GB2091003A - Magnetic seal system - Google Patents

Abstract

A seal frame position control system has displacement-sensing transducers (100) which sense changes in the sealing gap between a rotating seal frame 4 and the stationary axial end face 7 of a matrix (6) of a regenerative air preheater. The sealing gap varies due to thermal distortion of the matrix (6). When this happens, the transducers (100) control the energisation of electromagnetic devices 15 which are attached to the seal frame 4 and cause axial movement of the frame 4 relative to the axial end face 7 of the matrix (6). In this way the gap is maintained at a constant value, thereby minimising seal wear and leakage. There are a plurality of electromagnetic devices 15 spaced round the periphery of the frame, each one, or each group, with its own transducer which affects control by adjusting the triggering point of thyristors in bridges which supply the devices 15. The weight is partially supported by springs. <IMAGE>

Description

SPECIFICATION Magnetic seal system The present invention reiates to regenerative air preheaters and seal frame suspension control system therefor. It will here be described with reference to the type of air preheater in which movable air hoods rotate with stationary heating gas ducts on the axial end faces of a stationary, cylindrical, regenerative matrix, but the electromagnetic devices and associated control circuitry could equally well be used with other types of air preheaters such as air preheaters in which a cylindrical regenerative matrix rotates between stationary ducts.

The setting of the end seals which prevent leakage of heat-exchange media at the end faces of a regenerative heat-exchange mass is a matter of importance. Sealing frames are borne on one part, here the rotating hoods, and these rotate over a planar end face, in this case on a static regenerative mass. If the sealing frames are held clear of the end face there will be leakage; if they press together there will be wear. To relieve the pressure at this interface the sealing frames have been isolated from their supporting structure by spring-loading so that only a small portion of the weight of the frame (on an upper end face) or only a small component of force due to the springs (on a lower end face) is used to urge the two into contact.This type of system was either completely uncontrolled (so that with variation in temperature, or temperature gradients across the regenerative mass, there was a risk of unduly large clearances developing or unduly large pressures being exerted) or was controlled by a mechanical drive which adjusted the spring tension, either as a result of manual initiation of the drive, or automatically in response to temperature sensed in the regenerator.

The present invention proposes providing electromagnetic drive means which drive that frame axially clear of the end face of the mass, and an automatic control for that drive means. The control is responsive to deviations from the demanded position resulting in changes in clearance, displacement or position of the seal relative to the mass and is arranged so that the relationship is that which is desired in any one of a number of control strategies.

This is an invention therefore which does away with the problem of avoiding excessive clearance and excessive wear by electromagnetically adjusting the setting of the sealing frame as a result of signals generated in the regenerator by the very elements to be controlled.

Broadly, we provide a seal frame position control system for a regenerative air preheater which system has a plurality of electromagnetic devices to be operatively attached to a sealing frame of the preheater to cause movement of the frame in an axial direction, and electrical control means including position of displacement-sensing transducers associated with the sealing frame and being arranged and adapted to control operation of the electromagnetic devices to tend to control the relationship in the axial direction between the seal frame and axial end face of a regenerative matrix of the preheater. We provide also a preheater equipped with such devices and control means at a plurality of positions about the frame.

Preferably the electrical control means will independently control devices at respective positions.

The number of individual devices needed will depend primarily upon the size of the preheater.

The operation of the device is such that the sealing gap between the sealing frames and the end face of the matrix is maintained at a constant value at all points of the sealing circumference, irrespective of any thermal distortion of either the stationary or rotating components of the preheater. The sealing gap between the end face and the outer circumferential parts of the sealing frames should ideally be maintained at a value of nor more than 1 mm.

The preheater may be one of the stationary matrix type, in which case the seal frames are borne on hoods rotating at at least one of the end faces of the matrix.

An electrical control system preferred from the point of view of reliability is one which responds to the position of a transducer relative to the axial end face to tend to withdraw the seal frame.

Preferably the electro-magnet drive means act in conjunction with an arrangement of mechanical springs to provide an integrated sealing frame suspension system. The spring settings would normally be such that, for maximum gas temperature and the resulting thermal distortion light contact would result between seal and axial end face giving minimum leakage and minimum wear. For lower gas temperatures, and a lesser degree of thermal distortion, the sealing gaps and seal pressures change at the hot and cold ends depending on the shaft, bearing and matrix (masts) arrangements of the particular preheater design involved, but giving increased leakage and increased wear in all cases. In order to rectify this situation and thus minimise seal wear and leakage the relative position of the seals and end face is sensed and one of a number of control strategies is initiated.The objective common to these control strategies is to maintain minimum clearance between seal and axial end face, the electromagnetic drive means being required to provide a minimum force equal to the seal weight minus the appropriate spring force (or in some cases, equal to the seal weight plus the appropriate spring force).

