Since Forced Draft produces a pressure greater than atmospheric, its use is confined principally to forcing air through a thick bed of fuel on the grates. It is used largely in connection with certain kinds of stokers, and frequently with hand fired boilers using low grade coals. Forced draft is not suitable for steam atomized oil fired boilers, because there being no fuel bed on the grates to offer resistance, the positive pressure from the fan discharge would be carried up into the boiler setting. This would cause the gases to leak out into the boiler room, and in some cases would result in excessive furnace temperature and

FIG. 167.-Exterior view of San Francisco's new high pressure salt water pumping plant, showing chimney design for fuel oil practice.

Here is a view of the housing for the Townsend Street high pressure salt water pumping station at San Francisco. The chimney stack as shown illustrates the best and most permanent type of design for fuel oil practice. No artificial means of producing a draft is employed in this installation. It is a plant kept in eternal readiness, should disaster ever again visit San Francisco as happened during April, 1906, in the days of the great fire. burning out of the brickwork. For satisfactory operation of stationary boilers it is necessary to keep the pressure of the gases within the boiler setting slightly lower than atmospheric pressure. When forced draft is used the positive pressure should not extend beyond the ash pit. Forced draft is used extensively with Mechanical Atomizing oil burners, where owing to the small area for air admission it is impossible to get into

the furnace enough air for high overloads by means of natural draft.

Induced draft can be used instead of natural draft wherever desired. It is cheaper to install than a high stack, but the power required to drive the fan makes it more expensive to operate. It is of especial advantage where the gases escaping from the boiler are passed through an economizer to absorb some of their heat, before they are allowed to reach the chimney. The economizer introduces added frictional resistance to the gases so that extra draft is required. Besides this, the economizer reduces the temperature of the gases to such an extent that to obtain sufficient draft without a fan would require a stack of excessive height. By installing an induced draft fan between

FIG. 168.-Exterior view of San Bernardino Plant of Southern Sierras Power Company where artificial draft is employed.

the economizer and the stack, ample draft can be obtained regardless of the height of the stack or the temperature of the gases.

Induced draft is also of value, even when economizers are not employed, in cases where it is impracticable or undesirable to build a stack of normal height. An example of this is found in the power plant of the University of California, where a high unsightly stack would seriously interfere with the architectural features of the university buildings. By building a stack only 50 ft. high, and supplementing it with an induced draft fan, this difficulty was overcome.

CHAPTER XXXI

CHIMNEY GAS ANALYSIS

We have found in preceding discussions that for practical purposes the gases passing out through a chimney from the central station boiler are usually considered to be composed of carbon dioxide, oxygen, carbon monoxide and nitrogen. Since these constituents are usually determined volumetrically we shall represent them by the symbols V1, V2, V3, and V4, respectively. We shall now proceed to a discussion of the usual methods employed in determining the flue gas analysis during the boiler test.

The Taking of the Flue Gas Samples and Analysis.-Certain solutions have been found in the chemist's laboratory that will absorb carbon dioxide and will not absorb oxygen, carbon monoxide or nitrogen. Again another solution has been found that will absorb oxygen but will not absorb carbon monoxide or nitrogen. And still a third solution has been found that will absorb carbon monoxide but will not absorb nitrogen. If then a contrivance can be set up so that a flue gas sample may be successively washed in these solutions, a means is provided for determining an analysis by volume.

Orsat Apparatus.-Let us then see how the flue gas analysis is taken.

The apparatus commonly called the FIG. 169.-A carbon dioxide Orsat Apparatus (see Fig. 171) con

recorder.

pipettes, P', P", and P"". In addition a leveling bottle A and connecting tube T are also provided.

The tube E is connected to the point in the flue at which the sample is to be taken. The instrument is first set in

FIG. 170. A recorder for combustion operation.

From the discussion in the text it may be inferred that a knowledge of the carbon dioxide component of the flue gas enables us to judge concerning the combustion taking place in the furnace. The principle involved in the type of carbon dioxide recorder as shown is that a change of volume in a gas produces a change of pressure. A continuous sample of the flue gas enters at A and in passing through the absorption chamber the carbon dioxide is absorbed and consequently a reduction in pressure takes place. By the calibration of suitable manometer tubes the instrument may be made to read the carbon dioxide component direct.

is

operation by closing the stop-cocks f, g, and e, d being open. By lowering the leveling bottle A, a sample of the gas drawn into the burette B. This preliminary sample is then

expelled to the atmosphere by raising the bottle A and allowing the gas thus put under pressure to pass out through a by-pass at d. This process is continued until it is considered that an average sample has been drawn into the burette B. The leveling bottle A is next lowered so as to cause the water in burette B to come to its zero mark. By raising the bottle A the water is again forced into burette B and the gas sample expelled through stopcock e into the pipette P', in which there is a chemical solution

The Orsat apparatus is a portable instrument contained in a wooden case with removable sliding door front and back, as shown in its simplest form in this illustration, taken from the report of the Power Test Committee of the American Society of Mechanical Engineers. It consists essentially of a measuring tube or burette, three absorbing bottles or pipettes, and a leveling bottle, together with the connecting tubes and apparatus. The bottle and measuring tube contain pure water; the first pipette, sodium or potassium hydrate dissolved in three times its weight of water; the second, pyrogallic acid dissolved in a like sodium hydrate solution in the proportion of 5 grams of the acid to 100 cc. of the hydrate; and the third, cuprous chloride. These chemicals are sold by most of the large dealers. Details of how this apparatus is used to determine the chimney gas analysis were set forth in a previous discussion.

that absorbs carbon dioxide, but will not absorb oxygen, carbon monoxide or nitrogen.

To Ascertain the Carbon Dioxide Content of a Flue Gas.Exactly 100 cc. of gas were originally drawn into the burette B. If now the leveling bottle A is again lowered to draw the gas back through stop-cock e, the volume in the burette will be found to have lessened in quantity so that instead of reading zero it now reads N, which indicates directly the volume of car