GB2212916A - Sealed systems - Google Patents

Abstract

A flame failure valve (20, 22, 24, 25) operates to close a gas line (17, 18) when a temperature experienced by phial (10) drops below that consistent with the presence of a flame. The phial (10) forms part of a sealed system (14) in which a flexible container (13) expands to open the said valve. The sealed system is charged with alkaline water, the pressure of which expands the flexible container. In order to achieve a rapid response to extinction of a flame the phial is designed to cool quickly by providing a heat sink or sinks. This may be done by increasing the wall thickness of capillary (12) so that it can conduct the heat away. Other heat sinks, e.g. a mounting, may also be provided. <IMAGE>

Description

SEALED SYSTEMS.

This invention relates to temperature sensing devices of the kind known generally as sealed systems, comprising a phial and a flexible container with a capillary tube interconnecting them, the system being charged with a gas or liquid. When the phial is placed in a flame or other hot area of which the temperature is to be sensed, the pressure of the internal charge increases, so expanding the flexible container.

When the sensed temperature drops, the container contracts, and the movement of the container may then be used to operate a warning or other controlling means. The time taken for the warning or other controlling means to operate is called the response time.

When, for instance, a flame supervision device having a sealed system of this kind is used to detect the extinction of a gas flame, the movement of the container is used to shut off the gas supply in order to prevent build up of an inflammable gas. It is important that the device should respond rapidly to the extinction of the flame, i.e. that its response time should be short.

This invention is concerned with predetermining the response time.

According to one aspect, the invention provides a method of designing a sealed system se as to determine its response time by providing heat sink means to conduct heat away from the phial at a rate selected to produce the predetermined response time. It is now realized that in a sealed system the capillary tube itself is well positioned as a heat sink, since one end is in the hot area and the other end normally in a cooler location. However, the cross-sectional area is not in known designs sufficient to act as an adequate heat sink. Nor is the material selected with its heat conductivity in mind.

Thus, the invention according to another aspect provides a sealed system in which the capillary tube operates as a heat sink by making it of a material of good heat conductivity (e.g. copper) and with the walls thick enough to conduct heat away from the phial at a rate selected to produce a predetermined response time.

The required rate of heat conduction may be achieved by providing other or alternative heat sinks. In one arrangement, a mounting for the phial is designed of material of good heat conductivity, having an area of contact with the phial which provides a path for heat conduction. The mounting may be shaped or located to extend into cooler areas or include vane means for directing a cooler air stream over the mounting so as to aid its operation as a heat sink.

Known flame supervision devices, for instance as described in our patent No. 1,344,260, comprise sealed systems having stainless steel capillary tubes charged with mercury ar,d operating to close a gas valve when a flame is absent at the location of the phial. Such devices are commonly used in gas ccokers, but the presence of mercury in conjunction with food presents a hazard which it is desired to avoid. Attempts have been made to replace the mercury with a harmless charge, for instance of an inert gas or water. These can be made to operate at flame temperatures, but it has been found that their response time on flame extinction has been ur.acceptably slow.

According to another aspect, the invention provides a flame supervision device comprising a sealed system for operating a gas valve, charged with water or a water-based solution, and heat sink means for conducting heat away from the phial on extinction of a flame at a rate appropriate tc achieve an acceptable time. The heat sink means mey comprise the capillary tube and/or a mounting for the phial.

Acceptable response times are usually laid down by cooker manufacturers, and are typically between 40 ard 50 seconds. Under British Standard BS5386 Pt.3 1980 (EN30) time to close should not exceed 60 seconds.

In one arrangement the charge is alkaline water, the capillary tube is of copper with a wall thickness of a nominal 0.25 nit, and the mcunting is of aluminium or ccpper or brass.

A specific embodiment of the invention is now described with reference to the accompanying drawings, in which: Figure 1 is a diagramnatic section through a flame failure device for a gas cooker, and Figure 2 is a detail of a sealed system for use in the device of Figure 1.

The flame failure device includes a gas valve (19, 20) in the flow line between a gas inlet (18) and an outlet (17). The valve head (20) is biased by a spring (21) toward the closed pcsition and is opened by a sealed system (14), which is described in detail with reference to Figure 2.

The sealed system has a phial (10) which in use is located in a flame which is fed with gas through the valve (19, 20). A thick walled capillary tube (12) at one end is connected into or integral with the phial (10) and at the other end is sealed in communication with a flexible container (11) formed from two corrugated metal diaphragms (13). Valve head (20) carries a sealing O-ring (22) which is compressed against the valve seat (19) when the valve is closed. A spider-shaped guide member (24) has spaced legs (25) which locate within the valve opening to maintain the valve head aligned throughout its opening and closing movement. The guide member (24) is rivetted to the centre of the valve head and also carries a short push rod (28) which extends toward the flexible container (11).

At lower temperatures experienced by the phial (10) the flexible container is compressed and dces not contact the push rod (28). As the temperature rises, consequent expansion of the flexible container brings one side of it into contact with push rod (28), further movement then urging the valve head off the seat against the resistance of spring (21).

Figure 2 shows a detailed design of the sealed system for the flame failure device of Figure 1, with the same numerals as are used in Figure 1.

The thick walled capillary (12) is formed of copper tc British Standard 2871 (99.85% minimum quantity of phosphorous deoxidised non-arsenical copper). The tube dimensions are 1.5 nit nominal outer diameter and 0.5 rm7 nominal inner diameter, giving a wall thickness of 0.5 nit. The tube at one end is brazed to the flexible ccntainer (11). The phial (10) is about 55 mn long, of outer diameter 2.5 mm and inner diameter 1.5 itm, formed of the same copper as the capillary, and is brazed to the other end of the tube (12). The phial me,y be protected by plating (e.g. with nickel) from the corrosive effects of high temperatures. A cylindrical ferrule (26) is crimped at a convenient pcint along the phial to mount the phial in the path of a flame. The ferrule is a sliding fit over the phial and is between 5 and 10 imi long. The inner surface area of the ferrule is thus in intimate contact with the outer surface of the phial providing for the transfer of heat to the ferrule.

