GAS HEATERS AND CONTROL THEREOF

Abstract: A gas-fired apparatus having a solenoid valve (30) which is controlled by a control circuit which produces a control signal (111, 112, 113) the duty cycle of which is dependent upon the difference in temperature sensed by a thermistor (101) and a selectable reference (100). The apparatus may comprise a gas-fired air heater for domestic purposes.

Description: GAS HEATERS AND CONTROL THEREOF This invention relates to control of gas burning apparatuses. The general object of the invention is to increase the effectiveness of control of gas burning apparatuses. According to the present invention there is provided a method of controlling gas burning apparatus having a gas supply line which includes an electrically operated valve therein, said method comprising the steps of setting a reference temperature, sensing the temperature at a control point to generate a monitored temperature signal, generating a difference signal between the monitored temperature signal and the reference temperature, generating a periodically variable sequencing signal, comparing the difference signal to the sequencing signal to generate a control signal, applying the control signal to the valve whereby the duty cycle of the valve during each period of the sequencing signal is dependent upon the temperature difference between the reference temperature and the temperature sensed at the control point. The invention also provides a solenoid valve comprising a body, a movable armature, a coil which when activated moves the shaft in the first direction, a valve seat, a valve element movable with the armature to sealingly engage or disengage the valve seat in accordance with the position of the armature, and position limiting means for limiting movement of the shaft in said first direction, said limiting means comprising elastomeric material. The use of elastomeric material cushions the movement of the shaft and effectively eliminates the t sharp clicking sound which normally accompanies operation of a solenoid valve. This is particularly important in the preferred embodiment of the invention where the output of the heater is controlled by frequent operation of the solenoid valve or valves provided in the heater. The invention also relates to a gas fired apparatus having a gas burner, a gas supply line,.an electrically operated valve in the gas supply line, and a controlled circuit for controlling the solenoid valve, said circuit including a selectable reference temperature generator, a temperature sensing element, means for generating a difference signal between the output of the reference temperature generator and the temperature sensing element, sequencing means for producing a time variable sequencing signal, a comparator for comparing the difference signal to the periodically variable sequencing signal, the output of the comparator comprising a control signal which is coupled to control the valve the arrangement being such that the duty cycle during each period of the sequencing signal is dependent upon the difference in temperature between the reference temperature and the temperature sensed by said temperature sensitive element. The invention will now be further described with reference to the accompanying drawings, in which: FIGURE 1 is a schematic cross-sectional view through a flueless gas heater; FIGURE 2 is a schematic view of the heater with the front louvres removed FIGURE 3 shows a solenoid valve of the invention? FIGURE 4 shows a modified form of solenoid valve of the invention; FIGURE 5 shows a control arrangement utilizing two solenoid valves; FIGURE 6 shows diagrams of energy inputs as a function of time for the control arrangement shown in Figure 5; FIGURE 7 is a schematic arrangement of a modified form of control arrangement; FIGURE 8 shows diagrams of energy input as a function for the control arrangement of Figure 7; FIGURE 9 illustrates diagrammatically a controlled circuit for use in the apparatus of the invention; and FIGURES 10A, 10B, IOC and 10D show waveforms of signals of the circuit of Figure 9. The heater shown in Figure 1 comprises a housing 2 having a base 4, rear wall 6, front louvres 8 and top louvres 10. Located within the housing is a heat exchanger 11 which is preferably formed from stainless steel or other heat resistant material. Within the heat exchanger 11 is a burner 12 which in use produces two rows of flames 14 heat the inner surfaces of the heat exchanger 11. The front face of the exchanger is provided with a plurality of openings 16 to provide inlet air for the burner 12, as indicated by arrows 18. The top of the exchanger 11 has openings 20 which permit combustion products to escape from the interior of the exchanger. A cylinder fan 22 is located beneath of the exchanger 11 and is located rearwardly of a partition plate 24. The outlet of the fan being connected to an opening 26 in the plate. The arrangement is such that the fan 22 draws air inwardly through the front louvres, particularly those louvres near the top of the housing, the air is then drawn over the top of the heat exchanger 11 then down the rear face of the exchanger 11 and then discharged through the opening 26 and then through the lowermost louvres at the front of the housing. The air is heated as it passes adjacent to the outer surfaces of the exchanger 11. Further, the combustion products which escape through the openings 20 in the exchanger are entrained into the stream of air being