Modern Machine-Shop Practice Part 255

When one cylinder is placed above the other, as in Fig. 3395, R being the high pressure and S the low pressure piston, no receiver is employed, the steam pa.s.sing direct from the high pressure cylinder through the pipe P to the low pressure steam chest _c_. The high pressure steam valve V and the low pressure valve V are on the same stem, a cut off valve V' being provided for the high pressure cylinder.

3. Triple expansion engines have three cylinders, a high pressure, an intermediate, and a low pressure cylinder.

In a triple expansion engine the intermediate cylinder receives the steam that is exhausted from the high pressure cylinder, and expands it further. The low pressure cylinder receives its steam from the exhaust of the intermediate cylinder, and exhausts into the condenser.

[Ill.u.s.tration: Fig. 3397.]

[Ill.u.s.tration: Fig. 3398.]

In the ill.u.s.trations from Fig. 3396 to Fig. 3406 are represented the triple expansion engines of the steamship _Matabele_, constructed by Messrs. Hall, Russell & Company, of Aberdeen, Scotland. Fig. 3396 is a cross sectional view of the vessel showing the engine and its connections, and Fig. 3397 a similar view, showing the boilers. Fig.

3398 is a back elevation of the engine, showing the boilers also, and Fig. 3399 a plan of the same. Fig. 3400 is a sectional view, and Fig.

3401 an end view of the boilers. Fig. 3402 is a plan, Fig. 3403 an end elevation, and Fig. 3404 a front elevation, partly in section, of the engines. H P is the high pressure cylinder, I C the intermediate cylinder, and L P the low pressure cylinder. The high pressure cylinder has a piston valve, the steam chest being shown at A. The intermediate cylinder is provided with a double ported flat valve as shown at B, and the low pressure cylinder is provided with a similar valve whose weight is counterbalanced by the small piston at E; at F are the relief valves for relieving the cylinders of water.

[Ill.u.s.tration: _VOL. II._ =COMPOUND MARINE ENGINE.= _PLATE x.x.xVI._

Fig. 3393.

Fig. 3394.]

[Ill.u.s.tration: Fig. 3399.]

[Ill.u.s.tration: Fig. 3400.]

[Ill.u.s.tration: Fig. 3401.]

[Ill.u.s.tration: Fig. 3402.]

Each steam valve is provided with a link motion that may be used for varying the point of cut off (and therefore the expansion) as well as for reversing purposes.

The link motions are all shifted from one shaft, which may be operated by hand or by steam, the construction being as follows:

For shifting by hand, the wheel W is operated, its shaft having a worm driving the worm wheel G, Fig. 3403, which operates rod H, and through the lever J and rod K shifts the link L, one pair of eccentric rods being shown at N and P.

The shaft of the wheel W is, however, a crank shaft, and at M is a small engine, which may be connected or disconnected at will to shaft W. The lever J operates a shaft R in Fig. 3404, which connects (by a rod corresponding to rod K in Fig. 3403) to each link motion; hence all the links reverse together, and the ratio expansion of one cylinder to the other cannot be varied, or in other words, the point of cut off will be alike for each cylinder, let the link motion be shifted to whatever position it may.

The beam S, Fig. 3403, for working the air, circulating and feed pumps, is driven from the cross head of the intermediate cylinder.

The boilers are of the Scotch pattern that is usually employed for high pressures, as 160 or more lbs. per square inch, and have Fox corrugated furnaces and stay tubes.

Each cylinder requires a starting valve (which is sometimes called an auxiliary valve or a bye pa.s.s valve), which is used to warm the cylinder before starting the engine, and also (when there is no vacuum in the condenser) to admit high pressure steam when the high pressure piston is on the dead centre, in which case, there being no vacuum and no admission of steam to the low pressure cylinder, the engine would not have sufficient power to start.

[Ill.u.s.tration: Fig. 3403.]

In some cases the high pressure cylinder has no starting valve, the reversing gear being used to admit steam to one end or the other of the high pressure piston, and the starting valve being used to admit enough live steam to the low pressure cylinder to compensate for the absence of the vacuum.

When the vacuum in the low pressure cylinder is maintained while the engine is standing still, its starting valve obviously need not be used, except for warming purposes, before starting the engine; as soon, however, as the engine has started, the starting valve must be closed.

Each cylinder is provided with a relief valve, both at the top and at the bottom, to relieve the cylinder from a heavy charge of water, such as may occur if the boiler primes heavily.

