20 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[January, 



To determine the horse-power necessary to drive the fan wheti 

 discharg-injj air, the velocity of the tips of the vanes not to exceed 

 -jSj of the theoretical velocity, having given the density of air 

 required, also the cubic feet. 



First find the hor.se-power, as directed in former examples, when 

 the fan is not dischargintf air. 



IVicii nniltiply ^ part of the weight of air to be discharged by 

 the fan per minute in pounds by -j% of the theoretical velocity, and 

 divide by 33000. The quotient will give the horse-power necessary 

 to discharge this quantity of air, which add to the horse-power 

 necessary to drive the fan when not discluiryiuij uir, for the answer 

 required. 



Example. — Let D be the density of air rc()uired = 4 oz. A, a 

 column of mercury equal to the density ;= '5 aiul W = the weight 

 of air to be discharged = 220 lb. per minute, and V ,?j the velocity 

 of fau in feet per minute. 



:= 9-5 ^ P = the pounds acting on the vane. 



16 



Then by former rule, V 



9315-0 X 9-5 



= 2"67 horse-power 



33000 

 necessary to drive the fan without efflux. 



Now a cubic foot of common air at 60° Fahrenheit weighs r209 

 oz., therefore a cubic foot of the given density will be equal to 



,220 X 16 

 1-511 oz., and - . _.— = 2330 feet = the cubic quantity 



1-511 



of 



discharged per minute. And 



220 

 "60 



3-66 X 9315-0 



33000 ^ '■*' ^'"''- 



to discharge the given weight of air, and 

 the total horse-power required. 



power necessary 

 1-0 + 2-67 = 3-67 



When the velocity of the tips of the vanes is to move equal to 

 the theoretical velocity, then we proceed as in the last examples, 

 only we take -^ instead of ^ (as in former examples) of the 

 weight of air discharged, which added to the horse-power requisite 

 to drive the fan when no efflux takes place. 



We should here again remark, that when the fan is moving at 

 this velocity, that it is capable of discharging 4.80 lb. of air per 

 minute without any falling off in density. 



In a recent set of experiments, the inlet openings in the sides of 

 the fan chest were contracted from Ui, the original diameter, to 

 12 and 6 in. diameter, when we obtained the following results. 



First, that the power expended with the opening contracted to 

 12 in. diameter, was as 2| to 1 compared wiih the opening of 

 17^ in. diameter; the velocity of the fan being nearly the same, as 

 also the quantity and density of air delivered. 



Second, that the power expended with the opening contracted 

 to 6 in. diameter, was as 2g to 1 compared with the opening of 

 17^ in. diameter ; the velocity of the fan being nearly the same, 

 «nd also the area of the efflux pipe, but the density of the air 

 decreased one-fourth. 



These experiments show that the inlet openings must be made 

 of sufficient size, that the air may have a free and uninterrupted 

 action in its passage to the blades of the fan, for if we impede this 

 action we do so at the expense of power. 



(Papek No. 2.) 



In resuming the subject of the fan blast, I shall endeavour, as 

 far as I conveniently can, to avoid detailed statements of the pneu- 

 matic laws involved in its consideration, as they would occupy 

 more time than would be consistent with the present occasion; and 

 shall proceed to remark on the most important points connected 

 with the construction of the fan, viz. : the adoption of such forms 

 and proportions, as shall insure the greatest results with the least 

 expenditure of power; and effect a diminution of the intolerable 

 noise that generally arises from the working of the fan. And al- 

 though I have not been able to carry out such leading principles 

 to the fullest extent, I trust that I have furnislied materials that 

 will be found of value to those members whose greater leisure may 

 enable them to do so. 



From a contemplative view of the action and apparent effect of 

 that very useful ajiparatus, a fan blast, it would appear tliat the 

 air in the fan case is impelled by the vanes along the transit pipe, 

 or channel, to the chest provided for the blast ; and that the con- 

 tinuous rapid motion of the vanes, compresses air in the pipe and 

 chest, to a degree that may be shown and accurately measured, by 

 a water, or mercurial gage, attaclied to the blast chest. 



In my first communication, the principal investigation rested on 

 a theoretical question, viz. : whether the tips of the blade should 

 partake of the same velocity as a body falling freely a certain 

 height, such height being governed by the density of air required. 

 Recent experiments (the results of which accompany this paper) 



justify the conclusions then made, as will he seen on examining 

 tables Nos. 2 «, 3 a, and 4 a. 



