This invention relates to modifications to air heaters, in particular regenerative rotary air heaters used to preheat combustion air using heat in exhaust gases.

In conventional power plant boilers, ' Ljungstrom' type regenerative rotary air heaters are often used to preheat the combustion air using the heat in the exhaust gases. A schematic view of such a regenerative rotary air heater is shown in Figure 1 of the accompanying drawings. The rotor 1 is circular in construction and rotates around a normally vertical axis in a cylindrical casing 2. An air inlet 3 and a gas outlet 4 are usually connected to the bottom of the casing 2, and an air outlet 5 and a gas inlet 6 are usually connected to the top of the casing. The air flow is therefore usually upwards and the gas flow usually downwards. The rotor 1 is driven by a motor 7. The air heater rotor 1 is constructed with radial plates 8 to which are attached radial seals.

As shown in Figure 2 of the drawings, which is a more detailed perspective view of the known type of air heater shown in Figure 1, there are sub-division plates 9 between the radial plates 8 which are normally shorter than the radial plates and are not normally fitted with

seals. Air heater baskets which form the heat transfer surface and storage medium are inserted into the spaces between the radial plates and sub-division plates.

At any one instant exhaust gas flows through one side of the rotor 1, transferring heat to the rotor plates 8 and internals. As the rotor rotates the plates previously heated by the hot exhaust gas pass through that side of the air heater through which the air flows. Heat is given up from the hot plates of the rotor and transferred to the combustion air. After approximately one half a revolution the rotor plates move back into the gas side to be heated once more by the exhaust gas. This, therefore, is a continuous process.

As shown in Figure 3 of the drawings, which is a cut-away perspective view of the central part of the rotary air heater shown in Figures 1 and 2, top and bottom sector plates 10, 11 are provided to separate the air and gas sides of the .casing and to minimise leakage between the high pressure air side and the low pressure gas side. The sector plates are usually divided up into three parts. For example, the bottom sector plate 11 comprises a central part 11a and two outer parts lib hinged to the central part 11a to enable the sector plates to conform approximately to the shape of the rotor as it deforms due to the temperature differential.

Radial seals, fitted to the radial plates 8 on the rotor 1, minimise the leakage from the air to gas side. At the outer edge of the rotor, at both the top and bottom, circumferential seals are fitted to seal between the rotor and the casing, and to prevent air or gas by-passing the heat exchange surfaces. Axial seals are also fitted in line with the radial plates. The circumferential, axial and radial seals can be seen in Figure 8 of the drawings described hereinafter.

Because of the temperature differentials occuring in the rotor between the top and the bottom the diameter of the top expands more than the bottom. The rotor shaft has a fixed point at the top so that there is also a downward expansion. The rotor takes up a shape generally as shown in Figure 4 of the accompanying drawings, wherein a broken line 12 indicates the expansion profile of the rotor.

The main problem with rotary air heaters is the air leakage from the forced draft (air, high pressure) side to the exhaust gas side, causing reduced boiler efficiency.

The purpose of the modifications described in this invention is to improve the sealing performance between rotor and casing. Four modifications are proposed which may be used separately or together.

The present invention is thus concerned with a regenerative rotary air heater comprising a generally circular rotor arranged to rotate in a cylindrical casing, which casing has an air inlet and an air outlet for combustion air to be preheated and has a gas inlet and a gas outlet for exhaust gas, the rotor being constructed with radial plates having radial seals. Such an air heater is hereinafter referred to as " a regenerative rotary air heater of the type described".

In a first aspect the invention provides a regenerative rotary air heater of the type described, wherein top and bottom sector plates provided to separate air and gas sides of the casing each comprise a central part and outer parts hinged to the central part, wherein balance weights are provided to enable the sector plates to follow rotor deformation.

As stated, top and bottom sector plates are normally made in three parts on large air heaters and arranged so that they can move up and down on hinges in order to conform approximately to the top and bottom curvature of the rotor. This is normally effected by manual or electrically operated screw j cks.

