EP0047914A1 - Apparatus for extracting heat from a hygroscopic pulverulent material with a temperature lower than 100 degrees centigrade - Google Patents

Abstract

Spray nozzles (16, 18, 20) inject just sufficient water to produce the desired cooling of hot powdered substance by evaporation of the water. The water is sprayed finely and uniformly onto said substance to avoid wet lumps agglomerating. The substance comes from a hopper (2), and flows through the apparatus in the opposite direction to a drying gas flow from inlet (21) to a condenser (22) where the heat is recovered from the water vapor. Application to recycling foundry sand and, more generably, to using sand which has been heated.

Description

The invention relates to a device for extracting heat from a hydrophilic pulverulent material below 100 ° C.

This material can for example be foundry sand which has just been used and has been cooled to 100 ° C. by spraying with water, or sand which has been dried with a flame and maintains a temperature of 100 ° C. To continue handling this material it is desirable to start by cooling it from 100 ° C to 40 ° C.

It is known that very hot sand can be cooled to around 100 ° C by spraying water. As for cooling below 100 ° C, it is conventionally carried out without spraying water to obtain very dry sand.

• - soit l'air atmosphérique en contact direct avec le sable,
• - soit l'eau en contact indirect à travers la paroi d'un échangeur de chaleur.

For example, to cool foundry sand from 100 ° C to 40 ° C and recover heat, several known methods can be used using as heat transfer fluid:

• - either atmospheric air in direct contact with the sand,
• - or water in indirect contact through the wall of a heat exchanger.

In these two cases, the heat transfer coefficients are quite low, of the order of a few tens of kcal / hm 2. ° C while the transfer coefficients by direct evaporation and condensation of water are of the order 1000 kcal / hm ^2. ° C.

It is therefore advantageous to use water as a heat transfer fluid; but, as in some cases the sand to be cooled is sand which has been dried in a rotary kiln, it is desirable to cool it while obtaining a sand at 40 C perfectly dry for its use in a foundry mold or for its shipment . In addition, it is often useful to recover the heat from the sand.

The object of the present invention is to provide a device for extracting heat from a hydrophilic pulverulent material below 100 ° C. enabling a cool and dry material to be obtained in a simple manner and for recovering the heat. extracted.

The invention therefore relates to a device for extracting heat from a hydrophilic pulverulent material below 100 ° C, characterized in that it comprises means for turbulent movement of the material, means for spraying water on the surface of this moving material in an amount just necessary for the desired extraction of heat by subsequent evaporation of the water removed from the grains, and means for circulating a gas upon contact with the moving material so that the deposited water evaporates before contact with neighboring grains has been able to create a possibility of aggregation of these grains, said gas ensuring the evacuation of the water vapor thus produced.

In a preferred embodiment, the means for setting in motion the pulverulent material comprise a shaker conveyor, said conveyor carrying a succession of ramps, the shaking of this conveyor being chosen to successively climb all these ramps by the grains leaving them fall down each time they have reached the top of a ramp, the means for spraying the water being arranged in the vicinity of the zones where the grains fall from the top of a ramp.

• - on met le matériau à refroidir en mouvement turbulent - on pulvérise de l'eau sur la surface de ce matériau en mouvement en quantité juste nécessaire à l'extraction souhaitée de chaleur par évaporation ultérieure de l'eau déposée sur les grains,
• - et on fait circuler un gaz au contact du matériau en mouvement de manière à faire évaporer l'eau déposée.

This device implements the following method:

• - the material to be cooled is put into turbulent movement - water is sprayed on the surface of this moving material in an amount just necessary for the desired heat extraction by subsequent evaporation of the water deposited on the grains,
• - And a gas is circulated in contact with the moving material so as to evaporate the water deposited.

• - on pulvérise l'eau de manière que 90% au moins des gouttelettes formées aient un diamètre inférieure à 70% de celui de 90% au moins des grains du matériau pulvérulent.

