=Fuel.=--We have discussed the subject of fuel in a preceding chapter (Chapter XIII) and have indicated the conditions to be fulfilled by low grade or anthracite coal best adapted for use in suction gas-generators.

It may here be added that the coal used in the generator should be as dry as possible and in pieces of from 1/2 inch to 1 inch. Very small pieces, and particularly coal dust, are injurious and should be removed by preliminary screening as far as possible. Screened coal is thrown in with an ordinary grate shovel.

=How to Keep the Plant in Good Condition.=--In regard to the generator, apart from the cleaning of the grate and of the ash-pit, which may be done during operation, it is necessary to empty the apparatus entirely once a week, if possible, in order to break off the clinkers adhering to the retort. These clinkers destroy the refractory lining, form rough projections interfering with the downward movement of the fuel, bring about the formation of arches, and reduce the effective area of the retort. At the time of this cleaning, tests are also made as to the tightness of the doors of the combustion-chamber, of the charging-boxes, etc.

The vaporizer should be cleaned every week or every other week, according to the more or less bituminous character of the fuel and the greater or smaller content of lime in the water used. Lime deposits may be eliminated, or the salts may be precipitated in the form of non-adhering slimes, by introducing regularly a small amount of caustic potash or soda into the feed-water. If the deposits or incrustations are very tenacious, the use of a dilute solution of hydrochloric acid may be resorted to. Tar which may adhere to the conduits, pipes or gas pa.s.sages, is best removed while the apparatus is still hot, or a solvent may be employed, such as kerosene, turpentine, etc. The connections between the vaporizer and the scrubber are particularly liable to become obstructed by the acc.u.mulation of tar or dust carried along by the gas.

It is advisable to examine the several parts of the plant once or twice a week by opening the covers or the cleaning-plugs.

The lower compartment of the washer keeps back the greater part of the dust which has not been retained in collectors or boxes provided especially for this purpose. The dust takes the form of slime, and, in some arrangements of apparatus, tends to clog up the overflow pipe, thus arresting the pa.s.sage of gas and causing the engine to stop. This portion of the washer should be thoroughly cleaned once or twice a month.

If very hard blast-furnace c.o.ke is used in the washer, it may be kept in use for over a year without requiring removal. In order to free the purifying materials from dust and lime sediments carried along by the wash-water, it is well to let the wash-water flow as abundantly as possible for about a half-hour at least once a month. At the time of renewing the purifying material the precautions indicated in the section dealing with these matters should be observed, and care should be taken to have shelves or gratings on which the material is supported in layers not too thick, so as to avoid any resistance to the pa.s.sage of the gas.

In a general way it is advisable to test the drain-c.o.c.ks on the several apparatus daily, and to keep them in perfect condition. If, when open, one of these c.o.c.ks does not discharge any gas, water, or steam, a wire should be introduced into the bore to make sure it is not clogged up.

=Care of the Apparatus.=--Each producer-gas plant will require special instructions for running it, according to the system, the construction, and the size of the plant. Such instructions are generally furnished by the manufacturer. However, there are some general rules which are common to the majority of suction gas-producers, and these will here be enumerated.

=Starting the Fire for the Gas Generator.=--This operation calls for the presence of the engineer of the plant and an a.s.sistant. The proper procedure is as follows:

First: Open the doors of the furnace and of the ash-pit. Then open the outlet flue and make sure that the grate of the generator is clear of ashes and clinkers. It should also be seen to that the parts of the charging-box work well and that the joints are tight.

Second: Ascertain whether there is the proper amount of water in the vaporizer, in the scrubber, etc., and that the feed works properly.

Third: Through the door of the combustion-chamber introduce straw, wood shavings, cotton waste, etc.; light them and fill the generator with dry wood up to one-quarter or one-half of its height; then add a few pailfuls of coal.

Fourth: Close the doors of the ash-pit and of the combustion-chamber and start the draft by means of the fan. As soon as the draft is started, it must be kept up without interruption until the engine begins to run, which may be ten or twenty minutes after lighting the fire.

