US2815175A - Control system for gas burners - Google Patents

Description

1366- 7 P. F. SWENSON CONTROL SYSTEM FOR GAS BURNERS Filed June 27, 1952 INVENTOR. P401. F SWEA/SOA/ A T7'0/?/V v nited States Patent CONTROL SYSTEM FOR GAS BURNERS Paul F. Swenson, Cleveland Heights, Ohio, assignor, by mesne assignments, of thirty-seven percent to Otto Wanek, two percent to Kenneth J. Kitchen, ten percent to Paul F. Swenson, ten percent to Marcus 0. Swenson, four percent to Myron T. Cooperrider, eleven percent to Wilma G. Stupka, eleven percent to Wilma G. Stupka, trustee, ten percent to J. A. Weeks, and five percent to Earl P. Schneider, trustee Application June 27, 1952, Serial No. 295,850

Claims. (Cl. 236-80) The invention relates to certain improvements in auto matic controls and specifically to an efiicient and safe non-electrical control system and apparatus for gas burners.

The objects, aside from those already indicated, include the provision of an automatic burner control system which operates efl'iciently and safely to obtain, subject to simple adjustment, either complete modulated-heat operation or on-off operation of a burner or a combination of both; a gas burner control system wherein subatrnospheric pressure maintained by flow of the gas through a pilot burner apparatus is utilized effectually and safely fully to control the rate of flow of gas for main burner operation and for shutdown in event of derangement or malfunctioning of the burner apparatus or failure of the gas supply.

The burner mechanism disclosed herein as adapted for control by the present system is on the order of those shown in Swenson et a1, Patent 2,572,675 issued October 23, 1951, and Swenson et a1. application Ser. No. 37,347 filed July 7, 1948, now Patent No. 2,664,153.

Specific objects will be indicated in connection with the following description of the illustrative forms shown in the drawing, wherein:

Fig. 1 is an assemb y, largely schematic and partly sectional view showing the principal parts of the burner and of the elements invol ed in its control.

Figs. 2 and 3 are detail fragmentary, partly sectional, views of a metering valve and pilot thermostat protective feature taken as indicated at 2--2 and 3-3 on Fig. 1.

Fig. 4 is a sectional view showing a type of non-metallic tubing which the present control system is uniquely adapted to utilize as a signal-transmission line.

The gas burner assembly A of Fig. l is arranged for use in an updraft boiler or other combustion unit. A typical installation for such use comprises an upright main burner head B at the base of a combustion chamber structure C, the burner head forming the outlet portion of an L-shaped gas and primary air mixer tube T suitably mounted within a burner enclosure box or air supply duct E connected to the bottom combustion chamber wall around the burner head. All the air needed for combustion is admitted to the duct E through a single air inlet opening controlled by an automatically adjusted damper or door D. Variably adjustable gas metering valve M through a single nozzle orifice 0, see Fig. 2, introduces main burner gas into the flared mouth of the mixer tube, approximately centrally of the inlet end of the tube, as a concentrated substantially constant velocity single stream. Appropriate amounts of primary air from within the box E are thus drawn into the tube T,

are there mixed with the gas and the mixture is con-- trollably admitted to the combustion chamber througha variable flame port F formed between a movable lid or cap L and the adjacent rim of the upright enlarged end portion of the mixing tube. Secondary air from the box or duct E, admitted by damper D, enters the combustion chamber through a sleeve portion Q of the box around the burner head.

Gas admitted through a manually operated cock PG from an assumed regulated source supplies a pilot-light assembly P adjacent to the burner head. Another manual cock G, beyond pilot gas cock PG in the direction of gas flow, admits gas from said source to the inlet side of a burner input or main flow control or throttle valve V, shown as a flexible diaphragm. Valve V is the output element of a vacuum-signalled relay valve mechanism R and, in connection with the illustrated burner mechanism, the valve V acts as a master in that, in accordance with its position, a gas-pressure-actuated slave motor S operates, through a rod S and connected linkage, automatically to adjust or set gas metering valve M, variable flame port F and the air inlet control damper D as will be more fully explained later.

The control system, as shown, includes an aspirator device 10 of the pilot flame assembly P for establishing a partial vacuum in a signal transmitting vacuum line, represented by a tube 11, as a function of gas flow to the pilot burner; various valve devices such as 12, 13 and 14 which are operable to admit air under normal pressure into the tube 11 (vacuum varying or breaker valves) and a vacuum-responsive or signal-receiving spring biased diaphragm mechanism 15 (upper right Fig. 1) which is the input (pilot valve adjusting means) of relay valve unit R in automatically controlling the position of valve V.

