In the case of the first, i e, the W. P. of the bolts, we have first to find the amount of area of the furnace crown sheet supported by one bolt, which we do as follows:

The distance apart, or pitch of the bolts, is 8 inches, and each bolt supports half this distance, or half the pitch, which is 4 inches. Similarly, the width apart of the girders is 8 inches, and each bolt supports one half of this distance, or 4 inches.

By multiplying the pitch in inches one way by the pitch in inches the other, we get the area of plate the bolt supports, which in this case will obviously be 16 square inches, hence the working pressure of the bolt is as follows:

For the stress on each cross-sectional square inch of the bolt stay (measured at the bottom of the thread) we have:

Each cross-sectional square inch of the stay therefore sustains a stress of 916 lbs.

SUPERHEATERS.

Superheaters are used instead of receivers when it is desired either to dry or to superheat the steam after it has left the boiler. A common form of superheater being shown in Fig. 28. It consists of an inner and outer cylindrical shell, the heat and flame passing up through the centre on its way to the funnel, and the steam entering from the boiler at S, and emerging on its way to the engine at E. At D is a pipe for conveying away the water of condensation, and at V a safety-valve is fit ted. Stay plates p are secured by angle irons a a. and spaced at intervals all around the shell.

The drain cock of the superheater should always be at the very bottom,

to prevent the accumulation of water in the superheaters when the engines are standing, and when the boiler primes badly.

If, as is usual, the superheater drains into the main boiler immediately beneath it, the end of the drain pipe should be above the highest. water level, so as to prevent the water from the boiler from being drawn up into the superheater.

Superheaters are not usually applied to boilers carrying pressures above 100 lbs. per square inch.

Superheaters are regarded, so far as inspection is concerned, as the most important part of a boiler, as they are subject to rapid deterioration, first, because they have heat on one side and steam on the other side of the plates, and are subject to excessive heat, and second, because they are liable to be attacked by the oil used to lubricate the engine cylinders and valves.

In some cases in which the superheater is not subject to great heat, some oils may protect the surfaces of superheaters, but these are exceptional occurrences, and the effect the oil may have on the superheater always requires watching.

For these reasons the Board of Trade require their surveyors to ex

amine superheaters that have been in use, inside and out, and steel is not permitted to be used in their construction, because it has been. found liable to become brittle and to crack very easily when tested for soundness by hammer blows.

The Board of Trade rules concerning superheaters are as follows:

66

The strength of the joints of cylindrical superheaters and the factor of safety are found in a similar mamner as for cylindrical boilers and steam receivers, but instead of using 47000 lbs. as the tensile strength of iron, 30000 lbs. is adopted, unless where the heat or flame impinges at, or nearly at right angles to the plate, then 22400 Ibs. is substituted.

When a superheater is constructed with a tube subject to external pressure, the working psessure should be ascertained by the rules given for circular furnaces, but the constants should be reduced as 30 to 47.

In all cases the internal steam pipes should be so fitted, that the steam in flowing to them wiil pass over all the plates exposed to the impact of heat or flame.

Superheaters should, as regards survey, be deemed to be the most important parts of the boilers, and must be inspected inside and outside; those that cannot be entered on account of their size and arrangement must have a sufficient number of doors through which a thorough inspection of the whole interior can be made.

Special attention should be paid to the survey of superheaters, as with high pressure the plates may become dangerously weak, and not give any sound to indicate their state when tested with a hammer, the plates should therefore be occasionally drilled. Before commencing the survey, it is prudent to question the Engineer Officers as to the tendency of the boilers to flame; if flaming is a frequent occurrence extra care must be taken in the survey, and in fixing the pressure to be allowed, as the tensile strength of the plate, when heated, is often reduced to about four tons per square inch. Drain pipes must in all cases be fitted to superheaters in which a collection of water in the bottom is possible.

Superheaters that can be shut off from the main boilers must be fitted with a parliamentary safety-valve of sufficient size, but the least size passed without special written authority should be 3 inches diameter."

