1. It is, without any exception, the best coal-burning method. 2. When properly operated, it is absolutely smokeless with any coal. 3. Will burn any and every kind of coal. 4. Never troubled by clinkers. 5. Not ever necessary to interrupt the performance for the purpose of cleaning fires. which is not applicable to low-pressure heating plants, and it can not burn all grades and kinds of fuel. NO SMOKE WITH A PROPERLY FIRED HEATER. The second advantage, that of smokelessness, is a feature that must be considered seriously in connection with the mode of firing. If the heater is properly fired there can be no smoke, but it may easily be so fired that it will smoke outrageously. As the fuel lies quietly on the upper 6. Will produce most economical grate, it burns absolutely without smoke. combustion. The disadvantages are two in number as follows: 1. First cost is higher than common inefficient apparatus. 2. Successful results depend upon proper manipulation, or in other words, it is a labor problem. The requirement for efficient combustion of bituminous coal, is that the volatile matter be thoroughly mixed with the air at high temperature. This occurs with the down-draft principle, as the volatile which is distilled from the upper layer of freshly-ignited coal, must pass in many finely-divided streams between the incandescent coals in the bottom of the fire. At the same time, the air must take the same path, with the result that it is brought intimately and immediately in contact with all the volatile carbon, oxidizing it to carbon dioxide, or CO2. This performance differs from that of an automatic mechanical stoker, with which coal is supplied at a uniform rate, and from which the combustible gas rises in the form of masses of flames, which must be kept hot by means of a combustion chamber, until their combustion is completed. Thus the stoker does not produce immediate complete combustion at, or in, the fuel bed, the burning of the gases being a supplemental or secondary performance. The complete mechanical stoker furnace, of course, gives excellent results, but it is a machine The fuel bed, however, after a time becomes caked and fused together, with the result that the rate of combustion drops below requirements. When this occurs, the fire should be loosened up. by running a long slice bar under the fire along the water bars clear to the back of the furnace. The bar should then be pressed down just enough to pry up the fuel bed, sufficient to crack the caked masses, just enough to open air passages, but not enough to disturb the condition of the fire. This slicing should be extended across the furnace till the entire bed has been freed and made porous. The insertion of the slice bar will cause some incandescent coals to fall down to the lower grate, where they will be burned later. The slice bar will also dislodge the accumulated ash which will also drop to the bottom grate. After the slicing is finished, the hot coals on the bottom grate will rest in irregular piles, these should be leveled off with a rake or hoe, so that the fire on this grate will be perfectly level and uniform. After this has been done, the fuel bed on the top grate should be replenished by the addition of fresh fuel, which will complete a full cycle of operation. The frequency of these cycles will depend on the load on the boiler; when much steam is required, they should be performed at correspondingly frequent intervals, when little steam is needed, at correspondingly long periods. EXAMPLES OF IMPROPER FIRING. Having described proper firing, it is necessary, to explain how the furnace may be, and is often improperly fired. Frequenty the slice bar is only run part way back, then the fire is much broken up and turned upside down on the front, with the result that much green coal falls to the lower grate where it lies in piles, and, burning there, makes much smoke. The back of the fuel bed, not being disturbed, cakes up and at the same time allows the ashes to accumulate which fuse into clinkers. This results also in reducing the capacity of the furnace and producing a mass of clinker which must be dug out, sometimes with much labor. In other instances the entire fire is violently sliced, the fuel bed from front to back turned upside down, with the result that much green coal drops to the lower grate, producing a great quantity of smoke. Sometimes the fuel bed is not sliced at all, or, if it is, only at long intervals. The result of this is that the fire seals up, clinkers form and cover the water bars till not enough steam is made to satisfy demands. Then fresh coal is shoveled on the lower grate and the furnace is operated the same as any plain grate. It is under this latter condition that enormous volumes of smoke are made. Many of the readers of this article may question the accuracy of the statement that the down-draft is a smokeless and efficient furnace when properly operated. Their reason for doubting is possibly because their ideas of its capabilities are based only upon improper operation. ABILITY TO USE ANY KIND OF FUEL A DECISIVE FEATURE. Ability to use any kind of fuel that may be available is always desirable, especially under present