GB1601298A

GB1601298A - Cooling method

Info

Publication number: GB1601298A
Application number: GB2361478A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-10-28
Also published as: AU527112B2; AU4743879A

Description

(54) COOLING METHOD (71) We, BOC LIMITED, of Hammersmith House, London W6 9DX, England, and English company, 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: The present invention relates to a cooling method. In particular it relates to a method of cooling damp particulate material being fed to a grinding chamber (or other device or apparatus).

The grinding of particulate materials in order to produce even smaller particles is a well known industrial process. Frequently, the particles to be ground need to be cooled to a temperature below ambient so as to decrease their resilience. The cooling is commonly effected by means of liquid nitrogen.

Typically, the particles to be ground are pre-cooled in a hopper or screw conveyor from which they are fed into the grinding chamber.

Sometimes, the particles are damp. The cooling of liquid nitrogen tends to cause the moisture to freeze, thus giving rise to clusters of agglomorated particles. Such clusters tend to form blockages, particularly in the outlets of hoppers and between the flights and the casing of a screw conveyor.

According to the present invention there is provided a method of cooling damp particulate material being fed to a grinding chamber (or other device or apparatus), which method comprises the steps of feeding the particulate material into the upper end of a tunnel which is inclined with respect to the horizontal, and which is in communication with the grinding chamber (or other device or apparatus), rotating the tunnel so as to cause the particulate material to flow through the tunnel under gravity. introducing into the tunnel cryogenic liquid (as herein defined) which vaporises to produce such cold gas (or vapour). and causing the cold gas (or vapour) to pass in heat exchange relationship with the particulate material through the tunnel in the direction opposite to the direction of flow of the particulate material and then to be vented from the tunnel, the introduction of the cryogenic liquid being controlled such that the gas (or vapour) as it leaves the tunnel has been sufficiently raised in temperature as not to cause freezing of the moisture at the inlet (for particulate material) to the tunnel.

Preferably, the temperature of the cold gas (or vapour) is controlled such that the gas or vapour leaves the tunnel at a temperature greater than 0 C. However, if the residence time of the damp particulate material in, say, an inlet shute to the tunnel is relatively short, say, about 5 minutes, freezing ofthe moisture can at times be avoided even if the exit temperature of the gas or vapour is below 0 C or even as low as, say, --20"C.

A hopper is preferably used to feed the particulate material into the upper end of the tunnel. The hopper preferably has means of controlling the feeding of particulate material into the tunnel. Such means may typically take the form of a rotary valve or a sliding plate, grid or gate at the bottom of the hopper.

Typically, the hopper is connected to the upper end of the tunnel by a connecting passage.

Typically, the tunnel is inclined to the horizontal at an angle of 5 degrees. However, the angle of inclination may be less, or may be greater (e.g. up to 10 degrees).

The cryogenic liquid is preferably introduced into the tunnel close to its lower end.

Liquid nitrogen is generally preferred as the cryogenic liquid. However, it is possible to use one or more other cryogenic liquids instead of or in addition to liquid nitrogen.

For example, liquid argon or liquid helium could be used. In some instances it may also be possible to use liquid carbon dioxide, though this cryogenic liquid has a higher boiling point than the others mentioned herein, and in consequence, it does not have the cooling capacity of the other cryogenic liquids. For the avoidance of doubt, by the term "cryogenic liquid" as used herein is meant a liquid which has boiling point of minus 50"C or below at atmospheric pressure.

Preferably a fan or like device is used to draw the cold gas (or vapour) along the tunnel in the opposite direction to the flow of particulate material. It is to be appreciated that if a cryogenic liquid is introduced into the tunnel it will evaporate rapidly. particularly if introduced through one or more spray nozzles or through a spray header.

