Document 0gLVEV9kxYZmLyzjDKgbxr1ym

Genirifupct fan i With zero air flow, preaauro difference In horizontal duet between A and B equals taro *!--r i i FrnMM aw * pit* * *"** *t> solar** *'-Ctntr{fvgat fan 2 With taro air flow, pressure difference In vertical duct from A to B equals air weight 3 For flulde with high Reynolds numbers the friction fac'.or depends only on pipe or duct diameter and absolute roughness as correlated In this sir chart 4 Calculating chart for air-pressure dropa In piping. Example: Per air at atmospheric pressure and 60 F, flowing at a 75-fps velocity through a duct with a relative smoothness of 200. follow the trace and And a pressure drop of 0.03Mn. water through distance equivalent to that of one-pipe diameter 4 NOMENCLATURE a .................pip* Inclination angle, da d .......................fluid deadly, Ibpcrcufl d ...................water density, lb pet eo ft 0 ................................... pipe diameter, ft t ......................................... friction factor ...................gravitational uteritrailsa. ft pet eec per ett h ...................... preeture drop. In. water S ................................. pipe roughne**, ft p ....................... .absolute prature, pelt Ap ..................................... drop, pal R ........................................... gaicoenant R .................................. Reynold* number r ..........................................(ripe rodto*. ft S ......................................... pipe length. fi I ............................. flidd temperature. P u ................................... fluid velocity, fpe v .............fluid vtaeoiliy. lb tec per tq ft TABU II--RANGE OF ABSOLUTE ROUGHNESS Colculoted From Teit 0oo Kind ot pip* Absolute r*tluiat, b. In. Cold-drawn ptpv. breta, copper, lead New ttcl pip* New cait-lron ptpa a.ooos too.ooso 0.003 toO.Oil o.oartoo.oo How to Determine Reliable Air-Flow Data for Design of Duct Systems To overcome wide disagreements between air-pressure drops as found on test and as predicted in handbooks. Arthur H Korn here describes method for finding data rationally that may be used with confidence in specifying both duct and fan sizes the meon value for the new-steel-pipe range in Table II. Friction factors, given In Design of Industrial Exhaust Systems by Alden, yield pressure drops of only one-6fth to one-seventh of those given by the previous references. These wide dis crepancies call (or development of a rational approach to the problem. Designers with no experience in lay ing out specifications for air systems find themselves ot a serious disadvantoge trying to find reiiablo bosio data. Charts, giving pressure drops for air flow through galvanized pipe and round sheet-metal ducts given in various ref erence hand-books, differ by enormous amounts. Data on air flow through metal ducts, riveted and lock-seamed, are apparently nonexistent since none have been published. Absolute roughnesses. Table II, have been calculated from test data com piled by Kcmler, but they show too great a range to be reslty useful. In Kent's Mechanical Engineer's Hand book--Power, a diagram for pressure drop in heating and ventilating duels assumes a constant relative amoothness of 200 regardless of pipe diameter. Mark's Mechanical Engineer's Hand book gives a similar chart for velocities between 1200 and 2400 fpm and o con stant absolute roughness of about k = 0.006, also regardless of diameter. This seems low since it is at the low end for new steel pipe. Table II. The number of joints generally determines the re sistance, and sheet-metal-duct sections are usually only Vfy ft long. The abso lute roughness is probebly greater than RATIONAL DESIGN BASIS Pressure loss o( a fluid, caused by friction with the pipe or duct through which it it moving, is generally small compared with the fluid's sbtoluto pres sure. The general case for a fluid flowing upward in an inclined pipe, forming an angle a with the horizontal, has a pressure drop* of: ftp -f di sitio (I) The last member of the equation repre- * fr MiuUen *rtHlo tm 'On.* Eqntlm W nils Slaran And Dm Plm." b A H Kin. htdti Mr till. as ll-ll. 43 O0| POWER January 14*8 entt the pressure caused by the weight of a column of fluid of the height, s tin a. It can be neglected for air at atmospheric conditions. Since this pres sure difference in a vertical pipe al ready exists. Fig. 2, before air flow starts, it does not have to be overcome by the fan. Since bp s d.A/12, equation (1) can be written lor air flow os: *t. ... w Friction.factor. The frietlon factor / varies with air velocity, pipe size, air viscosity, air density and the pipe roughness. These quantities, excepting pipe roughness, are all related to the Reynolds number as in the following: (5) Cos density depends on its pressure and temperature aa given by the universal gas equation, d = p/K(r + 460). Using this relation and Ironsforming iV to give the pressure drop in a pipe of s length equal to one diameter: <'> For ,tni(i,phcr|c lit, p = 2117 pi/, and a 53.3. Water density is 62.43 lb ** h. and g ss 32.17 ft per sec per Substituting these numerical values: *OW|R January 1948 (3I 93