92 MOTIVE POWER IN MANUFACTORIES 



2. Externally fired boilers, when a masonry firebox is found underneath a boiler traversed by a 

 large number of tubes. Gases pass first to combustion chamber at rear end and then through 

 tubes baci< to front. 



To this class belongs the water tube boiler, with inclined tubes, a horizontal top vessel and 

 vertical tail tubes, creating a continuous circuit of water. 



(b) Pointers about boilers :- 



1. Twelve square feet of heating surface of boiler furnish one horse-power. 



2. Each horse-power requires one cubic foot or 7 ' ., gallons of water per hour. Nominally, the 

 conversion of 34'5 lbs. (=41 gallons) of water into steam at 212 degrees is the equivalent 

 of one indicated horse-power. 



3. Mud drum at base of boiler to receive impurities deposited by water. Where no mud drum 

 exists, boiler should be blown off weekly through a bottom valve (mud cock). 



4. Steam and water capacity must be sufficient to prevent any fluctuation in pressure or water 

 level. If fire is fed from mill refuse, steady heat and steam can be retained only with boilers 

 of large water capacity. The larger the boiler, the greater the fuel economy. 



5. A large water surface (horizontal versus upright boilers) prevents steam from carrying along 

 particles of water. Usefulness of dome is doubtful as a means to secure the return of watery 

 particles to the boiler. 



6. Water should occupy three quarters of boiler space. 



7. Combustion chamber should allow of full combustion of fuel and gases. 



8. Draft area should be one eighth of grate area. Heating surface should be as nearly as possible 

 at right angles to the current of escaping gases. 



9. Very best water gauges, safety valves, injectors and steam gauges are prerequisites. All boiler 

 fixtures should be readily accessible. 



10. Safety valves must be tried once daily. The water level should be controlled by gauge cocks, 

 glass gauges alone being unreliable. 



11. Cold water should not be fed directly into boiler and should never come in direct contact with 

 the boiler metal. Steam injectors will not lift hot water so well as cold water. 



12. Steam pressure gauge must stand at zero when pressure is off. 



13. In case of low water and danger of explosion, cover fire with wet earth. 



II. Engines :- 



(a) Engines in common use are designated as:- 



1. Condensing (low pressure) or non-condensing (high pressure) engines. 



2. Reciprocating (single or double acting) and rotatory engines. 



3. Center crank or side crank engines; center crank engines are preferable for small portable 

 sawmills, since they allow of exchange of flywheel and main driving pulley. 



4. Single cylinder or double cylinder engines; double cylinders are more effective than single 

 cylinders, especially if not hitched tandem fashion. This arrangement, however, allows of using 

 one piston rod for the pair of cylinders. 



(b) Pointers about engines :- 



1. Horse-power of engines is: — 



Sectional area of piston in square inches times pressure times velocity in feet over 550. 

 Deduct 10 to 20 per cent for friction. 



2. Pressure on the piston is not much over one half of pressure in the boiler (60 per cent). 



3. Interdependence between size of cylinder and horse -power actually developed is approximately: — 



Diameter, inches ... 8 ... 9 ... 10 ... 12 ... 12 ... 12 ... 14 ... 16 

 Length, inches ... 15 ... 15 ... 15 ... 15 ... 20 ... 24 ... 24 ... 30 

 Horse-power 12 ... 15 ... 20 ... 25 ... 30 ... 35 ... 50 ... 85 