Each cylinder is also provided with drain c.o.c.ks, to permit of the escape of the ordinary water of condensation in the cylinders when the engine is started, and also for use if the boiler primes.

The low pressure relief valve also prevents the acc.u.mulation of too great a pressure in the low pressure cylinder, which, from its large diameter, is not strong enough to withstand high pressure.

The oiling apparatus for the cylinders is arranged as follows:

In some cases pumps, and in others automatic or self-feeding devices are used. Oil is fed to the steam pipe of the high pressure cylinder, and this lubricates both the valves and the cylinders, but in many cases it is also fed to the steam chest, so as to afford more perfect lubrication to the valve.

For the low pressure cylinder the oil is fed into the receiver, and usually at a point near the slide valves.

Large marine cylinders are usually constructed with a separate lining, which may be replaced when worn or otherwise required.

A surface condenser consists of a cast iron sh.e.l.l or chamber forming the back of the engine frame. At each end of this chamber is a short part.i.tion, so that the condenser is divided lengthways into what may be called three compartments, of which the middle one is the longest and contains a number of thin bra.s.s tubes about 5/8 or 3/4 inch in diameter, the ends of these tubes being held in the plates or tube sheets forming the part.i.tions. The object of providing tubes of small diameter is to obtain a large area of cooling surface.

The exhaust steam from the engine generally pa.s.ses into the sh.e.l.l or body of the condenser, filling the middle part.i.tion and surrounding the tubes.

The condensing or circulating water pa.s.ses through the tubes, and by keeping them cool condenses the steam and forms a vacuum or partial vacuum in the condenser, which, having open communication with the low pressure cylinder, therefore gives a corresponding degree of vacuum on the exhaust side of the low pressure piston.

In some designs, however, the steam pa.s.ses through the tubes and the circulating water fills the middle compartment of the condenser. As, however, there is no pressure to counterbalance the weight of the water, it is preferable to have the water inside the tubes, so that they are subjected to a bursting pressure, in which case they may, for a given strength, be made thinner, because the strength of the tube to resist bursting is greater than its strength to resist collapsing, hence the circulating water usually pa.s.ses through the tubes. The chamber at the ends of the condenser permits the water to distribute through all the tubes.

In some cases the chamber at one end is divided horizontally into two compartments, so that the water is compelled to pa.s.s through one half and return through the other half of the tubes.

The water of condensation falls to the bottom of the condenser, from which it is removed by the air pump, which delivers it to the hot well.

The hot well is situated on the side of, and extends above, the pump, whose upper end it covers, thus water sealing the top of the air pump and preventing air from pa.s.sing into it through a leaky valve or bucket.

The top of the hot well is provided with a _vapor pipe_, which permits the air and gases to pa.s.s overboard. This pipe emerges through the side of the ship above the water line, and as there is no valve between the hot well and the sea, no pressure can possibly acc.u.mulate in the hot well.

The boiler feed is taken from the hot well either by the feed pump or by injectors, as the case may be.

In case the boiler feed should stop working, however, the hot well is provided with a pipe of large diameter, and called the overboard discharge pipe, so that the water of condensation may not acc.u.mulate a pressure in the hot well if the boiler feed ceases.

This overboard discharge pipe is provided with a weighted valve (placed at the side of the ship), which is constructed after the manner of a safety valve, relieving the hot well of pressure if the water acc.u.mulates, and preventing the sea water from entering the hot well.

To prevent loss of fresh water, the exhaust steam from the various engines and pumps (if any) about the ship pa.s.ses to the condenser and is pumped into the hot well.

In some cases, however, a separate and independent condenser is used for the smaller engines about the ship.

An independent condenser is one whose air pump and circulating pump are not worked from the main engine, and can therefore be operated when the main engine is standing still.

If the main condenser is independent, it may be started so as to form a vacuum before the main engine is started, and thus obviate the use of the starting valve on the low pressure cylinder except to warm the cylinder before starting.

Feed water for the boilers when the engine is standing is obtained by a pipe from the bottom of the condenser, so that the water of condensation of steam blown through the engine cylinders, and from the exhausts from the smaller engines about the ship, may be pumped or forced direct from the bottom of the condenser to the boiler.

This feed from the bottom of the condenser is necessary when the air pump is not working, and the water of condensation is not pumped into the hot well.

If the water thus obtained is not enough to keep the boilers supplied, an auxiliary or salt water feed admits extra water from the circulating water to the inside of the condenser to supply the deficiency.

This secondary suction pipe is provided with a valve because it must be shut off before the engine is started.