Document 11437oEEqLpb96BBnnjm9vRE

328 W. N. WITHEE1DQB (5) type of motive power (AC, DC, compressed air, or heat engine)*;^ or belt driven, and whether drive must be protected against air' coST heat, or excessive moisture; (7) kind of contaminants in air to pass" (8) quantity of contaminants in air to pass through fan; (9) con| the atmosphere surrounding the fan and motor, including potential'll fire hazard; (10) temperature of air or gas to pass through fan; (fT ture of atmosphere surrounding fan and motor; (IB) possibility of'co within fan housing or scroll; (18) whether to be located before $ cleaning equipment; (14) size and shape of available space; (IsM charge direction for centrifugal fan to avoid unnecessary bends in;'.cf e. Fan laws. These relationships or physical laws apply wf-p fan and ventilating system are operated together under variablh'llf fan speed and air density: Variable Ian speed--constant air density U) Air volume (c.f.m.) varies directly with fan speed (r.p.m.). () Static, velocity, and total pressures (SP., VP., TP.) vary directly with tMi speed (r.pjn.)". (S) Horsepower (HP.) varies directly with the cube of the fan speed (r.p.mv)* Constant fan speed--variable air density U) Air volume (cfan.) remains constant as air density varies. (5) Static, velocity, and total pressures (S.P., V.P., TP.) vary directly with air.ljnlj (6) Horsepower (HP.) varies directly with air density (d). -y,. Constant weight oj air delivered ' %' (7) Fan speed (r.p.m.) varies inversely with air density (1/d). (8) Static, velocity, and total pressures (S.P., VP., T.P.) vary inversely'wrEi d/d). (9) Horsepower (HP.) varies inversely with the square of air'density (1/d*)^ ,/r Standard air. Unless conditions require otherwise, the ventfff|] neer usually assumes a standard air density of 0.075 pound per ppbuT is the approximate weight of air at sea level barometric pressurei>i.;2^ Hg and 70 P. from 0 to 50 per cent relative humidity. g. Air horsepower. The horsepower output of an air-mbyiiffj known as "air horsepower." It is easily estimated by the formula,:^. AHP = 6350 where AHP = horsepower, c.f.m. = cubic feet of air per 'minut^^ pressure in inches of water. , j-ui-uiv Air'horsepower is sometimes further qualified as eitheF^tofflypiMifiM^tel. or "static air horsepower," Either of these may be computedyrfiqin^'hjfi equation by using for head (h) the "total pressure" or "static. pr|ss-iy.ygk[ tively. The choice between these two methods of rating must depeuifenjAviyB the over-all fan efficiency is to be expressd as "total cfficieficSfls VENTILATION 329 &il"5iency"), or "static efficiency," inasmuch as fan manufacturers fern their detailed statements of performance. 1 B. Ejectors, Venturi Ejectors, or Syphon Jets prfiiejector is a low-efficiency air-moving device selected as a last ||l\>entilating system must handle excessive temperatures, corrosive ||j|jyapors, or viscous or sticky air contaminants that would quickly ^Mtiveness -of axial or centrifugal fans. The air-moving efficiency Stye to the physical proportions of the outer conduit, and the ^pir-velocity through, the inner jet. Both straight-tube and venturi&4used, but the latter can be made more efficient. Efficiencies range ||||$ent. ||who are in a position to compute and specify a high-quality H>asis of the best available aerodynamic data are referred to the .reatment prepared by McElroy.19 Less precise information H|$I by other writers, which is satisfactory for applications iKfijjiVij^i'^eiocities can be produced (8000 to 10,000 l.f.m.) and where one is ^"^^liVgJScfficioncies in the range of 5 to 10 per cent. The "straight-tube" s. diameter one sixth of the straight duct diameter can be used H success when the connected system has a very low resistance. Jm1s Ch .of water. .5. Jesign of Ducts for Exhaust Systems Sial forms and charts have been created and published for the militating the aerodynamic design of ductwork for ventilating Ijjpmately, some of these devices consume more time than they fojjb-if the designer or estimator is not in constant practice, tlform or procedure is offered here as superior to all others, but er is advised to adopt one that best suits"fiis requirements. If Scalculations are infrequent, special tabulating and computing important than a complete list of individual steps for.the essential ffipferations. SfclSicigant-isoeqnu.ence4s. now. outlined, and'...'this.:.isi:foll'owed-.,;.by.~a,,.brief P* l&PROCEDURE for design of exhaust systems , ictwork and Design the Hoods , ....................... *" ;||teketch or layout of the exhaust system; use single lines for ducts if simplicity 4 a&out is more helpful if approximately to scale, although subsequent steps in ||i|es can be carried out even with a very poor drawing. ||||hft^t.ype of hood that will do the best possible job with the smallest amount .of.. ffljlitir&v. U. S. Bur. MineB Tech. Paper No. 678, 1945. *