Fig. 3364.
A diagram for a condensing engine is shown in [Fig. 3364], which corresponds to [Fig. 3363], except that the line of perfect vacuum or no pressure is marked in.
It represents a perfect vacuum, and must be marked on all diagrams from which the consumption of steam is to be calculated, because the quantity of steam used obviously includes that which is used in counter balancing the pressure of the atmosphere.
Learners often get confused on this point, hence it may be more fully explained as follows:
Suppose the engine piston to be blocked, in the middle of the cylinder, and has on one side of it a pressure of 20 lbs. of steam by steam gauge, and on the other the pressure of the atmosphere, and we might pump out the steam, thus leaving the cylinder empty on that side of the piston.
The atmosphere would then exert a pressure of about 141⁄2 lbs. per square inch on one side of the piston, and if we slowly admitted steam again, it would have to get up a pressure of 141⁄2 lbs. per square inch before the atmospheric pressure would be counterbalanced and the piston be in equilibrium.
But the steam gauge would at this time stand at zero, and not show that there was any steam in the cylinder, because the zero of the steam gauge is atmospheric pressure.
When, therefore, the steam gauge showed a pressure of 20 lbs. of steam in the cylinder, there would actually be a pressure of 341⁄2 lbs. of steam per square inch.
The clearance line and the vacuum line must both, therefore, be marked on the diagram when the quantity of steam used is to be computed from the diagram, and also when the proper or theoretical expansion curve is to be marked on the diagram.