Device for heat transfer

Abstract

A device for heat transfer comprising: - a chamber including a fluidized bed of solid particles, the solid particles exchanging heat with a metal semi-product, the solid particles circulating along a circulation direction, - a gas injector to inject gas within the chamber, - a heat exchanger having a circulating transfer medium, the heat exchanger being in contact with the fluidized bed so that the solid particles exchange heat with the transfer medium, - means for gathering scale particles released from said metal semi-product into said chamber, - means for removing scale particles from said chamber. The invention also relates to a method of removing scale from a device for heat transfer related to the invention.

Description

DEVICE FOR HEAT TRANSFER

[0001] The invention relates to a device for heat transfer. In particular, the invention relates to a device for heat transfer, with an improved design, made for cooling or reheating steel semi-products.

[0002] In steel production, there are several plants where steel semiproducts such as slabs are cooled after being manufactured to be stored for a period as there may be delays between the manufacturing of a semi-product and its hot rolling. Due to this, the stored semi-products must be reheated before hot rolling. These steps of cooling then reheating leads to a direct loss of energy. Thus, methods have been developed to reduce energy losses caused by these steps.

[0003] EP3821 171 discloses a heat transfer equipment for cooling slabs, wherein a slab is put in a chamber in contact with a fluidized bed of solid particles to capture the heat released by the slab and transferring it to a transfer medium such as water.

[0004] WO2023 / 1 1 1760 describes a method for reheating a semi-product, such as a slab, using a similar equipment, by transferring heat from a transfer medium to the slab using a fluidized bed containing solid particles.

[0005] However, as steel semi-products are cooled and reheated, they produce scale. Using this type of equipment, solid particles of the fluidized bed remove scale from the product when they come into contact with it. This produce scale particles that mix with the solid particles of the fluidized bed and can alter the operation of the device when present in excess. Scale particles can also be generated by the action of a thermal gradient between the steel semi-product and the solid particles as well as the action of devices, such as slab tongs, that place and remove the semi-products inside the fluidized bed.

[0006] The present invention discloses a device for heat transfer for cooling or reheating a steel semi-product with an improved design, allowing a good scale management .

[0007] A first object of the invention is a device for heat transfer comprising:- a chamber 1 including a fluidized bed of solid particles 2, the solid particles exchanging heat with a metal semi-product 16, the solid particles 2 circulating along a circulation direction,- a gas injector 3 to inject gas within the chamber 1 ,- a heat exchanger 4 having a circulating transfer medium, the heat exchanger 4 being in contact with the fluidized bed so that the solid particles 2 exchange heat with the transfer medium,- means for gathering scale particles 5 released from said metal semiproduct into said chamber 1 ,- means for removing scale particles 5 from said chamber 1 .

[0008] The device for heat transfer according to the invention may also have the optional features listed below, considered individually or in combination:- the gas injector 3 is composed of a plurality of fluidization nozzles 6 comprising an opening facing the bottom of the chamber 1 ,- the means for gathering scale particles 5 include means for blowing gas towards said scale particles 5 to move them,- the means for gathering scale particles 5 include a recess 8 located at the bottom of said chamber 1 ,- the means for blowing gas is composed of a plurality of gas nozzles 9 comprising an opening facing said recess 8,- the means for blowing gas is composed of a plurality of nozzles 14 having two openings, a first opening facing said recess 8 and a second opening facing the bottom of the chamber 1 ,- the bottom of the chamber 1 has a slope shape to further facilitate the accumulation of said scale particles 5,- the bottom of the chamber 1 comprises channels 15 to further facilitate the accumulation of said scale particles 5,- the bottom of the chamber 1 forms a tiled roof to facilitate the accumulation of said scale particles 5 and wherein the means for blowing gas is located under the tiles,the means for removing said scale particles is a tube with an endless screw 1 1 .

[0009] A second object of the invention is a method for removing scale from a device for heat transfer according to the invention, wherein scale particles 5 are gathered then removed from the chamber 1 of said device using means for accumulating scale particles and means for removing scale particles and wherein the solid particles 2 from the fluidized bed removed with the scale particles are recycled back into said fluidized bed.

[0010] The method for removing scale according to the invention may also have the optional features listed below, considered individually or in combination:- scale particles 5 are accumulated into a cavity 8 at the bottom of the chamber 1 by blowing gas onto said scale particles 5 towards said cavity 8 through nozzles having at least one opening towards said cavity 8,- the blowing of gas is pulsed,- the blowing of gas by the nozzles is alternated between a first opening towards the cavity and a second opening towards the bottom of the chamber.

[0011] The invention will be described, in a non-limitative way, in reference to the following drawings:- Fig 1 : general view of the device for heat transfer according to the prior art,- Fig 2: view of an embodiment of the device for heat transfer according to the invention,- Fig 3: view of a second embodiment of the device for heat transfer according to the invention,- Fig 4: view of a third embodiment of the device for heat transfer according to the invention,- Fig 5: view of an embodiment of the bottom of the chamber and the recess,- Fig 6: general view of a slab.

[0012] Fig 1 shows a device for heat transfer according to the prior art. The device comprises a chamber 1 wherein metal semi-products such as slabs 16 are placed. The chamber contains solid particles 2 and comprises gas injection means 3 which inject gas into the chamber 1 to fluidize the solid particles 2 and create a fluidized bed of solid particles 2 in a bubbling regime, circulating along a circulation direction. The metal semi-products are placed into the chamber 1 on support means, preferably so that their broad face is parallel to the direction of circulation of the fluidized particles. The device further comprises at least one heat exchanger 4 wherein a transfer medium is circulating, the heat exchanger 4 being in contact with the fluidized bed.

