GB2027869A

GB2027869A - Gas-heated appliance

Info

Publication number: GB2027869A
Application number: GB7926864A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1978-08-03
Filing date: 1979-08-01
Publication date: 1980-02-27
Also published as: DE2834025C2; GB2027869B; FR2432682B1; FR2432682A1; ES483088A1; DE2834025A1

Abstract

The appliance comprises a gas burner (12) for heating e.g. water in a heat exchanger (15), with the exhaust gases being force vented by a fan (17) driven by a motor (41) and the gas supply controlled by a signal representing heat demand by a solenoid value (11). A control circuit (Fig. 2, not shown) is provided comprising relays R1, R2 by which the split coils (42, 43) of the motor (41) can be switched into series connection or one (42) can be disconnected. The relays are operated by a threshold circuit (50) recessing the heat demand signal so that when the burner is operating at 2/3 rds its rated output, the fan speed is reduced thereby reducing exhaust (and thus air intake) to match the reduced operation of the burner. <IMAGE>

Description

SPECIFICATION Gas-heated appliance The invention relates to a gas-heated appliance comprising an air intake or exhaust gas fan in particular a water heater which operates according to the continuous-flow principle and is suitable as a heating source for central heating systems and/or for the heating of water for industrial uses.

An applicance of this type is already known (German Patent Specification 2 545 1 35), comprising a control system and in which, apart from a solenoid-operated opening and closing valve in the fuel supply line, the control system controls only a fan which acts to modulate the gas flow by means of a regulating valve situated upstream of the solenoid valve and controlled in dependence upon a pressure differential. An advantage of this arrangement is that, in compliance with safety regulations, gas is allowed to flow only when the fan is running. On the other hand, this type of control involves relatively substantial costs in order to provide infinitely-variable control of the fan speed.Moreover, in another known arrangement (German Patent Specification 2611292), the fan motor is an asynchronous motor having a primary winding and a phase-shifting winding connected in series with a capacitor, the phase-shifting winding being connected directly to the supply voltage, the primary winding, on the other hand, being connected, in series with a bridge rectifier and a transistor in the control system, to the secondary winding of a step-down transformer.

In another known appliance (German Patent Specification 2429 501), the control system acts upon the fuel flow, and the gas pressure at the burner influences the effective output of a constant-speed fan. For this purpose, an aperture, which is provided in a waste-gas collecting hood provided over the heat exchanger, is controlled by a by-pass valve operated by the gas pressure, and allows a proportion of the fresh air indued by the fan to by-pass the combustion zone and to be discharged, together with the waste gas, directly to atmosphere.In another variant of this type of control, the gas pressure at the burner influences a flap valve arranged downsteam of the fan in the waste-gas flue, the position of the flap valve forcing a proportion of the waste gas back through a by-pass passage, which by-passes the fan, into the waste-gas collecting hood, and thereby varying the effective output of the fan for a constant fan speed. However, such mechancal means for influencing the volume of air supplied to the burner involve a relatively substantial amount of equipment, and are disadvantageous, since adjustment of the air-to-fuel ratio to suit the particular type of gas used is complicated and, in some cases, can be accomplished only by the replacement of mechanical control elements.Where a by-pass valve is arranged in the waste-gas collecting hood, there is also the disadvantage that, when the output of the appliance is throttled down, the waste gas is cooled additionally by the increased proportion of fresh air, which renders the connection of the appliance to a flue problematic.

There is provided by the present invention a gas-heated appliance having a mounting chamber which houses the gas burner, a combustion chimney connected to the exterior of the chamber via an exhaust duct, and a heat exchanger, and is connected via an air intake duct to the exterior of the mounting chamber; the heater also having a fan located in the exhaust duct or the air intake duct, and having a control system which influences a fuel delivery valve in proportion to the heat requirement of a thermostat, and matches the effective output of the fan to the burner output, wherein the fuel valve is a valve which is controllable by means of an electrical regulator, serving as a modulating means, and there is associated with the fan a switching device for influencing the effective output of the fan; said switching device being connected, in parallel with the modulated fuel valve, to the regulator.

In contrast to the known appliances, the appliance according to the present invention has the advantage that no additional mechanical equipment is required in order to control the fan speed, and also that infinite variability of the fan speed is not a prerequisite for modulated control of the fuel flow, and, consequently, of the output of the appliance. The ratio of the fuel flow to the air flow may easily be adjusted, according to the type of gas used and the gas pressure, by the operation of a potentiometer, for acting, for example, upon a positioning element of the control system.

It is particularly advantageous if the fan is provided with an electric driving motor operable at at least two speeds, and if the switching device associated with the fan is provided with a voltage-dependent threshold-value switch for controlling its speed. This proposal is based on the known fact that, upon a decrease in the loading of the appliance, an insubstantial increase in excess air does not affect hygienically satisfactory combustion, and also that the efficiency of the appliance is not therefore significantly impaired. In view of this fact, in addition to control of the fuel flow, which may easily be performed and controlled by means of a solenoid valve, stepped control of the fan speed is proposed, whereby the fan motor, and its switching device for influencing the speed, are connected, in parallel with the solenoid valve, to the control system.

