GB2044147A - Cooling method - Google Patents

Abstract

In making resin-bonded moulds or cores for use in the foundry, it is important that the sand is not at a temperature at which the resin cures. With reference to Figure 1 of the drawings, sand recovered from resin-bonded moulds or cores, or obtained from quarries is cooled in a fluidised bed cooler having a cooling chamber 112. Air is supplied to the chamber 112 through a plenum chamber 110 via conduits 126. Sand is introduced into the chamber 112 from an inlet 116 and is fluidised by the air, thus being cooled, and the cooled sand leaves the chamber 112 through an outlet 120. Liquid nitrogen is sprayed into the air entering the conduits 126 so as to reduce the temperature of the air to below ambient, and thereby enable the sand to be reduced in temperature to below ambient. <IMAGE>

Description

SPECIFICATION Cooling method This invention relates to a cooling method. In particular, it relates to a method of cooling sand or other particulate or granular material from which moulds or mould cores can be made in the foundry.

It is well known in the foundry to make moulds and cores of moulds from refractory sand or other particulate or granular refractory material by ramming a mixture of the sand (or other material), a thermo-setting resin and a catalyst into a chosen shape at a temperature at which the catalyst will cause the resin to cure rapidly thereby bonding the individual particles of sand (or other material) together to give the mould or core its necessary integrity.

It is important to avoid premature setting of the resin. To this end, foundries commonly specify that the sand must be supplied at below a chosen maximum temperature, typically in the order of to 28or.

The sand is obtained by quarrying. Since the sand so obtained is wet it is necessary to heat the sand so as to dry it. Typically, this is done by fluidising the sand with hot air. This raises the temperature of the sand. It is thus necessary to cool it. This is conventionally done by means of a fluidised bed cooling technique. The warm sand is fed into a chamber and fluidised by ambient air. This enables intimate contact to take place between each grain of sand and the air. Sometimes, the cooling the air provides is supplemented by passing water through heat exchange tubes within the fluidised bed.Since the air is at ambient temperature, the sand will effectively be cooled to below the chosen temperature in all but hot summer weather unless a longer period of heating than usual is required to dry the sand, thereby increasing the temperature of the sand to above that at which it normally enters the fluidised bed. Once the sand has been cooled it is passed into a large storage hopper. From time to time regular deliveries of the sand from the storage hopper are made to foundries in a suitable vehicle. The thermal insulation properties of sand are such that a hot bath of sand (i.e. a batch of sand above the chosen temperature) will remain at that temperature for long periods. Thus, when such a batch of sand reaches the bottom of the hopper it will still be at an undesirably high temperature and will be delivered at that temperature to the foundry.

An analogous problem is encountered when recovering foundry sand from used resin-bonded moulds or used resin-bonded cores. Sometimes, the resin is removed by burning to yield substantially pure sand. Alternatively, the resin is removed by washing the sand with water. Generally, it is necessary to burn the resin and then wash away the residue. If it is necessary to wash the sand, subsequent drying, for example in a rotary kiln, will be required. Thus, hot sand is produced. Since the ambient temperature in a foundry is often above 28"C, it is often difficult to achieve the necessary degree of cooling in a foundry.

It is an aim of the present invention to provide a method of cooling the sand (or other material) which enables it to achieve a suitable temperature irrespective of the ambient or climatic conditions at the time the cooling is performed.

Accordingly, the invention provides a method of obtaining cooled sand or other granular refractory material, comprising the steps of quarrying the sand (or other material) and drying it, or recovering sand (or other granular refractory material) from moulds or cores formed of resin-bonded sand (or other granular material), fluidising the dried or recovered sand (or other material) with air taken from the atmosphere, and reducing the temperature of the air by heat exchanging or mixing it with an atmospheric gas in liquid or solid state, or the cold vapour of such gas.

Preferably, liquid nitrogen is sprayed or otherwise introduced into the fluidised bed. Instead of liquid nitrogen it is possible to use other liquefied atmospheric gases, for example, liquid argon or liquid carbon dioxide. It is also possible in the example of carbon dioxide to blow carbon dioxide snow, or other form of solid carbon dioxide, into the fluidised bed.