We may calculate the working pressure of a superheater, such as shown in Fig. 28, thus:

THE OUTER SHELL.

Internal diameter of the outer shell.

= 8 feet.

Thickness of plate...

1 inch.

Percentage of strength of the longitudinal rivetted seam....... 75

The construction being of the highest class and therefore entitled to a factor of safety of 5.

The working pressure due to the outer shell of the superheater, is therefore found to be 146 lbs. per square inch.

THE FLUE OR INTERIOR SHELL.

We have now to calculate the working pressure due to the interior shell, flue, or tube, which is under collapsing pressure, and the rules for which are as follows, being the same as for circular furnaces.

"Circular furnaces with the longitudinal joints welded or made with a butt strap. 90000 X the square of the thickness of the plate in inches

(Length in feet + 1) × diameter in inches.

working pressure per square inch, provided it does not exceed that found by the following formula:

The second formula limits the crushing stress to 4000 lbs. per square inch. The length is to be measured between the rings if the furnace is made with rings. If the longitudinal joints instead of being butted, are lap jointed in the ordinary way, then 70000 is used instead of 90000, excepting only where the lap is bevelled and so made as to give the flues the form of a true circle, when 80000 may be used. When the material or the workmanship is not of the best quality, the constants given above must be reduced, that is to say, the 90000 will become 80000, the 80000 will become 70000, the 70000 will become 60000, when the material and the workmanship are not of the best quality such constants will require to be further reduced, according to circumstances and the judgment of the Surveyor, as in the case of old boilers. One of the conditions of best workmanship is that the joints are double rivetted with double butt straps, or single rivetted with double butt straps, and the holes drilled after the bending is done and when in place, and the plates taken apart, the burr on the holes taken off, and the holes slightly counter-sunk from the outside. "* * The following examples will serve to show the application of the constants for the different cases that may arise:

Furnaces with butt joints and drilled

rivet holes.

90,000 where the longitudinal seams are welded.

90,000 where the longitudinal seams are double rivetted and fitted with single butt straps.

80,000 where the longitudinal seams are single rivetted and fitted with single butt straps.

90,000 where the longitudinal seams are single rivetted and fitted with double butt straps.

Now suppose that the workmanship of the flue or tube of the superheater is of the highest class of coustruction, and its constant will be 57695, which is obtained as follows:

Under the rules as above quoted, the strength of the iron for superheaters is taken at 30000 lbs. per cross-sectional square inch, instead of the 47000 lbs. allowed for circular furnaces, and we take this into ac count in the case of the flue, by reducing the constant of 90000 allowed for the highest class of construction in the same proportion, which is done by multiplying it by 30 and dividing by 47 thus:

Having found the constant, we may proceed to calculate the working pressure allowed for the flue, and supposing its thickness to be inch, its diameter 36 inches, and its length 6 feet, the calculation will stand thus:

As, however, this is a short length of flue to collapsing pressure, we must be careful to apply the limiting formula in order to ensure that the crushing stress does not exceed 4000 lbs. per square inch.

In applying this formula to superheater tubes, we have to reduce its constant in the proportion as 30 is to 47 for the same reason that we reduced the constant 90000 in the first formula), thus:

Constant for
furnaces.

8000 X 30

Constant for superheater
flues

47 5106

{85,000 where the longitudinal seams are double rivetted and fitted with single butt straps.

75.000 where the longitudinal seams are single rivetted and fitted with single butt straps.

85,000 where the longitudinal seams are single rivetted and fitted with double butt straps.

Furnaces with (80,000 where the longitudinal seams are double rivetted and bevelled.

75,000 where the longitudinal seams are double rivetted and bevelled.

"and not bevelled. and bevelled "and not bevelled

In the case of upright fire-boxes of donkey or similar boilers, 10 per cent. should be deducted from the constant given above, applicable to the respective classes of work.