GB2057686A - Thermal Actuators - Google Patents

Abstract

An actuator used with a flame- failure cut-off valve comprises a pressurised gas filled system consisting of a small bulb 10 locked in a pilot gas flame and connected by a capillary tube 11 to a capsule 20 in the form of a pair of close fitting corrugated plates. The capsule is connected to operate a valve plate 16 with an adjusting element 21 and the cap and the valve plate are prestressed in the closed position. Dimensions of the bulb and capsule and pressures for the gas are given. <IMAGE>

Description

SPECIFICATION Thermal Actuators This invention relates to temperature sensing thermal actuators intended to produce an output function in response to changes in temperature.

There are many existing types of thermal actuator designed for different purposes. For example, a bimetallic actuator is used for many purposes, changes in temperature acting on a bimetallic strip to generate movement which can be used to operate an output element. Such devices are valuable in many applications, but suffer from certain limitations and disadvantages.

If exposed to extremely high temperatures for long periods they tend to malfunction. Another commonly used thermal actuator uses a sealed fluid system containing mercury and including a bulb to be located at the thermal sensing position, a flexible capsule for generating an output movement, and an interconnecting capillary tube.

Mercury is chosen for its well-known properties of remaining liquid over a wide range of temperatures and in particular having a high boiling point. For use in sensing flame temperatures, for example as a flame failure device, a mercury filled system can generate an appreciable output movement and appreciable output forces during the transition from the liquid to the vapour phase, or vice versa.

A mercury filled system, however, suffers from a number of disadvantages. Mercury being highly toxic is not desirable and sometimes not acceptable for various applications, but particularly in domestic or living conditions.

Mercury is also extremely expensive and highly corrosive at high temperatures, which increases the risk of leakage. Furthermore if there should be a blockage in the capillary tube the risk of bursting a component of the system is appreciable, and this risk is aggravated by the need to restrict the volume of the components and hence the diameter of the capillary to minimum values.

Theoretically it is possible to use other liquids within such a sealed system, but in practice all known liquids suffer from various disadvantages.

For example at 360by the vapour pressure of water is over 18,600 kPa., and at flame temperatures the vapour pressure would be wholly unacceptable for most designs.

To use a gas instead of a liquid within the system might appear to be most undesirable. The thermal expansion co-efficient is inherently low and the output movement and the output force generated must be very much smaller than in the case of a mercury system. Surprisingly, however, it has now been discovered that a gas filled system can, by careful control of the parameters, produce valuable results.

By careful choice of the filling pressure at which the system is sealed coupled with design of the relative volumes of the components of the system and the stiffness of the flexible capsule, a gas filled actuator of this type can be used to provide effective control of an output element over a useful range of temperature changes. The gas will expand according to the normal gas laws approximately on a linear basis which facilitates design and calculation. A suitable gas can be chosen which will be inert or non-corrosive at the operating temperatures. The gas can be completely non-toxic and will be in any case relatively inexpensive. Filling presents little problem and the actuator can therefore be relatively simple and inexpensive to manufacture.

Gas filled thermal actuators according to the invention can be used for a wide variety of purposes. They may function, for example, as automatic gas control flame failure devices in gas burning equipment such as domestic stoves or boilers. They may also be used as flame detecting devices to operate sprinkler valves or switches in electric warning circuits. In general they may be used in any application where an output function is required automatically from a change in temperature.

Broadly stated the invention consists in a temperature sensitive actuator comprising a sealed system including a temperature sensing bulb, a flexible capsule and a small bore capillary tube interconnecting the bulb with the capsule, the system being filled with a gas at a pressure above atmospheric at room temperature.

Preferably the system is filled at an absolute pressure of 100 to 1 500 kPa, or more preferably 110 to 400 kPa, for example at 250 kPa. The filling pressure needs to be selected with various related factors in consideration. If the filling pressure is too great the pressure increases in operation and may cause the capsule to distort permanently. The pressure thus depends partly on the strength and stiffness of the capsule. The filling pressure should also take allowance for possible variations in altitude. Even a flame failure device for a domestic gas burner may be installed at any altitude up to, perhaps, 10,000 ft.

According to a preferred feature of the invention the gas is monatomatic and preferably inert (Group 8a of the Periodic Table) such as argon, helium, neon or krypton. These gases do not penetrate through metal envelopes at high temperatures. Helium has special advantages since it enables leaks to be detected, and is cheap and has high thermal conductivity which improves the speed of response. The inert gases are much preferred but may not always be essential. A gas such as CO2 or N2, which is relatively non-corrosive at lower operating temperatures, might possibly be used. It is also possible to use mixtures of gases. However, in general, it is not possible to use any organic hydrocarbon gases and, of course, the selected gas must not liquify at the lower end of the temperature range and must not explode at the higher end.

