July 13, 1888.] 



SCIENTIFIC NEWS. 



27 



THE BLACKMAN AIR PROPELLER. 



'"THE importance of ventilation as one of the hygienic 

 conditions necessary for health should be a truism. 

 We regret to say it is far from being so, especially in 

 crowded workshops and places of public resort, such as 

 theatres, concert halls, churches, etc., and even when the 

 need for ventilation is recognised, how inadequate are 

 the means often adopted for effecting it. In the majority 

 of cases it is admittedly difficult to ventilate efficiently, 

 i.e., to produce a constant supply of fresh air without in- 

 convenient draughts, and without occasional reversals 

 of current or downdraughts, caused by wind, or by the 

 temperature of the external air being higher than that of 

 the air inside the building to be ventilated. 



of coal-mines, where the resistance to be overcome is con- 

 siderable. Blast fans are also very useful for producing 

 the currents of great intensity required for smiths' forges, 

 foundry cupolas, etc. The Blackman propeller is specially 

 designed to move large volumes of air at low speed, and 

 is therefore well suited for the ordinary purposes of ven- 

 tilation. The eight blades of this air propeller, as will 

 be seen in the accompanying figure, are curved in a 

 special manner, not only to catch the surrounding air, 

 but to draw it towards the centre, and then finally to 

 push it forcibly through the propeller itself. Their 

 peculiar construction also prevents the escape of air radi- 

 ally, which is a common defect in most other rotary fans 

 The following table, supplied by the makers, gives the 



Without doubt, the most sure and efficient way of ven- 

 tilating large buildings is to forcibly exhaust the hot and 

 vitiated air by mechanical means. This is usually effected 

 by rotary fans driven by water or engine power. Much, 

 however, depends on the shape of the fan, and although 

 the Blackman is doubtless known to many, it has some 

 distinctive features which some of our readers may be 

 glad to have brought to their notice. In the first place it 

 exhausts or delivers a large volume of air without mate- 

 rially raising its pressure, whereas the other fans in use 

 generallydrive in air at a considerable pressure. The latter 

 system is the best when a strong current of air is required 

 as for instance in the ventilation of the complicated working 



work done, and the power required to do it, for the dif- 

 ferent sizes of the propeller, when there is a free inlet as 

 well as a free outlet for the air. 



Diameter in 



Revolutions per 



Cubic feet of Air 



Actual horse 



inches. 



minute. 



moved per minute. 



power required 



14 



1,000 to 1,500 



1,500 to 2,500 



Fractional. 



18 



700 „ 1,200 



2,000 „ 4,000 



Ditto 



24 



500 „ 900 



3,000 „ 6,000 



i to I 



36 



40O ,, 650 



7,700 „ 14,000 



i „ i* 



48 



300 „ 600 



13. 5°° 1. 3°,°o° 



T 2^ 

 1 »t ^2 



60 



240 ,, 450 



21,000 „ 43.00O 



ii „ 3* 



72 



200 „ 350 



30,000 ,, 58,000 



2 „ 5 



