US2579884A

US2579884A - Flame failure safeguard

Info

Publication number: US2579884A
Application number: US791054A
Authority: US
Inventors: Thomson E Craig; Giuffrida Philip
Original assignee: Comb Control Corp
Current assignee: Combustion Control Corp
Priority date: 1947-12-11
Filing date: 1947-12-11
Publication date: 1951-12-25
Anticipated expiration: 1968-12-25

Description

Dec. 25, 1951 M O T A 2,579,884

FLAME FAILURE SAFEGUARD Filed Dec. 11, 1947 Allllllll fivnfnrs E. CRAIG THOMSON PHILIP GlUFFRIDA fly T1] (2: 611 I Patented Dec. 25, 1951 FLAME FAILURE SAFEGUARD E. Craig Thomson, Boston, and Philip Giufirida,

Lawrence, Mass, assignors to Combustion Control Corporation, Cambridge, Mass., a corporation of Massachusetts Application December 11, 1947, Serial No. 791,054

8 Claims.

This invention relates generally to furnace control and in particular to flame failure controls for industrial furnaces, the present invention being an improvement on the device disclosed in the application of E. Craig Thomson, Serial No. 754,435, filed June 13, 1947.

The device disclosed in the Thomson applica tlon herein referred to is an electronic flame failure control in which a photocell, a flame electrode, or both simultaneously may serve as the flame detecting elements. The object of the present improvement is to extend the life and utility of such a control under conditions which may occasionally be encountered when the flame rod or-the photocell and flame rod combination is used on certain types of furnace installations. In particular, it has been found that on burners ignited by a high voltage spark, sharp voltage pulses may occur in the flame rod circuit because of the abrupt breakdown of the ignition air gap each half cycle as the voltage on the ignition electrode rises to spark value. Such pulses are liable to shorten the life of the amplifier tube to the grid of which the flame rod is connected. Furthermore, the relatively large current flowing in the flame rod circuit, because of the intense ionization of the flame caused by the high voltage across the ignition electrodes, may so charge the control condenser as to give rise to an excessive time delay in the response of the device to flame failure.

The objects of this invention are to reduce the voltage pulses across the grid and cathode of the amplifier tube, to maintain the input voltage to the amplifier tube within safe limits under all operating conditions likely to be encountered, to increase the reliability and adaptability of a furnace control of this type and to accomplish these improvements with the addition of a minimum of electrical components.

This invention can best be understood by reference to the accompanying circuit diagram. As explained in the Thomson application previously referredto, the control is designed to energize a relay upon detection of flame. The control system is energized by a transformer !6 having secondaries l9, l8, and 54, and a primary H which is connected to an alternating current source at terminals 20 and 2|. The device employs a single amplifier tube I of the pentode type having an anode ll, cathode I2 and three control grids I3, l4 and i5, conventionally designated the control screen and suppressor grids, respectively. When suppressor grid l5 V is at cathode potential, or positive with respect to cathode [2, current flows in the anode, or plate circuit, by way of

taps

32 and 24 of secondary l8, resistance 26, and secondary IS. The potential drop across resistance 26 makes control grid I 3 sufliciently negative with respect to cathode i2 so that flow of electrons to screen I4 is substantially cut off. Relay 25, which is connected in the screen circuit, is then de-energized. The relay controls a normally open contact 66 which is in the energizing circuits of burner motor 61, main fuel valve 89, pilot valve 9| and primary 52 of ignition transformer 86. The relay may control additional contacts for operating other control and alarm devices. A push button 51 or other suitable device may be provided for shunting contact 66 to permit starting of the burner. In addition, a push button 90 or similar device may provide for a time delay in opening the main fuel valve.