The charge in the system is de-ionized water containing additive material to reduce corrosion. The additive may be potassium hydroxide in sufficient quantity to render the pH value of the water just on the alkaline side of neutral, e.g. pH8 - 9. Another additive may be potassium bromide for reducing the freezing point of the water. The end (27) of the phial is crimped and sealed closed after the system is charged.

A mounting (not shown) holds the phial by means of ferrule (26) in a flame. With this arrangement, it is found that the sealed system shuts down the valve in 40 seconds from flame extinction. Because the heat conduction away from the phial is increased to make it cool more rapidly, it will be appreciated that the phial also heats up more slowly when the flame is first ignited, i.e. start up time is increased as the response or shut down time is decreased. However, in any case during start up the flame failure supervision has to be over-ridden by other means not shown here in order for the flame to be lit.

The spring (21) exerts a load on the charge, such that it is always under more than atmospheric pressure. Thus the boiling point of the charge is increased. In this example, a spring load of 3 lbs is used, 0 which produces a boiling point of 130 C. It will be appreciated that the higher the boiling point, the more rapid the shut-down time, because that part of the charge which has vapourised will condense when the cooling phial reaches the boiling point terperature of the charge, producing a sudden decrease in internal pressure.

It is further found with this example that, the phial being of copper connected to a gocd heat sink, the temperature which the phial reaches and maintains when located in a flame is considerably less than it would otherwise be. Thus, the amount of heat to be dissipated on extinction of the flame is less, and this effect is considered to contribute to the rapid shut-down achieved.

It will be appreciated that the heat conduction rate from the phial is influenced by the difference in temperature of the ends of the heat sink as well as the heat conduction coefficient of the material and the cross-sectional area of the heat sink. Thus when calculating dimensions it is necessary to be aware of the particular environmental temperatures to which the whole of the sealed system is to be subjected. The dimensions of the capillary may be calculated using known heat conduction factors. It is, for instance, known that copper is a better heat conductor than steel so that thinner copper capillary may be used to achieve the same shut down time as a thicker steel capillary.However, trial and error in the particular cooker for which the system is intended may be used, starting from the example given above and substituting thicker or thinner capillary according to the results found. It will be clear that other material than copper can be used. If a capillary tube of stainless steel to AISI 304 having a nominal outer diameter of 1.25 mm and a nominal inner diameter of 0.5 mm is substituted for the copper capillary described above, then the valve closes in 60 seconds.

To reduce this time the capillary tube may be made with a larger outside diameter, e.g. 1.7 mm. Thus the time taken for the sealed system to respond to temperature changes can be either increased or decreased.

Either in place of, or additionally to the use of the capillary as a heat sink, the mounting for the phial may also be designed to operate as a heat sink. For this purpose the mounting is made of material of good heat conductance, e.g. aluminium, copper or a copper based alloy or brass. The mounting is designed to have a good area contact with the ferrule, and may be extended to contact the phial and/or the capillary. The temperature gradient across the mounting is important, and in some arrangements the mounting is extended so as to contact a cooler air stream. The mounting may have vanes or heat exchanging ribs to dissipate the heat. This heat dissipation may be sufficient in itself to produce the desired response time, or may be additional to the use of the capillary tube.

Claims (17)

CLAIMS:

1. A method of designing a sealed system so as to determine its response time by providing heat sink means to conduct heat away from the phial at a rate selected to produce the predetermined response time.

2. A method as claimed in claim 1, comprising designing the capillary tube to function also as said heat sink means.

3. The method as claimed in claim 2, comprising designing a mounting for the phial to function also as part of said heat sink means.

4. A sealed system in which the capillary tube operates as a heat sink, being formed of good heat conductivity material and with walls thick enough to conduct heat away from the phial at a rate selected to produce a predetermined response time.

5. A sealed system as claimed in claim 1, in which the capillary tube is formed of copper, or of stainless steel.

6. A sealed system as claimed in claim 4 or claim 5, wherein said walls are between 0.25 mm and 0.75 mm thickness.

7. A sealed system as claimed in any of claims 1 to 6, comprising mounting means for the phial formed of good heat conductivity material and adapted to contribute to the production of said predetermined response time.

8. A sealed system as claimed in claim 7, wherein said mounting means includes a metal ferrule closely embracing said phial so that a surface of the ferrule contacts the outer surface of the phial and a mounting member holding said ferrule and adapted for connection to other structure.

9. A sealed system as claimed in claim 8, wherein said mounting member has vanes or heat dissipating ribs adapted to increase the heat sink operation.

10. A sealed system as claimed in any of claims 1 to 9, wherein said sealed system is charged with water.

11. A sealed system as claimed in claim 10, wherein said water contains a corrosion reducing additive.

12. A sealed system as claimed in claim 11, wherein said additive is potassium hydroxide in sufficient quantity to render the water alkaline.

13. A flame supervision device having a sealed system as claimed in any of claims 1 to 12.

14. A flame supervision device as claimed in claim 13, having spring means adapted to increase the pressure on the charge in the sealed system.

15. A sealed system substantially as described hereinbefore with reference to the accompanying drawings.

16. A flame supervision device substantially as described hereinbefore with reference to the. accompanying drawings.

17. A method of designing a sealed system substantially as described hereinbefore.