As the cold gas (or vapour) flows along the tunnel, typically countercurrently to the particulate material, so the particulate material gives up heat to the cold gas (or vapour) and is itself cooled. The temperature to which the particulate material is reduced may be controlled by means of a temperature sensor located at or near the outlet (or lower end) of the tunnel and be adapted to generate signals which automatically control the flow of cryogenic liquid or cold gas (or vapour) into the tunnel. For example, a solenoid valve may be used to control the flow of the cryogenic liquid, the position of the solenoid being governed by signals received from the temperature sensor.For a given rate of flow of particulate material through the tunnel, the temperature of the gas (or vapour) leaving the upper end of the tunnel depends on the efficiency of the heat exchange between the cold gas (or vapour) and the particulate material. The tunnel can thus be so designed that the temperature of the gas leaving the tunnel will never be less than 0gC.

However, if desired, the exit temperature of the gas may be sensed, particularly just upstream of the fan, and the rate of rotation of the tunnel adjusted, if necessary, so as to ensure that the exit temperature of the gas never falls to 0 degrees C or below. If desired, this may be effected automatically.

In order to promote good heat exchange between the particulate material and the gas, the tunnel may have situated therein heat transfer devices such as metal chains or metal fins.

The method according to the invention may be employed to cool and feed to a grinding chamber any damp particulate material. The method makes it possible to deliver the material to the grinding chamber as free-flowing particles.

The method according to the invention will now be described by way of example with reference to the accompanying drawing which is a diagrammatic representation of a combined feeding and cooling apparatus for performing the method according to the invention.

Referring to the drawing, a hopper 2 is intended to be fed with damp particulate material such as damp rubber. The hopper 2 has a sliding plate 4 which in one extreme position closes an inlet chute 6 which serves a rotary tunnel 8 and which may be slid out of such extreme position to provide a opening of chosen size through which particles in the hopper 2 can enter the chute 6. Instead of a sliding plate 4, a vibratory feed device may be used instead.

The rotary tunnel 8 is inclined at a small angle to the horizontal. The tunnel 8 therefore has an upper end 10 and a lower end 12.

The chute 6 terminates in the upper end 10.

The tunnel 8 carries in its outer surface one or more rings (not shown) which may be engaged by belts (not shown) which are driven so as to effect rotation of the tunnel.

Located near the lower end 12 of the tunnel 8, but above the surface of the tunnel immediately therebelow, is spray header 16.

The spray header 16 communicates with a source of liquid nitrogen 18 via a pipeline 20.

In the pipeline 20 is located solenoid valve 22. The lower end of the tunnel 8 communicates with an outlet chute 24 which is adapted to feed particulate material into a rotary grinding mill 26. A temperature sensor 28 is located in the outlet chute 24 near to the lower end 12 of the tunnel 8. The temperature sensor 28 is adapted to generate electrical signals which are relayed to a temperature controller 30 which controls the opening and closing of the solenoid valve 22.

In communication with the inlet chute 6 is a passage 32 which leads to a fan 34 having an outlet 36 communicating with the atmosphere.

In operation, damp particulate material is loaded into the hopper 2. The sliding plate 4 is set so as to give a chosen rate of introduction of solid material into the tunnel 8. The tunnel is rotated at a chosen rate and liquid nitrogen is sprayed into the tunnel through the spray header 16. The liquid nitrogen rapidly evaporates. Operation of the fan 34 causes the cold nitrogen to be drawn along the tunnel in the opposite direction to the particulate material which as the tunnel rotates is given a tumbling motion and passes alang the tunnel under gravity.

Thus, heat exchange between cold nitrogen and the particulate material takes place.

The particulate material may typically be cooled to a temperature of minus 150 degrees C in the tunnel, the cold nitrogen leaving the tunnel at a temperature of plus 5 degrees C.

The temperature of the particulate material or rather the gas in the chute 24 (which is approximately at the same temperature) is sensed by the temperature sensor 28 and relayed to the temperature controller 30. This is programmed so as to adjust the setting of the solenoid valve 22 such that the temperature sensed by the sensor 28 is always within plus or minus 2 degrees C of minus 150 degrees C when the apparatus is being operated. The length of the tunnel is chosen so that when the temperature of the particulate material leaving the tunnel is so controlled that the temperature of the gas leaving the tunnel in the opposite direction from its end 10 is always above 0 C and generally close to plus 5 degrees C. This gas passes into the chute 6 and is drawn into the passage 32 by the fan 34 which vents it to the atmosphere.

