GB2283565A

GB2283565A - Fuel burning appliance

Info

Publication number: GB2283565A
Application number: GB9322783A
Authority: GB
Inventors: Richard Arthur George Kinge
Original assignee: BLUE CIRCLE HEATING Ltd
Current assignee: BLUE CIRCLE HEATING Ltd
Priority date: 1993-11-04
Filing date: 1993-11-04
Publication date: 1995-05-10
Anticipated expiration: 2013-11-04
Also published as: GB2283565B; GB9322783D0

Abstract

A fuel burning appliance, typically in the form of a gas fired central heating appliance comprises a fuel burner 1, a fan 3 for supplying combustion air 2 to the gas burner 1, and a control arrangement 4 including a single threshold air flow sensor 5, for increasing the speed of the fan 3 in steps or continuously until adequate air flow is sensed for light-up purposes. The control arrangement comprises a timer 7 operated by a command signal, a latch device 8 operated by the timer and a fan speed controller 9 operated by the latch device. When the command signal is initiated the fan operates at a low speed and the latch device is operated by the timer after a predetermined time if the on/off airflow sensor is not operated to cause the fan speed controller to increase the fan speed. In an alternative embodiment, (Fig. 2) not shown, the fan speed is continuously variable. <IMAGE>

Description

Fuel Burning Appliances This invention relates to fuel burning appliances and more particularly to such appliances in which combustion air is provided by an electric fan. The invention is particularly applicable to gas fired central heating appliances.

In gas fired central heating appliances which incorporate a fan for supplying combustion air to the gas burner, it is necessary to provide an air flow sensing device which, during the start-up procedure, senses that the fan is operating before gas is supplied to the burner. The amount of air required to be supplied to the gas burner may vary from time to time dependent upon particular operating conditions and may vary between different installations dependent, for example, on the particular flue conditions which may exist. Because of this, in the past it has been necessary to use an air flow sensing device which affords an output which is continuously variable with air flow. Such air flow devices are expensive, which expense is reflected in the overall cost of the appliance.

It is an object of the present invention to provide a fuel burning appliance which incorporates a simple on/off (i.e. single threshold) air flow sensor for controlling combustion air to the burner thereof. Such on/off air flow sensors are inherently less expensive than continuously variable air flow sensors and can result in cost savings.

According to the present invention there is provided a fuel burning appliance comprising a fuel burner, a fan for supplying combustion air to said burner, an on/off air flow sensor for sensing the flow of combustion air from said fan, and a control arrangement for increasing the speed of said fan dependent upon the output of said air flow sensor.

In one arrangement in accordance with the invention, it may be arranged that said fan has at least two speeds, and said control arrangement is effective for initially setting said fan to a low speed, and is effective for setting said fan to an increased speed if said on/off air flow sensor is not operated.

In carrying out said one arrangement, it may be arranged that said control arrangement comprises timer means which is operated by a command signal, latch means which is operated by said timer means, and fan speed control means which is operated by said latch means, whereby when said command signal is initiated said fan is caused to operate at said low speed, and said latch means is caused to operate by said timer means after a predetermined time if said on/off air flow sensor has not operated, for causing the fan speed control means to increase the speed of said fan.

In another arrangement in accordance with the present invention, it may be arranged that the speed of said fan is continuously variable, and said control arrangement is effective for initially setting said fan to a low speed, and is effective for increasing the speed of said fan until said on/off air flow sensor operates.

In carrying out said another arrangement, it may be arranged that said control arrangement comprises pulse generator means which is responsive to a command signal, counter means operable on the output of said pulse generator means and fan speed control means which is operated by said counter means, whereby when said command signal is initiated said fan is caused to operate at said low speed and said pulse generator means is caused to be operated to increase the count of said pulse counter and to control the fan speed control means to cause the fan speed to be increased until said on/off air flow sensor operates.

Advantageously, said control arrangement also comprises ignition and fuel controlling means operated by said on/off air flow sensor for controlling the operation of said fuel burner.

The invention is particularly applicable to gas fired central heating appliances.

Some exemplary embodiments of the invention will now be described, reference being made to the accompanying drawings, in which: Fig. 1 is a block schematic diagram of part of a fuel burning appliance in accordance with the present invention; and Fig. 2 is block schematic diagram of part of an alternative form of fuel burning appliance in accordance with the present invention.

In Fig. 1 of the drawings there is depicted part of a fuel burning appliance in the form of a gas fired central heating appliance. The appliance of Fig. 1 comprises a gas burner depicted diagrammatically at 1 which is supplied with combustion air indicated by the arrow 2 by a variable speed electric fan 3 which is caused to operate at a low speed or a high speed under the control of a control arrangement 4.

The control arrangement 4 comprises a single threshold, on/off type air flow sensor 5 which in practice is disposed in the air flow path from the fan 3.

The air flow sensor 5 is essentially an arrangement which signals when adequate air is passing to permit safe and efficient combustion. This component may take many forms including electronic, thermoelectric and calorimetric devices but most typically will comprise a venturi tube or a simple restrictor in the path of the moving air, producing a pressure related to flow which is applied to a mechanical air pressure switch with microswitch contacts as shown diagrammatically in Fig. 1.

The air flow sensor 5 is connected to a command or switch-on signal input 6, which typically, but not essentially, will also be the electrical supply to the system. The command signal 6 may, for example, be derived from a room or appliance thermostat which instructs the control arrangement 4 to attempt light-up of the burner 1.

