An IMPROVED DESIGN
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J. .M. KRAPPE Purdue University, Lafayette, Indiana
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UR efforts to improve the operation of gas torches have all been directed along the lines of making their operation more flexible and convenient for the user. The experimental work was done with various types of torches, some of them especially converted to use natural gas and others of entirely new design. As a result of this work, a new variety of torch having a combination of features not found on"any torch available on the market was developed. With regard to types of flame, we desired a torch which would be as flexible as it is possible to make it. It was realized that every type of flame, from a sharp needle-point flame to a large brush flame, was required to handle the work being done in a typical laboratory. Furthermore, the torch had to have flexibility with regard to flame temperature, since all types of glass, from those having a very low softening point to those having the highest softening point, were being used for the purpose of repairing old glass apparatus, or for building new apparatus. The performance of the new design showed that it was possible to obtain good flames with air and gas alone and higher temperature flames with air, gas, and oxygen mixtures. ~. 1 Torch designed by J. M. Krappe. Associate in Gas Engincerm g and F. I. Mer"tt, Department of Chemistry, Purdue University.
FLAMES FOR WORKING GLASS
The principal use found for laboratory torches is in the field of glass working. A wide variation in types of flames is required to perform the various operations. For example, when it is desired to make a glass "T," a sharp needle-point flame is essential. This flame heats a portion of the tube about one-fourth of an inch in diameter with the hottest spot a t the center. The glass is heated to a higher temperature than that required for ordinary forming, and a hole is blown in the wall of the tubing. The large bushy flames are required for heating large areas and for forming large-size tubing. The importance of being able to obtain a flame temperature only slightly higher than the working temperature for the particular kind of glass being used can be readily appreciated. If a very hot flame, such as that given by a gas-oxygen torch, is used, the outside surface of the glass melts and runs before the heat has soaked into the interior. This results in the work being spoiled or very poorly shaped. It is essential to have the glass evenly heated before i t is formed or worked, and for this purpose a "soaking heat" is preferred. Table 1 shows the softening tem~eraturesof various types of glasses. For each main class there is a range temperatureS since a pf in composition results in a slightly different softening
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temoerature. However, the main classes of elass materialsmay be seen from the table; and the softening temperature, although lower than the actual working temperature, gives. an. indication of the relative flame temperatures requlred TABLE 1 SOITBNINO TSMP=XATU.BS OF V*ErOUS
GLASPZO
D~grrslC.
Dcgr~lrP.
FEATURES OF THE NEW TORCH
~h~ advantages of the improved construction include flame stability and the ability to either a needleDoint flame or a larEe bushv flame as desired, With ihe torch developed; small- flames were obtained as follows: a one-eighth-inch diameter needle flame with air and gas alone; a one-sixteenth-inch diameter needle flame with air, gas, and oxygen. With gas and oxygen
Lend glass= Limcsadn g l a o w Bo-ilicate glasses Barium glaiuer "None." glass* -Pyrex'' glass* Purcd ~~- silica., elear
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* Temperafvre~at which the logarithm of the vineosityir about 4.5,
r
The flame temperatures obtained on the Purdue torch, starting with a soft air-gas flame, are shown in Table 2. The air-gas-oxygen flame with only a slight amount of oxygen was sufficiently high in temperature to work pyrex glass. In fact, actual experience with
Air-gas name. soft. no inner mnr* Yellow flame* Air-aar flame. distinct inner eooe t Air-gas-oxygen flame, distinct iooer eooe; ratio, air to oxygen. 28 to 1 Airgas-oxygen flame, distinct iooer cone: ratio, air t o oxygen. 4 to 1
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400-840 920 1580-1590
760-1550 1680 2870r-2900
165CL1730
3000-3150
Above 1820 Above 3300
*
Measured with a chromel vs. alumel thermocouple. t Measured with a platinum, platinum-rhodium thermocouple. The thcrmacouole bead was inserted in the name just above tiD of inner cone for the high temperature flmmes. The range in flame temperatures given mey he attributed h part t o variations in placing the thermocouple in the Same and in part to the variation in velocity of the burning air-gas mixture. It was noted that high velocities tended to give higher temperatvrcr due to the higher rate of heat tramfpr to the thermocouple bead.