conditions and particularly so under conditions that will exist in the near future. This is a strong point of the down-draft furnace. It can use successfully bituminous lump, egg, mine run or screenings, as well as semi-bituminous, all on the top grate in the regular way. The only fuel that can not be used is dubb as it is so small that it will fall through the bars. The smaller sizes of washed coal are the least desirable, but they may be used. Coke may be used as well as coal. Anthracite may be burned on the lower grate if it is too slow for the upper fire, as it does not smoke. Under such conditions, the fire door for the upper grate must be closed tight. Wood and like refuse that accumulates at an apartment house and must be disposed of, may be burned on the lower grate. CLINKER TROUBLE AN EVIDENCE OF BAD FIRING. One of the frequent complaints that coal dealers receive from users of the down-draft boiler, is that their coal clinkers. There is no reason whatever for such complaint. Clinker trouble is always an evidence of bad firing. With proper manipulation no clinkers are formed on the top grate, even with the dirtiest of screenings. If the fire is properly sliced, all the ash will be dislodged and fall to the bottom grate, where it does, to some extent, fuse together in small clinkers, but these are readily removed as this fire is thin and easily cleaned. BEST METHOD OF CLEANING FIRE. The down-draft fire is very easily cleaned, in fact, the easiest to clean of all fires. As all of the ash falls to the bottom grate, it may be easily raked out without in any way disturbing the main fire, which is on the water bars above. The best method of cleaning, however, is to remove a portion of the ash each time the fuel bed is sliced. Just before slicing, the small clinkers should be raked out from the bottom grate, and depending on the amount present, a portion of the ashes may also be removed. The remaining ashes and fire should be well spread and leveled. Then when the top fire has been sliced, the hot coals which have fallen from the top fire, should be well spread and leveled on the bottom grate. It is better not to remove all the ashes from the grate; about an average amount should be left, because if they are all cleaned off, too much air will flow through the bare grate and reduce the efficiency. So it is best to maintain a thin bed of ashes on the grate to exclude this excess of air. CONTROL OF AIR SUPPLY AND ITS BEARING ON ECONOMICAL OPERATION. carbon is absorbed by the oxygen. This fact simplifies the analysis of combustion gases, because if our analysis shows 18% CO2, we know immediately that we have used all of our air in combustion. This would be a perfect condition, the maximum efficiency of combustion, which could be no better. If, however, our analysis shows 9% CO2, we know that we have supplied twice the amount of air that we have used; in other words, that air is in excess by 100%. If we get 4.5% CO,, we know that we have supplied four times the air needed or 400%. If 2.25% CO2 eight times or 800%. In heating a boiler, practically all the heat which does not go into the boiler. HEAT LOSS IN PERCENT 100 60 40 20 The foregoing has largely dealt with the matter of securing complete combustion, and the handling of the ash. The matter of the control of the air supply, the feature which applies to the sixth advantage of the down-draft furnace, is 80 the one which has the most important. bearing on economy. This requires some detailed explanation. Air contains 21% oxygen and 79% nitrogen. It is the oxygen that does the work, it enters into combination with the carbon and hydrogen of the fuel to produce combustion. The nitrogen takes no part in the process, it acts simply as a dilutent of the products of combustion. Therefore our interest centers in the oxygen, of which about three parts are used for combustion of the hydrogen of the coal. The combustion of hydrogen results in the production of water, which, at furnace temperature, is in the form of steam, and for this reason not easily determined in a chemical analysis of the combustion gases, so is not directly considered in studying air supply. As three parts of oxygen are used for the hydrogen, there remains 18 parts for carbon. The result of the combustion of carbon is that the volume of the CO2 produced is the same as the oxygen which entered into the combination. In other words, the oxygen is of the same volume after the carbon is added, as it was before. It is heavier, of course, but not larger. It may be said that the 2 4 6 8 10 12 14 16 18 20 PERCENT CO2 IN GASES FIG. 1-HEAT LOSSES TO CHIMNEY FOR escapes in the hot gases to the chimney. Therefore if we use eight times as much air as necessary, we have eight times. more gases to go to the chimney, consequently eight times as much heat wasted. Fig. 1 is a curve illustrating heat losses to the chimney for different percentages of CO2. It shows that when CO2 gets down to about 1.5%, the coal being burned does no work at all. An extreme condition, of course, but one which for comparatively short periods often exists. While the foregoing illustrates the fact of the great importance of proper proportions between air and