GB2147390A - Control mechanisms - Google Patents

Abstract

A damper valve (11<1>) controls an air inlet to a solid fuel boiler. A lever (14<2>) operates the valve under the control of temperature sensitive means comprising a phial (20<1>), capillary tube (21<1>) and expansible capsule (22<1>). Temperature adjustment means may be used to set the temperature at which the temperature sensitive means opens and closes the valve. The mechanism includes fail-safe means for ensuring that in the event of failure the mechanism maintains the boiler in a safe condition by closing the valve. The fail-safe means may be a second temperature sensitive means operating in parallel with the first such means. The two capsules (22<1>) bear on a lever (40) which is linked to the lever (14<1>). In normal use the lever (40) moves in a straight line to pivot the lever (14<1>). Upon failure of one of the temperature sensitive means, the lever (40) tilts and is disconnected from the lever (14<1>), allowing the latter to move to a safe condition under the action of a spring. The mechanism is also used to control an electric switch. When a single temperature sensitive means is used this is made fail-safe by providing a sub-ambient pressive within the capsule so that it expands if a leak occurs. <IMAGE>

Description

SPECIFICATION Control mechanisms This invention relates to control mechanisms in which a mechanical movement is used to maintain a desired condition.

The invention provides a control mechanism comprising a first expansible capsule, means for subjecting said capsule to a variable pressure related to a desired physical condition whereby said capsule expands or contracts, means for translating the consequent movements of the capsule into a control for said physical condition so as to maintain it at a desired level, and fail-safe means for ensuring that in the event of failure the mechanism fails to safety maintaining said physical condition at a safe level.

The mechanism may be for use in a solid fuel boiler controlling an air supply to the boiler and being subject to variations in flue pull due to wind conditions.

Preferably said capsule comprises two metal discs secured together around their peripheries and where lower than atmospheric pressure fill is used, the diameter is more than 2 cms.

There may be another capsule providing said fail-safe capability.

Specific embodiments of the invention are shown in the accompanying drawings, in which: Figure 1 is a representation of part of a damper valve and thermostatic control means therefor, Figure 2 is an alternative valve and thermostatic control means, Figure 2a is a side view of the valve and control means of Fig. 2, Figure 2b is a side view showing the control means of Fig. 2 operating a switch, and Figure 3 is another thermostatic control means.

The damper valve (11) controls an air inlet to a boiler, situated for instance beneath a fuel burning bed (not shown). The valve comprises a seat (12) and a movable valve plate (13). A lever (14) is pivoted at one end to a stem (15) secured to the plate (13) so that movement of the lever as described hereinafter moves the plate on and off its seat.

At its other end lever (14) is pivoted to a temperature adjustment means (17) comprising an adjustment screw. Alternatively, the temperature adjustment means may be a cam device or as described in Patent No.

1,141,842. Rotation of the adjustment screw aiters the temperature at which the valve (11) opens and closes, by altering the position of the lever's fulcrum.

Beneath the fulcrum, lever (14) is pivotally secured at (18) to temperature sensitive control means for pivoting the lever and thus opening and closing the valve (11). This comprises a temperature-sensing phial (20), located at a position to sense the temperature being affected by the boiler, e.g. a water temperature or a room temperature. The phial (20) is connected by a capillary tube (21) to an expansible capsule (22) of diameter 2.5 cms. The capsule (22) comprises two flexible metal discs secured together around their circumferences. The phial, capillary and capsule together form a sealed system partially filled with a liquid, such as carbon tetra-chloride, of a low viscosity comparable to that of water. A vapour space is formed in the system, the vapour pressure in which is a function of the temperature sensed by the phial.This pressure determines the expansion of the capsule (22).

Attached to the capsule (22) is a push rod (23) providing the pivot (18), whereby the lever (14) is moved forward and back by expansion of the capsule in response to temperature variations at phial (20). Thus, when the damper valve (11) is open, an increased air supply to the firebed increases the heat output of the boiler, so heating the phial (20).

This causes the capsule to expand and slowly close the damper valve until, when a temperature determined by the setting of the temperature adjustment means (17) is achieved, the damper valve (11) is fully closed. The air supply is then reduced and the heat output of the boiler reduced to a low rate.

