US3443799A - Air heating systems - Google Patents

Description

y 13, 1969 D. H. THORBURN 3,443,799

AIR HEATING SYSTEMS Filed Dec. 19. 1966 Sheet of 2 lNVLiN'IOA. DONALD H. THORBURN BYMI/7MA PATENT AGENTS y 1969 D. H. THORBURN 3,443,799

AIR HEATING SYSTEMS Filed Dec. 19. 1966 Sheet 2 of 2 FIG.2

IN VEN'] 0R. DONALD H. THORBURN BY My PATENT AGENTS United States Filed Dec. 19, 1966, Ser. No. 692,832 Int. Cl. F23l 9/04; F24d /04 U.S. Cl. 263-19 10 Claims This invention is concerned with improvements in or relating to air heating systems, and especially to such systems of the so'called direct-fired type, wherein the products of the combustion that produces the heat are released into the heated air.

A particular application of a direct-fired system in accordance with the invention involves its incorporation in the ventilating system of an enclosed space, such as a mine. In such a ventilating system a minimum quantity of air must be supplied at all times, to ensure adequate dilution and/ or removal of noxious gaseous products that are produced, for example, by seepage from the surrounding strata and/ or operation of the mine equipment. This air is therefore taken from the ambient atmosphere, passed through the mine, and thereafter is returned to the atmosphere without recirculation. It is a requirement to supply the air to the space to be ventiltaed at a temperature not lower than about 35 F., and heating means must be provided, preferably automatically operated, to heat the air when its temperature falls below the required value of about 35 F. A direct-fired system generally is the most economical in first cost of equipment, and fuel consumption, but, since the heated air is breathed by the occupants of the mine, such systems may not be used unless it can be shown that the heated air contains no detectable amounts of noxious gases, particularly carbon monoxide which is cumulative in its effects.

It is therefore an object of the present invention to provide a new direct-fired air heating system. It is a particular object to provide a new direct-fired air heating system which is capable of operation so that heated air does not contain an amount of carbon monoxide detectable by the commercial detectors generally available at the present time.

One of the special problems often encountered with direct-fired air heating systems is that of maintaining stable flame conditions under the widely varying ambient air conditions encountered. For example, in a mine ventilation system as referred to above, it frequently is known for combustion air taken from the ambient air to be at temperatures of 40 F., or lower. To avoid the production of noxious gases it is preferred that the air supply be as generous as possible, and this is very effective over the upper part of the ambient temperature range encountered. However, at the lower end of the range the large volumes of extremely cold air supplied to the burner may be suflicient to cause partial, or even complete, quenching of the flame, causing ineflicient erratic operation, and frequently the production of the unwanted noxious gases that the system was intended to eliminate.

It is therefore a further object of the invention to provide a new direct-fired air heating system which is opera tive without appreciable flame quenching over the full ambient temperature range encountered.

In accordance with the present invention there is provided a direct-fired air heating system comprising a burner, means for supplying to the said burner a fuel gas/ primary air mixture of a gas/air ratio to permit at least substantially complete combustion of the fuel gas in said primary air, shroud means receiving the combusted gases from the burner and confining them in a combustion path capable of ensuring complete combustion of said fuel gas,

and means for supplying secondary combustion air to the said combustion path.

A direct-fired air heating system which is a particular preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein:

FIGURE 1 is a general perspective view of an air moving and heating unit intended for use in a ventilation system, and

FIGURE 2 is a section through the heating unit only, taken on the line 22 of FIGURE 1.

The complete air moving and heating unit of the ventilation system comprises an enclosing structure 10, divided by an internal wall 11 into a control chamber 12 and an air chamber 13; ambient air enters the latter chamber through inlet air louvres 14 in one end wall 15, and discharges therefrom through an outlet port 16 in the opposite end wall 17 to a conduit (not shown) by which it is led to space to be ventilated. The air moving unit is of any conventional form comprising (for example) a variable pitch fan 18 driven by a motor in conventional manner.

