GB2222015A

GB2222015A - Flame failure devices

Info

Publication number: GB2222015A
Application number: GB8816237A
Authority: GB
Inventors: Michael Clift
Original assignee: Teddington Controls Ltd
Current assignee: Teddington Controls Ltd
Priority date: 1988-07-07
Filing date: 1988-07-07
Publication date: 1990-02-21
Anticipated expiration: 2008-07-07
Also published as: GB2222015B; GB8816237D0

Abstract

A flame failure device comprises a temperature-sensing device (15 Fig. 1, not shown) located in a flame operating as an electrical switch to control the energization of a solenoid (18). Gas valve (14) supplies gas to fuel the flame and is maintained open when the device (15) senses a high temperature commensurate with the presence of a flame. The device comprises a pair of conductors 26, 27 bridged by a slug 29 of soda glass which has the property of conducting when a temperature of 425 DEG C is reached. The conductors are preferably nichrome wires. In an alternative embodiment, Fig 3 (not shown), one conductor is a nichrome wire and the other conductor (21) is a stainless steel cylinder surrounding the wire. <IMAGE>

Description

FLAME FAILURE DEVICES.

This invention relates to flame failure devices for detecting the extinction of gas flames.

It is a known characteristic of certain materials that they are normally highly electrically resistant, but when heated to known temperatures undergo a change to an electrically conducting state.

Such materials will be referred to hereinafter as heat sensitive conductors. One example of a heat sensitive conductor is glass.

This invention provides a flame failure device comprising a pair of spaced electrical conductors bridged by a heat sensitive conductor located at or near the tip of an elongated cover, whereby when said tip is exposed to a flame it causes the heat sensitive conductor to conduct.

Said tip may comprise only a minor portion of the device, the remainder operating as a heat sink.

Preferably said heat sensitive conductor comprises a slug of material between 3 and 5 rrrr long. There may be a distance of between 3 and 11 rtrr from the tip of the elongated cover. The device may be between 20 mm and 40 itrn long.

The heat sensitive conductor may comprise soda glass.

The spaced electrical conductors may be two wires or a wire and a metal sheath. Ceramic beads may be used to insulate the conductors and to define the location of the heat sensitive conductor.

In use, the tip of the device is located in or near a flame so that the heat sensitive material is heated above its known changeover temperature and completes an electrical path between the spaced electrical conductors. The conductors are connected in an electrical circuit whereby when the electrical path is completed a gas valve is opened.

Specific embodiments of the invention are now described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic layout of a cooker burner having a flame failure device, Figure 2 is an electrical circuit for the flame failure device of Figure 1, Figure 3 is a section through a temperature sensor for use in the flame failure device of Figure 1, and Figure 4 is a section through another form of temperature sensor for use in the flame failure device of Figure 1.

A gas cooker burner (11) as shown in Figure 1 is supplied with gas along line (12). A solenoid-operated gas valve (14) is connected in the line (12) and is adapted to close, shutting off the gas supply, if the gas at burner (11) is not ignited. It will be appreciated that if the gas were not burning, then a build-up of unburnt gas would occur, which could then cause an explosion. To prevent this happening, a flame failure device detects that the gas at burner (11) is not burning, and closes the valve (14) by releasing the solenoid.

The flame failure device includes a temperature sensing device (15) located in or adjacent a flame from burner (11), and operating as an electrical switch, as described in greater detail hereinafter. The sensing device as shown in Figure 2 is connected in series in the mains electrical supply to a full wave rectifier (17) which is connected to energise the coil (18) of solenoid valve (14). When the temperature sensor (15) senses a low temperature, it presents an open circuit condition so that the coil (18) is not energised and the gas valve (14) remains closed. When the sensing device (15) senses a higher temperature commensurate with the existence of a flame, it presents a closed circuit condition, coil (18) is energised, and the gas valve (14) is opened.

The temperature sensor (15) may be either as shown in Figure 3 or in Figure 4. Referring first to Figure 3, a nichrome wire (20) extends along the axis of a stainless steel cylinder (21) of outer diameter 3.0 turn, leaving an annular gap therebetween. A single insulating ceramic bead (22) is threaded onto the wire, and is followed by a charge of powdered glass which is packed down to form a 5 mm long slug (23) which contacts both the cylinder (21) and the wire. Further cylindrical ceramic beads (24) are threaded onto the wire to extend the rest of the length of the cylinder (21). An insulating cover (25), e.g. of ceramic, extends over the cylinder (21). The length of the cylinder is about 30 mm, and the slug of powdered glass is located about 9 mm from its free end.

Electrical connections (not shown) are made to the nichrome wire (20) and the stainless steel cylinder (21). At normal ambient temperatures the glass does not conduct electricity, so that there is no electrical path between the wire (20) and the cylinder (21). Thus the circuit is open at normal temperatures, and no electrical power is supplied to coil (18).

