Chapter 4 FOUNDATION AND EXHAUST

The reader will remember from what has already been said that a gas-engine is a motor which, more than any other, is subjected to forces, suddenly and repeatedly exerted, producing violent reactions on the foundation. It follows that the foundation must be made particularly resistant by properly determining its shape and size and by carefully selecting the material of which it is to be built.

The Foundation Materials.-Well-hardened brick should be used. The top course of bricks should be laid on edge. It is advisable to increase the stability of the foundation by longitudinally elongating it toward the base, as shown in the accompanying diagram (Fig. 54).

As a binding material, only mortar composed of coarse sand or river sand and of good cement, should be used. Instead of coarse sand, crushed slag, well-screened, may be employed. The mortar should consist of 2?3 slag and 1?3 cement. Oil should not in any way come into contact with the mortar; it may percolate through the cement and alter its resistant qualities.

As in the construction of all foundations, care should be taken to excavate down to good soil and to line the bottom with concrete, in order to form a single mass of artificial stone. A day or two should be allowed for the masonry to dry out, before filling in around it.

When the engine is installed on the ground floor above a vaulted cellar, the foundation should not rest directly on the vault below or on the joists, but should be built upon the very floor of the cellar, so that it passes through the planking of the ground floor without contact.

Fig. 54.-Method of building the foundation.

When the engine is to be installed on a staging, the method of securing it in place illustrated in Fig. 55 should be adopted.

Although a foundation, built in the manner described, will fulfill the usual conditions of an industrial installation, it will be inadequate for special cases in which trepidation is to be expected. Such is the case when engines are to be installed in places where, owing to the absence of factories, it is necessary to avoid all nuisance, such as noise, trepidations, odors, and the like.

Fig. 55.-Elevated foundation.

Vibration.-In order to prevent the transmission of vibration, the foundation should be carefully insulated from all neighboring walls. For this purpose various insulating substances called "anti-vibratory" are to be recommended. Among these may be mentioned horsehair, felt packing, cork, and the like. The efficacy of these substances depends much on the manner in which they are applied. It is always advisable to interpose a layer of one of these substances, from one to four inches thick, between the foundation and the surrounding soil, the thickness varying with the nature of the material used and the effect to be obtained. Between the bed of concrete, mentioned previously, and the foundation-masonry and between the foundation and the engine-frame, a layer of insulating material may well be placed. Preference is to be given to substances not likely to rot or at least not likely to lose their insulating property, when acted upon by heat, moisture or pressure.

Here it may not be amiss to warn against the utilization of cork for the bottom of the foundation; for water may cause the cork to swell and to dislocate the foundation or destroy its level.

The employment of the various substances mentioned does not entail any great expense when the foundations are not large and the engines are light. But the cost becomes considerable when insulating material is to be employed for the foundation of a 30 to 50 horse-power engine and upwards. For an engine of such size the author recommends an arrangement as simple as it is efficient, which consists in placing the foundation of the engine in a veritable masonry basin, the bottom of which is a bed of concrete of suitable thickness. The foundation is so placed that the lateral surfaces are absolutely independent of the supporting-walls of the basin thus formed. Care should be taken to cover the bottom with a layer of dry sand, rammed down well, varying in thickness with each case. This layer of sand constitutes the anti-vibratory material and confines the trepidations of the engine to the foundation.

As a result of this arrangement, it should be observed that, being unsupported laterally, the foundation should be all the more resistant, for which reason the base-area and weight should be increased by 30 to 40 per cent. The expense entailed will be largely offset by saving the cost of special anti-vibratory substances. In places liable to be flooded by water, the basin should be cemented or asphalted.

When the engine is of some size and is intended for the driving of one or more dynamos which may themselves give rise to vibrations, the dynamos are secured directly to the foundation of the engine, which is extended for that purpose, so that both machines are carried solidly on a single base.

The foregoing outline should not lead the proprietor of a plant to dispense with the services of experts, whose long experience has brought home to them the difficulties to be overcome in special cases.

It should here be stated, as a general rule, that the bricks should be thoroughly moistened before they are laid in order that they may grip the mortar.

After having been placed on the foundation and roughly trimmed with respect to the transmission devices, the engine is carefully leveled by means of hardwood wedges driven under the base. This done, the bolts are sealed by very gradually pouring a cement wash into the holes, and allowing it to set. When the holes are completely filled and the bolts securely fastened in place, a shallow rim, or edge of clay, or sand is run around the cast base, so as to form a small box or trough, in which cement is also poured for the purpose of firmly binding the engine frame and foundation together. When, as in the case of electric-light engines, single extra-heavy fly-wheels are employed, provided with bearings held in independent cast supports, the following rule should be observed to prevent the overheating due to unlevelness, which usually occurs at the bushings of these bearings: That part of the foundation which is to receive such a support should rest directly on the concrete bed and should be rigidly connected at the bottom with the main foundation. When the foundation is completely blocked up, the fly-wheel bearing with its support is hung to the crank-shaft; and not until this is effected is the masonry at the base of the support completed and rigidly fixed in its proper position.

For very large engines, the foundation-bolts should be particularly well sealed into the foundation. In order to attain this end the bricks are laid around the bolt-holes, alternately projected and retracted as shown in Fig. 54. Broken stone is then rammed down around the fixed bolt; in the interstices cement wash is poured.

Air Vibration, etc.-Vibration due chiefly to the transmission of noises and the displacement of air by the piston should not be confused with the trepidations previously mentioned.