The heating unit comprises an air/gas mixer 19, and a burner 20, the mixer being fed with primary combustion air via a pipe 21, a motor blower 22, and another pipe 23, the inlet to the pipe 23 being located in the chamber 13 downstream of the heating unit. Fuel gas is supplied to the mixer through a sulpply system of any conventional type, indicated herein by a digrammatic block 24 disposed in control chamber 12, that will permit the reqired wide range control of the burner. Fuel gas is fed to this supply system via an inlet pipe 25 and thence by pipes 26 to the mixer. The electrical portion of the control system, which supplies power to the blower 22 and various valves of the system under the control of a thermostat (not shown) in the outlet 16, is indicated diagrammatically by the cabinet 27 in the control chamber.

As will be understood by those skilled in the art, the controls indicated by 24 and 27, are arranged to maintain in safe stable operation the number of burner tunnels necessary to provide the required amount of heat, as indicated by the sensing instruments, to maintain the output air temperature substantially constant in the neighborhood of the said minimum value. The usual safety devices will also be provided therein to take account of too high or too low fuel gas pressures, absences of flame upon ignition, failure of primary air supply, automatic purging etc.

The rate of flow of primary combustion air from the blower 22 is controlled by a butterfly valve 28, operated by a motor 29 under the control of the electrical system 27. This rate of air flow is correlated with the rate of flow of the fuel gas so that at all times the mixture supplied to the burner 20 has an air/ gas molecular ratio greater than 1:1 (e.g. a volume ratio greater than 32:1 for butane, 25:1 for propane, 10:1 for natural gas, etc.) and preferably about 1.05:1 or 1.10:1 to provide a small excess of air and ensure complete combustion of the fuel gas. As mentioned above, operation of the burner is not required until the entering air temperature drops below about 35 F., and it is not uncommon for its temperatures to drop to -40 F., or even -60 F. However, since this combustion air is obtained downstream of the heating unit, its temperature cannot be less than the output temperatures maintained automatically by the unit, and a stable, completely combustible mixture is always obtained. Since both the gas and electric control apparatus may be adversely affected by the lower range of temperatures encountered they are located in separate control room 12, which is heated to some predetermined temperature by supplementary heating means, such as 30, and/or by bleeding air near the outlet 16 into the chamber 12.

With this object of obtaining stable flame conditions and combustion, the burner 20 is of the type which provides one or more refractory-lined burner tunnels in which the combustion takes place; upon ignition of the fuel/air mixture in any of the tunnels, the tunnel quickly reaches an elevated temperature (e.g. usually about 2200 F.- 2400 P.) which ensures maintenance of stable combustion. At this time the gas/air mixture is not subjected to the effect of the main air stream, and accordingly the combustion is not affected by changes in the temperature of the stream.

The combustion gases from the burner 20 are discharged as a cylindrical stream into an elongated cylindrical shroud means 31 generally parallel to the longitudinal axis thereof, which axis is also generally parallel to the direction of flow of the air between inlet 14 and outlet 16. As illustrated, the header 20, the burner 19 and the shroud are mounted together on a metal framework 32, spaced from the floor of the chamber, so as to be surrounded completely by the air stream. The dimensions of the shroud transverse to the air stream are larger than the corresponding dimensions of the burner 19, so that an entry port 33 of generally annular form is formed leading to the interior of the shroud. The shape and arrangement of this entry port is such that air from the air stream entering the shroud is directed in the form of an annular stream which is interposed between the said cylindrical gas stream discharged from the burner and the inner cylindrical shroud wall.

Some of the air from the said annular stream is mixed with the burner combustion products to constitute, in effect, a secondary air supply therefor, and to further ensure that complete combustion takes place. Moreover, the length of the shroud is made such, in relation to the rate of flow of the gases through it, and to the rate of combustion propagation of the fuel/air mixture, that the resultant combustion path is long enough to ensure combustion is completed before the gases have travelled its full length. Preferably, the combustion is completed before they have travelled from /s to /4 of the said length. By use of such shroud means, in a particular example, the confined combusted gases were readily maintained at a temperature of about 16001700 F. for the full length of the shroud means, which is sufliciently above the usual minimum range of ignition temperatures of 950l200 F. for full combustion to take place. This suitable elevated temperature and consequent absence of quenching was obtained, along with the desired admixing of some secondary air, and despite the admission to the shroud of air over the said lower range of temperature.

