Oil Burners—Their Construction and Operation
Some Timely Data of Particular Interest to the Fire Chief—Safety Suggestions—Maintenance Hints
THE problem of the oil burner for domestic heating is a very live one to the fire chief, in view of the increased adoption of this method of heating private residences throughout the country. The fact that such heating plants are, in the great majority of cases, being operated by individuals who are unfamiliar with the principles of oil combustion and are rank novices in the handling of this type of healer makes the fire hazard all the greater. The following paper, prepared by experts in the United States Department of Agriculture, will provide the fire officers with many points of value in teaming householders of the risks to be avoided in the maintenance of their oil healing plants:
In contemplating the installation of an oil burner provision for storing fuel should be considered. For coal, the average home owner generally provides storage capacity ample to contain all the coal used during the heating season. With oil this is not usually the case.
Large storage tanks are installed in various ways and usually must conform to the ordinances which regulate such matters in the particular locality. From this tank the oil must be fed to the burner by suitable means, since regulations restrict the quantity of oil which may be stored above the burner level. A simple means of transferring oil from the main to an auxiliary tank is by vaporizing burner with automatic control. The ignition is by a continuous gas flame, which also serves as a source of heat for the vaporizing plate. In many types this transfer of the fuel from the storage tank to the burner is made automatically by means of a pump or by suction produced at tinburner itself. A typical full-automatic installation is shown in Fig. 1.
The primary function of an oil burner is to break up the oil into fine particles and so mix it with air that proper combustion will result under suitable conditions of temperature. A number of equally effective schemes arc employed, one of which is shown in Fig. 2. Combustion, or burning, is any kind of chemical combination in which heat is liberated. In domestic heating combustion is caused by the combining of the elements of the fuel with the oxygen of the air. When this combustion is completely carried out the maximum heating value of the fuel is realized.
There are on the market a number of burners for use in house heaters, varying in the method employed to prepare the fuel for combustion. In general they operate upon one or the other of two broad principles—namely, the vaporization or the atomization of the oil prior to burning. Burners may be classed, then, as either vaporizing, comprising the so-called gravity-feed type, or atomizing, including those in which the oil is broken up by mechanical or spray devices.
Gravity-feed Vaporizing Type
This is the simplest type of burner, very often consisting merely of one or two rough castings which are set inside the furnace (Fig. 3) and its initial cost is low. Some device, such as a “hot plate,” is essential for volatilizing the oil so that a vapor will be produced, as illustrated in bigs. 4, 5, 6 and 7. The air to support combustion is generally brought into the furnace by the natural draft produced by the chimney. Some rather ingenious means are used to induce an intimate mingling of this air with the vaporized fuel, but in general good combustion is not obtained by this method unless a highly volatile fuel is used. In the cheapest burners of this class the control is entirely manual. In some cases automatic control has been applied with apparent success.
The atomizing type of oil burners may be subdivided into those which break up the oil by purely mechanical means and those which atomize the oil by spray devices.
In one mechanical-type atomizing burner the oil is put under pressure and forced through a small opening to break it up into minute particles, and it enters the furnace as a vapor spray. Air is supplied by a blower or pump and so regulated as to bring about the proper combustion of the fuel (Fig. 8.)
In another type of mechanical atomizing burner the oil is broken up by being thrown from the periphery of a revolving disk or cup. The disk or cup speed is relatively high and is sometimes obtained by positive gear drive or by friction drive from an electric motor. Fig. 9 illustrates a motor-driven centrifugal atomizing burner.
The spray type of atomizing burner consists of air and oil nozzles so arranged that air supplied by a motor-driven compressor blows directly over the oil nozzle and creates a partial vacuum, as illustrated in Figs. 10 and 11. The oil is drawn up from the supply reservoir by the vacuum thus formed and is atomized or broken into minute particles by the air pressure, in preparation for ignition. The rate of feed is governed by the air pressure and the size of the nozzle openings. This principle is not unlike that of the ordinary carburetor employed in the gasoline engine.
The atomizing type is generally controlled automatically by a room thermostat which so regulates the burner as to maintain the desired room temperature. Fig. 12 is a sketch of one type of atomizing burner, the mechanism of which is practically entirely within the furnace. The mixing chamber is practically the only element of the burner proper which is exterior to the furnace.
Oil fuel used by domestic oil burners contains principally hydrogen and carbon, with much smaller quantities of oxygen, nitrogen, and sulphur. Of these elements, the carbon, hydrogen, and sulphur are the ones thatburn or combine with oxygen. The oil fuels that are now employed for domestic heating are very uniform in composition and contain roughly 84 per cent carbon and 13 per cent hydrogen; the oxygen, nitrogen, and sulphur taken together compose the other 3 per cent. A pound of fuel of this composition will require about 14 1/4 pounds of air for perfect and complete combustion.
Insufficient air supply is one cause of clouds of dense smoke and soot. Sometimes the draft prevailing under this condition is not sufficient to carry off the sooty, smoky products of combustion, and they permeate the entire house. This form of combustion is inefficient in that the fuel is not entirely consumed. It is not easy to supply precisely the amount of air theoretically required for perfect combustion and even if this were accomplished the intermingling of the air and oil probably would not be sufficiently complete to give perfect combustion. An excess of air is essential to insure that each subdivided bit of oil is provided with the amount of air necessary. In practice, possibly 25 to 30 per cent more air is supplied than that which is theoretically required.
