' Flow rate and velocity for both heating and cooling are the maintained by_the entrainment of th* room air into the.total air ' same for both outlet types in Fig. 2. The heating diagram for the strcain/lThe room air motion'betweea the stagnant zone and.the r;1 sidewall unit shows that under these conditions the total air does total ag.is relatively low and 'uniform. The highest air-motion !]; not'deacend along the walL Consequently, a condition;of over occurs in and near the total air streams.*- - -.f- ly' blow may hot be objtotiohable during hmtmg In fact, such a con- Group A"Outlet*' (Fig. 2): This group includes high sidewall Fli ditioo might be in eliminating the stagnant zone, grilles, sidewall diffusers, ceiling' diffusers, linear ceiling diffusers overblow causes some of the warm air to reach the floor level and and wmiTar outlets. High sidewall grilles and ceiling diffusers are i counteract the'stratification of the stagnant region. fflustratedm'F1g.-2.'- - *-E_ - - -r-` ,,i The heating diagram for the ceiling outlet again shows the effect The primary' air envelopes (isovels) show a horizontal'two-jet ' ^ of the natural convection currents. It results in a larger throw pattern for the high sidewall, and a 360 deg diffusion pattern for ' toward the cold exposed mil. It may be that the velocity the ceiling outlet. - Although variation of vane settings might of the total air toward the exposed wall complements the natural cause a discharge in one, two or three jets in the case of the side-'* convection currents. -However, it will be noted that the warm wall outlet, or obtain a smaller diffusion angle for the ceiling out total air its downward momentum at its terminal point, let, the general effect in both will be the same.' that buoyancy forces cause it to rise toward the primary air. During cooling, tire total air drops into the occupied zone at' some distance from the outlet, which depends on air quantity, Although these forces are complementary, the heating effect of.the, total air replaces the cool natural convection currents with warm supply velocity, temperature differential-between supply jamm!i .- total air , room air. deflection setting, ceiling effect, and the type of loading within the space. Analytical methods of relating some of these factors are given in the section Isothermal Axial Flow Jets. .The cooling diagram for the high sidewall outlet shows a condi Group B Outlet* (Fig. 3). This group includes floor registers, baseboard units, low sidewall units, linear type grilles in the floor or window sill and similar outlets. Fig. 3 illustrates a floor outlet adjacent toaninsidewalL.. . t tion' of 'eoerthroie which causes the total air to drop along the' opposite wall and Sow slowly some distance across the floor. This1 These outlets'have no deflecting vanes!"and consequently the primary air ia discharged in a single vertical'jet. When the total condition should be avoidecL because in this case the total air air strikes the it fans out in afl directions from the point of region becomes a region of high velocity accompanied by tem contact, and during cooling.follows the ceiling for some distanw perature depressions. Velocities in the order..of-10D-to;150,fpm) before dropping toward-the occupied sone. During heating, the may be' found near the wall, but will dissipate rapidly within a total air-flow follows the ceiling across: the room,-and tben.de- few inehes from the wall. - , !-.- cends the exterior wall for some length. , The cooling diagram-of the outlet shows that total- air, movement is somewhat counteracted by the rising natural convection currents on the wall, and therefore drops before reaching the heated wall. On the other hand. the total sir reaches' the inside wall and descends some distance along the wall. With; this type of outlet, temperature variations within the room are held to a mniiwi^ and there is hardly a stagnant region.' Since the. mowirmm velocity as' well as t-h* 'tninrtiiirn -temperature' variation occurs within 'anid dear the totabair envelope, the drop region (Equation 1) becomes highly important!' For this reason,' it'ts'imperative to know how far the air drops, before velocities . aiwi teniperaturto reach acceptable limit*. ' '^ The cooling diagram shows that a stagnant zone is formed out side the total air region above the terminal point of the total air. Below the 'stagnant zone, .the air temperature .is uniform, and complete' cooling is effectod within this region. Also,- the apace below the terminal point of the .'total air will be satisfactorily cooled. For example, if. the total,air flow is.projected upward for a distance, of 8 ft, the region from this level down to the floor will be satisfactorily cooled.' This* of course, does not apply to an ex tremely large' space.- Somejudgment is needed to determine the acceptable rise of the space outride of the total air. A distance of 15720 ft between the drop region and the exposed wall has been, found to be a conservative design value. 