Other less preferred cooling methods involve heat exchanging the fluidising air with a liquefied atmospheric gas upstream of the fluidished bed. If desired, the vaporised gas may afterwards be mixed with the fluidising air. It is also possible to introduce cold vaporised gas into the fluidising air after having vaporised the liquefied gas by heat exchange with a medium other than the fluidising air. This is not preferred, however, as it is wasteful of the latent heat of vaporisation of the gas.

A preferred method of cooling is to introduce the liquefied gas or solid carbon dioxide directly into the fluidising air upstream of the fluidised bed.

Preferably, the sand is cooled to a temperature in the range 20 to 26"C.

However the cooling is performed, it will not generally be necessary to cool continuously. Typically, the liquefied atmospheric gas will be suppplied through a pipeline. In the pipeline a valve is preferably disposed. The valve is preferably of an automatic kind. Desirably, there is at least one temperature sensor located in the bed (or its outlet). The arrangement is typically such that the valve will only be opened when the sensed temperature is at or above a chosen value. Once the temperature falls below the chosen value the valve will be closed and will remain in a closed position until the temperature rises again to the chosen value.

The aforesaid resin may be of the thermosetting or thermoplastic kind. The sand may, for example, be silica sand or zircon sand.

The method according to the invention has the advantage of being able to be performed on existing plant without the need to make any major modification to the plant. The only modification that will be typically necessary is the installation of the necessary equipment or plant for supplying the liquefied atmospheric gas (or solidified atmospheric gas).

The method according to the invention will now be described by way of example with reference to the accompanying drawings, of which: Figure lisa diagrammatic representation of a fluidised bed cooler for cooling sand, and Figure 2 is a schematic diagram illustrating means for cooling the air supplied to the cooler shown in Figure 1.

Referring now to the accompanying drawings, the cooler illustrated in Figure 1 has a housing 100. The housing 100 has a floor 102 and a roof 103, a pair of longer parallel sides 104 (only one shown in Figure 1)which shall herein be referred to as front and back of the cooler, and a pair of shorter parallel sides 106.

Spaced above the floor 102 and extending parallel thereto is a grid 108 which forms the roof of the plenum chamber 112 within the housing 100 is a cooling chamber 110 in which a fluidised bed 114 of grains of sand is able to be established in operation of the cooler.

The cooling chamber 112 has in one of the sides 106 of the housing 100 an inlet 116 in communication with a hopper 118 into which sand for cooling may be loaded located near to the other side 106 of the housing in the back 104 thereof, a relatively small distance above the grid 108 is an outlet 120 for sand.

This outlet may if desired communicate with another hopper in which cooled sand may be collected.

Dry air for fluidising the sand is able to be supplied from a blower 122 (see Figure 2) through an air main 124 and conduits 126 into the plenum chamber 110 in which the conduits 126 terminate. In each conduit 126 there is a balancing valve 128 operable to ensure that the fluidising air is equally distributed in use of the cooler. The pressure and flow rate of the air may be chosen so as to ensure that the sand can be adequately fluidised.

Referring again to Figure 1, the housing 100 has an outlet 130 for air. The outlet 130 is disposed in the roof 103. The outlet 130 communicates with a filter 132 of conventional design.

With reference to Figure 2, a spray head 134 is situated in the air main 124 downstream of the blower 22. The spray head 134 forms the outlet of a thermal insulated pipe 136 whose inlet is served by threethermally-insulated conduits 138(a), 138(b), and 138(c), all of which are placed in communication with a source 142 of liquid nitrogen by a thermallyinsulated pipeline 140. Automatic valves 144(a), 144(b), and 144(c), typically solenoid actuated, are located in the conduits 138(a), 138(b), and 138(c) respectively. Whether these valves are in open or closed positions is determined by a temperature controller-cum-indicator 146 is in turn operatively associated with a temperature sensor 148 located in the outlet 130 (see Figure 1).