By admitting the additional air in the form of an annular stream enclosing the combustion path, it has been found possible to use this stream to shield the shroud means from the hot gases of the path. In the said particular example, despite the maintenance of the said gas temperature of 1600-1700" F., the shroud temperature remained at about 1000-1100 F. It has therefore proven possible to use structural materials for the shroud means that might not withstand the higher temperatures of the combustion path, or might be more troublesome and expensive, both as to first cost and subsequent maintenance. In particular, it has been found possible to use a simple unlined tube of heat-resisting alloy steel, e.g. stainless steel, as the shroud means; at the temperatures of operation of the tube its life should be indefinite, whereas an expensive refractory lining of much shorter life would be required to withstand the direct action of the temperatures of the combustion path. The avoidance of the need for a refractory lining is of special importance in the application of the invention, in that the inevitable deterioration in use of such a lining is often accompanied by unstable combustion conditions that sometimes results in the production of noxious gases.

The gases discharging from the shroud means impinge on a diffusion baflie 34, also of a heat-resisting alloy steel,

which diverts them and directs them into the main air stream, so that all the streams are thoroughly mixed and a substantially uniformly heated stream of ventilating air passes out of the exit 16. The size and location of the baffie 34 is best determined by experiment for each particular installation.

In the said particular embodiment the burner 20 was of the refractory lined tunnel type, was capable of handling a flow of fuel gas to give a maximum heat output of about 5 million B.t.u. per hour, and was capable of maintaining stable combustion over a range of fuel input of at least :1, and usually :1, when supplied with the proper mixture of fuel and primary combustion air. The shroud was about 64 inches long and about 38 inches inside diameter, with an annular inlet port 33 of 4 inches radial dimension. With such a unit in full operation, and with a flow of ambient air of 65,000 c.f.p.m. at 40 F., completely stable operation was obtained and it has not been possible to detect carbon monoxide in the issuing ventilating air with the detection equipment at present generally commercially available. (i.e. with a sensitivity of 1 part in 1 million).

It is to be understood that a particular embodiment has been described herein, and that various changes and modifications thereto will be apparent to those skilled in the art, within the scope of the appended claims.

What I claim is:

1. A direct-fired air heating system for heating a stream of air comprising a burner discharging the combusted gases produced thereby into the air stream, means for supplying to the said burner a mixture of fuel and primary air of a ratio to permit at least substantially complete combustion of the fuel air mixture in the said primary air, shroud means receiving the combusted gases from the burner, confining them in a combustion path capable of ensuring complete combustion of said fuel air mixture and discharging them into the air stream for mixing therewith, means for supplying secondary combustion air to said shroud combustion path, means for controlling the said burner to maintain at a predetermined value the temperature of the mixture of the air stream and the combusted gases discharged therein from the shroud means, and means for obtaining heated primary air for the burner downstream of the discharge of the shroud means into the air stream.

2. A system as claimed in claim 1, wherein the said burner is of the refractory-lined tunnel type, and is capable of maintaining stable combustion over a fuel input range of at least 25:1.

3. A system as claimed in claim 1, wherein the said shroud means comprises an unlined elongated cylinder of a heat-resistant metallic sheet material having the said burner mounted adjacent one end thereof to form between itself and the inner wall of the shroud means an annular opening through which the said second air flows in the form of an annular stream enclosing the combusted gases from the burner and interposed between the last-mentioned gases and the shroud means.

4. A system as claimed in claim 1 and comprising a first chamber enclosing the said burner and shroud means, and an outlet from the first chamber downstream of the burner and shroud means, and wherein the said primary air obtaining means comprise pipe means connected between the said outlet and the burner and feeding heated air from the outlet to the burner.

5. A system as claimed in claim 1, and comprising a first chamber enclosing the said burner and shroud means, and an outlet from the first chamber downstream of the burner and shroud means, and wherein the said primary air obtaining means comprise pump means connected between the said outlet and the burner and pumping heated air from the said outlet to the burner 6. A system as claimed in claim 1, and comprising a first chamber enclosing the said burner and shroud means, and an outlet from the first chamber downstream of the burner and shroud means, and wherein the said primary air obtaining means comprise pipe means connected between the said outlet and the burner and feeding heated air from the outlet to the burner, and means in said pipe means for controlling the flow of air to the burner to control the said ratio of fuel and primary air.