The command signal connected to input 6 is applied to the fan 3 to cause it to be energised, and via the air flow sensor 5 to a timing device 7. The output from the timing device 7 is applied to a latching device 8 and thence to a fan speed controller 9 which initially sets the speed of the fan 3 to low speed.

The timing device 7 and latching device 8 are also conveniently energised from the command signal 6 as indicated by dashed lines 10.

In response to the command signal at 6, the fan 3 is energised at its low speed as set by the initial state of the latching device 8 in conjunction with the fan speed controller 9. Timing device 7, which allows for any delays in establishing air flow and for the acceleration time of the fan 3, commences timing in response to the signal from the air flow sensor 5.

If the air flow sensor 5 senses adequate air flow from the fan 3 before the timing device 7 has "timed-out", it is caused to operate thereby removing the input to the timing device 7, and maintaining the input to the latching device 8 and fan speed controller 9, so that the fan 3 is maintained in its "low speed" state. In operating, the air flow sensor 5 causes the command signal at 6 to be connected to a conventional arrangement of ignition and fuel controlling devices (such as spark generator and associated fuel handling solenoid valves, etc.) shown diagrammatically at 11, which supervise the safe light-up of the gas burner 1.

If the air flow sensor 5 does not sense adequate air flow from the fan 3 before the timing device 7 has "timed-out", then on "time-out" the timing device 7 will set the latching device 8 to a new state which causes the fan speed controller 9 to change the speed of the fan 3 to "high speed".

If with the fan 3 on high speed, the air flow sensor 5 senses adequate air flow, it is caused to operate as described above, to cause the fan 3 to be maintained at high speed and to cause the ignition and fuel handling devices 11 to be operated for light-up purposes.

If with the fan 3 on high speed, the air flow sensor 5 does not sense adequate air flow, the fan 3 is maintained in this position, but the air flow sensor 5 is not operated so that light-up of the burner 1 is not initiated. This condition is maintained until the command or switch-on signal 6 is removed.

In this way, a simple single threshold, on/off type air flow sensor 5 is used with a variable speed fan 3 in such a way that the fan 3 will always operate at the lowest speed compatible with the need for adequate air flow. This will typically offer reduced audible noise and greater combustion efficiency for the system and will allow a single design to automatically cater for wide field variations of ventilation.

In Fig. 2 of the drawings there is depicted a modified form of the gas fired central heating appliance of Fig. 1. Parts of the appliance of Fig. 2 which correspond to those of Fig. 1 have been given the same reference numeral.

In the appliance of Fig. 2, the control arrangement 4 has been modified to provide a clock pulse generator 12 which is operated by the air flow sensor 5, the output from the clock pulse generator 12 being applied to a counter 13 which controls the fan speed controller 9. It is arranged that the fan speed controller 9 is now of the continuously variable type where the speed of the fan 3 is set by the count of the counter 13.

In operation, this arrangement slowly increases the speed of the fan 3 after the command or switch-on signal 6 is received until a point is reached where the air flow sensor 5 indicates sufficient flow. At this point the clock pulse generator 12 is halted leaving the count on counter 13 fixed thus holding the fan speed at the level reached. Normal ignition and fuel burn will now commence via the control devices 11 whilst the fan 3 operates at the lowest possible speed required for adequate combustion.

It will be appreciated that the fuel burning appliances which have been described have been given by way of example only and may be applicable to other than gas fired central heating appliances. Also, although an air flow sensor 5 which has electrical contacts indicating its status has been used, other types of single threshold air flow sensor 5 may be used to increase fan speed in steps or continuously until sufficient air flow is sensed.

Claims

1. A fuel burning appliance comprising a fuel burner, a fan for supplying combustion air to said burner, an on/off air flow sensor for sensing the flow of combustion air from said fan, and a control arrangement for increasing the speed of said fan dependent upon the output of said air flow sensor.

2. An appliance as claimed in claim 1, in which said fan has at least two speeds, and said control arrangement is effective for initially setting said fan to a low speed, and is effective for setting said fan to an increased speed if said on/off air flow sensor is not operated.

3. An appliance as claimed in claim 2, in which said control arrangement comprises timer means which is operated by a command signal, latch means which is operated by said timer means, and fan speed control means which is operated by said latch means, whereby when said command signal is initiated said fan is caused to operate at said low speed, and said latch means is caused to operate by said timer means after a predetermined time if said on/off air flow sensor has not operated, for causing the fan speed control means to increase the speed of said fan.

4. An appliance as claimed in claim 1, in which the speed of said fan is continuously variable, and said control arrangement is effective for initially setting said fan to a low speed, and is effective for increasing the speed of said fan until said on/off air flow sensor operates.

5. An appliance as claimed in claim 4, in which said control arrangement comprises pulse generator means which is responsive to a command signal, counter means operable on the output of said pulse generator means and fan speed control means which is operated by said counter means, whereby when said command signal is initiated said fan is caused to operate at said low speed and said pulse generator means is caused to be operated to increase the count of said pulse counter and to control the fan speed control means to cause the fan speed to be increased until said on/off air flow sensor operates.

6. An appliance as claimed in any preceding claim, in which said control arrangement comprises ignition and fuel controlling means operated by said on/off flow sensor for controlling the operation of said fuel burner.

7. An appliance as claimed in any preceding claim, in the form of a gas fired central heating appliance.

8. A fuel burning appliance substantially as hereinbefore described with reference to the accompanying drawings.