Fifth: After the draft has been continued for a few minutes, the coal becomes sufficiently incandescent to start the production of gas, which may be ascertained by trying to light the gas at the test-c.o.c.k near the generator. Then the opening in the outlet flue is half closed for the purpose of producing pressure in the apparatus.

Sixth: Open the outlet flue adjacent to the engine for the purpose of purging the apparatus and the conduits of the air which they contain until the gas may be lighted at the test-c.o.c.k placed near the motor.

Seventh: Adjust the normal outflow of wash-water for the scrubber.

Eighth: As soon as the gas burns continuously at the test-c.o.c.k with an orange-colored flame the engine may be started.

The gas at first burns with a blue flame; this color indicates that it contains a certain amount of air. The opening of the test-c.o.c.k should be so regulated as to reduce the outlet pressure of the gas sufficiently to prevent the flame from going out. During the production of the draft, as well as during the ordinary running of the plant, the filling of the apparatus with fuel should be done with care to prevent explosions of gas due to the entrance of air. Particular care should be taken never to open at the same time the lid of the charging-box and the device, be it a c.o.c.k, valve, or damper, which controls the connection of the charging-box with the generator. All the operations which have been mentioned above should be carried out as quickly as possible.

STARTING THE ENGINE

The manner of starting the engine depends on the type of the engine and on the starting device with which it is provided, as we have already explained in connection with engines working with gas from city mains.

It is, however, important for the production of a good explosive mixture to regulate the amount of air supplied to the engine according to the quality of the gas employed. It is advisable to continue the operation of the fan until several explosions have taken place in the cylinder and the engine has acquired a certain speed so as to be able to draw in the normal amount of gas.

Naturally the gas-outlet tube near the admission-c.o.c.k should be closed after starting the engine, as well as the opening in the outlet flue of the generator. When the motor is running properly, the amount of water fed to the vaporizer and overflowing to the ash-pit is properly adjusted. The generator is then filled up to the level indicated by the manufacturer.

=Care of the Generator during Operation.=--As soon as the apparatus is running under normal conditions, it presents the advantage of requiring only very slight supervision and very little manual tending. The supervision consists:

First: In regulating and keeping up a proper feed of water to the vaporizer.

Second: In seeing to it that in apparatus provided with an overflow leading to the ash-pit, the water should flow constantly but without exceeding the proper amount.

Third: In keeping down temperature in the scrubber by properly regulating the feed of the wash-water. This apparatus may be slightly warm at its lower part, but must be quite cold at the top.

The manual tending to be done is limited to the regular filling up of the generator with fuel and to the removal of ashes and clinkers. The charging is effected at regular intervals, which, according to the various types of anthracite-generators, vary from one to six hours.

Charging the apparatus at short intervals entails unnecessary labor, while charging at too long intervals will often interfere with the uniform production of the gas.

It will be obvious that the amount of fuel introduced will be the larger, the greater the intervals between two fillings. This fuel is cold and contains between its particles a certain amount of air; furthermore, the layer of coal which covers the incandescent zone has become relatively thin. The excess of air impoverishes the gas, and the fresh fuel lowers the temperature of the ma.s.s undergoing combustion, so that again the gas in process of formation is weakened. Experience seems to show that as a rule it is best to fill up the generator at intervals of from two to three hours, according to the work done by the engine. It should be noted that the level of the fuel in the generator should not sink below the bottom of the feed-hopper.

The author wishes again to emphasize that in order to prevent the harmful entrance of air, the charging operations should be carried out as quickly as possible; and for this reason the fuel should be introduced not by means of the shovel, but by means of a pail, scuttle, or other appropriate receptacle.

Care should be taken to fill the charging box to its upper edge and to adjust its cover accurately before operating the device which closes the feed-hopper (valve, c.o.c.k).

The removal of the ashes and clinkers should be accomplished as infrequently as possible, since opening the doors of the ash-pit and of the combustion-chamber necessarily causes an inward suction of cold air which is harmful.

As a rule with generators employing anthracite coal, it is sufficient to empty the ash-pit twice daily; this should be preferably done during stoppages. However, the cleaning of the grate by means of a poker pa.s.sed between the grate-bars or over them in order to bring about the falling of the ashes, should be attended to every two to four hours, according to the type of the generator and the nature of the fuel. In order that this cleaning may be done without opening the doors, the latter should be provided with apertures having closing devices.