The noise of an engine is caused by two distinct phenomena. The one is due to the transmitting properties of the entire solid mass constituting the frame, the foundation, and the soil. The other is due to vibrations transmitted to the air. In both cases, in order to reduce the noise to a minimum, the moving parts should be kept nicely adjusted, and above all, shocks avoided, the more harmful of which are caused by the play between the joint at the foot of the connecting-rod and the piston-pin, and between the head of the connecting-rod and the crank-shaft.

Although smooth running of the engine may be assured, there is always an inherent drawback in the rapid reciprocating movement of the piston. In large, single-acting gas-engines, a considerable displacement of air is thus produced. In the case of a forty horse-power engine having a cylinder diameter and piston-stroke respectively of 133?4 inches and 213?5 inches, it is evident that at each stroke the piston will displace about 2 cubic feet of air, the effect of which will be doubled when it is considered that on the forward stroke back pressure is created and on the return stroke suction is produced.

The air motion caused by the engine is the more readily felt as the engine-room is smaller. If the room, for example, be 9 feet by 15 feet by 8 feet, the volume will be 1,080 cubic feet. From this it follows that the 2 cubic feet of air in the case supposed will be alternately displaced six times each second, which means the displacement of 12 cubic feet at short intervals with an average speed of 550 feet per minute. Such vibrations transmitted to halls or neighboring rooms are due entirely to the displacement of the air.

In installations where the air-intake of the engine is located in the engine-room, a certain compensation is secured, at the period of suction, between the quantity of air expelled on the forward stroke of the piston and the quantity of air drawn into the cylinder. From this it follows that the vibration caused by the movement of the air is felt less and occurs but once for two revolutions of the engine.

This phenomenon is very manifest in narrow rooms in which the engine happens to be installed near glass windows. By reason of the elasticity of the glass, the windows acquire a vibratory movement corresponding in period with half the number of revolutions of the engine. It follows from the preceding that, in order to do away with the air vibration occasioned by the piston in drawing in and forcing out air in an enclosed space, openings should be provided for the entrance of large quantities of air, or a sufficient supply of air should be forced in by means of a fan.

The author ends this section with the advice that all pipes in general and the exhaust-pipe in particular be insulated from the foundation and from the walls through which they pass as well as from the ground, as metal pipes are good conductors of sound and liable to carry to some distance from the engine the sounds of the moving parts.

Exhaust Noises.-Among the most difficult noises to muffle is that of the exhaust. Indeed, it is the exhaust above all that betrays the gas-engine by its discharge to the exterior through the exhaust-pipe. The most commonly employed means for rendering the exhaust less perceptible consists in extending the pipe upward as far as possible, even to the height of the roof. This is an easy way out of the difficulty; but it has a bad effect on the operation of the engine. It reduces the power generated and increases the consumption, as will be explained in a special paragraph.

Expansion-boxes, more commonly called exhaust-mufflers, considerably deaden the noise of explosion by the use of two or three successive receptacles. But this remedy is attended with the same faults that mark the use of extremely long pipes. The best plan is to mount a single exhaust-muffler near the discharge of the engine in the engine-room itself, where it will serve at least the purpose of localizing the sound.

Fig. 56.-Exhaust-muffler.

The employment of pipes of sufficiently large cross-section to constitute expansion-boxes in themselves will also muffle the exhaust. A more complete solution of the problem is obtained by causing the exhaust-pipe, after leaving the muffler, to discharge into a masonry trough having a volume equal to twelve times that of the engine-cylinder (Fig. 56). This trough should be divided into two parts, separated by a horizontal iron grating. Into the lower part, which is empty, the exhaust-pipe discharges; in the upper part, paving-blocks or hard stones not likely to crumble with the heat, are placed. Between this layer of stones and the cover it is advisable to leave a space equal to the first. Here the gases may expand after having been divided into many parts in passing through the spaces left between adjacent stones. The trough should not be closed by a rigid cover; for, although efficient muffling may be attained, certain disadvantages are nevertheless encountered. It may happen that in a badly regulated engine, unburnt gases may be discharged into this trough, forming an explosive mixture which will be ignited by the next explosion, causing considerable damage. Still, the explosion will be less dangerous than noisy. It may be mentioned in passing that this disadvantage occurs rarely.

A second arrangement consists in superposing the end of the exhaust-pipe upon a casing of suitable size, which casing is partitioned off by several perforated baffle-plates. This casing is preferably made of wood, lined with metal, so that it will not be resonant. The size of the casing, the number of partitions and their perforations, and the manner of disposing the partitions have much to do with the result to be obtained. Here again the experience of the expert is of use.

Various other systems are employed, depending upon the particular circumstances of each case. Among these systems may be mentioned those in which the pipe is forked at its end to form either a yoke (Fig. 57) or a double curve, each branch of which terminates in a muffler (Fig. 58).

Fig. 57.

Fig. 58.-Two types of exhaust-mufflers.

It should be observed that, under ordinary conditions, noises heard as hissing sounds are often due to the presence of projections, or to distortion of the pipes near the discharge opening. Consequently, in connecting the pipes, care should be taken that the joints or seams have no interior projections. Occasionally, water may be injected into the exhaust-muffler in order to condense the vapors of the exhaust, the result being a deadening of the noises; but in order to be truly efficient this method should be employed with discretion, for which reason the advice of an expert is of value.

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