STEAM BOILERS 139 



260-7-1,151.5 = 22.6% of the total. All heat taken from a 

 source of waste, therefore, that can be imparted to the feed- 

 water before it enters the boilers is just so much saved, not 

 only in cost of fuel but in boiler capacity. 



Types of Exhaust-Steam Feedwater Heaters. The impuri- 

 ties contained in the water will largely determine the type of 

 exhaust-steam heater to be used in any given plant. These 

 heaters are divided into two general classes, namely, open 

 heaters and closed heaters. 



An open heater is one in which the water space is open to 

 the atmosphere. In a direct-contact open heater, the exhaust 

 steam comes in contact with the water, which, by means of 

 some one of a number of suitable devices, is broken into spray 

 or thin sheets so that it will readily absorb the heat of the 

 steam. In a coil heater, the exhaust steam passes through 

 coils of pipe submerged in a vessel containing the water to be 

 heated, and open at the top. 



A closed heater is a heater in which the feedwater is not 

 exposed to the atmosphere, but is subjected to the full boiler 

 pressure. The steam does not come in contact with the water; 

 the latter is heated through contact with metallic surfaces, 

 generally those of tubes, that are heated by the exhaust steam. 



Selection of Heater. When the boiler feedwater is free 

 from acids, salts, sulphates, and carbonates, so that no scale is 

 formed at a high temperature, the closed feedwater heater will 

 be found satisfactory. Heaters of the coil type may be used 

 with pure water, but should not be used with water that 

 will precipitate sediment or scale-forming matter of any kind. 

 The coil heater is very efficient as a heater, as the water cir- 

 culating through the coils is a long time in contact with the 

 surface surrounded and heated by the exhaust steam. Heaters 

 of the closed type, with straight tubes and sediment chamber, 

 can be cleaned more readily than those having curved tubes, 

 but the curved tubes allow more freedom for expansion and 

 contraction. Heaters of the tubular type should have ample 

 sediment chambers and may be used with water that contains 

 organic or earthy matter, but not with water containing scale- 

 forming ingredients. Carbonate of lime is likely to combine 

 with earthy matter and form an exceedingly hard scale. 



