Thus far only street-gas or illuminating-gas engines have been discussed. If the engine employed be small—10 to 15 horse-power, for instance—street-gas is a fuel, the richness, purity and facility of employment of which offsets its comparatively high cost. But the constantly increasing necessity of generating power cheaply has led to the employment of special gases which are easily and cheaply generated. Such are the following:

• Blast-furnace gases,
• Coke-oven gases,
• Fuel-gas proper,
• Mond gas,
• Mixed gas,
• Water-gas,
• Wood-gas.

The practical advantages resulting from the utilization of these gases in generating power were hardly known until within the last few years. The many uses to which these gases have been applied in Europe since 1900 have definitely proved the industrial value of producer-gas engines in general.

The steps which have led to this gradually increasing use of producer-gas have been learnedly discussed and commented upon in the instructive works and publications of Aimé Witz, Professor in the Faculty of Sciences of Lille, in those of Dugald Clerk, of London, F. Grover, of Leeds, and Otto Güldner, of Munich, and in those of the American authors, Goldingham, Hiscox, Hutton, Parsell and Weed, etc. The new tendencies in the construction of large engines may be regarded as an interesting verification of the forecasts of these men—forecasts which coincide with the opinion long held by the author. Aimé Witz has always been an advocate of high pressures and of increased piston speed. English builders who made experiments in this direction conceded the beneficial results obtained; but while they increased the original pressure of 28 to 43 pounds per square inch employed five or six years ago to the pressure of 85 to 100 pounds per square inch nowadays advocated, the Germans, for the most part, have adopted, at least in producer-gas engines, pressures of 114 to 170 pounds per square inch and more.

High Compression.—In actual practice, the problem of high pressures is apparently very difficult of solution, and many of the best firms still seem to cling to old ideas. The reason for their course is, perhaps, to be found in the fact that certain experiments which they made in raising the pressures resulted in discouraging accidents. The explosion-chambers became overheated; valves were distorted; and premature ignition occurred. Because the principle underlying high pressures was improperly applied, the results obtained were poor.

High pressures cannot be used with impunity in cylinders not especially designed for their employment, and this is the case with most engines of the older type, among which may be included most engines of English, French, and particularly of American construction. In American engines notably, the explosion-chamber, the cylinder and its jacket, are generally cast in one piece, so that it is very difficult to allow for the free expansion of certain members with the high and unequal temperatures to which they are subjected (Fig. 22).

Some builders have attempted to use high pressures without concerning themselves in the least with a modification of the explosive mixture. The result has been that, owing to the richness of the mixture, the explosive pressure was increased to a point far beyond that for which the parts were designed. Sudden starts and stops in operation, overheating of the parts, and even breaking of crank-shafts, were the results. The engines had gained somewhat in power, but no progress had been made in economy of consumption, although this was the very purpose of increasing the compression.

High pressures render it possible to employ poor mixtures and still insure ignition. A quality of street-gas, for example, which yields one horse-power per hour with 17.5 cubic feet and a mixture of 1 part gas and 8 of air compressed to 78 pounds per square inch, will give the same power as 14 cubic feet of the same gas mixed with 12 parts of air and compressed to 171 pounds per square inch.

"Scavenging" of the cylinder, a practice which engineers of modern ideas seem to consider of much importance, is better effected with high pressures, for the simple reason that the explosion-chamber, at the end of the return stroke, contains considerably less burnt gases when its volume is smaller in proportion to that of the cylinder.

In impoverishing the mixture to meet the needs of high pressures, the explosive power is not increased and in practice hardly exceeds 365 to 427 pounds per square inch. With the higher pressures thus obtained there is consequently no reason for subjecting the moving parts to greater forces.