When a negative bias is applied to suppressor I 5, current in the plate circuit is cut ofi. Control grid I 3 then assumes the potential of tap 24, which, on the half cycles when the tube may conduct, is positive with respect to the cathode. Substantial current then flows in the screen circuit energizing relay 25. The negative bias on grid [5 may be supplied either by the photocell 30, which is arranged so as to receive light from the main flame 39, or by the flame rod 49, which is arranged to be in contact with pilot flame 4i, acting in conjunction with an input network which includes resistances 28 and and

condensers

29 and 64. When the pilot flame is burning, for example, current flows in the circuit including ground, flame 4|, rod 40, cable 43, the input network, tap 32, tap 24, resistance 26, and secondary [9. As is well-known, a flame has substantial rectifying properties, tending to conduct from its tip to its base more readily than in the opposite direction. A charge is, therefore, built up on

condensers

29 and 64 which is of such a polarity as to make suppressor 15 negative with respect to cathode l2. When flame' 4| becomes extinguished, breaking the flame rod circuit, this charge leaks off through resistance 28, restoring the suppressor to cathode potential and cansing relay 25 to become deenergized as previously suppressor 15. When light ceases to fall on the 3 photocell, the charge on condenser 29 leaks 05 through resistance 28.

The device may be used with the flame rod alone, the photocell alone, or with both the flame rod and the photocell connected as shown. In any case, a certain amount'of alternating current, arising from such factors as incomplete rectification in the flame and external leakage across the rectifying elements, ordinarily flows in the input circuit. This alternating current gives rise to an alternating potential across the input circuit which, on the half cycles when the tube may conduct, opposes the bias produced on suppressor l5 by the charge accumulatedon condenser 29 as a result of the direct current component in the input circuits. Under normal conditions, the alternating component is relatively small and suppressor [5 remains negative with respect to the cathode whenever flame is detected by either detecting element. Under abnormal conditions, however, if the alternating current resistance of the. flame rod should be substantially lowered, for example, by leakage due to accumulation of soot across the insulating sleeve 85, the alternating potential becomes large enough. to overcome the negative bias of suppressor F5 on the half cycles when the tube may conduct, Current then flows in the plate circuit, and current in the screen cirsuit is substantially cut off. Relay 25, therefore,

becomes de-energized on the occurrence of exces- 1 sive leakage in the flame, rod circuit.

A similar action occurs if; leakage across the photocell becomes excessive as a result of moisture accumulating on the'photocell base, or of a conductive deposit accumulating on the interior surface of the photocell envelope. It will be noted that photocell 3!! normally conducts so as to charge condenser 29 on the half cycle when tube is non-conductive. If leakage across the phototube has become excessive, the charge on condenser 29 is largely dissipated during the half cycle when tube in may conduct and the negative bias is removed from suppressorlS. "This action is generally knownlas a "safe failure and is of utmost importance in furnace safety control.

The impedance of condenser 64 is sufficiently high so thatthe alternating potential across the flame rod; input circuit, which consists of the parallel network of condenser 64 and resistance 28, is not shifted in phase enough to disturb the function of the circuit under theexcess leakage condition. Representative values are .001 microfarad for condenser 64 and 150 megohms for resistance 28. V v In a device of this type, it is usually desirable to introduce a certain time delay of the order of three or four secondsin the response to prevent false responses due to flickering of the flame. Since the capacity of condenser 64 is too small to provide this delay, a second condenser 29 of e o der of m cr fared s. co nec ed in a allel with resistancezil. This condenser serves both as a, second input condenser for the flame rod circuit, and'as the input condenser'for the photocell circuit. 'It is apparent that condenser 29. is of such high capacity that the differential in capacitance introduced 'by condenser 64 will not appreciably alter the time delay, and the delay is nearly the same for the flame rod and photocell circuits. In series with condenser 29 is a resistance 6501 the order of .20 megohms which makes the impedance of this branch of the circuitv high enough so as not'to afiect appre ciably the operation of the flame rod input network, consisting of 'comienser iM and resistance 28, under the excess leakage condition. Since resistance 65 is low as compared to resistance 28, resistance 65 may be varied without appreciably affecting the time delay. Variation of either condenser 54 or resistance 28, however, will shift the phase of the alternating potential applied to suppressor I5, and the value of the leakage resistance across the flame rod which will cause relay 25 to drop out may therefore be varied by either of these means substantially independently of the time delay.

