US2146410A - Heater for furnaces and method of operating the same - Google Patents

Description

. w39. -AHVAUGHAN l 2,146,410

`HEATER FOR FURNACES AND METHOD OF' OPERATING THE SAME Filed oct. ze, 1935 s sheets-sheet i HA; aan

Feb. 7, V1939. A. H. v/xLlGl-lm` 2,146,410

HEATER FOR FURNACAES AND METHOD OF OPERATTNG THE SAME Feb. 7, 1939'.

A. H VAUGHAN HEATER FOR VF'URNACES AND METHOD OF OPERATING THE SAME 3 Sheets-She'et 3 Filed Ost.4 26, 1935 l xNvENToR UNITED STATES PATENT OFFICE? HEATER FOB FURNACES AND METHOD OF OPERATING THE SAME Arthur H. Vaughan,'Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Application October 26, 1935, Serial No. 46,911

5 Claims. (Cl. 126-91) This invention relates generally to heat treating furnaces, and more particularly to heaters for such furnaces and to a method of operating such heaters.

Heat treating furnaces are often supplied with a protective atmosphere for preventing oxidation of the work during its heating and cooling. -Many of such furnaces containing a protective atmosphere in contact with the work being heated are heated by electrical resistors, since the use of fuel fired furnaces in which the products of combustion come into contactA with the work would lead to oxidation of the work. There are many cases, however, where it is advisable from 'i an economic standpoint to employ a combustible fuel rather than electricity for heating the furnace. It has been proposed heretofore to provide the heating chamber of the furnace with a double surface heat exchanger, usually in the form of a combustion tube, and to burn a mixture of fuel such as gas, and air within the heat exchanger in order to heat the furnace. present invention relates to this type of furnace in which the heating is accomplished by the combustion of fuel in a double surface heat exchanger or combustion tube.

In the accompanying drawings which illustrate by way of example two embodiments of my in' vention,

Figure 1 is a front elevation, parts being shown in section, of a heat treating furnace comprising a heating chamber and a cooling chamber and embodying my improved means for heating the heating chamber;`

Figure 2 is a transverse vertical section taken on the line II-II of Figure 1;

Figure 3 is a partial transverse vertical section taken in the same plane as that shown in Figure 2, but illustrating the double surface heat exchanger on an enlarged scale; and

Figure 4' is a partial horizontal longitudinal section through the furnace showing a modified form of the invention in which the heat exchanger is U-shaped an'd has both its inlet and outlet ends extending through the same wall of the furnace. v

ft has been proposed heretofore to supply an intimate mixture of fuel and air to one end of a combustion tube placed Within' a furnace and to cause the products of combustion to traverse the tube. In such cases, particularly where the combustion tube is ci' considerable length, it is difficult, if not impossible, to liberate the heat of combustion uniformly throughout the length of the tube, and this results in uneven heating The liberate heat. In this method, burners of the diffusion type are employed producing a long slow burning flame, the fuel andair passing through the tube at a low velocity. The fuel and air mix slowly and therefore burn slowly; but there is no positive control of the combustion reaction which takes place within the tube. Accordingly, it is dicult, vif not impossible, to produce uniform liberation of heat by thev combustion process if uniform distribution of heat is desired,A or to regulate the distribution of heat in different parts of the tube where that is desired. As a result, certain parts of the tube heat to a greater extent than other parts and, accordingly, deterioriate more rapidly.

My invention enables me to cause liberation of heat throughout the entire length of the tube if that be desired, or to control the liberation of heat along the lengthv of the tube so as to produce either uniform heat liberation or` a local excess of heat, as may be desired. Generally speaking, this is accomplished by introducing fuel such as gas, mixed with a deficiency of air into the interior of adouble surface heat exchanger or combustion tube. The products of partial vcombustion flow longitudinally through the tube,

mitted augments the air originally present in the mixture and enables substantially complete combustion of the fuel to take place. The secondary air is admitted at predetermined points and in predetermined amounts to the fuel and partial products of combustion as they flow along the tube. This produces a gradual increase in the burning of the mixture throughout the length of the tube. It provides for positive control of the points at which the secondary air is admitted and for the amount of secondary air. The process of combustion can therefore be accurately controlled so as to liberate heat where desired, in contradistinction with the method wherein a diffusion type burner is employed and in which the control of the burning process ceases upon the introduction of the fuel and air into the combustion tube.

