EP0248674A1

EP0248674A1 - Heat insulating panels

Info

Publication number: EP0248674A1
Application number: EP87304993A
Authority: EP
Inventors: William Robert Laws; Geoffrey Ronald Reed
Original assignee: Encomech Engineering Services Ltd
Current assignee: Encomech Engineering Services Ltd
Priority date: 1986-06-06
Filing date: 1987-06-05
Publication date: 1987-12-09
Also published as: GB8613841D0

Abstract

A heat insulating panel arranged to be exposed to high temperatures, in excess of 700°C, has a ceramic fibre core in two or more layers (8,10,12) with a metallic membrane (4,6) being interposed between the successive layers. The internal membrane or membranes inhibits the passage of shorter wavelength energy between successive layers so acting as a radiation barrier that increases the efficiency of operation of those layers to improve the insulating properties of the panel. For use at very high temperatures, the hot face (2) of the panel may comprise a ceramic material preferably with a glazed surface.

Description

This invention relates to heat insulating panels for high temperature applications, e.g. as heat shields in hot rolling mills and as walls of furnaces.
High temperature ceramic fibre insulation enables more compact furnace walls to be constructed because of its low thermal conductivity compared with traditional refractory bricks. To facilitate installation it is common practice to build ceramic fibre furnace walls with individual panels or with overlapping layers of ceramic fibre blanket held in place with studs. While this form of insulation is considerably better than conventional refractories in the form of brick, it is found however that it loses some of its relative advantage at higher temperatures, e.g. greater than about 800°C.
Panels comprising a ceramic fibre core have also found a useful application as heat shields arranged to surround lengths of hot metal being processed so as to prevent heat loss during transport and working of the metal. Thus, in the manufacture of hot steel strip a major loss of heat occurs while the hot transfer bar is on the roller table between the roughing mill and the finishing mills. Close-fitting insulating tunnels (see e.g. GB 1603428) using insulating ceramic and fibre panels with metal membranes have been developed specifically to reduce the heat loss in such circumstances. These heat shield panels provide fairly effective heat insulation in transient operations, i.e. when hot material is rolled with gap times of several minutes between successive transfer bars, but if the gap time is reduced to such an extent that the internal temperature in the panels rises excessively, as in the case of a high temperature furnace, there is a loss of efficiency.
The significance of this loss is magnified since it is becoming increasingly common to operate hot strip mills more intensively; even if production is not increased there is a tendency to operate for a smaller period but with reduced gap times so that the equivalent production is obtained in less operating time. During these periods of improved utilization, the insulating panels tend to approach more closely to steady state conditions than hitherto, i.e. their internal temperature increases towards the conditions that might be met in furnace wall structures.
The table that follows indicates the increase in thermal conductivity that can occur at very high temperatures in different insulating materials:
The first three lines of the table show conductivity values for solid insulating bricks of silica (SiO₂), of alumina (Al₂O₃), and of a mixture of these two substances. Ceramic fibres are typically made from the same substances or a mixture of the two and there follow examples of two different grades of ceramic fibre composed of 50% silica and 50% alumina. In addition, because ceramic fibre panels comprise mainly voids containing air, and because it is characteristic of small air pockets that little or no internal circulation or convection takes place, the final line of the table gives conductivity values for still air.
It is apparent that, whereas a 50% silica/50% alumina solid brick composition provides a conductivity which is almost constant with temperature, the conductivity of a similar composition ceramic fibre material of 64 kg/m density increases by a factor of 10 from ambient temperature to 1000°C. The loss of conductivity of the ceramic fibre material with increase of temperature that the table indicates cannot be explained by the change of thermal conductivity of air, which only increases by a factor of 3 in the temperature range given in the table.
The present invention is aimed at improving the construction of ceramic fibre heat-insulating panels so as to mitigate the increase of conductivity of the fibre material at high temperature and so reduce the heat loss of such panels.
According to the invention, a heat insulating panel is provided comprising a plurality of layers of ceramic fibre insulating material with a metallic membrane interposed between the ceramic fibre layers or at least two said layers. Preferably there is a plurality of such membranes, with a ceramic fibre layer separating successive membranes.
The invention is based on studies which show that, to a small extent, there is an increase of heat loss through a ceramic fibre material above 1000°C due to convection in the air-filled voids, but the main cause of heat loss is the transmission of radiant heat along the fibres themselves due to their transparency to radiation in the near infra-red and visible range. In fact, a property which has been exploited usefully in optical fibre applications hinders the effective use of ceramic fibres as a high temperature insulating material.
In the hot rolling mill heat shields of the earlier patent referred to above, the presence of a metal membrane at the hot face of a panel inhibits the passage of energy in the shorter wavelengths to the ceramic fibre core, and the thermal shock of rapid heating is taken by the metal membrane. As the membrane is heated by the hot material being processed, it modifies the radiant energy so that energy of longer wavelengths passes into the ceramic fibre core. The lengthening of wavelengths avoids significant heat loss by transmission of radiation through the fibres, but if the hot face of the panel is subjected to a high temperature for some period of time, the temperature of the membrane comes close to that of the hot material. Radiant heat losses through the ceramic fibres then increase. By placing a membrane within the fibre core, or a number of such membranes at spaced distances within the core from the hot face, in accordance with the present invention, the or each membrane can act as a radiation barrier so that the benefits obtained by use of the hot face membrane in the immediate transient condition are extended and it is also possible to obtain some advantage even under steady state conditions.
As in the earlier heat shield panels of GB 1603428, the hot face of a panel according to the invention may be formed by a membrane of a high-temperature alloy. Alternatively, however, and particularly at extremely high temperatures, it may be preferred to form the hot face by a ceramic coating laid directly onto the ceramic fibre, e.g. by spraying. This may, for example, take the form of a glazed coating.
For greater efficiency, the or at least one interposed membrane should have a reflective surface facing towards the hot face of the panel. In practice, the use of materials that can retain reflectivity at high temperatures may not be economic but, if the distance from the hot face is sufficient to ensure an appropriate drop in temperature, a reflective aluminium sheet may form the interposed membrane or an outer one or more of the interposed membranes.
By way of further illustration of the invention, reference will now be made to the accompanying drawings, in which:

Figure 1 is a graph illustrating the wavelength distribution of heat radiation from a number of metallic components at different elevated temperatures, and
Figure 2 shows a cross section through a heat insulating panel, to be used for example as a bottom panel of a hot rolling mill heat shield, in accordance with the present invention, and
Figures 2a and 2b on enlarged fragmentary views of two possible modifications of the panel in Fig. 2.

Referring to Figure 1, this illustrates the progressive shift of the wavelength distribution of the radiated heat from metal components at different temperatures, the peak occuring at progressively shorter wavelengths as the temperature of the component rises. In a hot rolling mill, a transfer bar at a typical temperature of 1050°C shows a peak below the 2.5 micron wavelength that is effectively an upper limit to the transparency of current ceramic fibres. Under steady state conditions, a heat shield panel having an outer metallic membrane facing onto the hot transfer bar might have that membrane at a temperature of 1000°C; the graph shows a slight increase in the peak radiation wavelength but it is notable that a substantial portion of the total heat radiation still occurs below the 2.5 micron limit. Only at substantially lower temperatures, such as the 700°C example also shown, does the wavelength peak drift much above that limit to give a marked drop in the proportion of energy radiated at shorter wavelengths.
The graph thus indicates how the outer metal membrane forming the hot face of a heat shield panel can quickly lose its effectiveness if it is exposed to a hot transfer bar long enough for the panel to reach steady state conditions.
In the example shown in Figure 2 a heat shield panel is shown in which, in addition to the hot face membrane 2 there are two further metallic membranes 4, 6 within the thickness of the panel, interposed between appropriately graded layers of ceramic fibre insulation 8, 10, 12. By their presence, the second and third membranes 4, 6 are able to modify the wavelengths of energy radiation through them from the hotter side of the panel, spreading the radiant flux to a longer mean wavelength, so as to improve the efficiency of operation of ceramic fibre layers.
The particular materials chosen for the membranes will depend upon the actual temperature conditions under which the panel is intended to operate. The hot face membrane 2, and, in normal circumstances the membrane 4 behind it, will be of high temperature alloy. The material will be more effective if it remains reflective at the temperatures encountered, but the high cost of the materials having this capability at very high temperatures may militate against their use. If the panel is to be exposed to extremely high temperatures, such as in a furnace, as it is required to minimise the risk of mechanical damage, it may be desirable to coat the hot face membrane or the ceramic fibre itself with a ceramic material (not shown), e.g. as a glaze or a sprayed-on coating 16 as indicated in Figure 2a, or even to deploy pre-formed ceramic fibre blocks or sheets 18 as indicated in Figure 2b, over the hot face of the panel.
Since aluminium can be made reflective and will retain a reflective oxide film, the second internal membrane 6 is made of aluminium foil. For this to be possible, it will be understood that the membrane must be placed at a position at which its temperature will be safely below the material melting point, i.e. preferably not more than 550°C.
The internal membranes of the panel described thus act as successive radiation barriers. The panel is given an improved insulating performance under transient conditions and there is a reduction in the steady state heat loss when the hot face is exposed to very high temperatures. Such constructions of panel can be applied in a wide variety of high temperature uses, e.g. in hot metal processing such as for rolling mill transfer table heat shields, and also in furnaces and ovens.

Claims

1. A heat insulating panel comprising a plurality of layers of ceramic fibre insulating material characterised in that a metallic membrane is interposed between the ceramic fibre layers or at least two said layers.

2. A heat insulating panel according to Claim 1 wherein there is a plurality of said membranes, with a ceramic fibre layer separating successive membranes.

3. A heat insulating panel according to Claim 1 or Claim 2 wherein the or at least one of the membranes has a reflective surface.

4. A heat insulated panel according to Claim 3 wherein the membrane or at least one of the membranes further from a hot face of the panel is of aluminium.

5. A heat insulating panel according to any one of the preceding claims wherein the panel has a face adapted to be disposed on the hotter side of the panel as a hot face, said hot face comprising a ceramic material.

6. A heat insulating panel according to claim 5 wherein said hot face is formed by at least one ceramic block or panel.

7. A heat insulating panel according to Claim 5 or Claim 6 wherein the ceramic material comprises a glazed coating.