Free flowing particulate material passes out of the end 12 of the tunnel into the chute 24 and thence into the grinding mill 26.

WHAT WE CLAIM IS: 1. A method of cooling damp particulate material being fed to a grinding chamber or other device or apparatus, which method comprises the step of feeding damp particulate material into the upper end of a tunnel which is inclined with respect to the horizontal, and which is in communication with the grinding chamber (or other device or apparatus), rotating the tunnel so as to cause the particulate material to flow through the tunnel under gravity, introducing into the tunnel cryogenic liquid (as hereinbefore defined) which on vaporisation produces a cold gas (or vapour), and causing the cold gas (or vapour) to pass in heat exchange relationship with the particulate material through the tunnel in the direction opposite to the direction of flow of the particulate material, and then to be vented from the tunnel, the introduction of the cryogenic liquid being controlled such that the gas (or vapour) as it leaves the tunnel has been sufficiently raised in temperature as not to cause freezing of the moisture at the inlet (for particulate material) to the tunnel.

2. A method as claimed in claim 1, in which the introduction of the cryogenic liquid is controlled such that the gas or vapour leaves the tunnel at a temperature greater than 0 C.

3. A method as claimed in claim 1 or claim 2, in which the damp particulate material is fed into the tunnel at a chosen rate from a hopper having in its outlet a flow control device.

4. A method as claimed in any one of the preceding claims, in which the cryogenic liquid is liquid nitrogen.

5. A method as claimed in any one of the preceding claims, in which the cryogenic liquid is sprayed into the tunnel at a region remote from that where the particulate material enters the tunnel.

6. A method as claimed in any one of the preceding claims, in which the tunnel has an inlet chute through which the damp particulate material passes into the tunnel.

7. A method as claimed in claim 6, in which a fan is operated to draw the cold gas (or vapour) along the tunnel in the direction opposite to that of the flow of particulate material, the fan communicating with the inlet chute.

8. A method as claimed in any one of the preceding claims, in which the temperature of the particulate material leaving the tunnel is sensed and the introduction of cold gas (or vapour) or cryogenic liquid is controlled such that the sensed temperature is kept at or near to a chosen temperature, the tunnel being long enough for the cold gas (or vapour) to be warmed to a temperature of above 0 degrees C as it passes through the tunnel in the opposite direction to that in which the particulate material flows.

9. A method of cooling particulate material being fed to a grinding chamber (or other device or apparatus), substantially as herein described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. from its end 10 is always above 0 C and generally close to plus 5 degrees C. This gas passes into the chute 6 and is drawn into the passage 32 by the fan 34 which vents it to the atmosphere. Free flowing particulate material passes out of the end 12 of the tunnel into the chute 24 and thence into the grinding mill 26. WHAT WE CLAIM IS:

1. A method of cooling damp particulate material being fed to a grinding chamber or other device or apparatus, which method comprises the step of feeding damp particulate material into the upper end of a tunnel which is inclined with respect to the horizontal, and which is in communication with the grinding chamber (or other device or apparatus), rotating the tunnel so as to cause the particulate material to flow through the tunnel under gravity, introducing into the tunnel cryogenic liquid (as hereinbefore defined) which on vaporisation produces a cold gas (or vapour), and causing the cold gas (or vapour) to pass in heat exchange relationship with the particulate material through the tunnel in the direction opposite to the direction of flow of the particulate material, and then to be vented from the tunnel, the introduction of the cryogenic liquid being controlled such that the gas (or vapour) as it leaves the tunnel has been sufficiently raised in temperature as not to cause freezing of the moisture at the inlet (for particulate material) to the tunnel.