GB2137978A - Heat Storage Blocks and Manufacture Thereof - Google Patents

Abstract

A heat storage block is manufactured from a heat retention material such as iron oxide or olivine by bonding with a bonding agent curing at a low temperature such as a phosphate bonding material e.g. sodium hexametaphosphate. This bonding material provides sufficient strength at ambient temperatures to permit storage, transport and installation of the block while being sufficiently stable up to 800 DEG C to maintain the strength of the block in use. A second bonding agent e.g. clay can be used which is effective at higher temperatures should these be achieved.

Description

SPECIFICATION Heat Storage Blocks and Manufacture Thereof This invention relates to heat storage blocks and the manufacture thereof, particularly heat storage blocks for domestic heating systems.

Heat storage systems, particularly for domestic use which are sometimes called heat storage radiators, have been long available and particularly commercial embodiments have been known in the United Kingdom for at least 20 years. In the usual embodiment blocks of a heat retention material are mounted within a metal casing, usually of mild steel, together with a means for providing heat to the blocks usually electric resistance heaters. Heat is inputted into the blocks with the resultant increase in temperature of the blocks. When the input of heat is ended, heat then transfers from the blocks to the air surrounding them and so passes into the room surrounding the heater. Release of heat can be expedited by a drawing air across the heated blocks, for example by means of a secondary fan mounted in the metal casing.In more sophisticated versions of the systems the release of heat can be controlled by means of dampers or vanes which control the rate at which air passes over the blocks.

Particularly in the United Kingdom the heating is by electricity and the heat energy is inputted during the night when electricity is available at lower tariffs. The system can then be adjusted to release heat during the day. The systems may provide all of the heat required to maintain ambient temperatures or can be used for background heating.

To provide sufficient heat storage the blocks must have a high density. In addition, in order to provide the necessary rates of input and output, the material of the block must be such as to conduct heat at a reasonable rate. Initial blocks were made from cast iron but these were costly.

Very dense fire-brick was then employed but to improve the heat capacity other heat retaining materials have been proposed such as olivine and iron oxide. To provide the necessary strength for storage transport and installation blocks of these materials have been formed by firing in a manner similar to production of refractory bodies.

It has now been found that a block of such a heat retention material is satisfactory even though bonded by a material which is curable at a relatively low temperature but which can resist temperatures of up 800--9000C. A bond of the type obtained by firing a ceramic composition to a temperature in excess of 1 0000C has not proved necessary.

According to the present invention there is provided.a heat storage block comprising a particulate heat retention material bonded by an agent which is curable to form a bond at a temperature up to 1 500C and which retains bonding properties up to 9000C.

The bonding agent can additionally comprise a further high temperature bonding agent which will form a bond at a temperature exceeding 8000C.

The particulate heat retention material is a material which is sufficiently dense so as to absorb considerable amounts of inputted heat and which has a thermal conductivity which permits ready input of heat and release of heat once the heat source is shut off. Relatively few materials have the necessary density and simultaneous thermal conductivity to be usable in such a heat storage block. For example, firebrick materials tend to be rather low in density. A very suitable material is a magnesium silicate particularly in the orthosilicate form, for example, olivine. Superior to olivine is iron oxide usually in the form of magnetite but which can contain a small proportion of haematite. Magnesia could also be employed although it tends to be somewhat uneconomic.

The suitability of material can readily be determined by review of the physical properties i.e. density and heat conductivity. The relevant density is of course that of the block after formation and not the particulate material in loose powder form. The preferred heat absorbing material iron oxide exists in various forms for example as magnetite and one must select an iron oxide which has the correct crystalline structure to provide the appropriate density. Some iron oxides may not have a sufficiently high density to be as efficient as the preferred magnetite with a small amount of haematite present. Thus appropriate densities are 3.6 to 3.9 g/cm3 but up to 4.2 g/cm3 can be obtained. Olivine has superior thermal transmittance properties but a lower density of 2.5 g/cm3 so that iron oxide is preferred as having adequate density and reasonably good heat conductivity.Prior to formation into the block the particulate heat retention material should have a particle size of grading such that it can be densified while being formed into the block. A particularly suitable particle size is 9 mm and finer down to dust preferably the size should be 5 mm and finer. Particle sizes up to 20 mm might be employable but with increasing difficulty.