At an elevated temperature the powdered glass changes characteristics to become a conductor. This establishes an electrical path between the wire (20) and cylinder (21), and provides for a power supply to coil (18) to maintain valve (14) open. The temperature at which this change occurs depends on the composition of the glass. Typical flame temperatures encountered are between 60CO - 85Ci C. The glass used in this application is soda-lime glass which changes its characteristic at around 425 0C. Thus, when the temperature sensor is placed in or adjacent a flame, as shown in Figure 1, the glass is heated to or above its characteristic temperature, conducts, and coil (18) is energised.

As an alternative to powdered glass, a short length of glass tubing may be used, slipped over the wire in the same way as the ceramic beads.

The glass may be heated after assembly to cause it to flow into any gaps.

A flame failure device for shutting off a gas valve is required to respond rapidly to the extinction of a flame, i.e. to reducing temperature, this being more important than its reaction to rising temperature. In order to aid the response, it is necessary that the temperature of the glass should drop as quickly as possible. For this to occur, it is now realised that the amount of heat taken in by the device can be reduced and the rate at which it is conducted away can be increased. Thus, only the tip of the device is located in the flame and the slug of powdered glass is concentrated in this area.

The e major length of the device is not directly in the flame and remains cooler, acting as a heat sink when the flame is extinguished. By this means a response time on extinction of the flame of 45 seconds is achieved. It will be appreciated that as soon as the glass temperature 0 drops below 425 C the changeover occurs to indicate flame failure.

The alternative design of temperature sensor shown in Figure 4 has two nichrome wires (26, 27) across which the circuit is made. Each wire is insulated by cylindrical ceramic beads (28). A ceramic sheath (30) of length 20 mm and outside diameter 4.4 mm houses the end parts of the insulated wires and a 4 itrn long slug of powdered glass. It will be seen that the glass is compressed into intimate contact with the ceramic sheath and both of the wires (26, 27). When the tip of the sensor is placed in a flame, the ceramic sheath is heated, heat is transferred to the glass slug, which, above its characteristic temperature, conducts. An electrical path is thus made between the two wires (26, 27), which then present a closed circuit to the power supply line seen in Figure 2.In this arrangement, the glass slug (29) is located between 5 and 11 mm from the tip of the device. The maximum diameter of the device is only 4.4 mn. Thus the heat capacity of the device is kept low, so that the heat to be conducted away is reduced. Only the last 11 mm of the device containing the slug of glass needs to be heated to the operating temperature, allowing the major portion of it to remain cooler and act as a heat sink.

In an alternative arrangement, the separate ceramic beads (28) for the two wires are combined into single beads each having two parallel holes through which the wires pass.

Although the above examples use soda-lime glass for its characteristic of rapid changeover to a conducting state when it reaches a known temperature, there are other materials which have similar characteristics, but at different temperatures. It is within the invention to select a different material to suit the expected temperature conditions.

In use, the flame failure device may operate in either a pilot burner or in the main gas burner. There may be over-ride means (not shown here) for allowing a gas supply sufficient for the burner to be lighted on starting up.

Although a full wave rectified energisation is described above with reference to Figure 1, it is clear that an A.C. valve operating device could be used without rectification of the supply.

Claims

CLAIMS:

1. A flame failure device comprising a pair of spaced electrical conductors bridged by a heat sensitive conductor located at or near the tip of an elongated cover, whereby when said tip is exposed to a flame the heat sensitive conductor conducts, indicating flame present, and when the flame is extinguished the heat sensitive conductor becomes nonconducting, indicating flame absent.

2. A flame failure device as claimed in claim 1, wherein said heat sensitive conductor occupies only a minor portion of the elongated cover, the remainder operating as a heat sink.

3. A flame failure device as claimed in claim 2, wherein said heat sensitive conductor is between 3 and 5 inn long.

4. A flame failure device as claimed in any of claims 1 to 3, wherein said conductor is formed of soda glass.

5. A flame failure device as claimed in claim 4, wherein said soda glass comprises powdered glass, or glass tubing.

6. A flame failure device as claimed in claim 5, wherein said glass is heated after assembly to cause it to flow into any gaps.

7. A flame failure device as claimed in any of claims 1 to 6, wherein said spaced electrical conductors are two wires or a wire and a metal sheath.

8. A flame failure device as claimed in claim 7, having ceramic beads insulating the spaced conductors, and defining the location of the heat sensitive conductor.

9. A flame failure device as claimed in any of claims 1 to 8, wherein said elongated cover is about 30 mm long and the heat sensitive conductor is spaced about 9 mm from its tip.

10. A flame failure device as claimed in any of claims 1 to 9, including a solenoid for operating a gas valve, and an electrical power connection, said heat sensitive conductor being connected as a switch allowing power to the solenoid to open the valve when a high temperature is sensed, and cutting off power to the solenoid when a low temperature is sensed.

11. A flame failure device substantially as described hereinbefore, with reference to Figures 1, 2 and 3, or to Figures 1, 2 and 4 of the of the accompanying drawings.