GB2098710A - Flue closure damper - Google Patents

Abstract

A damper to close the flue of fuel burning apparatus is of the kind having a fixed casing in which a blade 30 moves. A seal between the blade 30 and the static structure is provided by a rope 34 of ceramic fibre extending around the aperture 14 closed by the blade 30. The blade 30 makes limited pivoting movement as it closes onto the seal provided by this rope. Drive to open the blade 30 is conveyed by means of cranks connected by a gas spring which can be forced to contract if sufficient flue gas pressure forces the damper blade open; this then is detected by a plunger striking a microswitch connected so as thereupon to drive the damper fully open. <IMAGE>

Description

SPECIFICATION Damper This invention relates to dampers which can be employed to close the flues of fuel burning apparatus and so prevent the natural draught of the flue from drawing cold air through the apparatus and cooling it down, wastefully, when it is not firing.

Conventional dampers may also be employed to restrict the flue during normal operation of the boiler, and that is the case with this invention also.

The flues to which the dampers ofthis invention are applicable may lie in the range of 6 ins (15cam) to 3 feet or 1 metre diameter although the invention may be employed outside this range. Such dampers have hitherto generally employed flexible stainless steel strips to form a seal between the blade and fixed structure of the damper.

Broadly, in one aspect, this invention resides in employing ceramic fibre to form a seal in a damper.

Preferably a rope of ceramic fibre is secured on the fixed structure of the damper, around an aperture for the flue gas, and the blade of the damper closes against it. Preferably the ceramic fibre is fixed in place by ceramic fibre cement. The mounting of the blade desirably allows it to make limited free pivoting movement as it closes onto the ceramic fibre.

Use of a ceramic fibre seal allows the damper to have a round blade or a rectangular blade, as may be desired, whereas the stainless steel strips used hitherto have required the blade to be rectangular, even where the flue was of circular cross section.

Moreover corner joints have to be made between the stainless steel sealing strips and there is the possibility of leakage occurring at these joints. Additionally, fixing of the sealing strips to the blade entails drilling it to provide bolt holes, creating points of potential leakage.

By securing the seal on the fixed structure of the damper, rather than the blade, it can be kept out of the flue gas glow, so protecting it against abrasion by solid particles entrained in the flue gas.

In a separate aspect, the invention provides a damper having a blade movable between open and closed positions, wherein the blade is mounted at a generally central region to means for supporting and moving the blade, the mounting enabling pivoting movement of the blade as it reaches its closed position.

Dampers of the type to which the invention relates may be required to provide explosion relief, i.e. the blade of the damper must be forced open by excessive pressure in the boiler, such as would be caused by an explosion, relieving that pressure and preventing damage to the boiler.

In another separate aspect this invention provides a damper the drive to which is effected through a link resiliently maintained at a certain length, but whose length can be changed against the resilience by abnormal pressure tending to open the blade.

In a preferred construction the damper has (i) a blade carried by a pivotally mounted shaft with a crank mounted thereon to turn the shaft and so open or close the blade; (ii) drive means, such as an electric motor, arranged to drive a second shaft with a second crank mounted thereon; and (iii) a link of variable effective length connecting the two cranks and incorporating resilient means which normally maintain it at a constant length, opening pressure on the blade above a predetermined amount causing the blade to open and its shaft to turn, overcoming the resilient means and changing the length of the link.

In yet another aspect, the invention has means to detect forced opening of the blade of a damper, these means being operative to cause the damper drive to open the blade fully and hold it in its fully open position. The detecting means may comprise two co-operating parts attached to parts of the aforesaid link which move relatively to each other if the effective length of the link is changed.

Alternatively, the detecting means may respond to movement of the blade relative to the fixed structure of the damper when there is no corresponding movement of the damper drive. There may for example be a trip element carried by one part of the aforesaid link and a detecting switch positioned on the fixed structure of the damper so as to be operated by the trip element if the movement of the blade causes a change in the effective length of the aforesaid link, and yet remain clear of the path of the trip element during movement of the link without change in its effective length.

Am embodiment of this invention will now be described with reference to the accompanying drawings in which: Figure 1 is a downstream end view of the damper, when closed; Figure 2 is a side view, seen in the direction of arrow II of Figure 1; Figure 3 is a top view; Figure 4 is a vertical section on line IV-IV of Figure 3; and Figure 5 is a side view, showing a modification.

Referring the drawings, the damper has a housing in the form of a cuboidal box formed of steel plates.