The present invention, according to the first aspect, uses a system of balance weights, optionally assisted by

levers or pivots, to ensure that the sector plates remain in contact with the rotor radial plate seals without excessive contact load.

The top sector plate sits on the top of the rotor and the contact load is reduced by a balance weight to a level which keeps the sector plate in contact with the rotor and allows the sector plate to follow rotor deformation.

At the bottom the weight of the sector plate tends to move it away from the rotor. A lever is suitably used in this case so that the balance weight can act to keep the sector plate in contact with the rotor seals. In this case also this arrangement allows the sector plates to follow the deformation at the bottom of the rotor.

In the first aspect of the invention, the balance weights allow the sector plates to follow the deformation of the rotor without any undue loads being imposed.

In a second aspect the invention provides a regenerative rotary air heater of the type described, wherein top and bottom sector plates are provided to separate air and gas sides of the casing, and wherein either or both of the top and bottom sector plates are provided with

extensions so that the number of radial seals in contact with the sector plate is increased.

It is known that the leak flow rate can be reduced by increasing the flow resistance. A single seal having a leak area A and subject to a pressure difference p will have a leak flow rate proportional to A(p) 0' 5. If there is introduced a second seal, in series with the first, also of leak area A and geometrically similar to the first, the pressure drop across each seal is then p/2, because for continuity of flow must be subject to equal pressure drops. The leak flow rate is thus proportional to A(p/2) ' , i.e. 70% of the previous value.

This principle is utilised, according to the second aspect of the invention, by adding extensions of the sector plates so that the number of radial seals in contact with the sector plate is increased. For example, a standard sector plate has three radial plates in contact with the sector plate; by adding sector plate extensions in accordance with this aspect of the invention the number of radial plates in contact with the sector plates can be increased to four for example.

In a third aspect the invention provides a regenerative rotary air heater of the type described, wherein top and bottom sector plates are provided to separate air and

gas sides of the casing, and wherein generally circumferentially arranged sub-division plates are provided between the radial plates, and wherein at least some said sub-division plates extend the full height of the radial plates and are sealed against the sector plates.

It is known that, in some positions, some of the radial plates run off the sector plate allowing a cross (radial) flow. By extending the sub-division plates to the full height of the radial plates and by fitting suitable seals to seal against the sector plates the cross flow can partly be prevented, which has the effect of bringing part of another radial plate seal into effect, thus further increasing the resistance to leakage flow.

In a fourth aspect the invention provides a regenerative rotary air heater of the type described, wherein circumferential seals are provided between the rotor and the casing, which seals comprise a seal mounting bar inclined at a suitable angle to a seat and a seal secured to the mounting bar.

The type of seal generally used in the prior art is fitted with a predetermined cold gap between the seal tip and the rotor sealing surface; as the temperature increases and the rotor deforms the seal gradually comes

closer to a sealing bar. This is satisfactory at full load but boilers often have to operate at part load, in which case the rotor is not fully deformed and the seal therefore does not make contact with the sealing surface, thus impairing air heater efficiency.

In the present invention, according to the fourth aspect, the deformation pattern for the rotor can be determined for various load conditions up to full load. With this information a suitable profile for the top and bottom seats can be determined. A top circumferential seal preferably has the seat mounted on the rotor and the seal mounted on the casing, and a bottom circumferential seal preferably has the seat mounted on the casing and the seal mounted on the rotor.

In each case the seal preferably consists of a seal mounting bar, inclined at a suitable angle to the seat, and a stainless steel wool seal retained by two clamping plates and secured by bolts to the mounting bar.

The seat suitably consists of a folded steel section such that the inclination of the sealing surface follows the deformation profile of the rotor. At the top the seat is welded or fastened by other suitable means to the rotor, while at the bottom the seat is bolted or fastened by other suitable means to the casing.