Burners may be subclassified according to the manner in which the fuel is ignited. With the manually-controlled gravity burner, the hot plate is pre-heated by a wick which is saturated with oil and ignited by a torch. The plate must be heated to a temperature sufficient to vaporize the oil falling upon it. The heat of combustion is supposed to do this once the flame is started. The temperature of the house is maintained at a desired point by increasing or decreasing tbe intensity of the flame by means of a valve in the oil line, or the burner may be operated at a fixed intensity and then completely shut off as the conditions may demand. Whenever the burner is off for a few minutes the hot plate must be preheated again before the oil can again he vaporized and ignited.
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Oil Burners—Construction and Operation
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In some automatically-controlled vaporizing burners a gas flame is used for heating the hot plate and as a pilot light for igniting the fuel. This is shown in Figs. 6 and 7. The gas flame burns continuously and keeps the hot plate at such a temperature as to cause the oil to vaporize when it is admitted to the apex of the plate and trickles down over the corrugations shown. At the same time the pilot flame licks through holes drilled in the hot plate and ignites the mixture of vaporized oil and air. The automatic device in this case merely shuts off or opens a valve in the oil line to the burner.
With the atomizing type of burner it is necessary to introduce a flame or electric are within a region which is filled with an intimate mixture of oil and air in such proportions as to make it comparatively easy to ignite. Ignition methods used may be roughly classified as gas, electric, electric gas, or electric oil. With a gas pilot, ordinary illuminating gas is used to provide a continuous source of heat and is so placed as most effectively to bring the mixture of oil and air to such a temperature as to cause combustion. The pilot flame is sometimes caused to expand at the time the burner “comes on” and by this means the danger of extinguishing the pilot light is somewhat lessened and ignition is presumably hastened.
In the case of electric ignition, a spark is introduced into the region of the charge. Electric-spark machines are either continuous or intermittent. In the continuous-spark type the spark continues during the entire time the burner is in operation. In the intermittent type the spark is active only during the time necessary to ignite the charge, and then ceases.
In the electric-gas type a gas pilot is turned on, the gas being ignited by a spark. The pilot light then ignites the charge. Still another device is the electric-oil ignition in which an independent atomized mixture is ignited by an electric arc and is utilized as a source of heat energy to ignite the charge of the burner proper.
Automatic Devices for Control of Oil Burners
The oil flame is extremely rapid in heating and if not controlled in some manner will build up temperatures and pressures in the heating system which may prove dangerous. If the drafts of a coal furnace are inadvertently left open the worst that can happen is to burn up the coal then in the furnace. It is true that temporary overheating or increase in pressures might occur but in all probability no serious effects would be produced. With the oil burner, however, overheating would go on as long as the oil supply lasted.
The simple gravity-feed vaporizing type is generally manually controlled, and in order to maintain the desired room temperature the fuel admission is regulated by hand—at best an irksome job. When the house is deserted the fire is either extinguished or cut down so as to maintain a minimum temperature sufficient, at least, to prevent freeze-ups. Very frequently burners of this type operate unsatisfactorily when throttled so that it is safer to entirely cut off the flame. This, of course, introduces the danger of freeze-ups and accompanying damage. In some instances automatic control has been applied to the gravity-feed type of burner, and either the oil flame is increased and decreased as required, or the burner is rendered entirely inactive when the room temperature rises above a desired point. The power-atomizing type of burner best lends itself to a variety of automatic controls, and it is in this type that such controlshave been most successfully utilized.
It is essential that precaution be taken to cut off the burner in the event that ignition fails to take place. In such burners as permit, a drip bucket or sump is provided to catch the unburned fuel which Hows to it when ignition fails. This device trips when a certain quantity has been delivered to it and either cuts off the oil supply or breaks the power circuit, in either case rendering the burner inoperative as to the generation of heat and flow of oil. The machine must then be reset by hand before operation can be resumed. One of the chief objections to this control is the clogging of the line which delivers the unburned oil to the drip bucket or sump, owing to the accumulation of soot, scale, etc. Liberal passages offset this tendency to a great extent.
Another emergency control is designed on the assumption that so long as the pilot light burns the charge will be ignited and accordingly a thermostatic member which is exposed to the heat of the pilot light breaks the power circuit when the pilot light is extinguished.
When the general design of the burner is such as to make the catching of unburned oil and its subsequent delivery to a sump or drip bucket impracticable, the “stack control” is utilized. A thermostatic member is placed in the stack and if, after a predetermined period, it does not become heated—indicating that the burner has failed to function—the thermostatic member breaks the power circuit and stops the motor and supply of oil.
Low-water emergency controls are also applied in some installations and there are other devices, such as alarm bells, which are employed in conjunction with these emergency controls. Doubtless even more ingenuity will be manifested in this direction as design progresses.
