Document 930b04Z8mxyEb8EO3mBRZBGY3

300 CHAPTER 21 1960 Guide fig. 7.... Relation Between Velocity and Velocity Head for Standard Air fig. 8 .... Loss in 90-Deg Elbows of Rectangular Cross-Section Table 3 .... Pressure Losses Due to Elbows lAddHiooat fqwvafonf tom* in Cxctt* of Frhfioo to tntormetion of Center tine*} cm N-KC. .LLuSTiiAUCm comcxtions loss c* 1 l/o 1 Vw MCTANCULAft Oft OOUW; WITH Oft wrTM- JL *eo- VALUE KC ILU W4teouw sccnoM to-ocs. AECTAHOULAft section ea-cee. SOUARC StCTION WITH SW.O-TCA WKS wtu WITH ruwete WKS 553 wire* ft/0*O.S O.TS 1.50* 0 40 2.0 ft/w MITCH o. 0.23 0.7S 1.5 MITCft o.s o.s .s 0.24 0.14 1.25* 1.25 o-eo 0.57 0.14 .47 10 0.50 MITCH 0.5 1.0 0.75 1.0 15 urrtft 0.5 4.0 0.75 'VW > MrrtK 0.5 0.5 0.4 0.15 (.50 1.00 0.41 0.04 1.50 0.44 0. 17 0.14 0.70* 1.0 ' 1.0 1.3 UITtft 0.5 as 0.5 as 0.4 0.75 0.4 O.T 1.0 O.T t.O 1.s i.5 i.e run wo 0.15 0.1* 0.45 0.95* ranees wo O.IO* *5* 25 25 12 4 *4 40 * 75 50 21 no 05 45 ft so" 4 7.2 221* WITTES TIC WITH NB cowmen equal to * SCUILAft ELBOW. M>itn TCC * Vloe# based os / nines ctf epproxiimteiy 0.05. b Vstose s'flfnMrt (ran L/D sad LfW valoes el'Beferatce 0 for/ -- 0-C2. Note: Soperaoipt nnmbere refer to references at end of chapter. 2000 cfm, the ASHAE Friction Chabt, Fig. 3, gives a loffl of Ofi in. of water per 100 ft. of 12.4-in. diameter duct. Thus the loss from A to D is 0j6 X 58/100 -- 0348 in. of water. The use of elbows of radius ratio, R/W = 15, is considered good practice with respect to both installation and operation. In a given rectangular duct 6 by 24 in., for example, the elbow loss will be greater for a fiat bend where the aspect ratio, HfW = 1/4, than if the bend of the same radius ratio had been made in the plane of the narrow dimension giving an aspect ratio, H/W -- 4. Data now available for losses in compound bends/'' where two or more elbows are close together, do_not warrant re-._ finement of design calculations beyond use of the sum of the losses for the individual elbows. Actually, Urn losses are somewhat less than for two bends when they are placed to form a TJ bend, and somewhat more when they form a reversed, or S bend. Where angles of other than 90-deg bend are encountered, the loss may be considered as directly proportional to the Air Duct Design Table 4 .... Pressure Losses Due to Area Changes TYPE J/TA CON0I TIONS cocmecMT rroc LLUSTRA conoi- LOSS T>ON T0N3 COCFflCCKT ASftlftT CXPAM3KM A/A, Ci C, ai 0.2 0.5 04 aoi 11 ABRUPT A, 0.44 044 0.5* 10 3 511 eowrKAcnoi SQUARE COCC ~n* as 023 1.04 ao 014 0.45 i/A, 0.2 0.4 0.0 0.0 0>4>2 ass 0.25 O.I 0.04 004 0-10 ao 004 0j04 at ao> 031 50* 0.02* 45* 0.04 .r 00* 0 07 6AA0UAL EXPANSION 5* air* EQUAL AREA 0.22 10* 0.20 20* 045 V4, c ? <14* `ais'------- 40* ABRUPT A| jA, %Oi EXIT tAj*ft3 SQUARE (Oct OftVKC --"I A. /A, OO 02 Ol4 00 a s a?s ENTRANCE -- 0.54 e^Lce 2S0,J" ENTRANCE 220 1.40 sj: -- A> A./A* a.5 c 005* c. 00 1.54 .OO 40UANC iln** 0.0 250 1.40 OAK ACftOSS -bj OUCT e/o ass 0.50 07* 4.0 Oftiricc CKTftANCC if- 0.0 0 0 1.54 0.40 o.ei 0.54 PIPE ACftOS5 -N OUCT STRtAMLMt* STftt/T -M ACftOSS OUCT aio 0.25 e/o 0.10 azs 0.30 0.20* 053 *2.0 C act* 0.25 0-90 SOUK ORlftet IN OUCT A./A c. Lsli ~DT -- A, _fg. 0.2 0.4 0.0 O.ft 2.50** i.ae 1.21 0.04 0.20 0.0 Not* I: Subscript oo C indicate* crow eceticaat which velocity is calculated (See, for example. Equation 10 in text.) Note 5: Superscript numbers refer to references at end of chapter. 301 angle of bend. Losses' for elbows discharging air directly into a large space are higher than those given for elbows within duct systems. Turning vanes may be advantageously employed in elbows, both to reduce the pressure loss and to provide a more uni form velocity distribution downstream from the bend.*' " Vanes and concentric splitters are particularly recommended where miter elbows are used, because even the simplest vane forms will produce a substantial saving in pressure loss. PRESSURE LOSSES IN DIVIDED-FLOW FITTINGS Analysis" of available data for losses at branch take-offs indicates that the loss in the straight-through section is about * 35 percent of that for abrupt enlargement (1) involving the same ratios of velocities and (2) that the loss in the divertedflow section depends on the ratio of the velocity of diverted flow to total flow and on the angle the take-off nvOraa with the main, and is at a minimum for a natural relation that exists between these two variables. Some representative branch losses are given in Table 5. LOSSES DUE TO AREA CHANGES Area changes in ducts, generally unavoidable, are neces sitated frequently by the building construction or changes in the volume of air carried. Experimental investigations' of pressure changes, and pressure losses at changes of the area of duct cross-sections, indicate that the excess pressure loss over the normal friction loss is a dynamic loss due to a faster stream expanding into a slower stream, as determined by the actual areas occupied by the flow rather than the areas of the duct. No perceptible dynamic loss is due to the converging of the air stream itself where the flow is con tracted, but the air stream continues to converge beyond the edge of the contraction and reaches a minimum at the vena contracta. For contraction, therefore, the dynamic loss is caused by expansion from the vena contracta to the full area following the contraction- Abrupt contraction in area may therefore be considered as a special condition of abrupt enlargement. Fig. 11 illustrates (a) abrupt enlargement and (6) abrupt contraction. For a sudden symmetrical enlargement, a theoretical ex pression for the loss is '-('**)-(-am Table 5.... Ratio of Pressure Loss to Branch Velocity Pressure Ratio of Vefodfy in Branch to Vefodiy in Main Duef 0.4 06 as 1.0 1.5 2.0 3.0 90-deg 60-deg 45-deg 6.5 3.1 2.0 1-5 0.95 0.74 0.62 5.0 2.2 1.3 0.77 0.47 0.47 0.58 3.5 1.3 0.64 0.43 0.40 0.45 0.54 where Jf = pressure loss due to sudden enlargement, based on standard air, inches of water. . V, = velocity of standard air in the inlet duct, feet per min ute. fig. U .... Air Row at Abrupt Enlargement or Contraction of Air Stream