Cooling.—The increase in temperature of the cylinder-head and of the valves, due wholly to high compression, is perfectly counteracted by an arrangement which most designers seem to prefer, and which, as shown in the accompanying diagram (Fig. 76), consists in placing the mixture and exhaust-valves in a passage forming a kind of antechamber completely surrounded by water. The immediate vicinity of this water assures the perfect and equal cooling of the valve-seats. This arrangement, while it renders it possible to reduce the size of the explosion-chamber to a minimum, has the additional mechanical advantage of enabling the builder to bore the seats and valve-guides with the same tool, since they are all mounted on the same line. From the standpoint of efficiency, the design has the advantage of permitting the introduction of the explosive mixture without overheating it as it passes through the admission-valve, which obtains all the benefit of the cooling of the cylinder-head, literally surrounded as it is by water.

In large engines the cooling effect is even heightened by separately supplying the jackets of the cylinder-head and of the cylinder. In engines of less power the top of the cylinder-head jacket is placed in communication with that of the cylinder, so that the coldest water enters at the base of the head and, after having there been heated, passes around the cylinder in order finally to emerge at the top toward the center. The water having been thus methodically circulated, the useful effect and regularity of the cooling process is increased.

Notwithstanding the care which is devoted to water circulation, it is advisable to run the producer-gas engine "colder" than the older street-gas types, in which the more economic speed is that at which the water emerges from the jacket at about a temperature of 104 degrees F. It would seem advisable to meet the requirements of piston lubrication by reducing to a minimum the quantity of heat withdrawn by the circulating water. Indeed, the personal experiments of the author bear out this principle.

For street-gas engines, however, the cylinders should be worked at the highest possible temperature consistent with the requirements of lubrication. It should not be forgotten that, in large engines fed with producer-gas, economy of consumption is a secondary consideration, because of the low quantity of fuel required. The cost, moreover, may well be sacrificed to that steadiness of operation which is of such great importance in large engines furnishing the power of factories; for in such engines sudden stops seriously affect the work to be performed. For this reason engine builders have been led to the construction of motors provided with very effective cooling apparatus. Since the circulation of the water around the explosion-chamber and the cylinder is not sufficient to counteract the rise of temperature, it has become the practice to cool separately each part likely to be subjected to heat. The seats of the exhaust-valves, the valves themselves, the piston, and sometimes the piston-rod, have been provided with water-jackets.

Premature Ignition.—Returning to the causes of the discouragements encountered by some designers who endeavored to use high pressures, it has already been mentioned that premature ignition of the explosive mixture in cylinders not suited for high pressures is one reason for the bad results obtained. An explanation of these results is to be found in the high theoretical temperature corresponding with great pressures and in the quantity of heat which must be absorbed by the walls of the explosion-chamber. These two circumstances are in themselves sufficient to produce spontaneous ignition of excessively rich mixtures, compressed in an overheated chamber unprovided with a sufficient circulation of water. A third cause of premature ignition may also be found in the old system of ignition which, in most English engines, consists of a metallic or porcelain tube, the interior of which communicates with the explosion-chamber, an exterior flame being employed to heat the tube to incandescence. In tubes of this type which are not provided with a special ignition-valve, the time of ignition is dependent only on the moment when the explosive mixture, driven into the tube, comes into contact, at the end of the compression stroke, with the incandescent zone, thereby causing the ignition. This very empirical method leads either to an acceleration or retardation of the ignition, depending upon the temperature of the tube, the position of the red-hot zone, its dimensions, and the temperature of the mixture, which is determined by the load of the engine. Although this system, the only merit of which is its simplicity, may meet the requirements of small engines, there is not the slightest doubt that it is quite inapplicable to those of more than 20 to 25 horse-power, for in such engines greater certainty in operation is demanded. Even if only the more improved of the two types of hot-tube ignition be considered, with or without valves, it must still be held that they are inapplicable to high compression engines. The ignition-valve is the part which suffers most from the high temperature to which it is subjected. Its immediate proximity to the incandescent tube, and its contact with the burning gas when it flares up, render it almost impossible to employ any cooling arrangement. Although with the exercise of great care it may work satisfactorily in engines of normal pressure, it is evident that it cannot meet the requirements of high pressure engines, because the temperature of the compressed mixture is such that the charge is certain to catch fire by mere contact with the overheated valve. In industrial engines of small size, premature ignition has little, if any, effect except upon silent operation and economic consumption. This does not hold true, however, of large engines. Besides the inconveniences mentioned, there is also the danger of breaking the cranks or other moving parts. The inertia of these members is a matter of some concern, because of their weight and of the linear speed which they attain in large engines. Some idea of this may be obtained when it is considered that in a producer-gas or blast-furnace-gas engine having a piston diameter of 24 inches and an explosive pressure of 299 pounds per square inch, the force exerted at the moment of explosion is about 132,000 pounds. Naturally, engine builders have adopted the most certain means of avoiding premature ignition and its grave consequences.

