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9. What is a calorimeter, and for what purpose is it used?

10. By what other method may the moisture in the steam be estimated approximately?

II. What should be done with the percentage of moisture in the coal?

12. How is the chimney draft measured?

13. Describe a draft gauge.

14. Give the formula for ascertaining the efficiency. of the boiler and setting.

15. If the test is to determine the efficiency of the Doiler alone, what factors are used?

16. If a series of tests is made for comparing different varieties of coal, what should be done?

17. What is meant by equivalent evaporation?

18. Why should the results of all tests be computed from and at 212°?

19. What is a factor of evaporation?

20. How is it determined?

21. What is a boiler horse power?

22. How many heat units per hour is this equiv alent to?

CHAPTER VI

VALVES AND VALVE SETTING

Valves and valve setting-Importance of correct adjustment—The D slide valve-Single valve engines-Four valve enginesVarious positions of the slide valve during one revolution— Relative positions of the crank pin and eccentric during the stroke-Valve diagrams-Placing the engine on the centerAdjusting the length of eccentric rod-Measuring the inside and outside lap-Setting the valve-Fixed cut off enginesVariable automatic cut off-Factors affecting the distribution of the steam-Why the four valve engine is the most economical-Description of corliss valves and valve gear and directions for adjusting the same.

Valves and Valve Setting. It goes without saying that every man who aspires to be an engineer should endeavor to thoroughly acquaint himself with the principles governing the action of valves as well as the details of valve setting. But it must be remembered that this knowledge can not be acquired in a day or a week, or even months. True, a man may be able to learn some of the alphabet of valve lore in a comparatively short time, but the more practical experience he has in the work the more will he realize the supreme need of mastering all the details of the process.

The common D slide valve, simple as it appears, is capable of furnishing problems over which savants. have puzzled themselves.

The development of the full amount of power of which the engine is capable, its efficiency and economical use of steam, and its regular and quiet action are, in the largest degree, dependent upon the correct adjustment of its valve or valves.

There are many different types of valves for control. ling the admission and release of steam to and from the cylinders of engines, but the basic principles governing the adjustment of all, whether slide, poppet, rotative, piston, etc., are exemplified in the action of the common D slide valve, viz., the admission of the steam to the cylinder, its cut off and release, and the closure of the exhaust, each and all of which events are to take place at the proper moment during one stroke of the piston.

In order to properly perform these important functions the valve must have lead and lap. The various terms relating to valve action are plainly defined in Chapter VIII on "Definitions," and it is unnecessary to repeat them here. If the outside lap is increased admission will be later and cut off earlier, and if it be desired to keep the lead the same it will be necessary to move the eccentric forward, which will make the other events, cut off, release, and compression, earlier also. If the inside lap is increased the result will be an earlier closing of the exhaust and increased compression.

These propositions refer mainly to engines of the single valve variety in which one valve controls the admission and distribution of the steam for both ends of the cylinder. In engines of the four valve type, having a separate steam and exhaust valve for each end of the cylinder, each individual valve may be adjusted independently of the others, as will be explained later on, and in the case of engines having separate eccentrics, one for the steam and one for the exhaust valves, the adjustment becomes still more perfect.

We will first study the action of the D slide valve by

referring to Fig. 13, which is a sectional view of a valve, valve seat and ports. The valve is represented at mid travel or in its central position. S P, S P are the steam ports, and E P is the exhaust port. The projections marked X at each foot of the arch inside

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the valve, represent inside lap and may be added to o taken from the inside edges of the valve, according as more or less compression is desired. The dotted lines, O L, O L represent outside lap.

Motion is imparted to the valve through the medium

Lead'

FIGURE 14.

of the eccentric. If the valve had neither lap nor lead the position of the eccentric on the crank shaft would be just 90° or one-quarter of a circle ahead of the crank, but as more or less lap as well as lead is required, it becomes necessary to move the eccentric

still farther ahead of the crank, and this farther advance is termed angular advance, lap angle for lap and lead angle for lead.

Assuming the piston to be at the end of the stroke towards the crank, in other words, the engine to be

FIGURE 15.

2

on the dead center, the first function of the valve is lead or admission, illustrated by Fig. 14. Owing to the valve having both lap and lead, the position of the highest point of the eccentric will be assumed in this case to be 120° ahead of the crank, the position of the latter being at o°.

Exhaust opening has also occurred at the opposite

FIGURE 16.

end of the cylinder. The second function is full port opening, Fig. 15, the crank having moved through 60° and the eccentric is now at 180°, the farthest point of its throw in that direction, the valve being at the end of its travel. At this point it might be well to note a

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