Page:The New International Encyclopædia 1st ed. v. 18.djvu/605

STEAM. 619 STEAM. passes into the air, it is cooled and collects into small particles of water which are visible as a white cloud above the surface, and the phenome- non is called condensation. If all the particles of this white cloud were collected into one mass, there would be a volume of water equal to the volmue of water in the original vessel which had been converted into the steam forming the cloud. Boiling occurs only when the water in the vessel has reached a certain temperature. This tem- perature varies with the pressure. At the atmos- pheric pressure of 14.7 pounds at mean sea level, it is 212° F., but it would be somewhat less on the top of a high mountain and somewhat great- er at the bottom of a deep mine. See Hypso- METEK ; HYrfSOMETRY. The boiling temperature or boiling point of water thus varies with the pressure upon it. At a pressure of 5 pounds per square inch, it is as low as 162.3° ¥., and at a pressure of 100 pounds per square inch, it is as high as 327.58° F. Con- densation takes place at any temperature lower than the boiling temperature. To explain more fully the action of heat in the formation of steam, reference will be made to the accompanying diagrams. In Fig. 1, let the cyl- inder contain one pound of water at 32° F., and let the pressure of the atmosphere be represented by the weighted piston. Then if heat be applied to the bottom of the cylinder, the temperature of the water will rise higher and higher until it reaches 212° F. ; the piston will up to this point remain stationary except for the .small expansion of the water. On continuing the heat, the water shows no further rise in temperature, but steam begins to form and to force the piston u])ward as show-n by Fig. 2, and this continues until the last drop oi water is converted into steam and we have the condition illustrated by Fig. 3. i & & ± Fig. 1. Fig.S. Fig. 3. Fig. 4-. Before proceeding further, we must note first that no steam began to form imtil the water reached a temperature of 212° F., hence this is evident!}' the lowest temperature at which steam will form under norma! atmospheric pressure. Second, we must note that in the condition il- lustrated by Fig. 3 we- have one pound of steam occupying the least possible volume at atmos- pheric pressure. In actual figures this volume is 26.36 cubic feet. Steam in this condition is known as saturated steam. If now we continue to heat the steam in the cylinder Fig. 3, its tem- perature will rise above that of saturated steam, and the piston will move upward, and we will have siiperheated steam. If now we take the cylinder Fig. 3 and plunge it into a vessel o£ cold water, as shown by Fig. 4, the heat will be taken away from the steam and it will con- dense to water. When this water has cooled to 32° F., the whole heat taken away is exactly e(iual to the wdiole heat added during the opera- tions illustrated by Figs. 1, 2, and 3. The amount of this added and abstracted heat ni.ay now be considered. First, let lis assume that the cylinder Fig. 1 has an area of 1 square foot and that it contains 1 pound of water. The height to which the water rises in the cylinder is 0.016 foot. The pressure on the piston from the air is 14.7 pounds X 144 s<|uare inches = 2110.8 pounds. Xow on applying heat to the water it will at first gradually rise in tem- perature from 32° F. to 212° F. before evapo- ration commences. Then 212° — • 32° = 180 are the number of heat units required to raise water from 32° F. to the boiling point at atmospheric pressure. Steam now begins to form and the piston to rise until all the water is converted into steam at a temperature of 212° F. This .steam, as before stated, occupies a space of 26.36 cubic feet. The heat required to perform this operation is 006 units. Hence the total heat required first to raise the water from 32° F. to 212° F. and then to convert it into steam is 180 + 966=1146 units. It is quite clear how the heat required to raise the water from 32° F. to 212° F. has been expended, but it is not so clear how the 966° F. required to con- vert the water at 212° F. into steam at 212° F. has been expended. It will be observed that two things have happened in this last operation. First, the water has been converted into steam, which occupies 1644 times the space occupied by the water from wdiich it was generated. Second, the piston has been raised from the surface of the water in Fig. 1 to the surface of the steam in Fig. 3. Therefore, the heat has been expended in two ways: First, in overcoming the internal molecular resistance of the water in changing its condition from water to steam, and, second, in overcoming the external resistance of the pis- ton to the increasing volume of the steam dur- ing formation. The first task performed is called the internal work of the steam and the second task is called the external work. Now the share of the heat expended in each operation may be calculated as follows: The total heat expended is, as already stated, 1140 units. To raise the piston with a pressure on it of 2116.8 pounds thiough a height of 26.30 feet requires 55,799 foot-pounds of energy. As the energy of one heat unit is 772 foot-pounds, then 55,- 799 -;- 772 = 72.3 heat units expended in raising the piston. Adding this to 180, the number of heat units required to raise water from 32° F. to 212° F., we have 2.52.3 units consumed in heating the water and raising the piston. This amount subtracted from the total heat expend- ed gives 1146 — 252.3 = 893.7. which is the num- ber of heat units expended in internal work. We can now summarize the distribution of the heat as follows: In raJHiTij^ tPTnppratnre of water 180 units In (iohiK internal work 893.7 unite In diiinp; external work 72.3unlt8 Total 1146.0uuit3 Having gone through the phenomenon of steam generation in detail, we can summarize some of the general facts that have been brought out. The temperature of the water gradunlly rises until it reaches the temperature at which steam