The high voltage spark ignition used to ignite many types of furnaces is liable to cause intense ionization of the flame. Under this condition the direct current flowing in the flame rod circuit may become abnormally high, charging condenser 29 to a potential which would result in an excessive time delay in the response in case of flame failure, and applying a bias voltage to suppressor [5 which may be so high as to be destructive to. the tube.

To limit the voltage applied to suppressor 15 by the flame rod circuit to a safe value under all conditions, a, glow tube 63 is connected between suppressor l5 and cathode I2. This tube is of the two electrode type filled with a gas, such as neon, under low pressure. The tube remains substantially non-conductive until the voltage across it reaches the ionization potential of the gas, for example, about volts. Thereafter, the tube conducts at substantially constant voltage with increasing current. The presence of tube 63 has no effect on the operation of the circuit under normal operating conditions, when the voltage between the cathode l2 and the suppressor [5 re mains within a safe range of values. If the direct current in the flame rod circuit becomes unusually high, as it may during the ignition period, the tube 63 starts to conduct and prevents further increase in the voltage between suppressor and cathode.

In place of the glow tube, a resistance made of a suitable material such as-thyrite which has a non-linear resistance characteristic, offering a high resistance to the relatively low voltages at which the input circuit normally operates and dropping sharply in resistance above a certain voltage, may be used. From the foregoing description, it is apparent that this circuit, while retaining all the safe fail.- ure features and other advantages described in the Thomson application herein referred to, provides for safe andsatisfactory operation of the device under the conditions which may be en-: countered at the start-of the operating'cycle of certain types of furnaces. Also disclosed herein is a simple and eflicient method of adjusting the sensitivity of the device with respect to leakage across the detecting element without disturbing other operating characteristics. It will be further apparent that these advantages have been achieved by the addition of only a few simple 61B: ments in the circuit. Since certain changes may be made in the above-described article and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative only and not in a limiting sense.

What is claimed is:

1. In a flame failure control employing a dc: testing element adapted to form an electrical contact with the flame: a discharge device having an anode, cathode and control electrode; an input circuit including said cathode, said control electrode, said detecting element, and a network comprising a first capacitor, a first resistance connected in parallel therewith, and, also in parallel therewith, a second resistance connected in series with a second capacitance; and a gas filled diode of the voltage regulating type connected in parallel with said network.

2. A control device comprising: a discharge device having a first and a second current path and a control electrode adapted, upon application thereto of a control potential, to shift current from one to the other of said paths; a load in circuit with one of said paths; a flame detecting element; an input circuit for applying said control potential including said flame detecting element, said cathode, said control electrode, and a network comprising a first resistance, a first capacitance in parallel therewith, and, also in parallel therewith, a second resistance in series with a second capacitance; and a diode of the voltage regulating type connected in parallel with said network.

3. A device according to claim 2, said flame detecting element being an electrode disposed so as to make electrical contact with a flame.

4. A device according to claim 3 having a second flame detecting element connected in series with said second capacitance.

5. A flame failure control device comprising: a flame detecting element; a discharge device having an anode, cathode and control electrode; an input circuit including said cathode, said control electrode, saiddetecting element, and a network comprising a f rst capacitor, 9. first resistance connected in parallel therewith, and, also in parallel therewith, a second resistance connected in series with a second capacitance; and a diode of the voltage regulating type connected in parallel with said network.

6. A device according to claim 5, said flame detecting element being an electrode disposed so as to make electrical contact with a flame.

7. A device according to claim 5, having a second flame detecting element connected in series with said second capacitance.

8. A device according to claim 7, said second flame detecting element being a photocell.

E. CRAIG THOMSON. PHILIP GIUFFRIDA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS