Properties of Steam.—Before taking up in detail the valve and other mechanism of the steam pump it may not be out of place to consider briefly the action of steam and its expansive properties. Heat is identical with mechanical force and the one can be converted into the other. Aside from the means used in converting or developing the action a certain quantity of heat always produces a certain quantity of work.
Fig. 246.
The temperature of steam at atmospheric pressure (14.7 lbs. absolute) is 212° Fahr. As the pressure increases the temperature rises, but is always the same for a given pressure. The sensible heat required to raise the temperature of water from 32° to 212° is 180° and the heat absorbed by the water or latent heat at 212° is 996° making the total amount of heat expended 1176°. As the temperature rises the latent heat decreases in nearly the same proportion as the sensible heat increases. This number may therefore be taken as a constant to express the unit of heat in one pound of steam from 32° up to the temperature at which evaporation takes place. Then 1176 × 772 = 907,872 pounds raised one foot which represents the mechanical equivalent or maximum theoretical duty of the quantity of heat contained in one pound of steam.
One cubic inch of water if converted into steam at the pressure of the atmosphere (14.7 pounds) will occupy the space of 1642 cubic inches or nearly one cubic foot. As the pressure increases the volume is relatively diminished and if the same quantity of water is converted into steam say at 200 pounds pressure it will occupy a space of only 133 cubic inches. Assuming that no loss occurred by condensation, if released at this pressure it would expand and again occupy its relative volume at atmospheric pressure.
This is illustrated by the accompanying diagram, [Fig. 246], showing a cylinder with an internal capacity of 1642 cubic inches provided with a movable piston. A quantity of steam representing that formed from one cubic inch of water is forced into it, supposing the weight on the piston to be 200 lbs. per square inch, this weight will be raised until the space under the piston occupied by the steam will be 133 cubic inches. If the supply is now cut off (assuming that no condensation takes place) the piston will remain at this place supporting its load. If the load on the piston is diminished the volume of steam will expand and the piston will be correspondingly raised in the cylinder. This action will be continued until all the load is removed and only the weight of the atmosphere remains. The volume of steam under the piston will then be 1642 cubic inches. It will therefore be seen that the same quantity of steam has exerted a lifting pressure upon the piston due to its relative volume commencing at 200 pounds per square inch and gradually decreasing until the pressure of the atmosphere is reached.
The English unit of heat is that which is required to raise the temperature of one pound of water one degree Fahrenheit and is known as the British Thermal Unit, or B. T. U. Dr. Joule demonstrated by an ingenious device, [Fig. 247], in which a weight operated a paddle wheel agitating water in a closed vessel, that it required 772 foot pounds to raise the temperature of one cubic foot of water one degree, or, on the other hand, it was deduced that one unit of heat was capable of raising 772 pounds one foot high. The mechanical equivalent of heat is therefore accepted as 772 foot pounds for one B. T. U. based on Joule’s experiment.[A]