the torch showed that the use of excessive amounts of oxygen is undesirable. The last flame described in the table, that is, the one having an air-oxygen ratio of 4 to 1, was too hot for most operations on pyrex glass. FLAMES FOR BRAZING
Brazing alloys having melting points ranging from 1200°F. to 2100°F. are available for the joining of metals. An air-gas flame has sufficiently high temperature to melt the low temperature brazing alloys, but the melt in^ of the alloy is not the principal . -problem in this opera&. When the alloy is in contact with cold work, higher flame temperatures are required, since heat is conducted away from the flame into the work. Consequently, gasoxygen and air-gas-oxygen flames are required to heat the object being brazed. In the soldering operation it is necessary to preheat the work thoroughly before applying the solder. For this reason a large brush flame is generally employed. The ratio of air to oxygen required may be adjusted, depending on the size of the work being done. Light weight metal would not require as high a flame temperature as heavy sections.
alone, the diameter of the flame is a little over one sixteenth of an inch, but this flame has a very high temperature. Thus, the torch is suitable for doing the finest up to fairly large pieces. of glass work. The torch is also very flekible. I t may use any one of three mixtures without changing the burner head. I t is possible to use air and gas only; air, gas, and oxygen; or gas and oxygen, depending on the type of work being done. In general, higher flame temperatures are obtained by using large proportions of oxygen in the mixture. The sharp needle-point flames are used for spotheating small areas and have smooth contours showing a clear, distinct inner cone. This contrasts with the soft flame, which is less desirable due to its low flame temperature and large heating area. All of these types of flames may be obtained easily when using the torch on natural gas. As an additional feature, satisfactory flames may be obtained when holding the torch in almost any position. The flame stability is extremely good. The air valve may be opened suddenly without putting the flame out. In fact, i t is possible to turn the air pressure on full (up to fifteen pounds) without extinguishing the retaining flame. The flame is also very stable when the torch is moved around rapidly, as i t might be in ordinary handling on the job. The torch can be swung in an arc very rapidly and in a rough manner without blowing the flame out. The torch is relatively inexpensive, due to its simple construction. It is light in weight for comfortable handling when used as a hand torch. When used as a
bench torch a swivel or ball and socket stand clamps the torch 6rmly in the desired working position. The flame-head is designed in such a manner that there is no possibility of back-firing during adjustment. The mixture of gas and oxygen, which often gives difficulty on this score, is made a t the tip of the burner, and hence no back-firing can occur.
tention flames. The high pressure of the air-gas mixture is reduced and the mixture a t low velocity passes to the retention flame ports. (2) Purdue Torch.-The Purdue torch has been designed on an old principle of gas burning. A part of the air required for combustion is inspirated in a manner similar to that in an ordinary atmospheric-type gas burner. Thus, when the gas supply is turned on, a
TORCH CONSTRUCTION
( I ) Complete Premixing Torch.-The best picture of the newly developed torch may be obtained by comparing its construction with that of a complete premixing type of torch. A commonly available design has a chamber just in back of the bead in which all the air and gas are mixed. The air and gas pass through a large diameter port around which there are smaller flame retention ports. The mixture comes out of the flame retention ports a t low velocity and helps to keep the large flame from blowing away from the port. A
separate head with a single flame port is furnished for the use of oxygen and gas mixtures. However, neither the oxygen head nor the flame retention head is entirely satisfactory on a mixture of air, gas, and oxygen. When oxygen is used in the mixture, back-firing is very likely to occur during the adjustment. In some cases, this results in the head of the torch blowing off. Furthermore, this type of torch does n ~ give t the short, sharp needle-point flame required for heating very small * areas.