A particular embodiment of the invention will now be described with reference to the accompanying drawings wherein Figure 1 is a side view of a rotary regenerative air preheater.

Figure 2 is a graph of a desired relation between force due to the electro-magnets, spring force and gas temperature, Figure 3 is a section through an electromagnet, Figure 4 is a block diagram of the control system, Figure 5 shows in plan a distribution of control devices and spring pins around a rotatable pair of hoods, Figure 6 is a detail, partly in section, of one control device, and Figure 7 is a detail in part section, of a transducer.

In Figure 1, a rotary regenerative air preheater has a stationary cylindrical matrix 6 over both axial end faces 7 of which move hoods 3 which rotate about an axis 8 which is also the centre axis of tne matrix 6. The hoods are contained within stationary ducts 9 which direct the flow, shown by dark arrows, of a gas such as boiler exhaust gas which is to give up heat to a heat-exchange mass in the matrix 6. A medium such as air flows as shown by hollow arrows through stationary inlet and outlet ducts 1 and through the hoods 3 to pass through the heat-exchange mass and take up heat from it. To maintain separation of the two media sealed bearings are provided at 10 between the ducts 1 and hoods 3 and a sealing shoe 12 (see also Figure 6) borne by a seal frame 4 on the hoods slides on or just over the axial end faces 7 of the matrix.Expansion joint arrangements such as bellows 11 allow relative axial movement between the frames 4 and hoods 3.

The weight of the frames 4 and parts fast with them is almost balanced by spring-loaded pins 1 3 acting between the hoods and frames. Ancillary equipment such as soot-blowers 14 may also be rotatably borne on the central axle.

Thus far, the structure is conventional. The invention is concerned with actively controlling the axial relationship between the end faces 7 and the frames 1, and electromagnetic devices 1 5 are provided for this purpose. One embodiment of the electro-magnetic devices and control system by which control is achieved will now be described.

It is clear that variations in gas temperature, with consequent variations in the distortion experienced by the matrix, will affect the correctness of the setting of the frames if a constant force due to the springs is all that acts upon them. The force contributions of the springs of the spring loaded pins 1 3 against the contribution E due to the electro-magnetic devices to maintain theoretically a constant setting in a hot-end frame (the upper frame in Figure 1 ) where the weight to be supported is W is shown diagrammatically in Figure 2. The abscissa is temperature and the ordinate, total force. As can be seen, as temperature rises the spring force contribution S becomes progressively more important than the contribution E from the electro magnetic devices.

A preferred electro-magnetic device 1 5 which has been designed for the purpose described above is illustrated by Figure 3. It has an 'iron' (magnetic) circuit, part of which is a movable ferromagnetic armature 1 6 which is attached to a lug 1 7 welded to the seal frame 4 (Figure 6) the remainder of the magnet including its coil 1 8 being mounted on a frame 20 of the rotating air hood 3. The magnetic circuit includes also a housing 21 surrounding the coil 18 and an end plug 22 having a portion 23 projecting into a central passage of the electro-magnet.When the coil 1 8 surrounding the armature 1 6 is energised, the armature experiences an attractive electromagnetic force, the magnitude of which is a function of the dimensions of the magnet, the magnetic characteristics of the ferromagnetic material, the current flowing in the coil, and the variable air gap 'g' in the magnetic circuit (see Figure 3). The air gap 'g' is itself dependent upon thermal distortion and thus temperature because the armature is associated with the seal frame, and the coil 18 and end plug 22 with the air hood 3. The devices 1 5 and their control circuitry are designed so that at any temperature, the attractive electro-magnetic force produced when the magnet coils are energised, is sufficient to lift the seal frames so that their shoe parts 12 come axially free of the stator end face.

The coils 1 8 of the electro-magnet consist of a stack of rolls or coils of anodized aluminium strip, the aluminium oxide coating on the conductor being necessary for electrical insulation at the high operating temperatures of the preheater. Other insulation, such as that required between layers, between coils and former, and for the end connections is provided by woven ceramic material in the form of pads, tape or sleeving. The coils are positioned about a non-magnetic coii former sleeve 24 which also acts as a sliding bearing for the armature 1 6.

A number of magnets is used on any given preheater and these are placed at strategic points around the seal stator interface. One possible arrangement is seen in Figure 5 where the bladeshaped outline represents the circumference of one hood 3, hollow circles 13 represent the position of spring loaded pins 13 and full circles represent the position of electromagnets 1 5. The position of the magnets, relative to the springs which comprise the remainder of the suspension system, are carefully chosen, and one or more magnets may be situated in any particular control sector, the magnet or magnets in each sector preferably being controlled separately from those in any other sector. The resulting sectionalized system is then better able to adjust the clearance between the stator end face 7 and the seals 4, peripheral irregularities and uneven distortion then being more satisfactorily accounted for.

