1848.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



21 



TABLES OF EXPERIMENTS. 



In the 14 inch blade, the tip has a vek(city of 2-6 greater than 

 the lieel ; or, by the laws of centrifugal force, the air will have 2-6 

 times tlie density at the tip of the blade that it has at the heel. 

 The air cannot enter on the heel with more than atmospheric den- 

 sity, but in its passage along the vanes, it becomes compressed in 

 proportion to its centrifugal force. The greater the length of 

 vane, the greater will be the difference of the centrifugal force be- 

 tween the heel and the tip of the blade ; consequently, the greater 

 the density of the air. 



Reasoning, tlien, from these experiments, I recommend for easy 

 reference, the following proportions for the construction of the 

 fan : — Let tlie width of the vanes be one-fourth of the diameter of 

 tlie vanes. — Let the diameter of the inlet openings in the sides of 

 the fan diest be one-half the diameter of the fan. — And, let the 

 length of the vanes be one-fourth of the diameter of the fan. 



In adopting this mode of construction, the area of the inlet open- 

 ings in the sides of the fan chest, will be the same as the circum- 

 ference of the heel of the blade, multiplied by its width ; or the 

 same area as the space described by the heel of the blade. 



The following tables gives the sizes of fans varying from 3 to 6 

 feet diameter : — 



I recommend the proportions in table 1, for densities ranging 

 from 3 to 6 ounces per square inch, and for higher densities, viz. : 

 from 6 to 9, or more ounces, the sizes given in table 2. 



The dimensions of the above tables are not laid down as pre- 

 scribed limits, but as appro.\imations obtained from the best results 

 in practice. 



In some cases, two fans fixed on one spindle would be found pre- 

 ferable to one wide one, as by such arrangement, twice the area of 

 inlet opening is obtained, compared with a single wide fan ; and 

 they may he so constructed, where occasionally only half the quan- 

 tity of air is required, that one of them may be disengaged by a 

 clutch, and thus a sa\-ing of power effected. In a single fan of 

 great width, the inlet opening must either be made too small in 

 proportion to the width of the vane, or if it be made large enoiigli 

 for the width of the vnne, the length of the vane becomes so short 

 as to be quite incapable of producing air of the re(|uired density. 



It has been stated that the air from the fan diest is impelled by 

 the vanes along the transit pipe, to the blast chest, &c. : I beg at- 

 tention to the results of an experiment very recently made by me 

 with reference to the admission of air into tlie tr;insit pipe, and 

 which, I think, may lead to an important improvement in the fan. 

 The experiment alluded to, was made to enable me to ascertain the 

 result of varying the area of admission to the transit pipe, in pro- 

 portion to the quantity of blast required for use; and I'effected 

 this by adapting a segmental slide to the circular chest of the fan, 

 as shown in the accompanying section, by means of which, I vary 

 the width of the opening into the transit'pipe, from 12 to 4 inches. 



The object of this arrangement is, to diminish the transit pipe 

 opening at pleasure, in proportion to the quantity of air required, 

 and thereby to lessen the power necessary to woi-k the fan. The 

 results will be seen by experiments inserted in tables 1 b, 2 b, 3 b, 

 and 4 b. The inlet opening to the transit pipe having been con- 

 tracted from 12 inclies to 4 inches deep, so that the tip of the vane 

 and the bottom of the outlet opening were nearly in a direct Iiori- 

 zontal line, nearly the same quantity of air was'impelled, as with 

 the original opening; the noise produced by the fan had, however, 

 nearly ceased. It therefore appears, that the less this opening is 

 made — provided we produce sufficient blast — the less noise will 

 proceed from the fan; and by making the top of this opening level 

 with the tips of the vane, the column of air has little or no re- 

 action on the vanes. 



With respect to the degree of eccentricity which the fan should 

 have, with reference to the fan chest, ,'5 of the diameter of the fan 