In operation of the cooler shown in Figures 1 and 2 sand which has been quarried or which has been reclaimed from used resin-bonded cores or moulds by burning the resin and then washing out the residue from the sand is dried and then fed into the hopper 118. Typically, the material for drying is composed of silica sand, or other kind of sand, in the proportion of 90 to 95% by weight, substantially all the remainder being moisture. Typically, the moisture content of the dried sand is less than 0.1% by weight.

In order to form a fluidised bed, operation of the blower 122 is started so as to supply fluidising air at ambient temperature through the main 124 and the conduits 126 to the plenum chamber 110 and dried sand is allowed to fall under gravity from the hopper 118 into the cooling chamber 112. Fluidising the sand is effective to cool substantially each grain. As sand is continually passed into the chamber 112 so cooled sand leaves through the outlet 120. Its temperature is sensed in the outlet 120 by the temperature sensor 148. This temperature governs whether or not liquid nitrogen is sprayed into the air main 124. The temperature controller-cum-indicator 146 is programmed so as to translate electrical signals from the sensor 148 indicative of temperature into operating instructionsforthe automic valves 144.Electronic circuits for effecting this translation are well known in the art.

The controller-cum-indicator 146 may typically be "programmed" as follows. When the temperature is below a predetermined temperature, say 24"C, the valves 144(a), 144(b) and 144(c) are all in closed positions. Should the sensed temperature rise to 24"C, the valve 144(a) opens. Liquid nitrogen ata temperature of -196 C is thus sprayed into the air main 124 at a chosen rate. On contact with the air the liquid nitrogen evaporates thus cooling the air. Thus the fluidising air is reduced in temperature.Typically, this will cause the temperature of the sand to fall again to below 24"C. If after a chosen time the temperature has not fallen to below 24"C, the prog rammer-cum-controller 146 will generate a signal to open valve 144(b) so as to double the rate at which liquid nitrogen enters the main 124 and thereby further decrease the temperature of the fluidising air. If ater a further interval of time the temperature has not fallen to below 24"C, the programmer-cum-controller 146 will generate a signal to open valve 144(c) so as to increase again the rate at which liquid nitrogen enters the main 124 and thereby further decrease the temperature of the fluidising air. When the temperature falls below 24"C all the valves revert to their closed positions.

According to the sensitivity of the temperature sensor and the response time of the programmercum-indicator 146, the temperature at which the valve 144(a) is set to open may be, say 2"C below the maximum sand temperature that can be tolerated.

The cooled sand leaves the chamber 112 through the outlet 130. It may then be passed into a large storage hopper to await collection and transfer to foundries.

The fluidising air leaves the chamber 113 through the outlet 130. Some grains of sand are carried out of the chamber 112 in the air. These are removed from the air by the filter 132 and returned to the hopper 118.

Claims (9)

1. A method of obtaining cooled sand or other granular refractory material, comprising the steps of quarrying the sand (or other material) and drying it, or recovering sand (or other graular refractory material) from moulds or cores formed of resin bonded sand (or other granular material), fluidised the dried or recovered sand (or other material) with airtaken from the atmosphere, and reducing the temperature of the air by heat exchanging or mixing it with an atmospheric gas in liquid or solid state, or the cold vapour of such gas.

2. A method as claimed in claim 1, in which a liquefied gas is sprayed or otherwise introduced into thefluidised bed.

3. A method as claimed in claim 1, in which the liquefied atmospheric gas is introduced into the fluidising air upstream ofthefluidised bed.

4. A method as claimed in claim 2 or claim 3, in which the liquefied atmospheric gas is liquid nitrogen.

5. A method as claimed in any one of the preceding claims, in which the liquefied atmospheric gas is supplied through a pipeline, an automatic valve is disposed in the pipeline, at least one temperature sensor is located in the bed or its outlet, the arrangement being such that the valve will be open only when the sensed temperature is at or above a chosen value.

6. A method as claimed in claim 1, in which solid carbon dioxide is introduced into the fluidised bed.

7. A method as claimed in claim 1, in which solid carbon dioxide is introduced into the fluidising air upsteam of the fluidised bed.

8. A method as claimed in any one of the preceding claims, in which the sand is cooled to a temperature in the range 20 to 26"C.

9. A method of obtaining cooled sand, substantially as herein described with reference to the accompanying drawings.