EXPANSION OF WATER BY HEAT.] 



APPLIED MECHANICS. 



Temperature, 

 Fahrenheit. 

 60 

 120= 

 180= 

 212= 

 250 

 275 

 290 

 S0fr> 

 820 

 344 

 372 

 432= 



TABLE I. 



Pressure in Pressure in Pressure in Ibs. 



Atmospheres. Inches of mercury, above atmospheric. 



0-017 

 0120 

 0300 

 1000 

 2000 

 8-000 

 4-000 

 5-000 

 6-000 

 8000 

 12000 

 20-000 



05 

 3-6 



15 



30 



60 



90 

 120 

 130 

 180 

 210 

 360 

 600 



14! below. 



abo 



15 



30 



45 



60 



75 

 103 

 165 

 285 



We would remark, thai for temperatures below 180 

 and above 344 the results are somewhat uncertain. It 

 is within these limits, however, that any practical appli- 

 cation of the table can be required, for we seldom have 

 to do with steam pressing with elastic force of less than 

 half an atmosphere on the one hand, or with pressures 

 above eight atmospheres on the other. 



VOLUME OF STEAM. The great secret of the power 

 derivable from steam, lies in the fact that a small volume 

 of water is expanded by heat into a large volume of 

 elastic vapour, tending to occupy a greatly increased 

 space, and forcing any obstacle presented to its expansion, 

 with the pressure due to its elasticity. The volume of 

 steam produced from a given quantity of water has been 

 variously stated ; but we believe it may be very correctly 

 estimated at 1600 times that of the water when the 

 pressure is equivalent to 1 atmosphere. That is to say, 

 a cubic inch of water contained in a vessel open to the 

 air, when boiled off into steam, occupies 1000 cubic 

 inches of bulk, and forces the air contained in the vessel 

 away to the extent of that expanded volume ; or expands 

 with a force of 15 Ibs. on every square inch, which is 

 the measure of the atmospheric pressure, and therefore of 

 the force resisting the expansion of the steam. But had 

 the vessel containing the steam a volume of only 800 

 cubic inches, or half that to which the steam would ex- 

 pand at atmospheric pressure, then the density of the 

 steam being doubled or, in other words, the number of 

 particles crowded into the space being doubled the 

 pressure on every part of the vessel would be doubled 

 also. If the cover of the vessel were a movable piston 

 having one square inch area, it would in this case require 

 a load of 15 Ibs., in addition to the atmospheric pressure 

 upon it, to keep down the elastic steam within. 



Looking at the question generally, we see that the 

 volume occupied by steam from a certain quantity of 

 water is inversely as the pressure, because the pressure is 

 as the density, and the density is inversely as the volume. 

 The following is the rule for calculating the volume of 

 gteam at any pressure, produced from a given volume of 

 water. 



Rule. Multiply the volume of the water by 1600, and 

 divide by the pressure in atmospheres. 



Example 1. Required the volume of steam at 4 at- 

 mospheres (or having a pressure of 45 Ibs. per square 

 inch above atmospheric pressure) generated from 3 cubic 

 feet of water. 



3x1600 



1200 cubic feet of steam. 



Conversely, to find the quantity of water necessary to 

 generate a given volume of steam at a given pressure. 



Rule. Multiply the volume of steam by the pressure 

 in atmospheres, and divide by 1600. 



/.'" mple 2. Required the water necessary to generate 

 1200 cubic feet of steam at 4 atmospheres. 



IfiOO ** ^ cu bi 



of water. 



3. A cylinder 12 inches diameter and 20 

 inches long is filled 120 times per minute with steam, 

 having a pressure of 30 Ibs. above that of the atmo- 

 sphere ; required the quantity of water necessary to 

 generate the steam. 



The area of a circle 12 inches diameter is 113 square 

 inches, and the capacity of the cylinder is 113 X 20 = 

 22GU cubic inches. This capacity filled 120 times gives 



a volume of steam = 2260 X 120 = 271,200 cubic inches. 

 As the steam presses with 30 Ibs. above that of the at- 

 mosphere, or altogether with 45 Ibs., that is with 3 



atmospheres, the volume of water is 



271200 x 3 

 1000 



= 508J 



cubic inches. 



From these examples it is evident that a great force 

 can be obtained by subjecting water to the action of heat. 

 Just as a few grains of gunpowder on being ignited be- 

 come suddenly transformed into a large volume of elastic 

 gases, which by their expansive force propel a ball with 

 great velocity, or 'burst asunder the solid rock ; so a 

 small quantity of water heated above 212 is changed 

 into a perfectly elastic vapour, pressing upon the envelope 

 containing it, and forcing any body, opposing its expan- 

 sion, through a space sufficiently great to permit its 

 enormous increase of bulk. 



CONDENSATION. But not only to the expansion of 

 its volume does vaporised water owe its excellence as a 

 moving force, for its increased volume can be suddenly 

 reduced to a small bulk by the application of cold, or 

 the removal of the heat which is necessary to its vaporous 

 condition ; and whatever force the steam exerted in ex- 

 pansion, is returned again by its condensation. 



Thus if a vessel containing 1 cubic inch of water, and 

 799 cubic inches of air, were heated so as to turn the 

 water into steam at twice the atmospheric pressure, the 

 steam would force out the air and occupy its place ; act- 

 ing as it expanded with a pressure of 15 Ibs. on every 

 square inch above that of the atmosphere. Were the 

 vessel now cooled so as to reduce the 800 cubic inches of 

 steam to 1 cubic inch of water, the vacuum left by the 

 condensed steam would be immediately filled by air 

 pressed into it by the surrounding atmosphere with a 

 force of 15 Ibs. on every square inch. Were the vessel 

 fitted with a piston or partition capable of sliding up- 

 wards or downwards in it, without permitting the passage 

 of fluid round its edges, the eflect would be the same ; 

 Fig. I4i. for if the piston at A (Fig. 145) 



were in contact with the water be- 

 fore boiling, and raised to B by its 

 expansion into steam on heat being 

 applied, it must, in rising, have 

 been subjected to a pressure of 

 30 Ibs. on every square inch of its 

 under surface, so as to overbalance 

 the atmospheric pressure on its 

 upper surface by 15 Ibs. On cold 

 being applied so as to reduce the 

 steam to its original volume of 

 water, the pressure on the under 

 surface of the piston being removed, 

 that of the air on its upper surface 

 again forces it downwards from B to 

 A, its original position. Thus, both in the ascent and in 

 the descent there is developed a force, applicable to the 

 movement of machinery properly connected with the 

 moving piston. In order that we may attain some 

 notion of the amount of this force, let us suppose that 

 the piston has an area of 1 square foot, and can rise and 

 fall 2 feet, and that we have the means of generating 

 and condensing the steam in the vessel 50 times in every 

 minute. 



Since 1 squire foot = 144 sq. in., 



And on every square inch the pressure is . 15 Ibs. 



The total pressure on the piston is ... 2160 Ibs. 



Tin's force is moved 2 feet up and 2 feet down 50 times 

 per minute, or through a space of 200 feet per minute, 

 and is therefore equivalent to 2160 X 200 = 432,000 Ibs. 

 moved through 1 foot per minute. A horse-power being 

 reckoned at 33,000 Ibs. moved 1 foot per minute, the force 

 of the piston, as we have estimated it, is equivalent to 



432000 



"o-jnn/r" = 13 horse-power. From this example it will 



ooUUU 



appear that by increasing the size of the piston, the dis- 

 tance through which, it is moved, the rapidity of its 



