BOILERS AND GENERATION OF STEAM 319 



dampers in both directions by water pressure, and will close 

 or open them on a variation of one pound of steam. It makes 

 a partial stroke and stands at any point; that is, it will move 

 from the open position to one-quarter, one-half, three-quar- 

 ters, or fractions thereof, and come to and remain indefinitely 

 at a state of rest, and then return to the open position, thereby 

 making the only true and proper movement of the damper. 



360. Cleaning the Boiler. When the water is heated and 

 converted into steam, the sediment or suspended dirt remains 

 in the boiler and forms scales. These scales are composed 

 principally of mineral matter and affect the economical gen- 

 eration of steam by preventing the water from coming in 

 contact with the plates and tubes. The latter are then 

 heated to a much higher temperature than would otherwise 

 be necessary and to too high a temperature for the good of the 

 metal. Thick scales on the surface of a boiler cause unequal 

 expansion of the plates and tubes, resulting in leaky tubes and 

 seams, and largely accounting for blisters and bagging. 



Various methods have been invented for removing and pre- 

 venting scale. Kerosene oil removes oil scale very effectually. 

 About half a pint of kerosene oil per day fed continuously into 

 the feed water will be found sufficient to remove scale as fast 

 as it can be taken care of by cleaning the boiler, and without 

 danger of accumulating and causing serious overheating. 

 Scale may be to some extent prevented by the use of a good 

 compound, provided the water has been analyzed and the 

 compound which has been prepared particularly for that 

 water is used. Mechanical boiler cleaners may also be used 

 with good effect, but with any method a boiler should be 

 thoroughly cleaned at regular and frequent intervals. Boiler 

 tubes also should be cleaned often. The soot that collects 