The aspirator 10 is incorporated in a fitting 16, a sleeve 17 of which supports a pilot burner tube 18 having a ported pilot flame cap or nozzle piece 19 for maintaining a pilot flame adjacent to the main burner flame port F. One eflective end of the tube 11 is sealed around an aspirator-chamber-iorming bore 19' of the sleeve 17 and, as shown, the tube is communicated with the chamber through several small radial holes 20 intersecting the chamber. Pilot light supply gas issuing through a small orifice 21 at the base of the sleeve 17 draws air from the tube 11 and maintains subatmospheric pressure in the tube proportional to the rate of flow of gas to the pilot burner. Such rate may be assumed to be approximately constant and of the desired value at fully open position of the cock PG. Primary air for the pilot light is admitted through openings 22 in the sleeve 17.

Through the operation of mechanism not yet described the size of the flame at the main burner is maintained proportional to the degree of vacuum effective on the top side of a highly flexible or delicately acting diaphragm 23 forming a movable wall of a vacuum chamber 24 of signal-receiving mechanism 15 of relay R to which chamber a portion of the tube 11 is suitably connected. Diaphragm 23 is biased by a low rate modulating spring 25 in a direction to oppose the vacuum force which tends to lift the diaphragm.

Main flow control or throttle valve V of relay valve mechanism R, as shown, includes a suitable sealing disc 27 on a diaphragm 28 shown in Fig. 1 in closed position on its seat, i. e. the position which blocks movement of gas from the source to the outlet chamber of the valve V from which the gas issues to slave motor S and the metering valve M. A generally closed chamber 30 above the diaphragm 28 of valve V is established, in part, by a partition 31 and its supporting casing walls. A bleeder 32 conducts gas from the inlet side of valve V into the chamber 30 normally to maintain the valve V closed by pressure differential on the diaphragm. The under side of the vacuum-responsive diaphragm 23 carries a sealing plug or pad 36 in valving relation to the upper end of a venting tube 34 for the chamber 30 and which extends into a continually vented or open chamber or space 37. The passage through the tube 34 is continually open into chamber 30 and the lower end portion of the tube is designed to rest on a central rigid portion of the diaphragm 28 of valve V. An elastic double flanged seal 35 around the bent tube 34 blocks flow of gas from chamber 30 along the outer wall of the said tube into venting chamber 37 and also forms a pivotal support for the tube. Chamber 37, in order not to Waste gas vented through the bent tube 34 may be connected to the main burner mixer tube as by a pipe 38.

With the above described arrangement of relay mechanism R, it will be apparent that, as the vacuum-responsive diaphragm 23 is lifted by pressure differential at its opposite sides a sufficient distance to permit venting of chamber 30 through the control or pilot valve port at the top of the bent tube 34 at a rate exceeding the gas conductive capacity of the bleeder 32, the gas pressure below diaphragm 28 opens valve V, and that the bent tube 34 transmits the valve opening motion as feedback to the pilot valve in accordance with well established principles of relay or servomechanism operation- As one example of a detector device which is settable to vary the pressure difference inside and outside of the signal transmitting tube 11, or, for convenience, the relayoperating vacuum in the tube, the instrument 13 is shown as a room-type thermostat having a freely pivoted bimetal lever 1.3a carrying a valving element 13b in restricting or sealing relation to an orifice or control port of the tube (e. g. tube end as shown) until the bimetal element is subjected to a predetermined space temperature rise, and which then opens or varies the eflective area of the port or orifice in proportion to the amount of increase in space temperature. Thus, at a given setting of an adjusting cam 13c, the control port is variably restricted in accordance with space temperature decrease, and the relay-operating vacuum in the entire tube is proportionally increased.