The sides 2 and base 4 of the box can be formed from a single plate by folding, and the upstream end 6 and downstream end 8 welded on. The top of the box is closed by a cover 10 bolted on to flanges 12 projecting in from the sides 2 and ends 6,8 of the housing. The upstream end 6 and downstream end 8, seen in Figure 1, have an inlet hole 14 and outlet hole 16 respectively, surrounded by bolt holes 18 for attaching the end flange of pipework forming a flue.

The positioning of these bolt holes 18 is standard.

A gasket 19 provides a seal between the flanges 12 and cover 10. This may be of ceramic fibre, which will stand high temperature (at least 10000C) but where the flue gas temperature is sufficiently low an application of silicone mastic can be employed instead and is more convenient. Many silicone mastics will withstand up to 600C. Expandite Ltd make a mastic able to withstand 250"C.

A shaft 20 extends laterally across the housing and is supported at each side in a dry bearing 22 (this type requires no lubrication and is employed to withstand heat) sealed to the side 2 of the housing by a ceramic fibre gasket 24.

An arm 26 is secured fast on this shaft at a generally central position, by means of a through bolt 28. The blade 30 which closes the damper is a circular plate carrying two lugs 32 pivotally attached to the arm 26. Rotation of the shaft 20 swings the blade 30 from its closed position, shown in full lines in Figure 4to its open position, shown in broken lines (in which the blade 30 does not obstruct the passage between the holes 14).

A seal is positioned on the inside face of the upstream end 6. It consists of a rope 34 of ceramic fibre, bordering the inlet hole 14 and held in place by ceramic fibre cement 36. The blade, when closed, bears sealingly on this rope 34.

The pivotable mounting of the blade 30 on the arm 26 assists in forming a good seal as the blade closes.

The blade is held in approximate position relative to the arm 26 by means of a spring 38. It can be seen from Figure 4 that this spring contracts slightly as the blade opens.

As the blade closes its bottom edge region 40 contacts the rope 34 first, and the spring 38 extends slightly as the blade 30 comes to bear on the rope 34 all the way around the inlet 14.

A suitable material for the rope 34 is Kerlane 45 rope Electro-Refractaire (UK) Ltd. It can be secured in place with Gibfix adhesive, a ceramic fibre cement from Steetley Refractories Ltd. Ceramic fibre is used forthe gaskets 24, and may be used for the gasket 19 also, as mentioned. Kerlane sheet material from the same supplier, or ceramic fibre sheet from Steetley Refractories Ltd can be used for these gaskets also.

A support 42, welded to the base 4 of the damper housing, supports a motor 44 and gearbox 46 by means of which the shaft 20 is driven. The output shaft 48 of the gearbox projects at both sides of the gearbox. On the side nearer two the damper housing it carries a crank 50 fast with it. Another crank 52 is clamped onto a knurled end portion of the shaft 20 so as to be fast with that shaft. When the damper is closed both of the cranks project laterally as shown by Figure 2. The cranks 50,52 are connected together by a link 54 which is a gas spring, i.e. a piston and cylinder maintained in a normally extended condition by a permanent pressure of gas sealed into the cylinder. It therefore has a constant normal length, but when sufficient compressive force is applied to the ends of the gas spring 54 it will contract.

In normal operation of the damper the motor 44 is operated to rotate the shaft 48, thereby swinging the crank 50, so that the link 54 causes the crank 52 to swing and hence the shaft 20 is turned to open or close the blade. All of this is accomplished with the length of the link 54 remaining constant. The extent of movement is controlled by cams (not shown) which are mounted on the portion 56 of the gearbox output shaft, and cooperate with limit switches, also not shown.

Electrical controlsforthe normal operation of the motor 44 can be conventional, and may enable the damperto be operated from a local, our a remote, control panel and/or linked to the burner or forced draught fan of the boiler to open whenever the burner is firing, and close when it is not.

The shaft 20 may project at 57 as shown in phantom, and this projecting part may carry further cams co-operating with further limit switches. These would normally be provided if the damper was to be linked to a burnerorforced draughtfan,and are conventional.

The damper can be arranged to move to partially closed positions, determined by the cams and limit switches, if required.

If an explosion occurs while the blade 30 is closed or partially closed the force of the explosion pressure wave on the plate 30 is translated into force applied by the crank 52 onto the upper end of the gas spring 54 and tending to compress that spring. The pressure of gas stored in the gas spring is set so that a predetermined pressure on the closed blade 30 is necessary to overcome the gas pressure holding the spring in its extended condition, and so force the blade 30 open by compressing the gas spring 54.