The bonding agent is one which will provide sufficient strength by a curing process which should either be effective at ambierit temperature or with only slight heating for example up to 1 20cm or possibly 1 50O C. The bond should retain it strength to hold the block together up to temperatures of 8500C possibly 9000C which is above the normal maximum operating temperature for a domestic heat storage unit. A wide variety of bonding agents can be employed for example glycols,-starches, sulphite lye, sodium or calcium lignosulphates, waxes, resins, tars and polymeric bonding agents although some of the organic agents may not be altogether suitable if bonding is required up to the highest possible temperature because of the decomposition of the organic material. Inorganic agents such as silicates can be employed.The bonding agent should be of such a nature that, when heating of the blocks takes place particularly in initial use, there is no emission of noxious or corrosive fumes particularly as the temperature exceeds 5000C.

Particularly suitable has been found to be a phosphate bonding agent. Particularly preferred is a sodium hexametaphosphate. Other phosphates such as phosphoric acid or aluminium orthophosphates could be employed. Phosphates have been suggested as bonding agents for refractory materials. For example in European patent specification EPA 47728 phosphates are used to bond together materials such as magnesia or olivine and expanded perlite to form an insuiating material and other uses in the manufacture of refractories have been suggested. There has been no indication that such a bonding agent could be employed to form a block of heat storage material.

Although the normal upper limit of the elevated temperature for the block in normal use will be about 6500C it is possible for the block to reach higher temperatures particularly 800 or 8500C in normal operation perhaps even 1 000 0 C. At these higher temperatures it may be desirable to have a further or secondary bonding agent available which will form a bond at the higher temperature.

A particularly suitable for this purpose is a clay. A clay also has an advantage that in a bonding using a phosphate bond.the clay may co-act with the phosphate to strengthen the phosphate bond.

Thus the clay helps to react with the phosphate to form the bond and also as the temperature rises the clay may additionally react. An appropriate clay would be a refractory clay such as a ball clay or fire clay, and other clays used in high temperature firing could be employed.

The proportion of the bonding agent will depend on the nature of the bonding agent and whether or not a secondary bonding agent is also present. However, the proportion of bonding agent is readily determined from the known properties of bonding agents and the amounts needed to achieve a certain strength at the requisite temperatures. The amount of bond should of course not be so excessive as to interfere with the heat retention and heat transmission properties of the heat retention material in the block when finally formed. Should there be any doubt as to the requisite strength of the bond, it is easy to form a block and test the resulting density, porosity and crushing strength and the strength at different temperatures to determine its practical application.Particularly where a phosphate bond is employed the amount of phosphate based on the final weight of the block will be preferably 1% to 4%. If a clay is also employed then a content based on the final weight of the block of from 3 to 10% is desirable.

Too much phosphate leads to migration to the surface of the block and impares the density of the block.

The curing temperature required depends on the bond. For a phosphate bond temperatures of up to 1 500C may be required while as low as 800C may be effective. Usually a curing temperature between 100" and 1200C will be appropriate. To give strength below 1000 a sugar can be added to the bonding agent.

The block is usually formed by blending the heat retention material and the bonding agent or agents together preferably with small amounts of water to assist in the blending and compressing.

The mix is then compressed in a mould or die particularly a steel die under a sufficient pressure to achieve the necessary compression and formation into the final shape. Thus pressures of at least 4 tons per square inch (61.8 MPa) can be employed. Depending on the nature of the bond, the block can then be sufficiently heated to effect a low temperature cure. The blocks will than have sufficient strength for storing, transferring, transporting and installing.

The amount of water should be sufficient to form a slightly flowable material, particularly to permit flow into the die in which the block is compressed and from 1 to 4% preferably 3% by weight of dry mixture is preferred.

Generally the installation of the system involves the mounting or other installation of the metal framework or casing and then installation of the blocks in the casing mounted about the heat source which can be a mineral-insulated, incoloy sheath electric heating element. Because of the temperature of the block at the end of the heating period it is customary to have additional insulation in the unit so as to prevent loss of heat except by controlled access of air and also to prevent injury by reason of contact with the heated block. Since the block has to surround and encompass the heating element it will usually be of a specific predetermined shape. For example the block may have openings into the centre of the block to accommodate heating elements.

More customarily and because of simplicity in formation of the block the block will have such a design that two blocks can be assembled leaving a space between them into which the heating element can be fitted.

The invention will now be illustrated by the following examples in which the percentages are by weight of dry components.

