Part 35 (1/2)

It is especially necessary to reduce back-pressure and to obtain the most perfect vacuum possible without overloading the air-pump, if it is desired to obtain the maximum efficiency by expansion, and it then becomes also very necessary to reduce losses by ”dead-s.p.a.ces” and by badly-adjusted valves.

The piston-speed should be as great as can be sustained with safety.

Good engines should not require more than W = (200/sqrt(P)) where W = the weight of steam per hour and per horse-power; the best practice gives about W = (180/sqrt(P)) in large engines with dry steam, high piston-speed, and good design, construction, and management.

The expansion-valve gear should be simple. The point of cut-off is perhaps best determined by the governor. The valve should close rapidly, but without shock, and should be balanced, or some other device should be adopted to make it easy to move and free from liability to cutting or rapid wear.

The governor should act promptly and powerfully, and should be free from liability to oscillate, and to thus introduce irregularities which are sometimes not less serious than those which the instrument is intended to prevent.

Friction should be reduced as much as possible, and careful provision should be made to economize lubricants as well as fuel.

The Principles of Steam-Boiler Construction are exceedingly simple; and although attempts are almost daily made to obtain improved results by varying the design and arrangement of heating-surface, the best boilers of nearly all makers of acknowledged standing are practically equal in merit, although of very diverse forms.

In making boilers, the effort of the engineer should evidently be:

1. To secure complete combustion of the fuel without permitting dilution of the products of combustion by excess of air.

2. To secure as high temperature of furnace as possible.

3. To so arrange heating-surfaces that, without checking draught, the available heat shall be most completely taken up and utilized.

4. To make the form of boiler such that it shall be constructed without mechanical difficulty or excessive expense.

5. To give it such form that it shall be durable, under the action of the hot gases and of the corroding elements of the atmosphere.

6. To make every part accessible for cleaning and repairs.

7. To make every part as nearly as possible uniform in strength, and in liability to loss of strength by wear and tear, so that the boiler when old shall not be rendered useless by local defects.

8. To adopt a reasonably high ”factor of safety” in proportioning parts.

9. To provide efficient safety-valves, steam-gauges, and other appurtenances.

10. To secure intelligent and very careful management.