This process moreover preferably meets the following characteristics:

• the water is sprayed so that at least 90% of the droplets formed have a diameter less than 70% of that of at least 90% of the grains of the pulverulent material.

In the case of a pulverulent material with a substantially uniform grain diameter, the water is sprayed so that the droplets formed have diameters substantially between 60% and 30% of the average diameter of the grains of the material.

With the aid of the attached diagrammatic figures, an exemplary embodiment of the available will be described below, without limitation. sitive of the invention.

The single figure is a schematic view of a preferred embodiment of the invention.

Before describing the device, some explanations will be given on the process it implements.

The quantity of water just necessary for cooling is uniformly sprayed on the surface of the sand, this quantity corresponding to approximately 550 calories / kg of evaporated water.

Evaporation of the water is ensured by a low gas circulation, possibly under reduced pressure, which decreases the partial pressure of water vapor at the level of each grain and thus allows evaporation before the grains have been able to agglomerate. The gas also allows evacuation by entrainment of the water vapor produced.

A uniform spraying of water on the surface of the sand is necessary and, preferably, 3% of cold water is injected by weight relative to the sand.

Commercial sprayers produce relatively large drops compared to the particle size of the sand, drops of 200 for a sand having an average particle size of 130 (sand between 40 and 200) so that 3% of water tends to form small balls of wet sand which must be dispersed by mechanical means (rotating drum, agitator, etc.).

It is best to moisten the sand as regularly as possible with only 3% water. For this, it is possible to use sprayers of the "ultrasonic" type producing a "mist" of very fine particles, for example 20. Compared to droplets of diameter 200, there are 1000 times more drops of 20, so the sand is moistened much more regularly, so its cooling by evaporation will be faster and more uniform.

The device shown in the figure includes a hopper 2 for the supply of pulverulent material, for example hot foundry sand. Sand falls on shaker conveyor comprising a table 4 set in motion by a mechanism 6 of known type. On this table are arranged three ascending ramps 8, 10 and 12 that the sand successively climbs under the action of the tremors applied by the mechanism 6.

At the top of each of the ramps the sand falls back onto the start of the next ramp, to be finally brought to an output receiver 14.

'' In the vicinity of each of the ramp tops is arranged a water sprayer, respectively (16), 18) and (20), which directs the water onto the sand in the areas where the layers of sand mix in the vicinity from the top of the ramp, and where the sand falls.

Of course the shaker conveyor can be horizontal, inclined or helical.

The gas which, as indicated above, ensures evaporation is preferably air; it is sent, via a pipe 21, preferably in the opposite direction to that of the sand, and meets each grain which falls from the ramps, thus ensuring the evaporation of the water and the transport of the water vapor produced.

The air taken up in the vicinity of the hopper 2 is then hot and humid; it is sent to a condenser 22 which recovers the heat.

Claims (3)

1 / Device for extracting heat from a hydrophilic pulverulent material below 100 ° C, characterized in that it comprises means (4) for turbulent movement of the material, means (16, 18 , 20) for spraying water onto the surface of this moving material in an amount just necessary for the desired extraction of heat by subsequent evaporation of the water deposited on the grains, and means (21) for circulating a gas in contact with the moving material so that the deposited water evaporates before the coming into contact of the neighboring grains could create a possibility of aggregation of these grains, said gas ensuring the evacuation of the vapor of water thus produced. 2 / Device according to claim 1, characterized in that the means for setting in motion the pulverulent material comprise a shaker conveyor, said conveyor carrying a succession of ramps (8, 10, 12), the shaking of this conveyor being chosen to successively climb all these ramps by the grains, letting them drop each time they have reached the top of a ramp, the means (16, 18, 20) for spraying the water being arranged in the vicinity of the zones where the grains fall from the top of a ramp. 3 / Device according to one of claims 1 and 2, characterized in that the direction of circulation of the drying gas is opposite to that of the movement of the material.

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