GB1603428A

GB1603428A - Heat shields

Info

Publication number: GB1603428A
Application number: GB31500/80A
Authority: GB
Original assignee: Encomech Engineering Services Ltd
Current assignee: Encomech Engineering Services Ltd
Priority date: 1978-05-04
Filing date: 1978-05-04
Publication date: 1981-11-25

Description

(54) IMPROVEMENTS IN OR RELATING TO HEAT SHIELDS (71) We, ENCON ENGINEERING SERVICES LIMITE, a British Company of 729 London Road, Hounslow, TW3 1SE, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- This invention relates to heat shield arrangements, particularly, but not exclu sively, for use in metal processing, and to thermal insulation panels for use in heat shield arrangements.

Moder engineering techniques are increasingly demanding materials with improved properties, which involve closer control of their manufacture and processing. As a par ticular example in steel mill processing whether of billets, strip or sections, the metallurgical qualities of finished product are closely related to the accurate control of temperature of the material during the hot rolling process.

A modern hot strip mill producing steel coil can be up to a kilometrc in length and tvpically steel slabs or billets are reduced from 25 cms thickness to 0.2 cms using several roughing rnill strands and five or more finishing mill stands. During the rolling process considerate heat losses occur so that the slabs have to be hcated initially well in excess of the temperature requirement at the end of the process but a par ticular problem has been that the heat losses from slabs passing along the mill depend upon the time taken. If the slabs are delayed, excessive heat losses occur and the steel strip does not have its required rolling temperatures, so that it may have to be downgraded or even scrapped. Tn many long modern rolling mil0s the delay of one length of strip at the finishing end has an effect on the several lengths of material which are simultaneously at various preceding stases of rolling. Thus with more astringent quality specifications it is becoming more important to reduce the rate of heat loss from the material during transport between stands.

There is an added dimcutty in this because during the final reduction stages the back end of the strip takes longer to pass through the finishing mills and so there is a temperature"run-down"along the steel strip due to the cumulative time delay along the length of the strip. The effect of temperature"run-down"is to some extent ameliorated by accelerating the finishing mills during the rolling of each individual slab or strip but nevertheless it remains a problem.

Attempts have been made in the past to reduce the heat loss from the top surface of a hot strip during transport from the rough- ing mills to the finishing mills. Because radiation is a major source of heat loss at the temperatures involved (around 1060 C) reflectors have been used with some success to reduce temperature"run-down"from 40 C to around 25 C. However maintenance problems limit the usefulness of reflectors which become inefficient as soon as they become dirty. Such reflectors cannot in any case be fitted under a strip because hot scale and steam would rapidly damage them. Tn addition, the aluminium reflectors which have been used for their high reflectivity and relatively low cost can reach their melting temperature if their reflectivity decreases. As a result reflectors have been abandoned as a practical means of reducing heat losses from the top surface of a hot strip.

According to the present invention there is provided a composite thermal insulation panel, which may be arranged to limit heat loss from an adjacent hot surface, compris ing a core of fibrous ceramic material providing a heat-msulating layer and enclosed in a protective casing, a cover p'ate forming or overlying a main face of said casing being adapted to form the hot face of the panel and to radiate back heat received from the hot surface, the arrangement being such that the temperature of the cover plate can be brought close to that of the hot surface and that the cover plate is capable of relative thermal expansion with respect to adjoining regions of the casing. Series of such panels can be used in a heat sliield arrange- ment for hot material.

Such an arrangement is able to rely on re-radiation of the heat transmitted to it from the adjacent hot surface in order to reduce the cooling of that surface. If the cover plate is made relatively thin to have a low thermal capacity, at least a part of its area may be provided with dimples, corrugations or other locally contoured formations to increase the heat emissivity of its surface and such contouring may also be arranged to contribute to the strength of the cover plate and its ability to withstand thermal stresses, as well as to allow a measure of independent thermal expansion by flexure.