This cleaning has for its chief object to allow the free pa.s.sage of the air for supporting combustion and to keep the incandescent zone in the apparatus at the proper height. The acc.u.mulation of ashes and clinkers at the bottom of the retort will shift this zone upward and impair the quality of gas.

=Stoppages and Cleaning.=--After closing the gas-inlet to the engine, the damper in the gas-outlet flue of the generator should be opened and the c.o.c.ks controlling the feed of water to the scrubber and to the vaporizer should be closed.

If it is desired to keep up the fire of the generator during the stoppage so as to be able to start again quickly, the ash-pit door should be opened so as to produce a natural draft which will maintain combustion. While the door is open, the clinkers which have acc.u.mulated above the grate may be removed, as they are much more easily taken off the grate when they are hot.

At least once a week the fire in the generator should be put out and the generator completely cleaned--that is, when ordinary fuel is employed.

For this purpose, as soon as the apparatus is stopped, a portion of the incandescent fuel is withdrawn through the doors of the combustion-chamber, and the retort is allowed to cool before it is emptied entirely. Too sudden a cooling of the retort may injure its refractory lining. In order to prevent explosions caused by the entrance of air, the feed-hopper should remain hermetically closed during the removal of the incandescent fuel through the doors of the combustion-chamber.

If the apparatus is placed in a room poorly ventilated, the cleaning should be attended to by two men, so that one may a.s.sist the other in case he is overcome by the gas. In all cases there should be a strict prohibition against the use of any light having an exposed flame liable to set on fire the explosive mixtures which may be formed.

When the generator, after cooling, is completely open, the charging-box is taken apart, and, if necessary, the feed-hopper also; the grates are taken out, if necessary; and, by means of a poker inserted from above, the clinkers and slag adhering to the retort are broken off.

In the foregoing paragraphs the author has indicated how the several apparatus, such as the vaporizer, the washer, the conduits, etc., should be attended to and maintained in good working order.

CHAPTER XIV

OIL AND VOLATILE HYDROCARBON ENGINES

Although this book is devoted primarily to a discussion of street-gas and producer-gas engines employed in various industries, a few words on oil and volatile hydrocarbon engines may not be out of place.

Oil-engines are those which use ordinary petroleum as a fuel or illuminating oil of yellowish color, having a specific gravity varying from 0.800 to 0.820 at a temperature of 15 degrees C. (59 degrees F.), and boiling between 140 and 145 degrees C. (284 to 297 degrees F.).

Volatile hydrocarbon engines are those which employ light oils obtained by distilling petroleum. These oils are colorless, have a specific gravity that varies from 0.680 to 0.720, and boil between 80 degrees and 115 degrees C. (176 to 257 degrees F.). Among these "essences," as they are called in Europe, may be mentioned benzine and alcohol.

In general appearance, and the way in which they are controlled, oil-engines differ but little from gas-engines. Their usual speed, however, is 20 to 30 per cent. greater than that of gas-engines. Except in some engines of the Diesel and Banki types, the compression does not exceed 43 to 71 pounds per square inch. In volatile hydrocarbon engines, on the other hand, the speed is very high, often running from 500 to 2,000 revolutions per minute, while the speed of gas or oil engines rarely exceeds 250 or 300 revolutions per minute.

=Oil-Engines.=--Oil-engines are employed chiefly in Russia and in America. Because of the high price of oil in other countries they are to be found only in small installations in country regions and are used mainly for driving locomobiles and launches. The improvements which have been made of late years in the construction of gas-engines supplied by suction gas-producers for small as well as for large powers, have hindered the general introduction of oil-engines.

The characteristic feature in the design of many of the oil-engines of the four-cycle type now in use (to which type we shall confine this discussion) is to be found in the controlling mechanism employed. The underlying principle of this mechanism lies not in acting upon the admission-valve, but in causing the governor to operate the exhaust-valve in such a manner that it is held open whenever the engine tends to exceed its normal speed. Some engines, however, are built on the principle of the gas-engine, with an admission-valve so controlled by the governor that it is open during normal operation and closed whenever the speed becomes excessive.