268 PRINCIPLES OF ELECTRICAL DESIGN 



vent ducts, the cooling is not quite so good as when the armature 

 rotates, but a blast of air is driven through the ducts, and this 

 is effective in carrying off the heat. The difficulty in deter- 

 mining cooling coefficients that shall be applicable to all sizes 

 and types of machine stands in the way of obtaining great 

 accuracy in the calculation of temperature rise. It is, however, 

 suggested that the formulas (53) and (55) of Art. 34 (page 110) 

 be used, and that the temperature rise as calculated by the 

 application of these formulas be increased 20 per cent. A tem- 

 perature rise of 45 is usually permissible. 



In designing steam-turbine-driven machines with forced venti- 

 lation, the quantity of air required to carry off the heat losses 

 must be estimated (see Art. 34, page 112) and the size and con- 

 figuration of the various air passages must be carefully studied 

 with a view to preventing very high air velocities and consequent 

 increase of loss by friction. The average velocity of the air 

 through the ducts of machines provided with forced ventilation 

 is usually between 1,500 and 4,000 ft. per minute. This velocity 

 should preferably not exceed 5,000 ft. per minute; it is usually 

 possible to keep within this limit by carefully designing the 

 system of ventilation. 



High-speed machines such as turbo-alternators, when pro- 

 vided with forced ventilation, are usually totally enclosed, 

 the air passages being suitably arranged to prevent the out- 

 going (hot) air being mixed with the incoming (cool) air. Large 

 ducts must be provided for conveying the air to and from the 

 machine. A safe rule is to provide ducts or pipes of such a 

 cross-section that the mean velocity of the air will not exceed 

 2,000 ft. per minute. 