The casing may comprise a peripheral housing that supports the cover plate in a manner that permits said relative thermal expansion of the cover layer, and the op posite main face of the pancl forming its cold face may similarly be provided with a cover plate mounted in a manner permitting relative thermal expansion, so that thermal stresses on the casing are kept to a minimum.

Preferably, the casing provides a substan- tially dust-tight enclosure for the insulating core and venting apertures may be arranged in the casing for the escape of air and steam from within, the apertures being provided with valves for preventing or limiting a return flow through them.

The invention will be more particularly described by way of example, with reference to the accompanying drawings, which also appear in our copending patent ap plication No. 18760/77 (Serial No. 1603427) relating to another invention illustrated therein. In the drawings :- Fig. 1 is a longitudinal vertical section of a portion of the strip conveying path in a hot strip rolling mi ! ! showing upper and lower panels according to the invention used in a heat shield arrangement.

Fig. 2 is a perspective view of a part of a transfer conveyor of the mill showing on'y the upper panels in place, Fig. 3 is a graph illustrating temperature run-down of a slab in a hot strip mill and the iníluellee on this of a heat shield ar rangement according to the invention Fig. 4 is a transverse cross-section of a further form of heat shield arrangement according to the invention, Figs. 5 and 6 are schematic illustrations of alternative heat-insulating panels for the arrangement of Fig. 4.

Fias. 7 and 8 illustrate details of a modified front plate construction for a panel according to the invention, and Fig. 9 is a longitudinal section of a heat shield arrangement employing panels analogous to those described with reference to Fig. 7.

Referring to Fig. 1 of the drawings, a hot strip mill delay table is shown with a hot steel slab or strip 1 resting on rollers 2 and with low thermal mass insulation panels according to the invention disposed above and below the strip.

The upper panel comprises a casing containing a two-layer core of heat-insulating material consisting of hih temperature ceramic fibre board insulation 4 adjoining a bottom cover plate 3 forming the hot face of the panel, the insulation 4 being backed by lower temperature insulating board 5.

The cover plate 3 is formed by a thin sheet of temperature resistant material, e. g. stainless steel, which material can be made either heat reflecting or absorbing. The cover plate rests upon heat-resistant angle-section frame 7 of the casing, which allows free thermal expansion of the plate, and slots 31 in the frame also allow its relative thermal expansion. The top of the panel is sealed by a top plate 10 to which is fixed a mounting lug 11.

In use, as the hot strip 1 enters under the panel heat radiated from the strip will impinge on the cover plate. Initially the front end of the strip will cool at almost the rate it would achieve in free air while the cover plate is cool. If the plate is thin-for example, less than 1/50th of the thickness of the hot strip 1-then its temperature will quickly rise towards the strip temperature without a significant drop in strip temperature, and as heat loss into the panel is minimised by the thermal insulatin-core and the panel almost immediately begins to reradiate heat and approach thermal equilibrium with the strip.

The total effective emissivity of the cover plate of the re-radiating panel is increased by substantia ! corrugating or dimpling or like local contouring 6 of its surface, and since the cover plate is preferably very thin such contouring can serve to strengthen the plate and allow local flexing to facilitate relaxation of thermal stresses set up by rapid heating and cooling of the plate. i he cover plate is held from above against the horizontal flanges of the frame 7 by leaf springs 8 to help provide a seal between the plate and the frame and so minimise the intrusion of dirt and steam into the ceramic fibre core. Vents 9 can also be provided in the side walls of the mounting frame 7 to facilitate the escape of steam if, as during periods of inactivity, water is absorbed by the core and is subsequently vaporise by the heating that occurs when the panel begins to be used again. One vray flap volves 30 -, n the vents 9 help to prevent the ingress of steam from the surroundings. It should be noted that if the cover plate is very thin, extra retaining clips 13 may be required to hold it against the ceramic fibre material of the core.