7. A system as claimed in claim 1, and comprising a first chamber enclosing the said burner and shroud means, a second chamber immediately adjacent the first chamber, heating means in the said second chamber for maintaining the contents thereof within a predetermined range of temperature, an outlet from the first chamber downstream of the burner and shroud means into the second chamber, wherein the said primary air obtaining means comprise pump means in the second chamber connected by pipe means between the said outlet and the burner and pumping the heated air from the said outlet to the burner, the said pump means and pipe means being maintained by the heating means within the said predeterimned range of temperature.

8. A system as claimed in claim 1, and comprising a first chamber enclosing the said burner and shroud means, a second chamber immediately adjacent the first chamber, heating means in the said second chamber for maintaining the contents thereof within a predetermined range of temperature, an outlet from the first chamber downstream of the burner and shroud means into the second chamber, wherein the said primary air obtaining means comprise pump means in the second chamber connected by pipe means between the said outlet and the burner and pumping heated air from the said outlet to the burner, and control means in the second chamber between the said outlet and the burner for controlling the flow of air to the burner to control the said ratio of fuel and primary air, the said pump means, pipe means and control means being maintained by the heating means within the said predetermined range of temperature.

9. A system as claimed in claim -1, and comprising a first chamber enclosing the said burner and shroud means, a second chamber immediately adjacent the first chamber, heating means in the said second chamber for maintaining the contents thereof within a predetermined range of temperature, and control means for the said burner disposed in the said second chamber and maintained by the said heating means within the said predetermined range of temperature.

10. A system as claimed in claim 1, and comprising a first chamber enclosing the said burner and shroud means, a second chamber immediately adjacent the first chamber, heating means in the said second chamber for maintaining the contents thereof within a predetermined range of temperature, the said primary air obtaining means being disposed in the said second chamber, and control means for the said burner disposed in the said second chamber, the said primary air obtaining means and control means being maintained by the said heating means within the said predetermined range of temperature.

References Cited UNITED STATES PATENTS 2,654,219 10/1953 Zaba. 2,879,837 3/1959 Downs. 3,101,193 8/1963 Varvel. 3,240,478 3/1966 Marks. 2,518,364 8/1950 Owen.

JAMES W. WESTHAVER, Primary Examiner.

E. G. FAVORS, Assistant Examiner.

Claims (1)

1. DIRECT-FIRED AIR HEATING SYSTEM FOR HEATING A STREAM OF AIR COMPRISING A BURNER DISCHARGING THE COMBUSTED GASES PRODUCED THEREBY INTO THE AIR STREAM, MEANS FOR SUPPLYING TO THE SAID BURNER A MIXTURE OF FUEL AND PRIMARY AIR OF A RATIO TO PERMIT AT LEAST SUBSTANTIALLY COMPLETE COMBUSTION OF THE FUEL AIR MIXTURE IN THE SAID PRIMARY AIR, SHROUD MEANS RECEIVING THE COMBUSTED GASES FROM THE BURNER, CONFINING THEM IN A COMBUSTION PATH CAPABLE OF ENSUREING COMPLETE COMBUSTION OF SAID FUEL AIR MIXTURE AND DISCHARGING THEM INTO THE AIR STREAM FOR MIXING THEREWITH, MEANS FOR SUPPLYING SECONDARY COMBUSTION AIR TO SAID SHROUD COMBUSTION PATH, MEANS FOR CONTROLLING THE SAID BURNER TO MAINTAIN AT A PREDETERMINED VALUE THE TEMPERATURE AT THE MIXTURE OF THE AIR STREAM AND THE COMBUSTED GASES DISCHARGED THEREIN FROM THE SHROUD MEANS, AND MEANS FOR OBTAINING HEATED PRIMARY AIR FOR THE BURNER DOWNSTREAM OF THE DISCHARGE OF THE SHROUD MEANS INTO THE AIR STREAM.

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