The injection water enters the elbow at the top and is drawn through an annular opening into the condenser. This opening may be regulated by the small hand wheel shown at the top end of the stem.
The exhaust from the steam end flows into the condenser through the pipe as may readily be observed—or escapes into the atmosphere by throwing the switch valve.
Note.—Utilizing hot discharge water. In manufacturing establishments where large quantities of water are required, advantage can be taken of the fact that in condensing apparatus of this and similar pumps, the water, after performing useful work in the condensing chamber, can be elevated to a tank in any portion of the building, and used over again for another purpose, such as washing, cooling metal plates, rolling-mill rolls, etc. The fact that the temperature of this discharge water will range from 100° to 120° will, in many cases, be advantageous, and effect a saving in the cost of heating other water for purposes in which this discharge water will answer equally well. When the water is not required in the tank, the stop-valve may be opened, and the water allowed to escape into a drain, or any other convenient place.
A ball-float attached to an air valve is located at the right hand of the condenser so that in case the pump should fail to operate from any cause, the injection water will lift the ball-float, which in turn will open the air valve and by discharging the vacuum will prevent the flooding of the engine cylinder with water.
It is a well-known fact that the atmosphere exerts what is usually termed “back pressure” of 14.7 pounds per square inch upon the piston area of a steam engine, also that water converted into steam, may be converted into its original state by condensation. Now, if this back pressure, which is, in reality, the weight of the surrounding atmosphere, be removed from the piston of a steam engine, the steam on the opposite side of the piston would have that much (14.7 lbs.) less work to do.
Applying this to steam engines means conveying the exhaust, or expanded steam, which would otherwise be allowed to escape into the open air, into a closed chamber, where it is met by a spray of cold water, which so rapidly absorbs the heat contained in the steam that it ceases to retain its gaseous form, and is again reduced to its original bulk as water. A great change has now taken place, and the steam is reduced to its liquid form. As this water of condensation only occupies about 1⁄1600 of the space filled by the steam from which it was formed, the remainder of the space is vacant, and no pressure exists.
The difference in volume accounts for the atmospheric pressure on the outside of the chamber, and as the vacuum extends throughout the whole distance which the exhaust steam originally occupied, it, of course, is made available in the cylinder of the engine in the shape of a decreased pressure on the exhaust side of the piston; the atmospheric pressure remains constant, therefore we have the atmospheric pressure acting on one side of the piston, and absent on the other; the gain being 14.7 pounds per square inch, if a perfect vacuum could be secured. It amounts in average engineering practice to from 12 to 13 pounds, or 24 to 26 inches of mercury, as the graduations usually read on vacuum gauges.
Jet and Surface Condensers are further described and illustrated in a special allotted section of this work. The vacuum pump is usually of the reciprocating order, although other methods have been employed for emptying condensers, but not with equally satisfactory results.