The fan motor may advantageously be a split-pole motor with a split field coil, one coil element of which is disconnectible, or is con-- nectible in series with the other coil element; Switching from one speed of the fan to the-other may advantageously be performed at about 2/3 the rated capacity of the appliance.

An embodiment of the invention is shown in the accompanying drawings in which: Figure 1 is a cross-sectional diagrammatic view of the embodiments being a gas-heated continuous-flow heater; Figure 2 is the simplified electrical circuit diagram of the control system of' the appliance of Fig. 1; and Figure 3 is a functional diagram of the.

control system shown in Fig. 2; and Figure 4 shows the so-called water efficiendy g of the appliance as a function of its loading B.

The water heater is provided with a burner 12, to which gas is supplied via a fuel supply line 10 and a solenoid-operated gas valve 11, and above which there is arranged a combustion chimney 13, at whose upper end there is mounted a heat exchanger, in the form of a finned radiator 1 5, through which there extends a water-bearing tube 14. Over the heat.

exchanger there is provided a waste-gas collecting hood 16, at whose upper end there is mounted an exhaust fan 17, whose exhaust pipe 18 extends through the mounting wall 1 9 of the appliance to atmosphere. The de- scribed components are housed in an enclosed chamber 22, which communicates withthe atmosphere via an air intake pipe 23 arranged concentrically with the exhaust pipe 18.

The appliance has a control system which, as shown in Fig. 2, incorporates a regulator 30, in the form of an operational amplifier controlled by a first voltage divider 31 and a second voltage divider 32. The voltage divider 31 embodies a temperature-dependent resistor 34, serving as an actual-value transducer of a thermostat, in the pipe delivering hot water from the appliance, while the voltage divider 32 incorporates a set value transducer 35, in the form of a potentiometer, for setting the temperature the water is to be heated to.

From the difference between the actual value and the set value of the delivery temperature, the regulator 30 forms a voltage signal, which is applied to the base of a transistor 37, which operates as an amplifier, and whose collector-emitter path lies in the excitation circuit of the solenoid valve 11. This voltage signal is limited by a threshold-value switch 36.

To the output of the threshold-value switch 36 there is also connected the base of a transistor 38, in the form of a switch, whose collector-emitter path lies in the excitation circuit of a time-delay relay R1, which has a normally open contact r1, which controls a first connecting lead 40 of a driving motor for driving the fan 17, which motor is in the form of a split-pole motor 41, having two field coil.

elements 42 and 43. The arrangement, which is further described hereinafter, is such that the motor can be operated either with both coil elements 42 and 43, or with the coil element 43 only, the two coil elements being connected in series in the former case. The motor then runs at a high speed; and runs at a lower speed when the coil element 42 is disconnected.

The output of the regulator 30 is connected by way of a second threshold-value switch 50 to the base of a second switching transistor 51, which controls the excitation current of a second relay R2. The latter has a normally open contact r2, in a second connecting lead 52 of the motor 41, which lead 52 is connected to the junction of the coil elements 42 and 43. The relay R2 is also provided with a normally closed contact r22 in the connecting lead to the base of the transistor 38. The current of air created by the fan, or a pressure differential derived from this air current, actuates- a switch 60, whose switching contact lies in the excitation current circuit of the solenoid valve 11, and closes this circuit only when the fan is running. The signal voltage at which a change of speed of the fan takes place is selectable by means of a potentiometer 61 in the threshold-value switch 50.

The method of operation of the control apparatus is described hereinafter with reference to Figs. 3 and 4. In Fig. 3, the voltage signal V, delivered by the regulator 30, is shown as a function of the required output, or loading B, of the appliance. The regulator 30 provides a voltage signal of 24 volts when the appliance is to operate at its rated capacity to supply a maximum heat requirement As the heat requirement decreases and the actual value of the delivery temperature approximates more closely to the set value, the levet of the voltage signal V also decreases. The threshold-value switch 36 is so constructed that it responds when the voltage signal V drops to approximately 1 3 volts, which corresponds to 30% of the rated output of the appliance. Below this threshold, the appliance remains switched off, since, in this output range, satisfactory ignition of the burner is no longer guaranteed. The amplifying transistor 37 is so arranged that it is fully turned on at a base voltage of approximately 24 volts, and changes the solenoid valve 11 to its fullyopen position. On a drop in the base voltage, the collector-emitter resistance of the transistor 37 increases, whereby also the aperture cross-section of the solenoid valve 11 is increasingly reduced. At a base voltage of approximately 1 3 volts, the solenoid valve 11 throttles the fuel line 10 to such an extent that the burner operates at approximately 30% of its rated capacity.

The switching transistor 38 switches on when the threshold-value switch 36 responds.