In a modification of the first aspect of the invention, there is provided, according to a fifth aspect of the invention, a regenerative rotary air heater of the type described, wherein top and bottom sector plates provided to separate air and gas sides of the casing each comprise a central part and outer parts hinged to the central part, wherein the top and bottom sector plates are connected by rigid links to adjust and maintain the rotor/sector plate gap.

Adjustment is preferably carried out by means of a turnbuckle in the rigid link. Slipper shoes of suitable wear resistant materials, for example of cast iron, may be fitted to the outer edge of the sector plate, which bear on a corresponding rubbing surface fitted to the top side of the rotor cylinder, which requires increased clearance between the rotor and the sector plate. A modified type of radial seal is suitably fitted to accommodate the increased clearance between the rotor and sector plate. This arrangement has the advantage of reducing the contact loads between the rotor and sector plate slipper shoes. These contact loads occur because of the changes in pressure which occur during the air heater rotor rotation.

The modified radial seal referred to is fitted between a rotor radial plate and a top or bottom sector plate and is comprised of approximately 2 mm thick carbon steel.

The radial plate seal may have slotted holes therein, for adjustment.

The various aspects of the present invention as described above may be used separately or together in any combination thereof.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic perspective view of a known type of regenerative rotary air heater;

Figure 2 is a detailed perspective view of the air heater rotor shown in Figure 1;

Figure 3 is a cut-away perspective view of the central part of a known rotary air heater;

Figure 4 is a schematic view indicating the expansion profile of the rotor of a known regenerative air heater;

Figure 5 is a schematic side view of one-half of a regenerative rotary air heater, illustrating the first aspect of the present invention;

Figure 6 is a schematic view showing a sector plate of a regenerative rotary air heater, and illustrating the second aspect of the present invention;

Figure 7 is a schematic plan view showing a sector plate of regenerative rotary air heater, and illustrating the third aspect of the present invention;

Figure 8 is a cut-away perspective view illustrating typical axial, radial and circumferential seals as employed in a known type of regenerative rotary air heater;

Figure 9 shows a top circumferential seal to be applied between the rotor and the casing of a regenerative rotary air heater, in accordance with the fourth aspect of the present invention;

Figure 10 shows a bottom circumferential seal to be applied between the rotor and the casing of a regenerative rotary air heater, and also in accordance with the fourth aspect of the present invention; .

Figure 11 is a view similar to figure 5, and illustrates a modification of the first aspect of the invention; and

Figure 12 shows a modified type of radial seal, for use with the modification of figure 11.

In the accompanying drawings, like reference numerals indicate like parts.

Figure 5 shows a regenerative rotary air heater according to the first aspect of the present invention, which comprises a rotor 1 having radial plates 8 and rotatable within a casing 2. Top and bottom sector plates 10, 11 each comprise a central part 10a, 11a and outer parts 10b, lib hinged to the central part by hinges 13. Thus each of the top and bottom sector

plates 10, 11 are made in three parts and are arranged so that they can move up and down on the hinges 13 in order to conform approximately to the top and bottom curvature of the rotor.

As will be apparent there is used a system of balance weights and (in the case of the lower sector plate 11) levers to ensure that the sector plates remain in contact with the rotor radial plate seals without excessive contact load.

The top sector plate 10 sits on top of the rotor and the contact load is reduced by a balance weight 14 to a level which keeps the sector plate in contact with the rotor and allows the sector plate to follow rotor deformation. Reference numerals 15 indicate brackets and pulleys, and numeral 16 indicates a bellows.

The weight of the bottom sector plate 11 tends to move it away from the rotor. A lever 17 pivoted at 18 is used in conjunction with a balance weight 19 in this case to keep the sector plate 11 in contact with the rotor seals. In this case also this arrangement allows the sector plate to follow the deformation at the bottom of the rotor. Reference numeral 20 indicates a bracket and pulley associated with the balance weight 19.

Figure 6 illustrates the second aspect of the present

invention and shows schematically radial plates 8 and a sector plate 10 or 11 (numeral 10 is used in Figure 6). A standard sector plate has for example three radial plates 8 in contact with the sector plate; the sector plates are often assymetric, as shown. By providing the sector plate with extensions 21 as shown, the number of radial plates 8 in contact with the sector plate can be increased to four at each end.

