Page:Model steam turbines; how to design and build them (IA modelsteamturbin00harrrich).pdf/25

Rh itself, as each layer may be considered as acting as a perfectly free piston to the steam behind it, as in fig. 9. Similarly a projectile in a gun acquires velocity by the expansion of the gases generated by explosion. If, therefore, the work done on itself by steam in falling through a certain difference of pressure (the 'H' of the hydraulic analogy) is calculated, the velocity can be obtained as indicated by equation (5).

Fig. 10.

For example:

The work done on itself by steam expanding from 50 lbs. absolute (gauge pressure 35 lbs.) to $1/2$ lb. absolute (a vacuum of 29$1/2$ mercury) is 276,300 foot pounds approximately, and (5) becomes

or since the square root of 2 × 32⋅2 is 8 nearly,

There are two methods of evaluating the work done by steam in expanding from one pressure to another. One is to construct an 'indicator diagram' for one lb. of steam, as in fig. 10; or, alternatively, to