The threshold-value switch 50 is so arranged that it responds when the voltage signal V of the regulator 30 reaches approximately 1 7.4 volts, which corresponds to approximately 2/3 the rated capacity of the appliance. The transistor immediately switches on when the threshold-value switch 50 responds. The motor 41 of the fan 1 7 is so arranged that, at the lower speed, its effective output corresponds to the air flow prescribed by official regulations at 2/3 the rated capacity of the appliance.

If, during starting-up of the appliance, the thermostat calls for the full output of the appliance, first of all, all three transistors 37, 38 and 51 switch on. The relay R2 closes the normally open contact r2, and opens its normally closed contact r22, so that, by the time the transistor 38 is turned off, the time-delay relay R, has not responded, and therefore its switching contact r, remains open. The coil element 43 of the fan motor 41 is energised by way of the closed contact r2,, so that this motor runs at its full speed and delivers the amount of air required by the regulations, corresponding to the rated capacity of the appliance. As soon as the fan has started, the switch 60 closes, whereupon the solenoid valve 11 receives the full voltage and fully opens the gas line 11.The burner 1 2 is ignited by ignition means (not shown), and the appliance is started up.

This operating mode, corresponding to the rated capacity of the appliance, is denoted by point N in the diagram of Fig. 4. As the water temperature rises, the thermostat signals a reduced heat requirement, so that the output signal V of the regulator 30 decreases, and the opening stroke of the solenoid valve 11 is reduced. On the other hand, the transistor 51 remains fully switched on until the output drops to 2/3 the rated capacity, so that the fan motor 41 continues to run at its full speed, and the excess-air coefficient is slightly increased as compared with the value officially prescribed for the rated output; This is due to the fact that the water efficiency 17 of the appliance drops slightly, as shown by the line M in Fig. 4.

When, as the amount of heat required decreases, the output is throttled down to 2/3 the rated output, at point 0 the efficiency 11 has dropped slightly relative to the original value. At this output, the voltage signal V has reached the value of 1 7.4 volts, at which the threshold-value switch 50 blocks the flow of current and the relay R2 drops out. Consequently, the normally open contact r2, opens and the normally closed contact r22 closes, so that the relay R, then responds, and its normally open contact r, switches on both coil elements 42 and 43 of the fan motor 41.The effective output of the fan is thereby reduced to such an extent that the excess-air coefficient again reaches the officially prescribed value and the water efficiency of the appliance rises to the point P, which corresponds to the rated-capacity efficiency q,. On a further reduction of the heat requirement, the fuel flow is further throttled down; the fan, however, continues to be driven at the low speed. This again causes a slight drop in the efficiency of the appliance to the point R, which is reached at 30% of the capacity of the appliance. At this output, the threshold-value switch 36 responds and interrupts the connection to the regulator 30, whereupon both the transistor 37 and the transistor 38 block the flow of current. The solenoid valve 11 is closed, and the relay R, drops out, with the result that the fan motor 41 also stops. On an increase in the heat requirement, the described processes take place in the reverse sequence.

The diagram of Fig. 4 shows that the efficiency of the appliance falls only slightly within a limited heat-requirement range, which is readily acceptable, having regard to the advantages achieved.

Claims

1. A gas-heated appliance having a mounting chamber which houses the gas burner, a combustion chimney connected to the exterior of the chamber via an exhaust duct, and a heat exchanger, and is connected via an air intake duct to the exterior of the mounting chamber; the heater also having a fan located in the exhaust duct or the air intake duct, and having a control system which influences a fuel delivery valve in proportion to the heat requirement of a thermostat, and matches the effective output of the fan to the burner output, wherein the fuel valve is a valve which is controllable by means of an electrical regulator, serving as a modulating means, and there is associated with the fan a switching device for influencing the effective output of the fan; said switching device being connected, in parallel with the modulated fuel valve, to the regulator.

2. An appliance according to claim 1, wherein the control system incorporates an operational amplifier for converting the heat requirement of the thermostat into a voltage signal, and the fuel valve is a solenoid valve controlled by the voltage signal.

3. An appliance according to claim 2, wherein the fan has an electric driving motor which is operable at at least two speeds, and the switching device, associated with the fan, is provided with a voltage-dependent threshold-value switch for determination of the speed of the motor.

4. An appliance according to claim 3, wherein the theshold-value switch of the switching device for the fan responds and switches the fan from the reduced speed to the full speed when, on an increase in the heat requirement and in the fuel-valve aperture, the output of the burner reaches appro - mately 2/3 the rated output.

5. An appliance according to claim 4, wherein a threshold-value switch is serially connected to the regulator and releases the signal to the fuel valve and the fan motor only when it exceeds a lower threshold value corresponding to a 30% loading of the appliance.

6. An appliance according to any preceding claim, wherein the fan motor is a splitpole motor having a split field coil one element of which can either be disconnected or connected in series with the other element of the coil.

7. An appliance according to any preceding claim, wherein the signal voltage, at which a change in the speed of the fan takes place, is selectable by means of a potentiometer.

8. A gas heated appliance substantially as hereinbefore described with reference to the accompanying drawings.