When the vacuum-actuated mechanism 15 is adjusted for full range modulating operation the modulating spring 25, acting downwardly on its associated vacuumresponsive diaphragm 23, as through a flanged sheet metal cup 40, exerts a relatively small downward force sufficient to close the pilot valve port formed by the top end of the venting tube 34, so that, when gas is admitted to the system through cock G, the valve V will be closed by gas pressure action as already described. Now assuming the pilot light gas is turned on at valve PG, producing a partial vacuum in the tube 11 through aspirator 10, and that the bimetal lever 13a of the detector mechanism 13 has closed its vacuum line control port, or has restricted it a certain amount, it is evident that the diaphragm 23 of mechanism 15 will be lifted by atmospheric pressure in the chamber 37 until the increasing modulating spring force balances said pressure force. Thus the opening movement of the valve V is steadily limited by the feedback lever action of venting tube 34, to restrict the pilot valve port. Restricted venting through the tube 34 during modulating operation of the burner continues to take place substantially at the rate gas flows through the bleeder orifice 32.

Gas supplied to the burner mechanism A at a controlled rate through throttling relay valve V causes steady operation of the burner in a manner to satisfy the detected requirement for heat through operation of slave motor S and its connected mechanisms already identified and whose operation will be more fully explained later.

If, in vacuum-responsive mechanism 15, the modulating spring 25, through the intermediary of its supporting cup 40, is permitted to follow all movement of the diaphragm 23, as assumed in the above description, and the spring 25 is properly designed as to rate and scale true modulating burner operation takes place, which is to say that, within the capacity of the burner, only such an amount of gas is admitted through the metering valve as will continue to balance the heat loss in the space being heated. In cases such that, (e. g.) through limitations in fuel quality or pressure or design limitations of the installation served, modulating operating at very low flame setting is undesirable, the control system can be converted to on-01f operation at or below a desired minimum flame setting by temporarily holding the modulating spring out of action so that the vacuum-responsive diaphragm 23 has no effectual stabilizing force acting on it (no variable rate). Settable screw device 50 extending from the casing of mechanism 15 through the cup 40, the screw device 50 having a head portion 51 underhanging the top Wall of the cup, enables setting of the mechanism for such on-off operation as will be apparent. When set for on-oil?" operation the weight of the diaphragm or its weight combined with that of a helper disc 52 assures maintenance of the plug element 36 in sealing contact with the pilot valve port of the venting tube 34 when the burner mechanism is required to be shut down. As depletion of pressure in vacuum chamber 24 of mechanism 15 occurs, the diaphragm 23 snaps into contact with the lower rim of the cup 40, gas is admitted to the burner, and modulated operation then takes place only at higher than the minimum flame setting which would otherwise be obtainable.

Thus the apparatus affords variable on-off operation in combination with modulated operation, or, if desired, modulated operation only.

Instrument 12, is illustrative of one type of limit control which may be used to break the vacuum in control line 11 and terminate burner operation in the case of an emergency. A bellows 12a of a vapor generator, further represented by bulb 12b, operates to push a ball type plug out of sealing relation to a port in a wall of the vacuum line 11 thus causing shutdown of the burner. The ball is self returning to sealing position. The same (vacuum breaking) operation is performed by thermostat device 14 associated with the mixer tube T. Here a bimetal element 14a inside the tube displaces valve ball 14]) out of its port-sealing position in event of flashback of main burner flame into the mixer tube. Shutdown in that case continues only until the bimetal element 14a cools off, and the duration of the cycle is determined by design.

Referring to the burner pilot light apparatus (see Figs. 1 and 3) a safety device 55 associated with the aspirator 10 etc. prevents commencement of burner operation in event the pilot flame is extinguished when burner heat is demanded as by room temperature change detector instrument 13. A bimetal element 56 of the pilot light apparatus has as shown, a release finger positioned to move valve ball 57 off its seat on a vacuum-line-connected port when the bimetal element cools. An important feature of the device is represented by a pilot flame deflector and shield 59 for the bimetal element 55. The shield is carried and positioned by the bimetal element (when cool) in flame-directing position, (Fig. 3), in respect to one or more small lateral flame ports 18:; of the pilot burner tube which ports establish a flame ladder along the tube between the sleeve 17 and cap 19. Intimate association between the pilot flame and the bimetal is highly desirable at commencement of burner operation, (pilot just lighted) because the controls cannot function until the valve ball 57 is seated. However, thebimetal material deteriorates,

rapidly by oxidation if left in contact with flame. As

soon as the bimetal element becomes heated and the shield is thereby moved a short distance to the left from its flame-deflecting position (Fig. 3) into any of various positions it may occupy during main burner operation, the flame is cut ofl from access to the bimetal. A barrier 59 is recommended as indicated in Fig. 1 to shield the bimetal element 56 from main burner heat.