Control circuitry for a magnet or group of magnets is seen in Figure 4.

The magnets are energised with direct current derived from the a.c. mains via solid state rectifiers, the use of thyristors or other controllable semi-conductor devices being necessary. As stated above, the magnet or magnets in each control sector must be provided with a separate, controlled supply and an exclusive control signal if independent operation is to be achieved of the magnets or of the sectors of magnets, as the case may be. The coil or coils 1 8 in each control sector are therefore fed via an individual controlled bridge rectifier, or some other source of variable direct current, and the control signals are produced by a position-sensing transducer being dependent on and proportioned to the relative position, clearance or displacement from the demanded position of the hood 3 and the stator.

The position -- or displacement-sensing transducer is fed with low voltage alternating current via slip rings 2 mounted on or near the main axle, or some other form of sliding contact.

One embodiment of transducer is the inductance type gap sensor 100, Figure 7, which sensor is mounted on the sealing system to record changes in searing gap which result from deformation of the storage mass due to changes in operating temperature.

This is a continuous system of sensing and control for all changes of boiler loading. Adequate temperature compensation has been designed to eliminate temperature sensitivity of the sensors and actuators.

Figure 7 shows a section through the position sensor which essentially comprises a machined case 102 which has a sealing cap 101 locked firmly in place on one side of the sealing system by screw 107. The unit is rigidly fixed to the sealing system via mounting brackets 110. One end of the casing 102 is sealed using an insulating ring 105 and a specially machined aluminium disc 104. These are held using a circlip 1 06. The insulating ring supports the inductance coil 103 which has two suitably insulated and protected connectors which pass through 5 mm holes in the casing to a ceramic terminal block 108.

Connections from the sensor to the position sensing circuitry are made using the appliance socket outlet 109.

When clearance between the rotating seal and that stator end face becomes less or greater than is desired, the deviation is sensed by the position or displacement transducer and its associated circuitry. The error or deviation between the measured and demanded position signal is used to initiate some new adjustments to the state of the system, depending upon the control strategy in use.

The magnet control circuitry of one arrangement is illustrated in Figure 4. In this example only one coil 1 8 is situated in any given control sector, but the principle would be indentical if a plurality of such coils were in one sector to be controlled in common.

Power for the coils is supplied by a controlled bridge rectifier circuit 30 which applies current to the coil in accordance with the condition of a pulse generator 33 and a phase control circuit 34.

Excess clearance or deviation from a demanded position of the seal 1 2 and stator 7 applies a signal voltage to the thyristor phase control circuit which in turn provides a phase control signal being essentially dependent on, and proportional to, the deviation from the demanded position. The low voltage signal current is derived via an isolating transformer. Transformer 31 powers also, through rectifier bridge 35, the pulse generator 33. The phase control signal determines the proportion of a half cycle of supply voltage over which the thyristors receive triggering pulses generated by pulse generator 33, and thus controls the voltage applied to, and current flowing in, the coil 1 8. Loss of contact upon movement of the armature 1 6 is accompanied by the removal of the phase control signal and thus the thyristor triggering pulses.

Despite the large inductance of the coil 18, the thyristors are allowed to switch off by the presence of the 'fly-wheel' effect provided by the two diodes in circuit in the controlled bridge rectifier 30.

Various other control strategies can be adopted, devised, or envisaged, but the ultimate objective in all instances is to control the lift exerted by a specially designed electro-magnet and, likewise in all instances, the control depends for its efficiency upon a signal derived through the position or displacement transducer sensing the relative position of the seal and stator end face whose time and severity of deviation is to be minimised.

Claims (8)

1. A seal frame position control system for a regenerative air preheater which system has a plurality of electromagnetic devices to be operatively attached to a seal frame of preheater to cause movement of the frame in an axial direction, and electrical control means including position or displacement-sensing transducers associated with the seal frame and being arranged and adapted to control operation of the electromagnetic devices to tend to control the relationship in the axial direction between the seal frame and axial end face of a regenerative matrix of the preheater.

2. A control system according to Claim 1, wherein the preheater is a stationary matrix preheater, the seal frame being borne on a hood rotating at at least one of the end faces of the matrix.

3. A control system according to Claim 2, wherein mechanical springs act between the hood and the seal frame.

4. A control system according to Claim 3, wherein the electromagnetic devices act in conjunction with the mechanical springs.

5. A control system according to any one of the preceding claims, wherein the electrical control means is adapted to maintain a gap between the seal frame and the axial end face of the matrix at a constant value at all points of the sealing circumference.

6. A control system according to Claim 5, wherein the gap is not greater than 1 mm.

7. A seal frame position control system substantially as herein described with reference to and as illustrated in the accompanying drawings.

8. A regenerative air preheater having a seal frame position control system according to any one of the preceding claims.