Steam:
Its Generation and Use

Many boilers show a high apparent evapora- tion in consequence of furnishing “wet steam,” while practically they are anything but economi- cal. Parties have been known to claim an evap- oration of 19 to 20 pounds per pound of coal, where the highest practically possible is not over 13. Such boilers are dear at any price. The cause of priming may be either impure water, too much water, or improper proportions in the boiler. When a boiler is found to form wet steam with good water, carried at a proper height, it is a proof of wrong design. The amount of priming in different boilers varies greatly, and as yet there is not sufficient data to establish any definite ratio for boilers in ordinary use. The experiments of M. Hirn, at Mulhouse, showed an average of at least 5 per cent. ; Zeuner sets it down as approximately from 7% to 15 per cent.; the careful experiments at the American Institute in 1871 show in cylin- drical tubulars 7.9 per cent., and in the tests at the Centennial Exposition one boiler showed as high as 18.57 per cent, priming. In sixteen different tests of the dryness of the steam from Babcock & Wilcox boilers made by twelve different engineers, the average moisture in the steam was only 1.116 per cent. The high- est was 4.16 per cent., which was less than the same engineer with the same apparatus found in large two-flue boilers, working very lightly. SUPERHEATED STEAM. Steam which has a higher temperature than that normal to its pressure, is termed “super- heated ” or “gaseous.” Dr. Seimens found that when steam at 2120 was heated separate from water it increased rapidly in volume up to 2300, after which it expanded uniformly as a permanent gas. If this superheating could be carried to such an extent as to avoid the ‘ ‘ initial condensation within the cylinder of an engine, there would be a marked economy in its use, but this involves so high a temperature as to burn the lubricating material and destroy the engine in a short time. Dixwell found superheating so as to maintain in the cylinder a temperature of 400° with steam at a pressure of 70 lbs., to be the limit of possible lubrication. With a higher pressure that degree of superheating would not afford sufficient additional heat for the purpose. The present tendency to high pressures seems, therefore, to preclude the possibility of much gain through superheating, because the temperatures are already carried to very nearly the limit at which lubrication can be maintained. For other pur- poses the use of superheated steam adds little if anything to the economy, while it greatly increases the cost and the wear and tear. Where super- heating is required it should always be done by a separate apparatus, and pains must be taken to separate the entrained water from the steam before it enters the superheater. The use in any steam boiler of superheating surface exposed to the gases of combustion, is highly objectionable and is of doubtful efficiency. Attempts to super- heat steam by means of the waste gases, are usually failures because in a well proportioned boiler the low temperature of such gases necessi- tates an unreasonably large surface to produce the desired effect. Steam cannot be superheated when it is in contact with water. FEEDING BOILERS. The relative value of injectors, direct-acting steam pumps, and pumps driven from the engine, is a question of importance to all steam users. The following table has been calculated by D. S. Jacobus, M. E., from data obtained by experi- ment. It will be noticed that when feeding cold water direct to boilers, the injector has a slight economy, but when feeding through a heater a pump is much the most economical. Method of Supplying Feed Water to Boiler. Temperature of feed water as deliv- ered to the pump or to the injector, b0° Fah. Rate of evaporation of boiler, 10 pounds of water per pound of coal from and at 212Q Fah. Direct acting pump, feeding water at 6o°, without a heater....................... Injector feeding water at 1500, without a heater............. Injector feeding through a heater in which the water is heated from 150 to 2000...... Direct acting pump feeding water through a heater, in which it is heated from 60 tO 200°...................... Geared pump, run from the the engine, feeding water through a heater, in which it is heated from 60 to 200°.. Relative amount of coal required per unit of time, the amount for a direct acting pump, feed- ing water at 60°, without a heater, being taken as unity. Saving of fuel over the amount re- quired when the boiler is fed by a direct acting pump without heater 1.000 .0 \ •985 1.5 per ct. •938 6.2 “ .879 12.1 “ .868 13.2 ECONOMY OF HIGH PRESSURE STEAM. Higher steam pressure is the tendency ot the times, and with good reason, for the higher the pressure the greater the opportunity for economy in generating power. The compound and triple expansion engines of the present day, which have reduced the cost of power some 40 per cent, over the best performance of a few years ago, require higher pressure than can with safety be carried on shell boilers, but there is no difficulty in carrying any desirable pressure on a sectional water-tube boiler properly constructed. Babcock & Wilcox boilers in special cases, carry as high as 500 lbs. pressure in regular work.