Figure 7 illustrates the third aspect of the present invention and shows schematically radial plates 8 and sub-division plates 9, and a sector plate 10 or 11 (numeral 10 is used in Figure 7). Figure 7 also shows the sector plate extensions 21 previously described according to the second aspect of the invention.

As previously mentioned, it is known that, in some positions, some of the radial plates 8 run off the sector plate allowing a cross (radial) flow. By extending the sub-division plates 9 to the full height of the radial plates 8 and by fitting suitable seals (not shown) to seal against the sector plates the cross flow can be partly prevented, which has the effect of bringing part of another radial plate seal into effect, this further increasing the resistance leakage flow. Figure 7 shows two inner rows of sub-division plates 9, and shows at locations X how part of two radial seals are brought into effect. In a position with the rotor

rotated a further one half radial plate spacing, the effect is even more marked.

Figure 8 illustrates various types of axial, radial and circumferential seals as employed in a known type of regenerative rotary air heater.

Figures 9 and 10 illustrate top and bottom circumferential seals respectively, modified in accordance with the fourth aspect of the present invention.

The type of circumferential seal generally used in the prior art is shown in Figure 8. A seal is fitted with a predetermined cold gap between the seal tip and the rotor sealing surface; as the temperature increases and the rotor deforms the seal gradually comes closer to the sealing bar. This is satisfactory at full load but boilers often have to operate at part load, in which case the rotor is not fully deformed and the seal therefore does not make contact with the sealing surface, thus impairing air heater efficiency.

The deformation pattern for the rotor can be determined for various load conditions up to full load, and with this information a suitable structure for the top and bottom seats can be determined. The nature of these profiles is shown in Figure 4.

A top circumferential seal, according to the fourth aspect of the present invention, is shown in Figure 9 where the seat 22 is mounted on the rotor 1 and the seal 23 is mounted on the casing 2. A bottom circumferential seal is shown in Figure 10 where the seat 22 is mounted on the casing 2 and the seal 23 is mounted on the rotor 1.

In each case the seal 23 consists of a seal mounting bar 24, inclined at a suitable angle to the seat 22, and a stainless steel wool seal 25 retained by the clamping plates 26 and secured by bolts 27 to the mounting bar 24.

The seat 22 in each case consists of a folded steel section 28 such that the inclination of the sealing surface follows the deformation profile of the rotor. In the top circumferential seal shown in Figure 9 the seat 22 is welded or fastened by other suitable means to the rotor 1, while in the bottom circumferential seal shown in Figure 10 the seat is bolted or fastened by other suitable means to the casing 2.

Reference numeral 29 indicates lagging in Figures 9 and 10.

In an alternative embodiment of the first aspect of the invention, as shown in figure 11, the top and bottom sector plates 10, 11 are connected by rigid links 31 to

adjust and maintain the rotor/sector plate gap. Adjustment is carried out by means of a turnbuckle 32 in the rigid link. Slipper shoes 33 of suitable wear resistant material, such as cast iron, are fitted to the outer edge of the sector plate, which requires increased clearance between the rotor and the sector plate. A modified type of radial seal 34 is fitted to accommodate the increased clearance between the rotor and sector plate, as shown in figure 12. This arrangement has the advantage of reducing the contact loads between the rotor and sector plate slipper shoes. These contact loads occur because of the changes in pressure which occurs during the air heater rotor rotation.

The seal 34 shown in figure 12 is fitted between a rotor radial plate 8 and a sector plate 11 and is comprised of approximately 2 mm thick carbon steel. The radial plate seal 34 may have slotted holes therein, for adjustment. A similar seal will be fitted between the rotor radial plate and the upper sector plate 10.

The balance weight 19 shown in figure 11 allows the bottom sector plate 11 to follow the deformation of the rotor without any undue load being imposed.