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4. How large a flask will contain 1 lb. of Nitrogen at 3200 lbs. per sq. in. pressure and 70°F.?
6. How many lbs. of air does it take to fill 5600 cu. ft. at 15 lbs. per sq. in. pressure and 60°F.?
WATER AND STEAM
S we look about us in nature, we find that all inanimate creation presents itself to us in three distinct physical states. Certain bodies, for instance, of themselves readily maintain their shape while others, although non variant in density, nevertheless seem to have no particular physical configuration but seek, due to the force of gravitation, the lowest level attainable and consequently must as a rule be held in a containing vessel. On the other hand, a third class of bodies is found not only possessing no particular physical configuration, but which acFIG. 34.-Water and steam space tually seem inherently desirous of expanding to such an extent that
they must as a rule be completely housed, bottom and top, in a containing vessel.
In the class room or in the power plant, it is easy to find illustrations of these three general classifications. Thus, chalk, iron pipe, and coal are instances of the first division and are known as solids. Crude petroleum, water, and kerosene are instances of the second division, and are called liquids. Finally, air, steam, and producer gas illustrate the third division, and are called gases.
These States are Possible to all Bodies.-The most interesting thing about these socalled states of matter, and indeed the item of most importance to the engineer, is that by varying the pressure externally forcing itself against the sides of any one of these bodies and by adding or subtracting the heat that may be held in store within the body itself, any solid may be converted into a liquid and then into a gas, or any liquid may be converted
into a solid or a gas, or any gas may be converted into a liquid then into a solid.
The Fundamental Principle in Steam Engineering. It is this property of matter that makes the operation of the steam engine possible. For if we were not able to heat water and convert it into steam, it would be impossible to make use of this liquid for steam engineering purposes, although it is the most widely distributed in nature.
Again, since fuel oil must be converted into the gaseous state before it readily and efficiently burns beneath the boiler, it would certainly be cumbersome and impractical for its use in the great majority of central stations if it could not be conveyed through pipes or in oil tanks as a liquid from the oil fields to the place of consumption.
Steam Engineering Still Supreme. Since water is so widely disseminated in nature and since it can be readily and efficiently changed from one state to another, it is the working substance that today still drives the vast majority of power developing mechanisms in the industries in spite of the rise of the gas engine and the great modern evolution in water power development. Let us then trace the physical phenomena that accompany the transformation of water into the solid state which of course is necessary in the production of ice, and again from the liquid to the gaseous state which becomes necessary in the production of steam.
The Formation of Ice.-Let us first start with a pound of water at ordinary temperatures-say at 62°F. As we begin to lower the temperature, in other words to draw off heat, the volume slightly decreases. Thus the pound of water now occupies less space than formerly. Hence, if this water was on the surface of a mountain lake and the night was getting cooler, the surface water would sink to the lake bottom and allow warmer water from the bottom to rise only to be cooled at the surface to again drop to the bottom. This is what is known as water circulation and is very important in steam generation, as we shall see later.
When, however, the water under consideration lowers to a temperature of 39.4°F., a strange thing happens. Something develops in its internal structure that now makes the water expand as the temperature is further lowered. A unit volume of water now becoming lighter than formerly, no longer will it
sink to the lake bottom but remains on the surface. Hence when a short time later the water on the surface is lowered to 32°F. or freezing point, ice is formed on the surface only, since water is a poor conductor of heat. Nature thus protects the fish in the waters below.
Coming back from the mountain lake, however, to the formation of ice in the ice plant when the temperature has reached 32°F., although heat be now driven off, the water does not lower itself in temperature but remains at this temperature until it has all been converted into ice.
Latent Heat of Fusion.-The quantity of heat necessary to form one pound of ice at 32°F. from one pound of water at 32°F. is a definite measurable quantity and is known as the latent heat of fusion. By careful measurement, the latent heat of fusion for water has been found to be 142 B.t.u. That is, to convert one pound of water at 32°F. into ice at 32°F. requires the drawing off of as much heat as would approximately be required to lower one pound of water one hundred forty-two degrees in temperature.
When this pound of water is converted into ice, its volume still further expands. Hence, one pound of ice will float in water. This accounts, of course, for the floating of icebergs on the water surface, and furthermore this sudden increase in volume accounts for the rutpure in pipes and other nuisances that occur in severely cold weather.
Going back to our pound of water now converted into a pound of ice, let us again proceed to draw off heat. It is now found that we may lower the temperature of the ice much more easily than when it existed as a liquid. Indeed only about one-half the heat is required to be drawn off per degree lowering in temperature while its volume practically remains constant.
The Formation of Steam.-Let us now proceed to a consideration of the physical changes and phenomena that occur when water passes into steam. Starting with water at say 62°F., as we add heat the temperature increases at the rate of about 1°F. for every unit of heat energy added to the water. At the same time the volume slightly increases. Hence, if our pound of water under consideration be situated at the bottom of the well-known tea-kettle, the observation of which led James Watt to the invention of the steam engine, this pound of water becoming now less dense will rise to the top and cooler water at the top will sink to the bottom which in turn is passed again to the top as it becomes
heated to make way for more water from the top to be heated along the portion exposed to the heat application. Thus the water becomes warmer and warmer and the transference from bottom to top continues. The ease with which this transfer of heated bodies of water takes place has much to do with efficient operation of the steam boiler which may be likened to an enlarged
FIG. 35. The temperature heat diagram.
Here is graphically indicated the history of a pound of water in its relationship with temperature and heat. Beginning at 32°F. and atmospheric pressure, by drawing off heat the horizontal line ab is traced, showing that the temperature remains constant until the water is completely converted into ice, after which the temperature rapidly falls at the rate of about one degree for every half unit of heat drawn away By the addition of heat, however, at point a, the curve ae is traced, which indicates that the temperature rises with absorption of heat at the approximate rate of one degree for each unit of heat absorbed. At 212°F. and atmospheric pressure the horizontal line eg is traced until 970.4 B.t.u. are absorbed. After all the water is thus converted into steam the curve gh is traced for superheated steam, which rises at the rate of about one degree for every .47 of a B.t.u. absorbed.
tea-kettle with accessories and appurtenances to care for its increased responsibilities as compared to tea-kettle operation.
Latent Heat of Evaporation. The water in this manner continues to absorb heat until if under atmospheric pressure, it reaches a temperature of 212°F. At this point, however, vast quantities of heat may be added and still the water will remain at this temperature although it may now be observed that steam