« ПретходнаНастави »
foot of the hot gas is less than the weight per cubic foot of the cold air. Of course, no more air can enter the furnace than the size of the openings in the furnace doors and the ash-pit permit. The part then of the supply W that can be utilized is thus limited and must be of the same size as that which leaves the smoke-pipe, although its weight will be greater, say Wą. We then have the weight of a column of air of outside temperature (atmospheric temperature) pressing against the air in the furnace and ash-pit openings, and this weight, Wa, of cold air is opposed by the weight, W,, of a column of hot gases of the same dimensions. The difference in these two weights, W.-W, is equal to the natural draft and this difference is the pressure that causes the flow of the air through the grate bars, through the fire bed, through the furnace door openings, and then continues with the flow of the hot gases along the heating surfaces, tubes, of the boiler and to impart heat to them by coming into contact with their surfaces.
This motion of the gases is produced, therefore, by an excess of the pressure at any point over that at any other point towards which, however, the gases are flowing. As this pressure difference increases, the velocity of the flow increases. The air which has been pushed through the fuel bed into the furnace is partly modified by the combustion and is further modified as it flows through the combustion spaces, through the boiler, uptake and smoke-pipe to the outside air.
Sketch No. 12 shows the pressure drop. The total pressure drop is shown from the top of the smoke-pipe to the bottom of the grate bars. In the study of the absorption of the pressure drop through the boilers, it is not necessary to consider the total overall drop, but merely that for one boiler or for one boiler and its superheater as the case may be.
Sketch No. 13 shows a B. & W. boiler with superheater, and the rig used on the Texas for finding how the “ draft” was being divided in the boilers. It was found that when once regulated the magnitudes of the various "drops” would remain proportional, regardless of the " total draft," and that having determined the most economical series of steps in “ draft absorption " for one speed, it would likewise be the most economical for any other speed above“ steerageway.” The greatest difficulty was found in the correct laying of the grates and in maintaining it. This is the
most important part of the boiler when the efficient utilization of the draft is obtained. It is nearly always neglected and simply taken for granted as being a constant and of small moment. A few moments' thought, however, with the realization that the energy required to move the air through it is uselessly expended, will show that the air spaces through the grate (that is, the percentage of air spaces between the grate bars) should be as high as the mechanical construction and design will allow, and that they must be evenly distributed over the entire grate. Compare Sketches Nos. 14 and 15. The pressure drop through the grate
should be as nearly zero as possible, but the openings between bars must not permit excess falling of coal into the ash-pit. In this connection, therefore, an analysis of the ash must be made in order that the economical, efficient, "happy” working medium may be attained. The officers and men were made to realize the importance of correctly laying their grates, and it is felt that this contributed materially to winning the “E.” The pricker bar, so long laid aside, again came into play and the grate bar openings maintained while steaming. Likewise, immediately after hauling furnaces the grate bars were turned, dumped; they were also scraped of any fused clinker that might be adhering to them.
Returning to a consideration of Sketch No. 13. In order to obtain the pressure drop through the boiler the joint was broken at the point indicated and the manometer end thrust into the uptake. The difference in the two levels was measured as so many inches. This apparatus was used to make certain that the pressure drop through the grates of all the boilers was the same; and that the resistance of the grate when there was no coal in the furnace should not be more than one-tenth of the entire drop. This was obtained by running the forced draft blowers and measuring the total boiler drop from the ash-pit to the uptake. The pressure in the uptake was greater than atmosphere, and that in the ash-pit greater than that in the uptake. The first test of the grates showed marked variations and it was necessary to take out or put in grate bars to make them all homogeneous. By making the grates the same it was then fixed that equal division of the “ draft” would not be prevented by them.
It may therefore be readily seen that after steaming, as the fires get dirty, the grates become more or less clogged and the pressure drop through the grates will increase. The amount of pressure drop through the fire bed itself is of course a variable quantity, so variable that accurate measure of it is nearly impossible. It is clear, though, that as the fires become dirty its pressure drop will increase and if the total pressure through the boiler remains constant there will be less and less available above the furnace for pushing the hot gases through the boiler and immediately the capacity of the boiler decreases. This large pressure drop now experienced through the grates and fire bed does not mean that the rate of combustion is high, on the contrary it is most likely to mean the opposite. It is common knowledge that if it is desired to increase the rate of combustion either (1) the fires must be worked more in order to break them up and lower their resistance, which is bad practice; or (2) more boilers must be cut in, which is good practice; or (3) the forced draft blowers must be run, which is good practice up to the point demanding more boilers, and then again at full power, which is the point where the boilers are producing all the steam the engines will take.
(In considering the cutting in or out of boilers versus the use of forced draft, the coal used to prime furnaces and cut in the boiler must be compared to the power consumption of the fans and in connection therewith the time element of operation must be
considered. The benefit that may be derived by having cut in a clean boiler so that later a dirty boiler may be cut out must also be given its due weight before the order to do one thing or the other is sent to the engineer officer of the watch.)
SKETCH No. 14.—Showing Irregular Placement of Grate Bars.
NNN= No air flow—no fire.
H= Extreme air flow-hole occurs.
The rig shown in Sketch No. 13 may also be used as a dead check on the dampers. When measuring the total boiler pressure drop, one connection being made to the uptake, have the damper closed. The two zeros should be reached gradually as the pressures equalize, if after a few minutes they do not do so, inspect the dampers. This was done on the Texas and two dampers were found six inches open when “closed.”
The pressure (draft) gage was explained to the men and put into use when cleaning fires. Such a gage when connected to the furnace will give useful information as to the condition of the fire. For instance, after a man has cleaned a fire, if the pressure drop from ash-pit to furnace is very low, he may be sure that he has his fire too thin or that there are holes in it. With hand firing a gradual increase in the drop will be noticed and will indicate the formation of clinkers.
Of course, a drop through the fuel bed is adjudged high or low only after considering it in connection with the total drop through the whole boiler. It may happen, in the same boiler, with the same coal from the same bunker, and with the same total pressure drop, that much less coal is burned and a smaller amount of steam produced on one day than on another, although the pressure drop through the fuel bed is higher on the day of smaller steam production. The fireman may wonder why this is. The explanation may be drawn out thus: When coal is taken out of the side and bottom of a bunker, the larger pieces tend to flow out first, leaving the smaller pieces and dust in the far corners, which stay there to the last until all the coarser coal has been burned. When burning the finer coal, the resistance to the passage of air through the fuel bed is greater, and this greater resistance causes a higher pressure drop, that is, a higher " draft" above the fire; simultaneously, the smaller air supply results in a lower rate of combustion and a smaller steam production.
The drop of gas pressure from one part of the boiler to another varies, naturally, as some power of the resistance offered to the flow of the gases. Thus a great drop from ash-pit to furnace indicates a high resistance in the fuel bed, and a great drop from the furnace to the uptake indicates high resistance to the flow of gases through the boiler proper (baffles leaky or out of line). This law has been determined to be similar in some respects to Ohm's law as applied to problems involving the electrical resistance of conductors. It may be stated as follows:
(a) If the resistance to the flow of gases remains constant the pressure drop through any portion of the path of the gases will have a constant ratio to the total drop from the ash-pit to the uptake. Thus, for example, if the pressure drop through the fuel bed is 0.25 inch of water when the total drop is 0.50 inch, it will be i inch if the total drop is increased to 2 inches of water.