While any suitable pipe can be used to form the various portions of the control tube 11, installations requiring long go and return portions can be facilitated by the use of (e. g.) plastic twin tubing stock such as illustrated (cross section only) in Fig. 4. Such go and return tubing portions 11a of any appropriate shape, preferably circular, are made readily separable from each other as at a connecting frangible web portion 11b formed as a part of the twin tubing. One advantage, aside from cost, of using plastic tubing is that if, during installation, it is accidentally bent shut at any point, as in leading of it around corners, it is more apt to spring back than metal into properly functioning condition.

In the following description of the burner mechanism A, which involves frequent mention of relatively low and high B. t. u. content gas (or simply B. t. u. gas), reference is to differences in the essential chemical composition of the gases discussed.

For use of the burner A, as controlled by the above described system, interchangeably with relatively low and high B. t. u. content gas and in installations which differ from each other radically in design it is necessary, in order to avoid interchangement of parts and for reasons explained below, to provide for adjusting the operating ratio between the slave motor S and the metering valve and, separately therefrom, for adjusting the automatic operating effect of the slave motor on the common connections with the variable flame port and air control damper, and, additionally, for adjusting the damper independently of all other adjustments.

Low B. t. u. gas, throughout the control range, needs a relatively open metering valve aperture or area of orifice O as compared to the aperture areas which are suitable for high B. t. u. gas for a given heat output; the flame port must be more restricted for high B. t. u. gas because of the more rapid flame propagation rate of such gas particularly where a high turn down ratio is desired (stability at low flame settings as for automatic modulated control from a relatively high flame setting), and secondary air must be properly and accurately limited if the burner is to operate with fairly high efficiency i. e. be free from elfective heat output reduction (stack loss) by excessive secondary air. Proper mixture of gas with primary air is of course highly important, regardless of the type or grade of gas which is used, and the problem of obtaining optimum gas/ air ratio and mixture velocity at the flame port for all flame settings becomes more critical with higher B. t. u. gas i. e. the higher heat content.

The metering valve M and its operating slave motor S as herein arranged, constitute an inherently stable mechanism as will be seen from the following: Gas is supplied through suitable passages at throttled rates (function of variable position of valve V) from an assumed regulated or substantially constant pressure source to the diaphragm chamber 44 of the slave motor S and simultaneously to the metering valve chamber which is formed between a body 60 and metering valve plug 61. The diaphragm 44 of the slave motor, (shown as connected to the metering valve plug 61 by a lever 62 having a movable supporting fulcrum 63 and adjusting mechanism 65 for it), opens and variably restricts the metering valve. orifice O in accordance with the position of the diaphragm 44 in its chamber. Gas-pressure-responsive movement of the diaphragm in the meteringvalve-opening direction is opposed by an adjustable biasing means, shown in the form of a diaphragm-attached tension spring 66 having supporting and adjusting screw elements 67 and 68 one of which is connected to the spring. Since the diaphragm chamber 44' and the metering valve M have a common source of supply of gas, the action of the diaphragm 44 becomes stabilized, in a position determined by the adjusted force of spring 66, as increasing volume issuance of gas from the metering valve nozzle orifice 0 enables reduction of the net force acting on the diaphragm 44 to zero.

Largely to enable the burner to be operated with a stable flame at low gas setting, (high turndown ratio), and interchangeably with relatively high and low B. t. u. content gas, the metering valve M, as more fully explained in Swenson et a1. application Ser. No. 37,347, is so arranged as to preserve a nearly constant velocity and direction of the single stream of gas issuing from it into the mixing tube, assuming a generally constant supply pressure.

The velocity at low gas throttle settings is required in order to cause a local mixing and accelerating of a quantity of air of only that volume which is needed for combustion of the gas and the directing of such local gas-air-mixture within the mixing tube to the flame port F.

When the metering valve has only two parts, body 60 and plug element 61, an extremely effectual shape for the metering orifice, whereby the cross sectional form of the gas stream is uniform regardless of its size and the boundary surfaces defining the orifice can be minimized in gas-fiow-restricting effect is a triangle, (may be diamond shape, i. e. two triangles). The illustrated metering orifice O is of triangular shape and, because of it, the orifice dimensions vary progressively in two directions with opening and closing movement of the valve. Thus increments of metering orifice area increase as a function of equal increments of slave motor movement (non-linear functions of slave motor movement).