It will be clear that almost half the heat radiated by the strip is from its lower surface. The problems presented in placing a panel under the strip are greater than for the top panel and in general a modified construction will be required. The lower panel shown in Fig. I is supported on the table base plate 15 and can rest thereon either directly or through brackets 16. The cover plate 17 form ng the hot face of the lower panel has local contouring like the plate 3 but forms an integral part of an outer casing of welded construction with stainless steel side watts 18 and base 19 as below the strip greater protection is needed against the ingress of foreign matter. The contouring is preferably in the form of a series of parallel corrugations arranged particularly to accommodate thermal expansion of the hot face relative to the remainder of the panel, but because the cover plate is restrained at its edges, the casing is made relatively narrow in the direction of the corrugations, e. g. some half the length transverse thereto. The corrugations also increase the effective emissivity of the hot face, as mentioned above, and strengthen the relatively thin material.

The base of the casing has one or more corrugations 20 to allow differential thermal expansion and the panel is filled with a core of higher and lower temperature insulating ceramic fibre panels 21, 22 respectively, similar to the upper panel core. Small steam vents 23 are provided in the base of the panel to allow venting of trapped water and these are normally closed by one-way flap valves 30.

A number of lower panels are disposed side-by-side across the width of the strip and not only reduce the heat loss from the bottom of the strip, but by extending close to the opposed sides of successive rollers of the delay table they also reduce the thermal load on the rollers thereby improving their operating life and reducing the amount of water cooling required.

The effect of a series of such upper and lower panels as a heat shield in a rolling mill can best be understood by reference to Fig.

3, which shows the temperature of a strip or slab at entry to the roll train of a mill after it has passed along a delay table.

Curve A shows how the temperature of the strip, as sensed at the mill entry, runs down because of the increasing heat losses that have occurred at points progressively further back along the length of the strip due to the increasing delay hefore entry as dictated by the travel speed of the strip in the rolls. The initial temperature of the strip would be, say, 20I2 F and while the front end temperature loss might be some 100 F at mill entry (point F), further back along the strip the material arrives at the mill with a progressively lower temperature.

Curve B shows how the heat shield arrangement of panels according to the invention gives a similar temperature fail at the front end of the strip, while the panels are still cool, but they almost immediately radiate heat to reduce the temperature fall and very quickly approach the initial temperature of the strip and can reverse the temperature profile for the rear end of the strip, so as to give a more uniform temperature at roll entry.

In a sufficiently long run, the rear end temperature of strip can of course approach the initial strip temperature, but in general the form of the temperature distribution at points along the strip length at their entry to the mill can be controlled by choice of the storage and conductivity characteristics of the hot faces of the panels and the heat insulting cores. It is even possible to incorporate heating means in the panels adjacent their hot faces to give a more uniform exit temperature, but usually this would only be required for material having a relatively low thermal mass, such as bars and sections.

As already stated, the cover plates can be heat-renecring but in genera ! it will be found convenient to make them black to suit the dirty environment conditions and in this case they can operate better to improve the temperature distribution in the common condition of a strip with a higher temperature leading end in the manner shown in Fig. 3, or similarly where there is some time lag in the passage of the trailing end to a roll stand.

The mounting of the lower panels presents no problems but the top panels require to be mounted so that they can be readily lifted from over the hot strip, e. g. for maintenance or to allow the removal of the strip in the event of a long stoppage or to prevent damage to the panel if there is a buckled or bent strip. Fig. 2 illustrates a mounting means suitable for strip mill applications in which upper panels 32 are suspende over the tables in pairs by their top plates 10 and mounting lugs 11 from booms 12, although for narrow mills one panel width will be sufficient to span the roller table whereas on wide mills as many as four panels side-by- side may be required. The panelsarearranged in a row or rows extending along the stripconveying direction to make uu the required heat shield area. It will be understood that the smaller width of the plower panels dictated by thermal expansion considerations in this particular use will result in the use of a larger number of panels than the upper panels to cover the same width.