induced to the fan 22 so they too contribute to the heating capability of the heater. Of course, the combustion products are diluted to a considerable extent with the air which is induced into the heater. As seen in Figure 2, the cylinder fan 22 is driven by a fan motor 28 which is located near the side of the heater, the arrangement is such that the air which has been heated by the heat exchanger 11 is drawn towards the fan 22 and generally away from the motor 28 and accordingly the motor is not overheated by the hot air. It has been found that no partitioning is required in order to direct the hot air away from the motor 28. Figure 3 shows a solenoid valve 30 constructed in accordance with the invention and particularly useful for controlling the supply of gas to the burner 12 in the heater shown in Figures 1 and 2. The solenoid valve comprises a lower body 32, an upper body 34 and coil 36. The lower body is formed with inlet and outlet ports 38 and 40. The inlet port 38 communicates with a lower chamber 42 which is in communication with an upper chamber 44 formed in the upper body 34. The valve includes an armature shaft 46 which is mounted for reciprocal movement in a tubular guide member 47 mounted in the upper body 34 and extending into the coil 36. A resilient valve element 48 is mounted on the lower end of the armature shaft 46 and in use sealingly engages a valve seat 50 formed in the body 32 and communicating with the outlet port 40. In use, the valve element 48 is normally seated against the valve seat 50 so as to close the valve but upon actuation of the coil 36 the shaft 46 is drawn into the coil and unseats the element 48 from the seat 50 thereby permitting gas to flow from the inlet port 38 through the chambers 44 and 42 and then out the outlet port 40. The solenoid includes an abutment member 52 which is, in accordance with the invention, formed from a high temperature silicon rubber or elastomeric material and a compression spring 54 acts between the lower face of the member 52 and the armature 46 to bias the armature to the closed position. The elastomeric abutment member 52 limits the upward movement of the armature and because of its elastomeric properties cushions the deceleration of the armature, thus resulting in very silent operation of the relay valve. This contrasts markedly with known forms of valve where the abutment 52 comprises an iron slug or the like whereby each time the valve is opened and the armature shaft strikes the abutment, there is emitted a sharp clicking sound. With the solenoid of the invention, it is possible to control the gas heater in a manner which has very frequent operation of the solenoid valve which would not be possible with a normal solenoid valve because the noise level would be intolerable. Figures 7 and 8 show a typical control arrangement which is possible by using solenoid valve of the invention. Figure 7 diagrammatically shows the solenoid valve 30 in a gas line 60 which leads to the burner 12 of the heater. Waveform 62 (gas flow as a function of time) shows periodic closing of the valve at the end of predetermined periods which in the illustrated arrangement are of three minutes duration. The off periods are predetermined to be a minimum of say fifteen to twenty seconds. The waveform 62 thus corresponds to the maximum setting of the gas input to the heater. Waveform 64 has valve open periods which are shorter than those of waveform 62, the ends of the on periods being controlled in accordance with control signals from a thermostatic controller shown in Figures 9 and 10. It will be observed that the effective duty cycle for the waveform 64 is much lower than that for waveform 62. Because of the relatively high frequency of operation of the solenoid enables, the control to resemble the sort of control which is normally only available with a fully modulated gas regulator which of course is much more complex than the arrangement illustrated in Figure 3. Further, the thermostatic sensor can be arranged to have a low temperature differential, i.e. the difference between the temperatures at which the solenoid opens and closes. The bypass solenoid valve 65 illustrated in Figure 4 is essentially the same as that shown in Figure 3 except that a bypass passage 66 is formed in the lower body 32 to interconnect the inlet and outpprts 38 and 40. The arrangement is such that when the valve element 48 is seated upon the valve seat 50 a relatively low rate of flow of gas passes from the inlet to the outlet. Figure 5 illustrates the use of the bypass solenoid valve 65 in series with the on-off solenoid valve 30. Figure 6 shows typical waveforms for the energy input to the burner as a function of time. The waveform 68 (gas flow as a function of time) show periodic closures of the solenoid valve 65 at predetermined periods, as before. The level of gas flow at the closure, of solenoid valve 65 is determined by the size of the bypass passage 66. The solenoid valve 65 is controlled so as to alter its effective duty cycle to thereby produce the required average gas input to the heater. The valve 30 would only be closed if there is a power failure, over temperature, physical tipover of the heater, flashback retention or by operation of the on-off switch by the user. It would be possible of course to arrange for several bypass solenoid valves to be provided in series each having a different size bypass passage so that the resultant combination would more closely