soft blue flame which may he readily lighted exists a t the tip of the torch. The remainder of the air or airoxygen mixture is injected into this soft flame a t high velocity by means of a small air orifice. Figure 3 shows the essential arrangement of such a torch. A needle valve controls the gas supply to the torch, while an aircock or screw-clamp on rubber tubing is used to contra1 the air supply. An air shutter is supplied to control the amount of primary air to the'soft blue flame. The oxygen is mixed in with the air supply by means of a tee in the air line, and its flow is regulated by means of pressure reducer supplied with the oxygen tank. AIR-PRESSURE VARIATIONS
The flame retention head is a definite aid in maintaining the flame on the burner when a long flame is desired. Both the flame retention device and the flame port are designed for a given maximum rate of air flow, and the flame will go out if higher rates of flow are employed. In principle, the flame retention device is a restricting orifice placed in the air-gas mixture supply to the re-
Assuming that the operator has adjusted the torch controls for the type of flame be desires, air-pressure variation seems to be the main source of difficulty thereafter. Sudden variations are more disastrous than a gradual variation, since a sudden change in air pressure may cause the flame to go out. Gradual variations in air pressure also lead to difficulties in delicate glassblowing work. After the operator has adjusted his flame for certain flame characteristics which he knows
are desirable for the work he is doing, a change in air pressure will cause the flame to change its characteristics so that the operation cannot be finished until a readjustment of the controls is made. As long as the pressure variations are not too frequent and too violent it is possible to maintain the same characteristics of flame by making slight adjustments to the needle valve in the gas supply. Apparently, this is the easiest way to make small changes to compensate for variations in air pressure. Of course, the control on the air supply could also be operated, but the adjustment is not as easy to make, due to the higher air pressures available. In other words, a small movement of the air valve results in a very large change in the rate of air flow. As an example, experience with a small dental type of torch may be cited. A small air compressor when set for a low outlet pressure gave unsatisfactory operation due to pressure variation. The flame constantly changed its appearance and occasionally went out. However, when the operator adopted a technic of setting the compressor motor for a higher pressure and throttling the air supply down by means of the needle valve on the torch, his flame became much more stable. The operator now leaves the. needle valve controlling the air supply in a fixed position and obtains a soft, bushy flame or a sharp, pointed flame by merely adjusting the needle valve controlling the gas supply. Some experimental work was performed with contiol valves with the following results. With two control valves in the air line, a pressure variation of six per cent. in the supply was reduced to a variation of only one and one-half per cent. a t the torch. In another test with an entirely different setting of the valves, the pressure variation of six per cent. in the supply was reduced to one-tenth of one per cent. a t the torch. The results show that valves in theline, if throttled carefully, can reduce the pressure variation to almost nothing. In a thud test with an air-pressure regulator, a variation of six per cent. in the supply was reduced to onetenth of one per cent. a t the torch. c Gradual variations in air pressure also lead to difficulties on delicate glass-blowing work. After the operator has adjusted his flame to a fine needle point or for a certain flame appearance which he knows is desirable, a change in air pressure will cause the flame to
change its characteristics so that the operation cannot be finished until a readjustment of the torch is made. In general practice, air is delivered from two pounds to ten pounds per square inch in laboratory air lines. Actually, the pressure required a t the torch head is exceedingly small. For example, the torch using a premixing of air and gas in back of the head requires a pressure of only fifteen inches water column, or about one-half pound a t the torch tip. Ordinarily the high line pressure is reduced successively by a valve in the air line and by the valve on the torch. The high airline pressure, as cited, cannot be considered disadvantageous. Actually, there is a certain amount of pressure variation in service. For example, if one torch is in use on the line, the turning on of additional torches will interfere with the operation of the first one. These variations in pressure are due principally to the sudden drain on the air line when the additional torches are turned on. However, if the initial high pressure is throttled a t the torch. difficulties due to air pressure variation may be reduced. CONVERTED BLAST BURNERS
These laboratory blast burners originally used the nozzle-mixing principle. They were converted to use natural gas and air by huilding a by-pass into the barrel of the burner and gave fairly satisfactory results. This by-pass is an adjustable needle valve which is designed to allow a portion of the compressed air to mix with the gas in the barrel. With this device, satisfactory needlepoint flames may be obtained on natural gas. In comparison with the Purdue torch, i t m y be said that the converted blast burners are not as easily adjusted for the various needle-point flames required. The reason for this is apparent when examination of the construction is made. The amount of air flowing through the by-pass depends on the setting of the main air valve, and hence must be reset with evej-change in flame type. As a result, considerable manipulation of both the main air valve and the by-pass are required before the desired type of flame can be obtained. With the Purdue torch it is easier to adjust for the needle-point flame, since the air-gas mixture going to the burner head is independent of the compressed ajr supply.