The dead volume consisting of the space within the capillary tube and the collapsed capsule should be held to a minimum since this is relatively unheated and does not contribute to the expansion. Moreover, the dead volume tends to reduce the output movement from the capsule.

Preferably, therefore, the dead volume is restricted to not more than 30% of the total volume of the system including the bulb.

The volume of the bulb should be large enough to produce the required output, but excessive size is a disadvantage. Conveniently, the volume of the bulb is between 200 and 1000 mm3 and preferably between 300 and 500 mm3.

The capsule may take various different forms.

Theoretically, a bellows or a Bourdon tube may be used, but their design is complex and in some applications they are unsuitable. In any case it is desirable that the dead volume should be at a minimum and according to a preferred feature of the invention the capsule comprises two opposed walls which are capable of movement towards and away from one another and which in their collapsed condition define an internal volume of less than 50 mm3.

The two walls may be flat, but for flexibility it is preferred that they should be non-planar and of approximately the same shape to "nest" or fit closely one against the other.

At least one of the capsule walls is preferably prestressed in the collapsed or limiting position.

The resilience in the wall therefore acts partly as a return spring on the output element and, in addition, tends to restore this wall to its minimum volume collapsed state.

The invention also consists in a thermal actuator as defined in conjunction with a mechanical output element connected to a movable part of the capsule. The output element may be arranged to operate a fluid control valve or an electric or magnetic switch or some other external control device.

The invention may be performed in various ways and some details will now be given by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic sectional side elevation illustrating an automatic flame failure device for a gas burning appliance, Figure 2 is a diagrammatic view illustrating a thermal actuator according to the invention coupled to a micro switch forming part of an alarm circuit, and Figure 3 is a diagram illustrating the relationship between temperature and capsule extension for different filling pressures and capsule flexibility.

Referring to Figure 1, a stainless steel bulb 10 is located in the flame of a pilot gas jet (not illustrated) and is connected by a small bore capillary tube 11 to a gas control unit 12. This has a main gas inlet 13 and an outlet 14 with a passage controlled by a circular valve plate 1 6.

The capillary tube is set into a bore in the housing and located by a block 17 and adhesive 18 and is hermetically sealed to an expandable capsule 20 which consists of two flexible metal plates formed with mating annular corrugations. The top plate is effectively anchored and the bottom plate is secured to a screw threaded stem 21 attached to the circular valve plate, formed with a corresponding screw thread at its centre.

The sealed system including the bulb, capillary tube and capsule is pressure filled with helium at a pressure of approximately 250 kPa (absolute) at normal room temperature (1 50C). The valve plate diameter is approximately 38 mm, the capsule diameter is between 5 mm. and 50 mm. and preferably about 18 mm, and the bulb volume is approximately 500 mm2. In use the initial loading on the capsule 20 and valve plate 1 6 is adjusted by turning plate 16 on the stem 21 until the valve plate is tight closed and the capsule 20 is slightly pulled apart and thus prestressed.

It will be noted that the capsule is subject externally to the pressure within the gas flow outlet which in practice can be taken to be approximately atmospheric. Atmospheric pressure changes will therefore to some extent affect the operation. The inlet gas pressure acts upwards on the valve plate and this augments the valve closing force produced by the prestressed resilience in the capsule itself.

In the example of Figure 2, the temperature sensing system is similar to that described above and like parts are indicated by the same reference numerals. In this case the output element or stem 21' attached to the capsule 20' is arranged to act through a rocking lever 24 on a micro switch 25 forming part of an electrical warning circuit. It will be understood that the actuator may be arranged to produce a mechanical output function of any type for any particular application. The capsule may act directly as a mechanical actuator, for example to open a fire sprinkler valve, or it may act through a mechanical multiplying device such as a lever, or it may act on an amplifying or servo system through a pilot valve.

The stiffness or flexibility of the capsule is an important design parameter of the system, and preferably should be between about 5x10-10 meters per pascal and 5x 10-8 m/pa. The best practical value is approximately 3x10-8 m/pa.

The invention offers a number of very valuable and rather surprising advantages. By using a pressurised inert gas filling, as opposed to a liquid vapourfilling as with a mercury filled system, an actuator according to the invention can be used to sense temperatures in the range below 350 or.