Having satisfied myself with respect to the velocity a fan ought 

 to have, when a certain density of air is required, I purpose in this 

 paper to examine the fan under other varied conditions, the object 

 being to establish the best proportions of inlet openings in the 

 sides of the fan chest, and the suitable corresponding length of 

 vanes. For this purpose, I caused the iipeiiiugs in the sides of the 

 fan chest to be made of a large diameter, and I was enabled to vary 

 those openings, by fitting in rings of wood ; and I varied the fan 

 by attaching to its arms, vanes of corresponding lengths. The 

 experiments are classed in the follow iiig tables : — 

 luble Nu. 1 Oy CotUiiiHS the tirst sut oi txpi-niueiiis. 



„ 2 a, Experiiuents made witli an iiilel opening 30 inches diameter ; 



the length of vane being redui-ed to 8 inches. 

 „ 3 a, With an inlet opening of 2J| inches diameter, and the length 



of the vane 11 inches. 

 „ 4 a, With an inlet opening of 20 J inches diameter, and the length 



of the vane 13f inches. 

 „ 1 b, Shows the effect produced by narrowing the blades to 6 



inches, the length being 16 inches, with outlet to transit 



pipe 4 inches deep. 

 „ 2 6, 3 b, i b, are experiments showing the effect produced by 



contracting the outlet opening. The inlet opening, and the 



length of vane, being the same as the table under which it 



is classed. 

 In the concluding part of the first paper it was stated that, by 

 impeding the free admission of air into the vane, it was done at the 

 expense of power. Thus, by contracting the inlet opening to 12 

 inches diameter, we expended more than twice the power. This 

 led to an extension of the openings, the results of which will be 

 seen on comparing the former state of the fan, in table No. 1 a, 

 with the present tables Nos. 2 o, 3 a, and 4 a. 



In the first five experiments, no efflu.x of air takes place; and if, 

 in these experiments, we take the mean of the density of the air 

 and the horse-power, and call them unity, their proportions with 

 the corresponding experiments represented in tables 2, 3, and 4, 

 will stand thus : 



Table No. 1 1- Density of air. 1- Horse-power, 



2 -69 „ 1-21 



II 3 '° i» '9 If 



4 1- „ 1-10 



Here the results are in favour of the fan in its original shape, and 

 similar results appear when the fan is discharging air. 



I will now proceed to e.xamine the inlet opening, and the best 

 length of vane. 



From the experiments enumerated in the tables it will be seen 

 that the longer vane possesses a preponderating power over the 

 shorter one, in condensing air of the greatest density, with the 

 least proportion of power. Thus, with a vane 14 inches long, the 

 tips of which revolve at the rate of 236'8 feet per second, air is 

 condensed to 9-4 ounces per square inch above the pressure of the 

 atmosphere, with a power of 9-6 horses ; but a vane 8 inches long, 

 the diameter at tlie tips being the same, and having, therefore, the 

 same velocity, condenses air to 6 ounces per square inch only, and 

 takes 12 horse-power. 



Thus, the density of the latter is little better than -S^ of the 

 former, while the power absorbed is nearly 1-25 to 1. Althougli 

 the velocity of the tips of the vanes is the same in each case, the 

 velooity of the heels of the respective blades are very different ; for 

 whilst the tips of the blades in each case move at the rate of 236-8 

 feet per second, the heels of the 14 iiu-li blades move at the rate of 

 908 feet per second ; and the heels of the 8 inch move at the rate 

 of 151-75 feet per second; or, the velocity of the heel of the 14 

 inch, moves in the ratio of 1 to 1-67, compared with the heel of the 

 8 inch blade. The longer blade approaching nearer the centre, 

 strikes the air with less velocity, and allows it to enter on the blade 

 with greater freedom, and with considerable less force than the 

 shorter one. The inference is, that the short blade must take 

 more power at the same time that it accumulates a less quantity of 

 air. 



These experiments lead me to conclude that the length of the 

 vane demands as great a consideration as the proper diameter of 

 the inlet opening. If there were no other object in view, it would 

 be useless making the vanes of the fan of a greater width than the 

 inlet opening can freely supply.* t)n the proportion of the length 

 and width of the vane, and the diameter of the inlet opening, rest 

 the three most important points, viz. : quantity, and density of air, 

 and expenditure of jmwer. 



* The proportion a suction pipe bearg to a pump, Is an anagalous case ; for, if we 

 drive the bucket at a greater velocity than the suction pipe will supply it with water, 

 the consequence will be, that we shall not lift so much water, at the same time that we 

 absorb more power. 