resemble a fully modulated regulating valve. It is normally desirable for the fan 22 to be operated in synchronism with the valve 30 so that it is always operational whilst gas is flowing to the burner. Figure 9 illustrates schematically a controlled circuit for producing control signals for the solenoid valve 30 so as to produce the gas flow waveforms similar to waveforms 68 and 62 shown in Figures 6 and 8. The circuit includes a variable reference temperature element 100 which may comprise a rheostat which is adjusted by the user of the equipment in order to select a desired temperature for instance of a room. The circuit also includes a thermistor 101 (or other temperature sensitive element) which is located in the gas appliance or remote therefrom. The outputs from the reference element 100 and thermistor 101 are connected to the inputs of a differential amplifier 102 the output of which is connected to a comparator 107. The circuit also includes a ramp voltage generator 106 which is arranged to produce a sawtooth waveform 108 as illustrated in Figure 10A. The waveform 108 rises periodically from a lower limit V min to an upper limit V max corresponding to temperatures T min to T max. Figure 10A also shows waveforms 103, 104 and 105 which are representative of the outputs of the differential amplifier 102 in the following conditions: 1. Where the temperature sensed by the thermistor is lower than T max. 2. Where the temperature sensed by the thermistor 101 is intermediate of T max and T min. 3. Where the temperature sensed by the thermistor 101 is higher than T max. The waveform 104 is shown as decreasing slowly with time, but, of course, it could remain constant. Figure 10B shows an output waveform 111 of the comparator 107 in the instance where the differential amplifier 102 has an output waveform 103. In this case the voltage is continuously high and the solenoid will be continually open. It would be possible to arrange that the waveform 103 always coincides with the peaks of the sawtooth waveform 108 so that the solenoid would close briefly at the end of each period of the sawtooth waveform. Figure IOC shows an output waveform 112 of the comparator when the input from the differential amplifier is like waveform 104. It will be observed that the waveform 112 is high when the magnitude of the waveform 104 exceeds the magnitude of the waveform 108, thereby producing a pulsed output the duration of the pulses being related to the difference between the temperature sensed by the thermistor 101 to the reference temperature generated by the reference generator 100. In the illustrated arrangement the waveform 104 decreases with time and the pulses in waveform 112 vary in width. If waveform 104 were constant the widths of the pulses in waveform 112 would be the same. Figure 10D shows a waveform 113 which is produced when the output of the differential amplifier 102 has a waveform 105. In this instance the waveform 113 is continuously low and thus the solenoid valve 30 will remain closed continuously. In the control technique of the invention, the difference between T max and T min can be made low say for instance of the order of 2°C thereby avoiding undesirably large variations in the temperature to be controlled. This is more desirable from the user's point of view and moreover avoids large temperature variations in the heating apparatus which would be present with other forms of thermostatic control where there is a large difference between temperatures corresponding to T max and T min. This avoids problems caused by repetitive thermal expansions and contractions. Further, the thermistors are very accurate and therefore the temperature control is very accurate in accordance with the invention. Where the period of the sawtooth waveform is say three minutes and the temperature sensed by the thermistor 101 is such that the output of the differential amplifier 102 is intermediate of T min and T max, the temperature of the heat exchanger may vary, for example between 245°C and 250°C when the solenoid is closed and open. This may produce a variation in temperature from say 68°C to 70°C in the air heated by the heat exchanger. In a conventional thermostatic control the heat exchanger would normally be permitted to cool to ambient during off periods and may rise to as high as say 500°C during on periods thereby greatly increasing problems due to thermal expansions and contractions. In the control technique of the invention, it is preferred that the period T of the sawtooth waveform is about three minutes. However this period can be made very much shorter in which case the solenoid will be operated more frequently and the energy output of the apparatus will be more analogous to an apparatus having a fully modulated gas control. If' the period T of the sawtooth waveform were made of very short duration it is possible that the armature shaft 46 of the solenoid valve 30 will assume an equilibrium position as determined by the duty cycle of the voltage waveform applied to the coils 36 (tending to open the valve) and the biasing force of the spring 54 (which tends to close the valve) . In this way a more or less conventional solenoid valve can be made to operate in a manner analogous to a modulating valve. it will be appreciated however that the construction of the valve shown in Figure 3 or 4 is very much simpler than a conventional modulating valve. Many modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention.