The method of ignition which at present seems to be preferred to any other for producer-gas is that employing a break-spark obtained with the magneto apparatus previously described. Some builders of large engines, particularly desirous of assuring steadiness of running, have provided the explosion-chamber with two independent igniters. It may be that they have adopted this arrangement largely for the purpose of avoiding the inconveniences resulting from a failure of one of the igniters, rather than for the purpose of igniting the mixture in several places so as to obtain a more uniform ignition and one better suited for the propagation of the flame.

The Governing of Engines.—Various methods have been adopted for the purpose of varying the motive power of an engine between no load and full load, still preserving, however, a constant speed of rotation. These methods consist in changing either the quantity or the quality of the mixture admitted into the cylinder. Thus it may happen that an engine may be supplied:

1. With a mixture constant in quality and in quantity;

2. With a mixture variable in quality and constant in quantity;

3. With a mixture constant in quality and variable in quantity.

1. Mixture Constant in Quality and Quantity.—This method implies the use of the hit-and-miss system of admission, in which the number of admissions and explosions varies, while the value or the composition of each admitted charge remains as constant as the compression itself (Fig. 34). This system has already been referred to and its simplicity fully set forth. By its use a comparatively low consumption is obtained, even when the engine is not running at full load. On the other hand, it has the disadvantage of necessitating the employment of heavy fly-wheel to preserve cyclic regularity.

2. Mixture Variable in Quality and Constant in Quantity.—The governing system most commonly employed to obtain a mixture variable in quality and constant quantity is based upon the control of the gas-admission valve by means of a cam having a conical longitudinal section, as shown in Fig. 35. This cam, commonly called a "conical cam," is connected with a lever actuated from the governor. As the lever swings under the action of the governor, the cam is shifted along the half-speed shaft of the engine. The result is that the gas-admission valve is opened for a longer or shorter period.

In another system a cylindrical valve is mounted between the chamber in which the mixture is formed and the gas-supply pipe, the valve being carried on the same stem as the mixture-valve itself. The cylindrical valve is displaced by the governor so as to vary the quantity of gas drawn in with relation to the quantity of air.

When the engines are fed with producer-gas the parts which have just been described should be frequently inspected and cleaned; for they are only too easily fouled.

Engines thus governed should be run at high pressure so as to insure the ignition of the producer-gas mixtures formed when the position of the cam corresponds with the minimum opening of the gas-valve. Powerful governors should be employed, capable of overcoming the resistance offered by the cylindrical valve or the cam.

It may often happen that variations in the load of the engine render it necessary to actuate the air valve, so as to obtain a mixture which will be ignited and exploded under the best possible conditions.

3. Mixture Constant in Quality and Variable in Quantity.—In supplying an engine with a mixture constant in quality and variable in quantity, the compression does not remain constant. The quantity of mixture drawn in by the cylinder may even be so far reduced that the pressure drops below the point at which ignition takes place. For that reason engines of this type should be run at high pressures.

The variation of the quantity of mixture may be effected in various ways. The simplest arrangement consists in mounting a butterfly-valve in the mixture pipe, which valve is controlled by the governor and throttles the passage to a greater or lesser degree. A very striking solution of the problem consists in varying the opening of the mixture-valve itself. To attain this end the valve is moved by levers. The point of application of one of these levers is displaced under the action of the governor so as to vary the travel of the valve within predetermined limits. Under these conditions a mixture of constant homogeneity is introduced into the cylinder, so proportioned as to insure ignition even at low pressures.

In recent experiments conducted by the author it was proved that with this governing system ignition still takes place even though the pressure has dropped to 43 pounds per square inch. This system has the merit of rendering it possible to employ ordinary governors of moderate size, since the resistance to be overcome at the point of application of the lever is comparatively small. In the accompanying illustration the Otto Deutz system is illustrated.

CHAPTER XI

Furthermore, these constituents vary within certain limits. This is also true of the calorific value. Experiments made by the author have demonstrated that in the same place at an interval of a few hours, variations of approximately ten per cent. occur.

Composition of Producer-Gases.—The average chemical composition of producer-gases varies with the conditions under which they are generated and the nature of the fuel. The following are the proportions of its constituents expressed volumetrically:

Blast-furnace gas has been used for generating power by means of gas-engines for about ten years. At the present time it is used in engines of very high power, a discussion of which engines more properly belongs to a work on metallurgy, and has no place, therefore, in a manual such as this.

Producer-gas, in the true sense of the term, is generated in special apparatus either under pressure or by suction in a manner to be described in the following chapters.

Mond gas is produced in generators of the blowing or pressure type from bituminous coal, necessitating the employment of special purifiers and permitting the collection of the by-products of the fractional distillation of the coal. Mond gas plants are, therefore, rather complicated and can be advantageously utilized only for large engines. More exhaustive information can be obtained from the descriptions published by the builders of Mond gas generators.

Mixed gas is generated in apparatus arranged so that the retort is kept at a high temperature, thereby producing a gas richer in hydrogen than that made by producers. It should be observed that in practice the generators at present used yield a producer-gas, the calorific value of which fluctuates between 1,000 and 1,400 calories per cubic meter (3,968 to 5,158 B.T.U. per 35.31 cubic feet); and the composition varies accordingly, in the manner that has already been indicated in the tables for producer-gas and mixed gas. There is no necessity, therefore, for drawing a distinction between these two qualities of gas.

Water-gas should theoretically be composed of 50 per cent. carbon monoxide and 50 per cent. hydrogen, resulting from the decomposition of steam by incandescent coal. In practice, however, it contains a little nitrogen and carbon dioxide. The gas is obtained from generators in which air is alternately blown in to fan the fire and then steam to produce gas. Water-gas is employed in soldering on account of its reducing properties and of the high temperature of its flame. The great quantity of carbon monoxide which it contains renders it very poisonous and exceedingly dangerous, because it is generated under pressure. From the economical standpoint, its generation is more expensive than that of producer-gas, for which reason its employment in gas-engines is hardly of much value.

Wood-gas, the composition of which has already been given, is generated in apparatus of the Riché type, the principle of which consists in heating a cast retort charged with any kind of fuel, namely wood, and vertically mounted on a masonry base.

This apparatus should be of particular interest to the proprietors of sawmills, furniture factories, and the like, since it offers a means of using the waste products of their plants.

The relatively high proportion of carbon monoxide in producer-gas is objectionable from a hygienic standpoint, so much so, indeed, that it has attracted the attention of manufacturers. Carbon monoxide, the specific gravity of which is 0.967, is a gas peculiarly poisonous and dangerous. It cannot be breathed without baneful effects, and is even more dangerous than carbonic-acid gas, which eventually causes asphyxiation by reducing the quantity of oxygen in the air. For this reason, it is necessary to take the utmost precaution in efficiently and continuously ventilating the rooms in which the gas-generators and their accessories are installed. This suggestion should be followed, above all, when the apparatus in question are installed in cellars and basements. As a further precaution, where the plant is rather large a workman should not be allowed to enter the generator room alone.

Blowing-generators, or those in which the gas is produced under pressure, are more dangerous than suction-generators. In the former a leaky joint may cause the vitiation of the surrounding air as the producer-gas escapes; in the suction apparatus the same fault simply causes more air to be drawn in.

Dr. Melotte recommends the following procedure in cases of carbon monoxide asphyxiation:

Cases of poisoning by carbon monoxide are both frequent and dangerous. The gas is extremely poisonous, and all the more dangerous because it is odorless, colorless and tasteless. When it comes into contact with the blood, it forms a combination so stable that it is reacted upon by the oxygen of the air only with difficulty. It follows, therefore, that with each respiration of air charged with carbon monoxide, a certain quantity of blood is poisoned. In consequence of this, there is a possibility of poisoning in open air.

Symptoms.—The symptoms observed will vary with the manner in which the blood has been poisoned. There are two ways in which this poisoning can occur. The one depends upon whether the atmosphere contains an excess of carbon monoxide; the other whether the air breathed contains only traces of the gas.

Gradual, Rapid Asphyxiation.—At first a vague sickness is felt, rapidly followed by violent headaches, vertigo, anxiety, oppression, dimness of vision, beating of the pulse at the temples, hallucinations, and an irresistible desire to sleep. If at this stage the patient has a sufficient idea of danger to prompt him to open a window or door, he will escape death.

In the second stage, the victim's legs are paralyzed, but he can still move his arms and his head. The mind still preserves its clearness, and in a measure assists the further process of asphyxiation because of its impotency. Then follow coma and death.

Slow, Chronic Asphyxiation.—Slow, chronic asphyxiation is not infrequent. Its symptoms are often difficult to detect. Poisoning is manifested by weakness, cephalalgia, vomiting, pallor, general anemia, lassitude, and local paralysis. If any of these symptoms appear in the men who work in the vicinity of the producers, immediate steps should be taken to prevent the possibility of carbon monoxide asphyxiation.

It has already been stated that the oxygen of the air has no oxidizing effect upon blood contaminated by carbon monoxide. Only a liberal current of pure oxygen can oxidize the combination formed and render hematosis possible. This liberal current can be obtained from an oxygen tank of the portable variety, provided with a tube carrying at its free end a mask which is held over the mouth and the nostrils. The absorption of gas takes place by artificial respiration, which is effected in several ways. The most practical of these are the Sylvester and Pacini methods.

Sylvester Method.—The patient is laid on his back. His arms are raised over his head and then brought back on each side of the body. This operation is repeated fifteen times per minute approximately. The method is very frequently employed and is excellent in its results.

The Pacini Method.—Four fingers are placed in the pit of the arm, with the thumb on the shoulder. The shoulder is then alternately raised and lowered, producing a marked expansion of the chest. This method is the more effective of the two. The movements described are repeated fifteen to twenty times each minute very rhythmically.

One or the other of these two methods of treatment should be immediately applied in serious cases. Certain preliminary precautions should be taken in all cases, however. The patient should be carried to a well-ventilated and moderately heated room, stripped of his clothes, and warmed by water-bottles and heated linen. Reflex action should be excited, the peripheral nervous system stimulated in order to contract the heart and the respiratory muscles, and the precordial region cauterized. In addition to this treatment, the region of the diaphragm should be rubbed and pinched, the skin rubbed, cold showers given, flagellations administered, urtications (whipping with nettles) undertaken, the skin and the mucous membranes excited, the mucous membrane of the nose and of the pharynx titillated with a feather dipped in ammonia, alcohol, vinegar, or lemon juice. Rhythmic traction of the tongue is effective when carried out as follows: The tongue is seized with a forceps and kept extended by means of a coarse thread. It is then pulled out from the mouth sharply and allowed to reenter after each traction. These movements should be rhythmic and should be repeated fifteen to twenty times a minute.

All these efforts should be continued for several hours. When the patient has finally been revived, he should be placed in a warm bed. Stimulants such as wine, coffee, and the like should be administered. If the head should be congested, local blood-letting should be resorted to and four or six leeches applied behind the ears. It should be borne in mind that the various steps enumerated are to be taken pending the arrival of a physician.

Most of the coal used in generating producer-gas contains sulphur. Sulphuretted hydrogen is thus produced, which mixes with the gas and imparts to it its characteristic odor. In some gas-generators, purifiers are employed in which sawdust mixed with iron salts is utilized, with the result that a combination is formed with the sulphuretted hydrogen, thereby removing it from the producer-gas. In other forms of generators a more summary method of purification is adopted, so that traces of sulphuretted hydrogen still remain. Since this gas attacks copper, the employment of this metal is not advisable for the following apparatus: Generator (openings, cock for testing the gas); piping (gas-pressure cocks, drain and pet cocks); engine (gas-admission cock, lubricating joint in the cylinder, valves and cocks of the compressed-air starting-pipe).

The distillation of coal in generators results in the formation of ammonia gas. This also has a corrosive action on copper and its alloys; but owing to its great solubility, it is eliminated by the waters of the "scrubber" and does not reach the engine.

The quantity of gas produced in most generators varies from 6.4 to 8.2 pounds per cubic foot of raw coal burnt in the generator. The engine consumes per horse-power per hour 70 to 115 cubic feet of gas, depending upon its richness.

CHAPTER XII

Among these may be cited works in which producer-gas is employed as a furnace fuel or as a soldering or roasting medium. Still other cases are those in which the producer-gas must be piped to some distance from a central generating installation to various engines, in the manner rendered familiar in gas-lighting practice.

Most pressure gas-generators have been copied from the original type invented by Dowson. These include a generator in which the gas is produced; an injector fed by a boiler; a fan or a compressor by means of which a mixture of steam and air is blown under the generator-furnace; washing apparatus termed "scrubbers"; gas-purifying apparatus; and a gas-holder (Fig. 77).

Generators.—The generator consists of a retort made of refractory clay, vertically mounted, and cylindrical or conical in form. This retort is protected on its exterior by a metal jacket with an intermediate layer of sand which serves to reduce the heat lost by radiation. The fuel is charged through the top of the retort, which is provided with a double closure in order to prevent the entrance of air during the charging operation. The generator rests on a grid arranged at the base of the retort, upon which grid the ashes fall. The outlet of the injector-pipe opens into the ash-pit, and this injector constantly supplies a mixture of steam and air. The mixture is generally superheated by passing it through a coil arranged in the fire-box of the boiler, in the generator, or in the outlet for burnt gases. Sometimes the air is subjected to a preliminary heating by recuperating in some way the waste heat of the apparatus.

The chief features in the arrangement of generators which have received the attention of manufacturers are the following: Good distribution of the fuel in charging; easy descent of the fuel; reduction of the destructive action of the clinkers on the walls; means for cleaning the grate without interfering with the generation of gas; prevention of leakage. Many devices have been employed to fulfil these requisites.

A perfect distribution of the fuel during charging is attained chiefly by the form of the hopper, and of its gate, which is generally conical. In most apparatus the gate opens toward the interior of the generator, and the inclination of its walls causes a uniform scattering of the fuel in the retort. It is all the more necessary to disperse the fuel in this manner when the cross-section of the retort is small compared with its height.

The facility of the fuel's descent is dependent largely upon the nature and the size of the coal employed. Porous coal gives better results than dense and compact coal. It is therefore preferable to employ screened coal free from dust in pieces each the size of a hazel-nut. The various sections given to the interior, including as they do cylindrical forms, truncated at the summit or the base, partially truncated toward the base and the like, would lead to the conclusion that this question is not of the importance which some writers would have us believe. Still, it must be considered that if the fuel drops slowly, its prolonged detention within the walls of the hopper and its transformation into fusible slag may result in a disintegration of the refractory lining of the furnace.

The quantity of steam injected, greater or less, according to the nature of the fuel, renders it possible to obtain friable slags and consequently to prevent grave injury to the retort. Red-ash coal is in general fusible, containing as it does some iron. Its temperature of fusion varies between 1,832 to 2,732 degrees F.

Cleanliness is most important so far as the operation of the generator is concerned. It should be possible to scrape the generator during operation without changing the composition of the gas, when the incandescent zone is chilled, or an excess of air is introduced, or the steam-injector be momentarily thrown out of operation. Mechanical cleaners with movable grates or revolving beds have the merit of causing the ashes to drop without interfering with the operation of the apparatus. The same meritorious feature is characteristic of ash-pits having water-sealed joints.