Moreover, by appropriate choice or adjustment of the thickness and characteristics of the rubber seal at the seat of the valve 1 6 the opening temperature can be made to rise, fall or remain stable at different ambient temperatures. In other words variations in ambient temperature can be compensated by varying thickness or material of the valve seat.

The use of a pressurised gas filling is particularly important. Firstly, it enables leaks to be detected. Secondly, it greatly improves the performance of the system, i.e. the extent of valve movement for any selected temperature change.

Thirdly, it enables the resilience of the capsule 20 to be used as an automatic return spring so as to provide an additional "fail/safe" feature. The use of an additional return spring at extra expense is removed. In operation the sealed system 10, 11, 20 is initially filled at a pressure which causes the two walls of the capsule 20 to separate slightly and become slightly prestressed. The adjustment of the valve plate 16 on the stem 21 adds further stress to the capsule. If there is any failure in the system the natural resilience of the capsule will then pull the valve seat 1 6 tightly closed holding it stressed against the valve seat.

The invention is particularly useful as an automatic flame failure shut off valve to control å main gas valve in response to failure of the flame at a pilot jet connected to the same gas supply.

Flame temperatures may be of the order of 11 000k and the device may be set to close the gas valve at approximately 7000k.

The Diagram of Figure 3 The curves show the variation of the TOTAL capsule extension with phial temperature for thermal systems with the following characteristics:- Phial volume-500 mm3 Dead volume50 mm3 Capsule Diameter-1 8 mm The absolute fill pressure is shown on the curve, and curves are shown for different capsule flexibilities.

Notes 1. Increasing the fill pressure increases the gradient of the curve, and therefore improves the valve performance.

2. A particular design of capsule cannot be taken beyond the maximum extension without adversely affecting it. (For 302 steel, 1 8 mm diameter capsules, this is about 1.1 mm). In selecting a design this must be taken into account.

3. Stiffer capsules tend to give lower gradients, (hence poorer performance) even if the fill pressure is increased so that the initial extension is the same as a less stiff design (see 200 KPa, 5x10-9 m/Pa and 600 KPa, 10-9 m/Pa curves).

4. Thus for maximum performance, low stiffness capsules are to be preferred. However, in low stiffness designs the opening temperature is more sensitive to changes in atmospheric pressure. Thus the best compromise of stability and performance must be found.

Claims (13)

Claims

1. A temperature sensitive actuator comprising a sealed system including a temperature sensing bulb, a flexible capsule and a small bore capillary tube interconnecting the bulb with the capsule, the system being filled with a gas at a pressure above atmospheric at room temperature.

2. A thermal actuator according to claim 1, in which the system is filled at an absolute pressure of 100 to 1 500 kPa, or more preferably 110 to 400 kPa, for example at between 150 and 225 kPa.

3. A thermal actuator according to claim 1, or claim 2, in which the gas is monatomic and preferably inert such as argon, helium, neon or krypton.

4. A thermal actuator according to claim 1, or claim 2, in which the gas is relatively inert and non-corrosive at the operating temperatures, for example CO2 or N2.

5. A thermal actuator according to any of the preceding claims, in which the dead volume of the capillary tube and capsule is not more than 30% of the total volume of the system including the bulb.

6. A thermal actuator according to any of the preceding claims, in which the volume of the bulb is between 200 and 1000 mm3 and preferably between 300 and 500 mm3.

7. A thermal actuator according to any of the preceding claims, in which the capsule comprises two opposed walls which are capable of movement towards and away from one another and which in their collapsed condition define an internal volume of less than 50 mm3.

8. A thermal actuator according to claim 7, in which the two walls are non-planar and of approximately the same shape to fit closely one against the other.

9. A thermal actuator according to any of the preceding claims, in which the capsule has an effective diameter between 5 mm and 50 mm.

1 0. A thermal actuator according to any of the preceding claims, in which the capsule is resilient and prestressed in its limiting position and its resilience acts to return the capsule towards a collapsed state.

11. A thermal actuator according to any of the preceding claims, including a mechanical output element connected to a movable part of the capsule.

12. A thermal actuator according to claim 11, in which the output element is arranged to operate a fluid control valve or an electric or magnetic switch or some other external control device.

13. A thermal actuator according to any of the preceding claims in which the flexibility of the capsule is between 5x10-10 and 5x10-8 metres per pascal, or preferably between 10-9 and 10-8 metres per pascal.