156 THE APPLICATION OF SMALL STEAM TURBINES 



careful study and experimental work by the fan-builders, this has been increased 

 to 50 per cent at the present day. 



The main difficulties lay in the design of a low-head fan without casing. In 

 other words, the old turbine propeller problem was again present. The fan, of 

 course, had to be of the single-inlet type and the area of inlet large enough to 

 pass the required volume of air with the pressure governing the tip speed, and in 

 turn limiting the revolutions per minute of the turbine. 



One other great difficulty encountered in the early days was the true deter- 

 mination of the performance of the fan. With the exception of Captain Taylor's 

 experiments and apparatus for testing fans standardized by him, little had been 

 done in research work to determine the performance of fans of this kind. To 

 carry out Taylor tests was an expensive operation at best, and the figures given 

 by the fan-makers were more or less guesswork based on the more elementary ap- 

 paratus necessary for cased fans, which (as has later been found out) was in no 

 way applicable to the determination of the performance of open-type fans. 



The adoption of Terry-driven units for destroyers was about coincident with 

 the use of fuel oil under the boilers, and the correct amount of air required for 

 the oil fuel in torpedo-boat service had not as yet been definitely determined. 



Very careful tests were laid out and conducted by Mr. Charles F. Bailey, chief 

 engineer of Newport News Shipbuilding & Dry Dock Co., on board the first vessel 

 to be fitted with turbine blowers (the U. S. S. T. B. D. Roe), on April i^, 1910 

 (Journal A. S. N. E., Vol. XXIT, No. 2, May, 1910). On this test two fans de- 

 livered 44,000 cubic feet of air per minute to one boiler room, at an average speed 

 of 1,361 revolutions per minute and 3.25 inches static pressure in the fire-room. 

 The fuel oil used per hour, 2,400 pounds; total water evaporated, 102,700 pounds 

 per hour. 



Since that time the capacity and air pressure have gone up with the increased 

 horse-power of the new destroyers, so now the standard for destroyers Nos. 69 

 to 74 is 32,000 cubic feet per minute against 6 inches air pressure. This has been 

 obtained without increasing the diameter of ventilators, but by improvement in 

 turbine and fan efficiency, so that the total steam of the latest design is practically 

 the same as required on the U. S. S. Roe. 



The high static pressure in the fire-room and greater capacity have increased 

 the peripheral speed of the fan to such an extent that now the type of fan selected 

 is in part determined not only by its efficiency but by the noise it makes, as ex- 

 cessive noise, of course, is a tactical disajdvantage to any destroyer. Furthermore, 

 the first destroyers were fitted with mushroom-type ventilators, whereas the latest 

 class has the cowl type, which forms a most excellent megaphone for projecting any 

 fire-room noise. 



Again, as capacities have gone up, the weight of fan has increased, and it is 

 now generally conceded that the increased duty on the bearings when the destroyer 

 is in a seaway, due to the gyroscopic action of the turbine unit, makes it desirable 

 to limit the speed of these forced-draft sets to about 1,600 revolutions per minute. 