The booms 12 are mounted as cantilevers from shafts 24 on external floormounted frames at one side of the mill the shafts being supported in rotary bearing blocks 25 to be able to pivot the booms through substantially 90'between the horizontal position shown and a raised vertical or near vertical position. Columns 26 carrying the bearings blocks are positioned so as to provide access to the roller drive motors (not shown) of the mill. The panels and booms are counterbalanced on the shafts by weights 27 and the movement of the panels is obtained bv motors 28, e. g. hydraulically operated, which are designed to give positive drive both when raising and lowering the panels.

Operator controls (not shown) are provided for the motors 28 to allow the panels to be raised and lowered for maintenance or access to the mill. To prevent damage to the panels during operation, e. g. if there is a bent or buckled strip in the mill, a skid bumper bar 14 is fitted to certain of the panels or to the boom structure so that the bent strip will contact the bar to push the panels upwards. Preferably, the skid bumper bars are connected to pressure switches 29 which activate the raising movement of the motors 28 in such an eventuality, so that the panels are lifted independently of the force on them.

The invention is not restricted to installations in which the heated material is in strip form, and Figs. 4 to 6 illustrate the use of further panels according to the invention in an installation to control the temperature of bar of profiled section stock moving along a conveyor path.

Mounted on a fixed frame 71 are drive rollers 72 for the stock 73, shown here as a square section bar, driven by one or more motors 74. The conveyor path runs perpendicular to the plane of the figure. Mountings 76 pivotable about an axis 77 parallel to the conveyor path carry upwardly projecting side support rollers 78 for the stock and lower heat shield panels 80 of the composite construction already described. To gether with upper U-form pancls 8i of similar construction these lower panels 80 form a tubular enclosure for the stock. The upper panels 81 are supporte on pivot mountings 82 that allow them to be swung upwards to the broken-line position by rams 83, clear of the conveyor and the stock on it. When the upper panels have been swung away in this manner, the side support rollers 78 can be pivotecl downwards to give free access to the stock, in particular to allow bent or damaged stock to be removed more easily.

The pivoting of the mountings 76 for the bottom panels allows them to be tilted when the installation is being serviced to shake off any accumulation of mill scale or other foreign matter.

Because the top panel has a cross-sectional profile that increases its stiffness as compared with the flat panels so far described, it can be made considerably longer: it may have flat faces 84 or axial corrugations 85 can he provided to stiffen the inner cover plate, as has already been described.

This inner cover plate is allowed some freedom to expand axially and laterally and is retained in place by lipped edges 86 of the panel casing and heat-resistant ties 87 passing through the inner core of insulating material. A bumper bar 88 projects through the top panels to protect them from being damaged by out-of-shape material. As already described, the bar can be provided with impact sensors 89 arranged to trigger the operation of the lifting rams 83 for raising the top panels.

The cross-sectional profile of the panels can be varied, for example, to suit a particular cross-section profile material, as is exemplified in Figs. 5 and 6. In Fig. 5, the bottom panel 91 also has a U-form profile increasing its stiffness and the radiussing of the cover plates at the inner corners 92 of the panels can improve the uniformity of the temperature of the stock passing through the arrangement. The corners of rectangular stock or even of the outer edges of the flanges of the I section shown will normally radiate heat more freely and so cool faster, but the increased insulation of the radiussed corners of the panels has an opposite and therefore compensating effect.

A similar effect can be obtained with the configuration shown in Fig. 5, where both panels 93 have a semi-circular cross-section.

In some circumstances it may be required not only to conserve the heat in the material being processed but also to apply further heating. For example, this might be necessary if the material is to be held at a constant temperature. It is possible then to provide heating means in or on the insulation panels themselves, as is illustrated in Figs. 7 and 8. These show a multi-layer cover plate, with a front plate I01 forming the hot face of the panel, a first electrically insulatin layer 102, an electrical strip heating element 103 capable of operating at a temperature of 1100 C or more, a second insulating layer 104, and a back plate 105 that like the front plate is suitable for use at high temperatures.

