CHAPTER XLII SATURATED STEAM Q. What is saturated steam? A. That with just enough heat to keep it from condensing. Q. What is dry steam? A. Properly speaking, saturated steam; usually, however, that which has not over two per cent of entrained water. Q. What is superheated steam?* A. That which has been heated, when not in contact with water, hence has a higher temperature than saturated or dry steam at the same pressure. Q. What is the advantage of superheating? A. Carrying more of the heat of the fire into the cylinders, there to do mechanical work. Q. What are the disadvantages of using saturated steam? A. (1) Ordinary valve gears, with high speeds and early cut-offs (say 1-10 or under) have too much compression and too little initial pressure, hence too little power; (2) cylinder condensation increases with the diameter; (3) with large cylinders where the initial pressure is too high it cannot be sufficiently throttled; whereas at low boiler pressure the steam temperature is too low. Q. What is the disadvantage of very high pressures of saturated steam, used in either compound or non-compound engines? A. Too hard on the fire-box, and especially on the staybolts, thus increasing the cost of maintenance and repair. Q. Is the average temperature of the steam in the boiler the same as that of the water? A. It should be; and it is when the circulation is good; *See next chapter. but there may be short intervals when the water is colder than the steam, by reason of overfeeding, etc. Q. What is the temperature of saturated or ordinary steam at 140 pounds pressure? A. 361 deg. F. Q. How many British heat (or thermal) units does one pound of it contain? A. 1,192. Q. What is its volume? A. 2.92 cubic feet. Q. How many B. H. U. (or B. T. U.) would be necessary to heat it to 680 deg. F.? A. 153. Q. What would be its volume then if allowed to expand? A. 4.06 cubic feet. Q. Its pressure, if the volume remains the same? Q. Which increases, pressure or volume, in the locomotive boiler? A. Both volume and pressure. Q. What then would be gained by this superheating? A. By adding only 12.8 per cent more heat, 39 per cent of volume. Q. At what temperature is superheated steam now worked in locomotives? A. 660 deg. F. in the cylinders. Q. What is the highest "flash-point" of lubricant, attainable? A. 780 deg. to 790 deg. F. Q. How about the temperatures at pressures above atmosphere (boiler pressures)? A. Subtract 14.7 pounds (roughly, 15 pounds) from pressures reckoned from vacuum. Q. What have we to do with Centigrade degrees? and reports from other countries where this system is used, and if sent to such countries to understand their units. Further, the Centigrade system is making headway in America. Q. What is the rule for reducing Fahrenheit to Centigrade degrees? A. Subtract 32 and take 5, 9 of the remainder. Thus: 212 deg. F. (212 (212 — 32) × 5 ÷ 9 = 100 deg. C. Q. What is the rule for reducing Centigrade degrees to Fahrenheit? A. Take 9/5 of the sum and add 32. Thus: 180 deg. C. = (180 X 95 324) + 32 356 deg. F. = Q. What are the freezing points on these two kinds of thermometer? A. Fahrenheit 32 deg., Centigrade 0 deg. Q. The boiling points of water? A. Fahrenheit 212 deg., Centigrade 100 deg. Q. What are the temperatures of saturated or dry steam at various pressures from 75 pounds to 200 pounds per square inch? A. The following short table gives them both in Fahrenheit and in Centigrade (also called Celsius) degrees: Q. How do we figure the total heat contained in steam at any given temperature and pressure? A. By adding the latent heat of steam formation to the sensible or appreciable heat, as measured by the thermometer. Thus the total heat above 32 deg. F. is 1,083 deg. plus three-tenths of the observed temperature Fahrenheit. Q. How do we calculate the latent heat of steam? A. By adding the internal heat to the external or that required to increase its volume against external resistance. The rule is to take from 1,114, seven-tenths of the latent heat. Q. Figure the total, latent and sensible or appreciable heat of steam at 160 lbs. gage pressure, reckoning from 32 deg. F. A. The total heat equals 1,083 plus (0.3 X 371) 1,194.3 deg. The latent heat equals 1,114 minus (0.7 X 371) 854.3 deg. The sensible heat equals 371 32 deg. 339 deg. Q. How much of its weight does saturated steam lose by cylinder-condensation at usual cut-offs? A. 30 per cent. CHAPTER XLIII SUPERHEATED STEAM Q. What is superheated steam? A. That which has been raised in temperature and increased in volume by the addition of heat when no more water can be evaporated by that heat. Q. How high is that superheating carried in locomotive practise? A. Up to 600° F., from say 375° F. at 170 lbs. gage pressure. Q. How much is its volume increased? A. That at 170 lbs. is increased in volume over 32 per cent by 200° of superheating. Q. What are the advantages of superheating? A. Saving of coal by utilizing the heat otherwise wasted through the stack; saving of water; avoidance of priming, and of cylinder condensation (hence, possibility of using larger cylinders); increase of engine efficiency when forced; reduction of boiler repairs by use of lower pressures. Q. Does superheated steam follow the laws of perfect gases? A. When far removed from the temperature of saturation it does follow the laws of perfect gases very nearly, but near the temperature of saturation the departure from those laws is too great to allow of calculations by them for engineering purposes. Q. What is its volume compared with the saturated? A. About 25 per cent greater. Q. What is the disadvantage of superheating? A. Increased difficulty of lubrication as the steam of high temperature breaks up the ordinary oils. |