EXAMPLE 1 A commercial iron oxide comprising magnetite with a small amount of haematite and having the specification (in weight percent) Fe 68.5 Mn 0.06 P 0.05 S 0.03 CaO 0.6 MgO 0.7 Al203 0.6 Si202 2.3 TiO2 0.4 Na2O 0.12 K20 0.1 H20 2.5 (the iron oxides present are measured as indicated by reference to the iron content) sold commercially under the name FW1 and having a particle size 9 mm to powder as was blended with a commercial refractory ball clay and a commercial sodium hexametaphosphate sold under the name "Calgon" (Calgon is a registered trade mark) in the proportions 92% iron oxide, 5% clay, 3% sodium hexametaphosphate all by weight of mixture of dry components and 2 parts of water by weight of the dry mix components was then added.

The mix was pressed in a steel die under a pressure of at least 4 tons per square inch (61.8 MPa) to form a rectangular block 228x 192x47 mm having an indentation in one major face approximately 5 mm deep extending across the face. The density of the final compacted block was at least 3.6 g/cc.

The blocks were then cured at a temperature between 800C and 1000C after which the strength was such that they could be readily stored, transported and installed.

The blocks were installed in a domestic electric storage unit manufactured from mild steel metal casing with insulation between the blocks and the external casing. The storage unit was provided with at least one mineral-insulated incoloy sheath electric heating element which was installed between two blocks having the indented faces opposed to each other so as to provide a space between the blocks. The unit was also provided with a controllable thermostat and an adjustable room temperature sensing element and damper bars controlled by said thermostats. On operation of the heating elements the temperature of the blocks was raised over a period of 7 hours to 7300C in the faces adjacent to the heating elements and 61000 in the external faces. During this heating no noxious fumes or other undesirable gases were noted, there only being a slight release of water vapour. The system provided adequate storage of heat and release of heat for normal domestic use and on constant operation over an extended period of time no deterioration- was noted in the block.

EXAMPLE 2 A block was formed as in Example 1 excepting that instead of iron oxide there was employed olivine of the same particle size. The final block after pressing in the die had a density of at least 2.55 g/cc and the block was cured in a manner similar to that of Example 1.

The olivine block was installed as in Example 1 and showed the same characteristics except for a slightly lower heat capacity.

Claims (1)

1. CLAIMS.

   1. A heat storage block comprising a heat retention material bonded by a bonding agent which provides sufficient strength to the block front an initial curing of up to 1500C and which provides bonding strength when the block is subjected to elevated temperatures in normal use.

   2. A heat storage block according to claim 1 in which the bonding material is an inorganic phosphate.

   3. A block according to claim 2 in which the phosphate is sodium hexametaphosphate.

   4. A block according to any one of claims 1 to 3 in which the heat retention material is an iron oxide.

   5. A heat retention material according to claim 4 in which the iron oxide is principally magnetite.

   6. A block according to any one of claims 1 to 3 in which the heat retention material is olivine.

   7. A block according to any one of claims 1 to 6 in which a secondary bonding agent effective at a temperature above 80000 is present.

   8. A block according to claim 7 in which the secondary bonding agent is a bonding clay which forms a bond at temperatures exceeding 8500C.

   9. A block according to claim 8 comprising by weight 3 to 10% clay, 1 to 4% sodium hexametaphosphate and balance iron oxide or olivine.

   11. A block according to any one of claims 1 to 10 which is shaped with a recess in the block to accommodate an electric heating element.

   12. A block according to claim 1 substantially as hereinbefore described with particular reference to the Examples.

   13. A method of forming a heat storage block comprising blending a heat retention material, a phosphate bonding agent and a clay with sufficient water to make the mixture flowable, compressing into the desired shape of block, and curing at a temperature not exceeding 1 5000.

   14. A method according to claim 13 which comprises blending heat retention material which is iron oxide or olivine with clay and sodium hexametaphosphate in a weight proportion 3 to 10% clay, 1 to 4% hexametaphosphate and balance heat retention material and then adding from 1% to 4% by weight of dry components of water.

   1 5. A method according to either of the claims 1 3 and 14 in which the blocks after installation in a heat storage unit are heated to a temperature of at least 6000C.

   16. A method of forming a block substantially as hereinbefore described with particular reference to the Examples.

   1 7. A block when formed by method as claimed in any one of claims 13 to 1 6.

   18. An electric heat storage unit comprising an electric heating element located within or surrounded by heat retention blocks in which the heat retention blocks are as claimed in any one of claims 1 to 12 and 17.