The layers are secured together by hollow rivets 106 that may also serve to receive suspension ties 107 for the panel, although panels with cover plates of this construction are suitable for mounting both above and below the material being shielded. To maintain the material temperature, the element 103 may be of relatively low power, e. g. up to 5 kw/ml, but higher powers may be used when required. It will be clear without further illustration that the control unit 60 can also regulate the operation of the heating elements.

The use of panels with heating elements is shown in the arrangement in Fig. 9, although tubular sheathed elements 103a are illustrated. This example shows a further heat conservation measure, it being assumed in this example that the arrangement is being employed with an existing steel mill roller table, which will be equipped with large diameter rollers 110 of high thermal capacity, often water-cooled. The upper and lower thermal insulation panels 111, 112 have the general composite construction already described and both series of panels are arranged in pairs side-by-side, with a central skid bar, 113 and 114 respectively, between the panels of each pair. The lower panels are also so constructed that there are two or more successive panels along the distance between adjacent rollers 110, and between these successive panels transversely extending lifting bars 115 are interposed.

These bars, of a high temperature alloy and possibly also provided with heat insulation, can be raised, e. g. by fluid pressure rams 116, to the position shown and so lift the material 1 from the rollers 110. The mechanism can be used when the operation of the mill requires the material on the roller table to be stopped for any significant length of time. Because of the thermal mass and/or the cooling of the rollers, there would be the risk that cold spots would develop in the material at the regions of contact with the rollers, and that these would affect the uniformity of the final product.

Claims

WHAT WE CLAIM 1S :-

1. A composite thermal insulation panel comprising a core of fibrous ceramic material providing a heat-insulating layer and enclosed in a protective casing, a cover plate forming or overlying a main face of the casing beinn aclaptccl to form the hot face of the panel and to radiate back heat received from a hot surface adjacent saicl face of the panel, the arrangement being such that the temperature of the cover plate can be brought close to that of the hot surface, the cover plate being capable of relative thermal expansion with respect to adjoining renions of the casing.

2. A panel according to claim I wherein the cover plate has at least a part of its area provided with locally contoured formations.

3. A panel according to claim 2 wherein said formations comprise a serves of parallel corrugations and the panel is of elongate form with the corrugations extending transversely to its length.

4. A panel according to any one of claims I to 3 wherein the casing has a main face opposite to said cover plate main face formed by a further plate, that is arranged to permit thermal expansion with respect to adjoining regions of the casing.

5. A panel according to any one of the preceding claims wherein a substantially clust-tight enclosure for the core is formed by the casing, or by the casing and cover layer jointly, and vents to the exterior from the interior of the casing are provided with non-return closure means for preventing or limiting a return flow therethrough into the casing.

6. A panel according to any one of the preceding c ! aims wherein supporting elements for the cover plate extend through the core of the panel to said plate.

7. A panel accordng to any one of the preceding claims wherein the cover plate forms an integral welded construction with adjoining side faces of the panel.

8. A panel according to any one of claims 1 to 6 wherein the casing comprises a peripheral support frame against which the cover plate freely rests to permit relative thermal expansion between the cover plate and the casing.

9. A panel according to claim 8 wherein resilient means urge the cover plate against said support frame.

10. A panel according to any one of the preceding claims wherein the core comprises a plurality of heat-insulating layers, said fibrous ceramic material forming at least one layer of the core immediately adjacent said cover plate.

11. A panel according to any one of the preceding claims comprising heating means dispose immediately adjacent said hot face.

12. A panel according to any one of the preced. ng claims having an arched or channel transverse cross-section.

13. A heat-insulated tubular conduit formed by a plurality of composite thermal insulation panels accordino, to any one of the preceding claims in juxtaposition to each other.

14. A heat shield arrangement for material being processed in a hot-rolling mill, the arrangement comprising respective series of heat-insulating panels according to any one of claims I to 12 mounted above and below a path for the hot material through the mill.

15. A composite thermal insulation panel constructed and arranged for use substantially as described herein with reference to any of the illustrated embodiments.