Claims: CLAIMS: 1. A method of controlling gas burning apparatus having a gas supply line which includes an electrically operated valve therein, said method comprising the steps of setting a reference temperature, sensing the temperature at a control point to generate a monitored temperature signal, generating a difference signal between the monitored temperature signal and the reference temperature, generating a periodically variable sequencing signal, comparing the difference signal to the sequencing signal to generate a control signal, applying the control signal to the valve whereby the duty cycle of the valve during each period of the sequencing signal is dependent upon the temperature difference between the reference temperature and the temperature sensed at the control point. 2. A method as claimed in claim 1 wherein the period of the sequencing signal is fixed and is less than 5 minutes. 3. A method as claimed in claim 2 wherein the period of the sequencing signal is approximately 3 minutes. 4. A method as claimed in claim 1 wherein the variable sequencing signal comprises a sawtooth waveform. 5. A method as claimed in claim 1 including the step of sensing an over temperature in the apparatus and generating a cut-off signal which overrides said difference signal whereby the control signal will keep the valve closed when an over temperature is sensed. 6. A gas fired apparatus having a gas burner, a gas supply line, an electrically operated valve in the gas supply line, and a controlled circuit for controlling the solenoid valve, said circuit including a selectable reference temperature generator, a temperature sensing element, means for generating a difference signal between the output of the reference temperature generator and the temperature sensing element, sequencing means for producing a time variable sequencing signal, a comparator for comparing the difference signal to the periodically variable sequencing signal, the output of the comparator comprising a control signal which is coupled to control the valve the arrangement being such that the » duty cycle during each period of the sequencing, signal is dependent upon the difference in temperature between the reference temperature and the temperature sensed by said temperature sensitive element. 7. Apparatus as claimed in claim 6 wherein the sequencing means comprises a sawtooth waveform generator the period of which is fixed and is less than five minutes. 8. A gas fired apparatus as claimed in claim 7 wherein the period of the sawtooth is approximately five minutes. 9. Apparatus as claimed in claim 6 wherein the valve comprises a solenoid valve which includes an armature movable within an armature housing and wherein a resilient abutment member is provided in the housing so as to cushion movement of the armature and so limit noise generated thereby. 10. Apparatus as claimed in claim 6 wherein the valve comprises a solenoid valve which includes a body, a movable armature, a coil which when activated moves the shaft in a first direction, a valve seat, a valve element movable with the armature to sealingly engage or disengage the valve seat in accordance with the ' position of the armature, biasing means for biasing the armature to a position in which the valve element engages the valve seat and wherein the means for generating a variable sequencing signal has a relatively short period whereby the armature can assume an equilibrial position in which the extent.of separation of the valve element from the valve seat depends upon the duty cycle of the control signal. 11. Apparatus as claimed in claim 6 wherein the apparatus comprises a flueless gas heater having a housing, a heat exchange member which is, in use, heated by said burner, a fan which is arranged to draw air into the housing and past the heat exchange member to be heated thereby and expelled from the housing near a lower position thereof.

Inventors: TRIHEY JOHN MASSEY, TRIHEY, JOHN, MASSEY
Applicants: VULCAN AUSTRALIA, VULCAN AUSTRALIA LIMITED
Application year: 1985
Main classification: F23N1
IPC codes: F23N1