In order to maintain eificient combustion ratios of gas and air at all gas throttle settings it is necessary that the flame port and air supply areas shall at all times be properly varied in relation to the volume of gas being supplied through metering valve M. A simple manner of making the opening and closing movements of flame port cap L and air damper -D non-linear functions of diaphragm movement to about the same degree as the metering valve gas volume adjustment is non-linear comprises, as shown, conversion of the rectilinear motion of the diaphragm connected rod S into rotary motion of a crank 70 as by a rack and gear couple 71, 72 and to use such a portion of the crank throw for operation of a flame-port-adjusting link 73 and air-damper-adjusting link 74 that downward movement of the diaphragm 44 out of the illustrated initial position (metering valve, flame port and air damper closed) results in a progressively increasing rate of opening movement of the linkoper-ated parts substantially matching the rate of increased metering valve opening area with opening movement of the metering valve. In a properly designed arrangement (showing being largely schematic) the attachment of both links 73 and 74 to the crank at or close to the effective dead center initial position of the crank relative to the links and the crank supporting axis secures approximately optimum automatic coordination of air inlet, flame port and metering valve control.

For adjusting the amount of air which is variably admitted by the damper D independently of flame port and metering valve adjustment and control, and particularly to meet the requirement for less secondary air in some cases than in others, due to special draft effects of installation design or to the B. t. u. content of the gas which has to be used, as will be explained, the point of attachment of link 74 (pivot bracket 75) is settable toward and away from the damper hinge 76 by appropriate means. The motion of the link 73, as shown, is communicated to the flame-port-adjusting cap L through a counterweighted bellcrank 78 and pushrod 79 to which 75' latter the cap is secured.

Adaptation of the burner for use interchangeably with gases having widely different B. t. u. content involves changing the operating ratio between the slave motor S and the metering valve M so that (e. g. in going from low to high B. t. u. gas) a smaller metering valve orifice for a given heat output occurs when using such high B. t. u. gas as compared with the orifice size which is required for low B. t. u. gas and the same heat output, and, additionally, for efiicient combustion over a wide control range, reducing the flame port and air duct inlet areas as compared to those which are suitable for low B. -t. 11. gas.

Since the pressures, at which dilferent common types of gas (differing from each other in chemical composition) are used, normally increase with increasing B. t. u. content, the slave motor/metering valve operating ratio mentioned above can be adjusted according to a fixed scale (not shown) associated with the lever fulcrum adjustment means 63, 65. Thus, from a position of the fulcrum 63 appropriate to a typical low B. t. u. gas, the fulcrum is shifted toward the metering valve (e. g. as a first operation) for use with a known higher B. t. u. content gas. The automatic flame port (and air damper) control is then coordinated with the adjusted automatic metering valve movement by increasing the diaphragm-biasing force of the spring 66, usually by trial manipulation of the nuts 68 with the gas throttled to a desired low flame setting, until, due to the higher pressure of the higher B. t. u. gas acting on the diaphragm, the flame port and air inlet are sufliciently restricted to secure or generally insure a steady low flame. The higher pressure, hence kinetic energy, of the higher B. t. u. content gas assists, at low flame, in maintaining the necessary velocity of the single gas stream from the metering valve orifice O for localized entrainment of air in the mixing tube and conveyance of the mixture to the flame port.

It was indicated above that both installation design peculiarities and gas composition have pronounced effect on draft hence burner efliciency. This refers (e. g.) to stack pull variations in updraft types of equipment such as shown and to differences in draft-modifying flame intensely, varying with the B. t. u. content of the gas. Notwithstanding limitation of secondary air supply as by damper adjustment at 75 to a minimum for peak efficiency of the burner, the single source of air as controlled by the damper does not tend to cause the gasair mixture in the mixing tube to be ineffectual providing that the gas stream as it issues from the metering valve orifice has adequate air-inspirating velocity at all times (assured by the disclosed metering valve and its controls as already explained) and that substantially uniform air pressure is always maintained at the two ends of the mixing tube. Such uniform air pressure results from the fact that the restriction at the air damper determines the air pressure in the entire air duct E including that within the secondary air outlet sleeve Q and, of course, the inlet end of the mixing tube T which lies within the duct E in adequately spaced relation to its walls. An uncontrolled primary air source outside of the duct E to the inlet throat of the mixing tube would, under high draft conditions, frequently cause the mixture issuing from the flame port to be too lean for stable combustion.