1 ^ Since.these outlets dischargp.horisootally near the ceiling, the' "A tompariton of Figs 2 and 3 for heating shows that the stag-" warinest air in'the room is mixed immediately with the'cool pri- nant region is smaller for Group B than Group A outlets. This is - Space Air Distribution 541 t SETTING O-l t U*d by pcnmssoo from Jlefoenc* 4. Rg.,4 .... Air Motion Characteristics of Group .C Outlets Used by paraaooo fros Cafaem 4. Rg. 5'.... Air Motion Characteristics of .Group D Outlets due to the fact that the air entrained in the immediate vicinity'of the outlet is mainly taken from the stagnant region,'and thus it is* the coolest air in the room. This results in greater temperature equalization and fewer buoyant effects in the total air would occur'with Group A outlets. The temperature gradients for both outlet groups are about the except that the stagnant layer for Group B is lower than for Group A outlets. .Group C Outlet* (Fig. 4). This group includes, floor diffusers. mite wall diffusers, linear type diffusers and other outlets installed in the floor or window silL Although outlets of this group are related to Group B, they are characterised by wide spreading jets and diffusing action.-Never- thelea, total air and room aircharacteristics are very similar,,to those of Group B, although the stagnant sone formed is larger during cooling, and smaller during Meeting Diffusion of th* pri mary air usually causes the total air to fold back on the primary air and total air during cooling, instead of following the ceiling. This diffusing'action of the outlets melrea it more difficult to project the cool air, but it also provides a greater area for induo-' tion of room air. This is especiaUy beneficial during heetingj wine* the induced air comes from the rawer regions of the room. " '. . Group D Outlet* (Fig. 5). This group' includes baseboard and low sidewall registers and wimiUr outlets. ` These outlets discharge the primary air in a ginglH or in multiple fete However' since'the air is discharged horizontally across the' floor, the total air, during cooling, remains near the floor, and a' large stagnant zone is established in the entire upper region of the' room. --- During heating, the warm total air rises toward ceding, as a result of the buoyant effect of the warm air.- Also, the temperature variations are uniform with the exception of the total air region.- - Group B Outlet* (Fig. 6). This group includes refling dif- fusers, linear type grilles, sidewall diffusers and grilles, and similar1 outlets. Fig. o shows the heating and cooling diagrams for.a writing dourer with vertical downward projection. ''J During tooling, the total air projects to the floor, and follows. - r001) Thich results in a stagnant region near the wiling Dur ing heating, the total air flow; after reaching the floor, folds back' toward the ceiling. If the projected air does not reach the floor, a* stagnant sone will result near the floor. discharged from the grille by means of the vanes is impossible! If it is higher than two, further improvement is slight. A grille discharging air uniformly forward (vertical vanes in straight position), will have a spread of 14 to' 24 degrees, depending on type of outlet,' duct approach, and discharge velocity. Turning of the vanes will influence the direction and the throw of the discharged air stream." ' A grille with diverging tones' (vertical vanes witii uniformly inertosing angular deflection to a'maximum at each end of 45 deg) 'will have a spread of approximately 60 deg and the throw will-be reduced considerably. With increasing diver gence the quantity of air handled by the grille for.a given duct static pressure will decrease; "" A grille with' converging tones (vertical1 .vanes with uni formly decreasing angiibu- deflection) will have a slightly higher throw than a grille witii straight vanes, but the spread will be the garTM* for both settings. The air stream will .con verge' slightly for a' short distance in front of the outlet,' and then spread somewhat more than the air discharged1 from a grille with straight vanes. --In addition to vertical vanes -which normally spread the air horizontally, horizontal vanes may be usedto spread'the' air vertically. However, spreading the air vertically entails the risk,of-hitting beams ox other obstructions,, or of .blowing primary air at excessive' velocities into the occupied' zone. On the other hand, vertical deflection may increase the ad-, herence to the ceiling and reduce the drop..(See Chapter 30). General Factors Affecting Outlet Performance.. Effect of Vanes. Vanes affect grille performance if their depth' corresponds at least to the distance between the vanes. If the tone ratio is less than unity, effective control of the air stream.. Utod by permissioa horn ftmfmreae* 4. Rg.. 6 .... Air' Motion Characteristics of!Group"E!Outlets