Pressure gas-generators need not be as perfectly gas-tight as suction apparatus. Leakage of gas, which is usually manifested by a characteristic odor, results in a loss of consumption and renders the air unfit to breathe.

A generator should be provided in its upper part with openings through which a poker can easily be introduced in order to shake up the fuel and to dislodge the clinkers which tend to form and which cause the principal defects in operation, particularly with fuels that tend to swell, cake, and adhere to the furnace walls when heated. Many apparatus, moreover, are provided with lateral openings having mica panes through which the progress of combustion can be observed (Fig. 79).

Air-Blast.—The system by which air and steam are injected necessitates the employment of a steam-boiler of 75 pounds pressure. This method of blowing, which is rather complicated, has the disadvantage of varying in feed with the pressure of the steam in the boiler, which pressure is not easily maintained at a given number of pounds per square inch. Moreover, when more or less resistance is offered by the fuel in the generator the quantity of air which is injected is likely to be diminished in quantity while the quantity of steam remains the same. The result is a change in speed which follows from the modification of the proportions of the two elements. For these reasons some manufacturers have resorted of late years to the employment of fans and blowers.

Blowers.— The fans or blowers employed vary considerably in arrangement. Most of them are based on the Koerting system (Fig. 80), and comprise essentially (1) a tube through which the steam is supplied under pressure, and (2) a cylindro-conical blast-pipe. The tube is placed in the axis of the blast-pipe at its outer opening. As it escapes under pressure the steam is caught in the blast-pipe and draws with it a certain quantity of air, which can be regulated. It is important that these injection blowers should operate in such a manner that the pressure and the feed of air and steam can be controlled.

Fans.—Mechanical blowers have the advantage of dispensing with the employment of steam under pressure and the consequent installation of a boiler (Fig. 78). Driven by the engine itself or from some separate source of power, these apparatus are easily placed in position, require no great amount of attention, and utilize but little energy. They are either of the centrifugal type or of the rotary type, exemplified in the Root blower (Fig. 81). The latter system has the advantage of high efficiency, and of enabling comparatively high pressures—19 to 27 inches of water—to be attained, which, however, are used only for special fuels, such as lignite, peat, and the like. The air supplied by the blower, before reaching the fire-box, is superheated, either before or after it is charged with steam.

Compressors.—In some installations air is supplied by compressor under the high pressure of 70 to 90 pounds per square inch, and seem well adapted to the production of a gas of good quality. Moreover, neither tar nor ammoniacal waters are produced. The Gardie producer may be considered typical of this class of apparatus (Fig. 82). The chief feature of this producer is to be found in simple washing and purifying apparatus. It may be well to state here that the compression of air at high pressure occasions some complications, and a considerable expenditure of power.

Exhausters.—Some designers have invented devices which draw gas into the generator whence it is supplied to the engines, these suction apparatus being connected with the blowers or used separately. But with the exception of a few special instances, such arrangements are not widely used—at least not for the production of motive power alone.

Whatever may be the arrangement employed for the introduction of a mixture of air and steam under the grate of the generator, the blast-pipe as a general rule discharges toward the center of the apparatus. Still, in large producers it becomes desirable to provide a means for varying the quantity of air and steam within wide limits so as to regulate the heat of the fire. For that reason several outlets are symmetrically arranged below the fuel.

Washing and Purifying.—In pressure producers the gas is generally washed and purified with much more care than in suction apparatus. Given a sufficient pressure, the gas can be driven through the different apparatus and the spaces between the material which they contain without any difficulty. The gases emerge from the generator highly heated, and this heat is used either to warm the injection water or to generate the steam fed to the furnace. The gases then enter the washing apparatus, which most frequently consists of a succession of contrivances in which the gas is washed either by causing it to bubble up through the water, or by subjecting it to superficial friction against a sheet of water, or by systematically circulating it in a mass of continuously besprinkled inert material. The object of washing is to remove the dust contained in the gas and to precipitate it in the form of a slime which can be removed by flushing.