I claim:

1. In a gas burner control system and apparatus, a burner and a gas supply conduit leading thereto having gas inlet and outlet passages, a port between the passages and a movable valve member capable of closing and opening the port, means forming a generally closed gas reception chamber defined in part by said movable valve member, the reception chamber being in restricted communication with the gas inlet passage for inlet-gas-pressure-operated closing of said port, wall means forming a vacuum chamber and means remotely thereof and capable of inducing subatmospheric pressure-therein, signal means 8 remotely of the vacuum chamber and operative to vary subatmospheric pressure therein, one wall portion of the vacuum chamber being movable in response to the varying subatmospheric pressure in the vacuum chamber, and gas conduit means communicating with the gas reception chamber at one end and having its opposite end disposed to be sealed and unsealed by said movable wall of the vacuum chamber, said opposite end, when unsealed, being arranged to vent gas from said reception chamber to atmosphere at rates in excess of the restricted rate of reception of gas thereinto from the said gas inlet passage whereby to enable opening of said port.

2. Apparatus according to claim 1, wherein a burner gas flow rate modulating spring is operatingly connected to the movable Wall of the vacuum chamber so as to oppose with increasing spring force movement of said movable wall of the vacuum chamber in a direction to unseal said gas conduit means for communication with atmosphere.

3. Apparatus according to claim 2, having, additionally, adjustable abutment means associated with the spring in a manner to render the spring ineffective to oppose initial movement of said movable wall in its conduit-meansunsealing direction.

4. In a control system for a main gas burner having a pilot light burner with a gas supply independent of the control system, means forming a control passage, means operable to-maintain a partial vacuum in said control passage, gas control valve means for the main burner, and vacuum actuated means connected with the control passage and acting to initiate opening of the control valve means only when a predetermined degree of vacuum exists in the control passage, a main burner safety device comprising a normally closed and self closing vacuum-breaker valve in the passage, a bimetal element associated with the pilot burner, means for supporting the bimetal element so as to expose a freely movable portion of the element to pilot burner flame heat, said portion being operatingly disposed in relation to the vacuum-breaker valve so as to open that valve only when the bimetal element is unheated by pilot burner flame whereby the main burner will not be supplied with gas when the pilot burner flame is extinguished, and a deflector device carried by said freely movable portion of the bimetal element, said device, in an unheated condition of that element, being disposed in a position to direct heat of the flame toward the element, and, when the element has been heated to the point of permitting the vacuum breaker valve to close, is then interposed between the pilot flame and said element in a manner to shield the element from flame heat.

5. In a gas burner control system and apparatus, a burner and a gas supply conduit leading thereto including a hollow housing having gas inlet and outlet passages, a port between the passages and a movable valve member capable of closing and variably opening the port, the housing having a generally closed gas reception chamber of which the valve member forms a movable wall, the reception chamber being in restricted communication with the inlet passage for inlet-gas-pressure-operated closing of said port, said reception chamber having a fixed wall transverse to the axis of the port, means forming a vacuum chamber in fixed relation to the housing, signal means remotely of the vacuum chamber and operative to vary subatmospheric pressure therein, one wall of the vacuum chamber being movable toward and away from said fixed wall in response to the varying subatmospheric pressure, and a pilot valve mechanism for control of said movable valve member by said movable wall of the vacuum chamber, said mechanism comprising a substantially rigid tube in the form of a bell crank resiliently sealed to said fixed wall and passing therethrough for pivotal movement of lever arm constituting portions of the tube toward and away from the valve member and movable wall of the vacuum chamber respectively, valve means carried by the movable wall of the vacuum chamher and co-operating with the adjacent open end portion of the tube as a pilot valve to vent gas from said reception chamber at rates in excess of the restricted rate of reception of gas from the said gas inlet passage, whereby movements of said valve member are responsive to pilot valving movements of the movable wall of the vacuum chamber.

References Cited in the file of this patent UNITED STATES PATENTS 400,758 Easton Apr. 2, 1889 10 Gassett Dec. 5, 1893 Ljunglof Aug. 20, 1907 Campbell Sept. 5, 1933 Stockmeyer Feb. 27, 1934 Schuck et a1 Dec. 11, 1934 Adlam Sept. 8, 1936 Betz Nov. 15, 1938 Senninger Apr. 9, 1946 Higley May 6, 1947 Swenson et a1 Oct. 23, 1951

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