The piston 58 of the gas spring carries a plunger 60, clamped to it by a clamp 61 and arranged to cooperate with a microswitch 62 secured to the cylinder part 64 of the gas spring. The plunger 60 slides through a guide 66. When the gas spring is in its normal, fully extended, condition the plunger 60 is spaced slightly from the microswitch 62. When the spring is compressed, however, the plunger 60 operates the microswitch 62. This arrangement serves to detect compression of the gas spring, an event which otherwise might go unnoticed because once the pressure on the blade 30 has been relieved the stored pressure in the gas spring would return it to its normal length. The microswitch 62 is con necked to control circuitry for the motor 44, and when operated by the plunger 60 causes the motor 44to drive the blade to its fully open position.

The control circuitry can moreover be such that operation of the microswitch 62 sounds an audible alarm, or lights a warning lamp on a control panel.

Moreover it can be arranged that after the motor has driven the blade to its fully open position, the motor is unable to operate further until deliberately reset an action which should properly be preceded by inspection of the boiler.

A suitable gas spring is supplied Dictator Technik (Hydraulics). The pressure on the closed blade 30 to cause the gas spring to be overcome and the blade to be forced open is four inches (100 mm) water gauge but other pressures can be preset for particular applications, by suitably choosing the pressure of gas stored in the spring 54.

The arrangement described above, employing parts 60,62,66, would require attachment of an electric cable to a moving part. Figure 5 shows an alternative arrangement which avoids this. Apart from the means for detecting an explosion the construction is as previously described. However, on the side of the damper housing there is a fixed bracket 70 on which there is mounted a microswitch 72. The bracket 70 and microswitch 72 project out only a short distance from the side of the housing, so that they lie in the space between the housing and the gas spring 54 and do not impede movement of the gas spring.

On the cylinder 64 of the gas spring, there is secured a clamp 74 supporting a rod 76 which extends out from the clamp as shown, and then turns towards the damper casing, forming a trip element.

During normal operation of the damper, this arm of the rod moves along a path indicated by the curved chain-dotted line 80. It can be seen that this path clears the microswitch 72. However, if an explosion occurs while the damper blade is closed, the pressure on the blade swings the crank 52 from the position shown in Figure 5 but the crank 50 does not move. If this happens the trip element moves on the path shown by the dotted line 82, and as can be seen, operates the microswitch. This microswitch 72, analogously to the microswitch 62 referred to previously is connected to control circuitry for the motor 44, and causes the motor 44 to drive the blade to its fully open position. The geometry of the arrangement shown in Figure 5 is such that the microswitch will be operated if the damper blade is blown open eitherfrom afullyclosed position or from a partially opened position fairly close to its closed position. If an explosion occurs while the damper is fully open, the damper blade would not be caused to move in any event.

Once again, the control circuitry can be made such that operation of the microswitch 72 sounds an audible alarm and/or lights a warning lamp. The control circuitry should preferably be such that operation of the microswitch 72 as the gas spring returns to its normal length is without effect. Desirably, after the microswitch 72 is first operated the motor drives the blade to its fully open position and then is prevented from further operation until the circuitry has been deliberately reset.

Claims (6)

1. A damper to close the flue of fuel burning apparatus comprising static structure and a blade movable relative thereto between open and closed positions, characterised in that a seal between the static structure and the blade in the closed position of the latter is provided by means of ceramic fibre.

2. A damper according to claim 1 wherein the ceramic fibre is a rope of ceramic fibre positioned so that in the closed position of the blade the rope extends around an aperture through the static structure which aperture is closed by the blade, the rope being then positioned between the blade and the static structure.

3. A damper according to claim 2 in which the rope is secured on the static structure.

4. A damper according to claim 3 in which the blade is mounted so as to permit limited free pivoting movement as the blade closes onto the ceramic fibre.

5. A damper according to any one of the preceding claims wherein the blade is movable between open and closed positions and is mounted at a generally central region to means for supporting and moving the blade, the mounting enabling pivoting movement of the blade as it reaches its closed position.

6. A damper substantially as herein described with reference to the accompanying drawings.

6. Adamperto close the flue of fuel burning apparatus having a drive to open the damper which drive is effected through a link resiliently maintained at a predetermined length, but whose length can be changed against the resilience by abnormal pressure tending to open the damper.

7. A damper according to claim 6 having a blade carried buy a pivotally mounted shaft with a crank mounted thereon to turn the shaft and so open or close the blade; drive means arranged to drive a second shaft with a second crank mounted thereon; the said link connecting the two cranks and incorporating resilient means which normally maintain it at a constant length, opening pressure on the blade above a predetermined amount causing the blade to open and its shaft to turn, overcoming the resilient means and changing the length of the link.

8. A damper according to any one of the preceding claims having means to detect forced opening of the blade of a damper, said means being operatively connected to cause a drive of the damper to open the blade fully and hold it in its fully opened position.