Adjustment of Burner
After a burner has been installed in a boiler, it must be properly adjusted for efficient service. Assuming that the burner has been properly set and such details as impingement of burning gases directly against water-cooled surfaces have been guarded against, the next step is to establish the fuel and air rates.
The adjustment of the burner consists of regulating the quantities of oil and air admitted. The rate of oil consumption depends of course on the heating load, that is, size and character of house, temperature to be maintained, etc. The adjustment of the air admission depends upon the quantity of fuel to be burned. A definite quantity of fuel burned requires a definite quantity of air. The proper flame is the one which appears least attractive because of its slightly smoky appearance, it is well for the average owner to refrain from any attempt to adjust the burner.
With any oil burner, it is important to give considerable attention to the setting of the dampers, especially the stack damper. The underwriters have the following to say regarding this:
Dampers which may entirely close the chimney uptake are prohibited. Damper area should be carefully determined in each case but in no case shall it be greater than 80 per cent of the internal cross-sectional area of the uptake.
Very often, in the adjuster’s zeal to cut down the loss of heat up the stack, he will almost entirely close the stack damper, resulting in a leaking of burned gases into the rooms of the house. One simple method of determining whether or not this leakage is occurring is to place a candle flame, or flame of a match, near the fire door which has been slightly opened, as illustrated in Fig. 13. The flame should be drawn into the furnace; if it is not. there is indication of a pressure in the furnace caused by a blocked chimney passage.
The automatic controls very often require some little adjustment after installation in order that the proper room temperature may be maintained. Adjustments of oil burners had best be left to the installation men and generally there should be no tinkering on the part of the owner.
Care of Burner
As in the case of any other mechanism, a little care in the following of certain directions that are given in printed form by the manufacturers or agents of the burner will he well repaid. Oil-burner men are subject to service calls for minor ailments easily remedied by the owner if just a little attention were devoted to the burner. Accidental dropping of a drip bucket or the blowing of a fuse account for many service calls, and these could easily be remedied by the owner.
Instructions as to oiling and general care of the burner should be followed strictly. Various agents have different plans for the care of the burner. Some periodically overhaul and oil the moving parts at a nominal price, and thus each heating season is begun with a plant in good condition.
In the summer, the temperature of the basement is generally somewhat lower than the outside temperature and there is apt to be a general dampness which tends to corrode parts of the burner. As far as is feasible the removal of motors and other parts likely to be injured is recommended.
It is well for the owner to refrain from attempting adjustment. The prime requirement tor burning oil economically is that the proper proportions of air and fuel shall be admitted to the furnace.
A question of great importance in installing an oil burner is safety. Some commendable work has been done by the Underwriters’ Laboratories wherein various burners have been investigated from the standpoint of safety. The underwriters do not test burners for efficiency. To quote from a pamphlet issued by them:
Primarily, the questions to be settled are: Will this burner set fire to the building in which it is installed, or is there a possibility of its causing an explosion, or being instrumental in injuring the person giving it the ordinary amount of attention required?
Roughly speaking, there are three parts to the work of a complete investigation of any oil-burning equipment. These include a study of the design and construction from blue prints and the actual samples; tests of the samples and an investigation of the field service record.
As one of the important factors in the use of burners of this type is the selection of suitable grades of oil, at least part of the test work is done using the lowest grade of oil which the submittor claims is suitable.
In general, the listing of any oil-burning equipment by Underwriters’ Laboratories means that the equipment has been examined and tested, and found to comply with the minimum requirements in effect for its class, and that the manufacturer has agreed to maintain the standard established and to identify the listed product with the marking agreed upon. Particular attention should be paid to the exact form of marking used to designate the listed device as stated on each card and in the regularly published list under the heading of “Marking.” The listing of oil burners as standard does not mean that the products listed are necessarily equivalent in quality or merit. Any question with regard to the effect of the installation of an oil burner on insurance rates should be taken up with the company insuring the property or with local authorities having jurisdiction.
In addition to the safety of the burner itself there are to be considered the matters of placing tanks, installation of piping, the burner proper, and in some cases the electric wiring. In handling all of the items the installer should have in mind the National Board of Fire Underwriters’ rules and the national electric code.
The underwriters’ designation on a burner is looked for when considering the device from the standpoint of a fire hazard, but a burner should not arbitrarily be eliminated because it does not bear such approval. There are undoubtedly many burners not listed by the underwriters which present just as little fire risk as those listed. As a matter of fact many of the burners have not been submitted to the underwriters for test and still others may be on the waiting list.
Faulty Installation and Adjustment Cause Fire
Investigations made by the Department of Agriculture have disclosed the fact that a great number of the oil-burner fires can be traced to faulty installation and adjustment rather than inherent weakness in the burner.
Safety is of paramount importance in any device to be used in the home and, regardless of any advantages claimed, no one would be justified in installing a burner that is not reasonably safe. The Underwriters’ Laboratories have tested and listed many of the burners now manufactured as complying with certain standards of minimum hazard. Burners that have been so tested bear a mark indicating the fact. However, the absence of such marking does not necessarily mean that a burner does not comply with the requirements of the Underwriters’ Laboratories since there are on the market a number which have not been submitted for test or which are now on the waiting list,