Referring more particularly to the accompanyn .ing drawings, in which the invention is illustrated in connection with a heat treating furnace, there is shown a furnace indicated generally by the reference numeral 2 and comprising a heating section 3 and a cooling section 4. The heat treating furnace is of conventional type, the heating section 3 having a shell 5, an inner refractory lining 6, and an intermediate lining 1 of thermal insulation. The cooling section 4 is provided with a water cooled jacket 8. The work to be u bustion tubes. Each of these tubes is supported by a strap I6 and a hanger rod I1 extending through the roof of the furnace. Each of the combustion tubes comprises a main bod'y portion I8 having a length slightly greater than the width of the furnace chamber. An inlet pipe Il is connected to one end of the tube I5. 'I'he other end 20 of the inlet pipe I9 extends into a chamber 2I formed by a casting 22 provided with a cover 23. The connection between the inlet pipe and casting 22 is sealed by a gland 24 and packing 25, the gland being adjustable by means of a screw 23. The gland 24 is provided with openings 21 so that a mixture of fuel and air supplied through the pipe 23 can enter the inlet pipe I3 and thus Dass into the combustion tube.

The tube I5 is provided at its leit-handend as viewed in Figure 2, with an enlarged portion 33 which receives a helical heat absorber 3| best shown in Figure 3. The combustion tube is held ilrmly in place in the opening 32 by providing a bead 33 on the portion 30, which bead seats against a ring seat 34 and is held in place by a ring clamp 35. An elbow 33 connected to the end of the tube section 3l acts as a chimney for the products of combustion escaping from the com bustion tube.

A secondary air tube 31. located inside of the combustion tube I3 is in communication at its outer end 33 with the interior of the heat absorber l 3l. The secondary air tube is closed at its inner end 33 and is provided along its length with orices 40 for supplying secondary air to the space 4I between the tubes 31 and Il. The tube 31 is provided with feet 42 for spacing it from the combustion tube. Secondary air is supplied to the interior of the heat absorber through pipe 43.

The products of combustion, before passing out of the chimney 33, ilow along the helical passage 44 between the outer surface of the heat absorber 3| and the inner surface of the enlarged portion 33. The interior of the heat absorber is provided I with a core 4I closed at its ends and which forms with the walls of the absorber 3|, a helical passage 43 for the secondary air supplied through pipe 43. In this manner, heat from the products of combustion is transferred to the incoming secondary air as it passes through the heat absorber, and this heated air is thereafter delivered through the orifices 43 into the space V4I for combining with am unburned portions of the mixture of air and fuel.

Any suitable arrangement may be employed for required for perfect combustion. During normal 2 supplying a mixture of fuel and air to the combustion tube l5 and for supplying secondary air to the secondary air tube. One suitable means is illustrated in Figure 2. Air under pressure is supplied through a. pipe provided with a valve 5 5I. The air passes through a metering orifice 52 to an inspirator 53, to which is connected a gas pipe 54. The mixture of gas and air flows from the inspirator through pipe and branch pipes 56, each of which delivers the mixture to the 10 chamber 2i associated with one of the combustion tubes. The secondary air admitted into the combustion tube through pipe 43 is supplied y through pipes 51, 53 and 59. A portion of the air admitted into pipe 50 ows through e. meter 1,; ing orifice and a three-Way Valve (Si to the pipe 58. A by-pass pipe 62 is connected at one end tcthe valve 6I and at the 'other by a 'l' G3 to the pipe 55. Air normally hows through the pipe 62 orilg7 during the starting oi furnace, 2G as hereinafter described.

In operating the furnace, the inspirator 53 is adjusted to deliver a very rich mixture of gas and air, usually containing about 50% of the air operation of the furnace, this rich mixture enters the combustion tube I5 through inlet pipes i9 and burns inside the tube with liberation of a part of the potential heat of the gas. Generally, I use a mixture as rich as can be burned in the presence of heat Without depositing carbon. The products of partial combustion still contain relatively great fuel value and this is utilized by. causing further combustion to take place with liberation 0f additional heat by supplying-air through the 3J secondary air tubes 31 from which it issues through the orifices 40 and mixes with the partially burned gases flowing along the combustion tube. The waste gases, after being subjected to approximately complete combustion, escape fio through the end portion 30 of the combustion tube and the chimney 38. The air entering the secondary air tube is considerably heated by conduction through the walls of the secondary air tube and through the walls of the hes-.t absorber 3|. This results in the recovery and. reuse of heat from the waste products and thus reduces the amount oi fuel needed under given en? itions. For lighting up a cold furnace, spe dure is necessary as the rich mixture referred to will not usually burn exccp 11:1 c. surroundings. Accordingly, I place the threeway valve 8i in position to cause the air passing through the orifice 3B in the upper portion of pipe 51 to re-join the main stream at the T I3, thus providing a mixture in about the proportions for perfect combustion. The valve 5I is then ad- Justed to give a relatively small iiow and the mixture is ignited at the opening of the chimney 33. The flame burns back into the combustion tube 60 and finally reaches the discharge opening 65 of the inlet pipe I3. This pipe is proportioned so as lto give a sufficiently high velocity of mixture to -tube is employed having both legs extending,

throughopeningsinthesameturnaoewall. Inu' this embodiment, a fuel-,rich mixture of gas and air is introduced through pipe Il and flows through a fitting 1`l located in the left-hand end of the leg 12 of the combustion tube indicated generally by the reference numeral 13. A. secondary air tube 'Il extends through the fitting 1| and into the leg 12. The mixture of gas and primary airflowing through the fitting 1i passes through an orifice 1i formed between the fitting and an enlargement 'Ii on thesecondary air tube 1I. After the mixture of gas andair has passed into the leg 12, it is supplied with secondary air issuing through orifices 11. Another secondary air tube 'Il provided with orifices 19 is arranged in the legI Ill of the combustion tube. The tube 18 provides additional air for completing the burning of any unburned portions of gas passing into the leg Il. The products of substantially complete combustion pass the heat absorber Il, which is identical in construction with the Vheat absorber 3| shown in Figure 3, and escape through the chimney I2. The secondary air supplied to the tube 10 is heated by passing through the heat absorber before it is delivered from the orifices 18, as previously described.

While I have vshown and described certain means for supplying the combustion tubes with a mixture of fuel and air and for introducing secondary air, any other suitable arrangement may be employed. 'Ihe distribution of air orifices in ythe secondary air tubes may be made to suit particulary requirements, depending upon the characteristics'of the fuel used and upon the distribution of heat input desired. Even without physical changes in the apparatus, I have found it possible to adjust the rates and distribution of heat input through a considerable range over the length of the tube by adjusting the amount and ratio of gas-air mixtureand the amount of secondary air admitted. The invention provides for the positive control of the process oi combustion in the combustion tubes so that heat may be liberated as and where desired.

I have illustrated anddescribed two embodiments of my invention, but it will be understood that the invention may be otherwise embodied` or practiced within the scope of the following claims.

i' claim:

l. Heating means forfumaces comprisingatube type heat exchanger having an inlet in communication with fuel and air supplies and an outlet' for the discharge of products oi' combus-` tion, a secondary air tube extending through the outlet of the heat exchanger for supplying secondary air to the space between it and the heat exchanger, and means in the outiet of the heat exchanger for transferring heat from the products of combustion to the incoming secondary ail'. I

2. In a heat treating furnace, walls enclosing a heating chamber, a combustion tube traversing said heating chamber, a portion of the combustion tube within the heating chamber being imperforate, conduits communicating with the interior of said combustion tube at opposite ends thereof and extending exteriorly of said walls, means for introducing fuel with a predetermined amount of primary air insufilcient for complete combustion through one of said conduits and flowing products of vpartial combustion toward the opposite end of the combustion tube, a secondary air tube extending through the other of said conduits, the secondary air tube extending through the major portion of the effective length of the combustion tube and being provided with perforations throughout substantially all of its length within the eifectiveportion of the combustion tube, means for supplying through said secondary air tube predetermined amounts of secondary air for progressive further combustion of said products while so traveling, and means for varying the amounts of fuel, primary air and secondary air in substantially constant ratio to each other.

3. Heating means for the chamber of a heat treating furnace, comprising a combustion tube having its interior sealed from the atmosphere of the furnace chamber, means for introducing a mixture of fuel and a deficiency of air to said combustion tube adjacent one end thereof, for

partial combustion adjacent said end and for causing the products of partial combustion to ow toward the opposite end of the combustion tube, means for equalizing temperature throughout substantially the length of the combustion tube comprising a secondary air tube extending into the combustion tubethrough said opposite end, the secondary air tube extending through the major portion of the effective length of the' combustion tube andv being provided with Vperforations throughout substantially all of its counter-current to the now of products of coni-V bustion.

4. The process of burning a fuel which comprises intimately mixing the fuel with an amount of primary air insufficient for complete combustion, introducing the mixture into an elongated combustion zone adjacent one end thereof, fiowing the products of partial combustion toward the opposite end of the combustion zone and supplying secondary air to the combustion zone ina direction counter-current to the flow of the products of partial combustion and `along the major length of the combustion zone for the progressive further combustion of said prod-ots while maintaining the combustion sone out of communication with the atmosphere outside it.

5. The process of burning a fuel which comprises intimately mixing the fuel with primary air in` an amount approximately 50% lof that required for perfect combustion, introducing the mixture into an elongated combustion zone adjacent one end thereof, bowing the products of partial combustion vtoward the opposite end of f the combustion zone and supplying secondary air to the 'combustion zone inv a direction countercurrent to the now of the products of partial combustion and along the maior length of the combustion zone for the progressive further combustion of said products while maintaining the combustion zone out of communication with the atmosphere outside it.

ARTHUR n. vandaan.

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