fuel, it gives no hint as to how the air may be controlled. We do not see the air and it is difficult to realize that for each ton of coal supplied to the furnace, at least 12 tons of air is also supplied, and that the amount may be, under bad conditions, as high as 100 tons, for each ton of coal. Of course an analysis would give this information at once, but this is out of the question in the kind of service that is under consideration. So it is absolutely necessary that we find some other simple path which leads to the solution of the air supply matter. Fortunately there is such a path. It consists in eliminating from our problem the idea of air, and in its place substituting the idea of draft. Draft is something applying to dampers, ventilators in fire doors, etc., things that every janitor and fireman understand. AIR CONTROL EXPRESSED IN TERMS OF DRAFT. A strong draft causes a large flow of air into the furnace, a mild draft a small flow. When a large amount of steam is required, much air is needed, when little steam is needed, little air is sufficient. Thus adjustment of drafts takes care of the air supply matter, as draft and air supply in this connection are synonymous. Now as air supply, or, in other words, draft, must be in proportion to the amount of steam needed, it necessarily follows that fuel must also be supplied in sufficient amount to satisfy this air. The way to do this is to maintain at all times a fuel supply, or a fire condition sufficient to satisfy the maximum draft conditions. Then with strong draft the fire will burn fast, with mild draft it will burn slow. The efficiency of combustion will be uniformly high, and the rate of heat production in proportion to requirements for steam. To secure the results above described a sufficiently thick fire must be maintained on the upper grate, to insure that there shall always be an ample supply of combustible to satisfy whatever amount of air may flow through it. In this way the fire-bed becomes a fuelmagazine always ready to respond to any demand, or to lay semi-dormant. THICKNESS OF FIRE DEPENDS ON KIND OF COAL USED. The proper thickness of fire depends upon the size of the coal. Screenings, being a small fuel, makes a compact bed, consequently it need not be very thick. Lump coal is a large fuel, is not compact, as there are large interstices between the pieces. With its use the fire must be very thick, or else air will flow through the spaces without coming in contact with burning fuel. Mine-run coal, composed of both large and small sizes, should be maintained at a thickness midway between that for lump and screenings. Holes should never be allowed to form in the fuel bed. The fire on the bottom grate should be kept level and uniform. The draft-opening at the fire door of the upper grate should be adjusted to give the capacity required. The draftopening in the ash pit door below the lower grate, should be set so as to admit only sufficient air to uniformally burn the coal that drops to it, the idea being that the fuel on this grate should last from one poking period to another. too much draft is given, this fire will burn away and leave a bare grate, allowing excessive air to enter. If draft enough is not admitted, the fire will accumulate on this grate. If The draft-openings in the middle door, the one between the grates, should be closed except when wood or rubbish is burned. The manipulation of the drafts together with the stack damper, either by hand or automatically, to correspond with the load on the boiler, will always give an economical and satisfactory result, if the fire is kept at suitable thickness and in proper condition. The effect of good and bad firing is illustrated by the curve of CO2 in Fig. 2, careless poking and recoaling causes it to rise to 5. That it went no higher is because the fuel thickness was not enough, while there were holes in the fire and the bottom grate was not leveled. The result was that in an hour and a half the CO, had dropped again to a point at which no steam was produced in the boiler. This interval on the diagram is indicated as poor firing. What the loss for different periods of the hour and a half was is also shown. At the end of the three-hour period, after some clinkers had been removed from the bottom grate. the fire was properly sliced and coaled which resulted in a rise of CO, to 10. The coals. on the bottom grate were leveled and drafts set which put it to 15. After this combustion proceeded for about six hours, at which time the CO., had dropped to 9%, when the process was repeated. This shows good firing, with an average of 12 CO, and an effciency of 82%. While the poor condition averages 3 CO2, with an efficiency of about 35%. The down-draft boiler costs a little more than the common one but it has so many advantages that more than justify the additional cost. It also requires a certain higher standard of operation than would pass muster with a common boiler. This, however, does not mean more labor, because the hard part of operating a common boiler is the cleaning of the fire which is very easy with the down-draft. Its higher efficiency requires the use of less coal so that there is less to handle. |