If the discharge oil is trapped in the pipe or in an exhaust pot located close to the engine, it will accumulate until it is set afire.
The exhaust is always at a high temperature, around 800° Fahren heit, and frequently a flame blows through the exhaust ports and ignites this residue. Many fires, some of them serious, have resulted from the use of an exhaust pot or muffler. If the employment of a pot is decided upon, then a thimble should by all means be placed around the pipe where it passes through the roof.
In preference to the pot, a concrete exhaust pit should be con structed outside the building. It is a good plan to place the pit at least 3 feet from the building wall, although 5 feet is better. Means must be provided for draining away the residue that accumulates in the pit. If the contour of the land permits, the drain should have an open end. If not, it can be run to a smaller pit and a bucket placed in this pit below the drain. In this way the residue will collect in the bucket and can be removed.
Figure 333 shows a form of exhaust pit very generally adopted. It is provided with an extra exhaust-stack pit leading from the pit proper. While this is of assistance in deadening the noise, it is a refinement not actually required. The concrete walls should be 9 inches thick as a minimum, and reinforcing rods should be used. These rods will resist the ordinary strains to which the walls are subjected. The top can be made either of a boiler plate covered with earth, or of old iron rails with a concrete slab as a cover. A manhole should be constructed in the top, both for access to the pit and for safety in case a violent explosion occurs.
Another good form of pit is shown in Fig. 334. Here the ex haust pipe A enters below the layer of rock B which is supported by old rails or iron bars and serves to deaden the sound of the ex plosion. Such a pit is well-nigh noiseless and is as cheap to build as a less efficient one. It should be provided with a manhole in the side, below the layer of rock. This manhole should be fitted with a thin cover held in place by two small studs so that, if a heavy explosion does occur, the cover will blow off and pre vent damage to the pit.
Frequently a cylindrical exhaust pit like that shown in Fig. 335 is used. This, however, is not as good a design as Figs. 333 and 334 since it does not even deaden the noise of the exhaust.
Furthermore, as it has no drain, the residue cannot be removed readily. Figure 332, showing pipe .conduit, also outlines a type of exhaust pit, or pot, that is used in some installations. It is essentially a boiler plate tank with inlet and discharge openings.
It has the disadvantage of high initial cost and no advantage other than that it can be moved in case the power plant is ever transferred to another location.
In a two-engine installation the two engine exhausts can be run into one exhaust pit that is partitioned into two parts, each of which has a manhole and a stack. This reduces the cost of ex cavation.
There is a large variation in the plans of different engine build ers as to the necessary exhaust pit volume. Some call for entirely too great an outlay of concrete while others are equally as thrifty in the dimensions given. If the pit is designed on the basis of 1 cubic foot of volume for each horsepower of the engine rating, the pit will be ample in size and the cost reasonable.
The pit should be so located that its top will be a few inches below the ground level. The exhaust stack is best held to the pit by means of a cast-iron flange. If a flange cannot be procured, four lugs riveted on the stack will serve. The stack should have a reinforcing ring both at the top and bottom. It is advisable to have the stack considerably larger than the exhaust piping; for instance, if an exhaust pipe 8 inches in diameter is used, the stack should be at least 12 inches in diameter. Owing to initial cost it is customary to use a sheet-steel stack of from No. 8 to No. 16 gage. Corrosion in the stack is generally severe, and a light gage steel will not last long. The stack ought never be made of less than No. 10, and plate will prove the cheapest in the end. It is good practice to paint the stack each year with an asphaltum stack paint. This will keep down the corrosion and increase the good appearance of the plant. The question as to the length of stack is one that is governed by the location of the plant. A good rule is to always have its top at least 10 feet above any building close by. This serves to carry away all fumes and eliminates the effect of the air waves that so often cause windows, for blocks around, to rattle.