The sealed system is charged at less than atmospheric pressure, e.g. to 2 inches Hg vacuum, so that should the system leak or be fractured the internal pressure will increase and the valve plate (13) be closed, a condition in which the boiler burns at a slow rate and is safe.

It will be appreciated that the damper valve (11) is in a flow path which leads from the air inlet through the firebed into an exhaust duct and outlet flue. This path is subject to flue draught variations due to wind conditions around the flue outlet. This variation produces forces on the valve plate (1 3) tending to cause it to be pulled toward the air inlet or, worse, to cause oscillations of the plate. To counteract these forces, the capsule (22) is formed with a relatively high stiffness (load/movement ratio) as compared for instance with bellows units which tend to provide less resistance to expansion or contraction. This stiffness is chosen so that the expected fluctuations of pull on the plate (13) are insufficient to overcome the resistance of the capsule.

Thus, unwanted movement of the valve plate is reduced or eliminated.

However, this increase in stiffness of the capsule would have the effect of reducing the desired movement of the capsule due to sensed temperature variations. This reduction in desired movement is counteracted by increasing the effective area of the capsule, so that it is a minimum of 2 cms diameter, and may be up to 4 cms diameter. The force on the capsule due to its internal pressure is that pressure multiplied by the effective area, but the forces on the capsule due to the flue draught variations are not a function of the capsule's effective area. It is the realization of this fact which has enabled the capsule to be designed with a stiffness such as to resist the flue draught variations but without that stiffness reducing the movement of the capsule due to internal pressure variations.

The embodiment shown in Figs. 2 and 2a has a valve (11', 12') operated by a lever (14') similar to the embodiment of Fig. 1. The lever (14') is pivoted at (1 7'). Temperature sensitive control means for the valve comprises two sealed systems, each comprising a temperature-sensing phial (20'), capillary tube (21') and expansible capsule (22'). The capsules operate on each end of a lever (40), which engages through a stirrup extension (40') a bearing surface on a fulcrum pin (41) on the lever (14'). The engagement may be a sliding line as shown, a point contact or a toggle mechanism. Spring (43) urges stirrup (40') into contact with the capsules and fulcrum (41). When both sealed systems are operating correctly they move at the same time the same distance so that lever (40) moves bodily in a straight line without any substantial tilt.Stirrup (40') is thus moved to the right or left as seen in Fig. 2, urging fulcrum (41) similarly. Since lever (14') is only free to pivot about axis (17'), this results in tilting of the lever (14'), to open or close valve (11'). Axis (17') is located closer to fulcrum (41) than to valve (11') so that a small movement at (41) produces a larger movement of the valve. Should either phial fail for any reason then the sealed systems will not move their push rods by the same distance and lever (40) will tilt. Tilt more than a predetermined angle will cause the stirrup (40') to slip off its narrow resting point on fulcrum pin (41), thus freeing the lever (14') which is then pulled by a spring (not shown) to the closed position in which it remains. The boiler then maintains a low rate until a repair is effected, the system thus failing to safety.

In this arrangement the sealed systems are fully filled, for instance with hydraulic oil, the expansion of the oil causing the capsule expansion. The capsules may have diameters of between 1 and 2 cms.

Fig. 2b shows the same mechanism as that shown in Fig. 2, but instead of operating a valve, the lever (14') operates a micro-switch (42) so as to make or break an electrical circuit (not shown) under thermostatic control.

Fig. 3 shows a variation of the thermostatic control and adjustment means of Fig. 1 and has a lever (14) operating a valve (not shown) similar to that of Fig. 1. A housing (30) contains a capsule and temperature adjustment similar to those of Fig. 1 and not shown in the drawing. This arrangement has an additional safety feature in the provision of a second capsule (31) also having a phial (not shown) sensing a-boiler temperature. The second capsule operates one end of a second lever (32) pivoted to the housing at (33). The other end of the second lever bears on the lever (14) at screw (34), thus operating lever (14). The safety feature can be operated in a number of different modes. Both sealed systems are oil-filled.In a first mode, the temperature adjustment afforded by screw (34) is matched to that of the main capsule, so that lever (32) does not alter movement of lever (14) due to the first capsule. Should the main capsule fail, the second capsule, operating similar characteristics to the main capsule, continues a similar temperature control, moving the lever (14) appropriately.

In a second mode, the adjustment of screw (34) is changed so that the valve (11) is closed at a higher temperature when the second capsule is operating alone. Thus a user can see that something is wrong, when the second capsule is operating, although the higher temperature is still a safe temperature.

In a third mode, a pawl mechanism (35, 36) is made to keep the lever (32) operated with the valve (11) closed once the second capsule takes over control. Thus, again, a user knows something is wrong. The pawl (36) may operate a warning flag (not shown).

The second capsule and phial forming a sealed system may have a restricted filling, i.e. produce less vapour pressure than the first sealed system, so that the second system is subjected to lower pressure variations and has a longer life expectancy than the first system.

The opening movement available is, of course, also less, but since this is a fall-back or safety system this is not important.

The two phials for the two sensing systems may be located either to sense the same temperature or at different locations, e.g. the first one sensing water temperature in the boiler and the second flue gas temperature.

It will be noted that the thermostatic control mechanisms described above may be used to operate various devices including air supply valves, electrical switches or gas valves, and may operate either to open or close the respective device on increase of sensed temperature. Similar mechanisms may also be used as pressure control devices in which the pressure to be controlled is applied to a pair of diaphragms, the consequent movement of which controls a pressure varying means. The pressure connections to the diaphragms contain restrictions such that should either diaphragm leak the local pressure drops, the restriction being such as to prevent the pressure being restored before the fail-safe mechanism has operated.

Claims (14)

1. A control mechanism comprising a first expansible capsule, means for subjecting said capsule to a variable pressure related to a desired physical condition whereby said capsule expands or contracts, means for translating the consequent movements of the capsule into a control for said physical condition so as to maintain it at a desired level, and fail-safe means for ensuring that in the event of failure the mechanism fails to safety maintaining said physical condition at a safe level.

2. A control mechanism as claimed in claim 1, wherein said fail-safe means comprises a second expansible capsule subjected to a variable pressure related to the desired physical condition, the movement of said capsule being mechanically compared or balanced with that of the first capsule, and any substantial discrepancy effecting said failure to safety.

3. A control mechanism as claimed in claim 2, wherein said two capsules operate in parallel but spaced apart, on a single lever, such that the angle of tilt of the lever effectively compares the movements of the capsules.

4. A control mechanism as claimed in claim 3, wherein said lever engages an operating device through a linear or point contact such that when the angle of tilt of the lever exceeds a predetermined amount the lever slips out of engagement with the operating device, said operating device then being urged into a safe position.

5. A control mechanism as claimed in claim 4, wherein said lever has stirrup means providing a bearing surface engaging cylindrical surface on said operating device, said operating device comprising a pivoted operating lever.

6. A control mechanism as claimed in claim 2, wherein both said capsules operate directly or indirectly on a common main lever whereby on failure of the first capsule, the second capsule maintains a similar or modified movement of the lever.

7. A control mechanism as claimed in claim 6, wherein said second capsule engages a further lever which engages the main lever, and in normal operation does not affect its movement.

8. A control mechanism as claimed in claim 7, wherein there is a locking mechanism maontaining the main lever in a closed-down condition when the second capsule takes over operation.

9. A control mechanism as claimed in claim 1, wherein said capsule comprises part of a sealed temperature-sensing system which is charged with a liquid at less than atmospheric pressure such that leakage of the system causes the capsule to move in the sense to reduce the temperature sensed to a safe level.

10. A control mechanism as claimed in claim 9, wherein said capsule comprises two metal discs secured together around their peripheries and having a diameter of more than 2 cms.

11. A control mechanism as claimed in any of claims 1 to 10, wherein said physical condition is a temperature, said capsule forms part of a sealed temperature-sensing system, and said control for the physical condition is an air supply valve for a solid fuel boiler.

1 2. A control mechanism as claimed in any of claims 1 to 10, wherein said physical condition is a temperature, said capsule forms part of a sealed temperature-sensing system, and said control for the physical condition comprises an electric switching device.

1 3. A control mechanism substantially as described hereinbefore with reference to any of the accompanying drawings.

14. A thermostatically-controlled damper valve for a solid fuel boiler substantially as described hereinbefore with reference to any of the accompanying drawings.