[0013] The invention uses a similar equipment to reheat or cool metal semi-products 16. In case of cooling, the hot metal semi-products are immersed into the fluidized bed of solid particles 2, solid particles 2 which are then able to capture the heat released by the hot metal semi-products. This allows a homogeneous cooling of the semi-product, as all parts of the semi-product are in contact with the fluidized solid particles 2. The solid particles 2 are kept in motion by the injection of gas by the injection means 3 and come in contact with the heat exchanger 4 where they release the captured heat to the transfer medium circulating within. The flow rate of the transfer medium inside the heat exchanger 4 can be regulated to control the cooling rate, indeed the more medium is circulating inside the heat exchanger 4, the more heat is released from the solid particles 2. In case of reheating, the same mechanism is applied but the solid particles 2 capture the heat from the heat exchanger 4 and release the captured heat to the cold semi-products placed into the chamber 1 .

[0014] Hot steel is subject to oxidation on its surface. After continuous casting or during reheating, the steel semi-products produce scale. Inside the device, with the action of semi-products logistic devices, thermal gradient between the fluidized bed and the semi-products or with the repetitive contact of the fluidized solid particles 2 on the semi-products, the scale is removed from the semi-products and scale particles 5 are liberated into the chamber 1 and mixedwith the solid particles 2. This can cause perturbations in the fluidization thus reducing the efficiency of the heat transfer. It also causes an increase in the volume of the fluidized bed and can cause overflow. The more scale in the chamber 1 , the more important the perturbations can be. It is thus important to have a device for heat transfer that comprises means that can manage scale.

[0015] To achieve a good management of scale, the invention adapts the previous design, by adding means for gathering scale particles and means for removing the scale particles from the chamber 1 .

[0016] Fig 2 represents an embodiment of a device for heat transfer according to the invention. It comprises a chamber 1 comprising a fluidized bed of solid particles 2 to exchange heat with a semi-product, a gas injector 3, a heat exchanger 4 having a circulating transfer medium, means for gathering scale particles released from the flat metal product into the chamber 1 and means for removing scale particles from the chamber 1 .

[0017] In the preferred embodiment represented in Fig 2, the gas injector 3 is composed of a plurality of fluidization nozzles 6 comprising an opening facing the bottom of the chamber 1 . The fluidization nozzles 6 allow the formation of the fluidized bed.

[0018] There are several regimes of fluidization. Fluidization is the operation by which solid particles 2 are transformed into a fluidlike state through suspension in a gas or a liquid. Depending on the fluid velocity, behavior of the particles is different. In gas-solid systems as the one of the invention, with an increase in flow velocity beyond minimum fluidization, large instabilities with bubbling and channeling of gases are observed. At higher velocities, agitation becomes more violent, and the movement of solids become more vigorous. In addition, the bed does not expand much beyond its volume at minimum fluidization. At this stage the fluidized bed is in a bubbling regime, which is the required regime for the invention in order to have a good circulation of the solid particles 2 and a homogeneous temperature of the fluidized bed. Gas velocity to be applied to get a given regime depends on several parameters like the kind of gas used, the size and density of the particles or the size of the chamber 1 . This can be easily managed by a person skilled in the art.

[0019] The fluidization nozzles 6 are preferably disposed in several rows in the chamber 1 . This assure that the entirety of the solid particles 2 is put in a bubbling regime so that the fluidized bed occupies all the volume of the chamber 1 . The fluidization nozzles 6 are all connected to a main gas circuit 7.

[0020] The gas can be nitrogen or an inert gas such as argon or helium and in a preferred embodiment, air. It is preferably injected at a velocity between 1 and 30 cm / s which requires a low ventilation power and so a reduced energy consumption. The flow rate of the gas is preferably monitored by a main valve 12.

[0021] To manage scale into the fluidized bed, the method used in the invention is to remove the scale from the chamber 1. However, as scale particles 5 are released into the fluidized bed from every part of the flat products, it can be very difficult to evacuate the scale directly. The method thus comprises a first step of gathering the scale particles 5. The device for heat transfer according to the invention thus comprises means for gathering the scale particles 5.

[0022] In the embodiment represented in Fig 2, the means for gathering scale particles 5 include a recess 8 located at the bottom of the chamber 1 . As scale particles 5 have a density higher than the density of the solid particles 2, they naturally fall to the bottom of the chamber 1 . The invention uses this fact and the recess 8 allow the scale to gather naturally into it. The recess 8 is preferably located on one side of the chamber 1 , as represented in Fig 2.

[0023] The means for gathering scale particles preferably includes a means for blowing gas towards the scale to move them. As all scale do not fall naturally into the recess 8, this means for blowing gas allows to move the scale that falls to the bottom of the chamber 1 but outside of the recess 8. They are moved towards the recess 8 to make them fall into it so that most of the scale particles 5 are gathered into the recess 8.

[0024] As represented in Fig 2, the means for blowing gas is preferably composed of a plurality of gas nozzles 9 comprising an opening facing the recess 8. They are preferably disposed in several rows into the chamber 1 . The gas nozzles 9 are connected to a secondary gas circuit 10 different from themain gas circuit 7. The blowing of gas is preferably pulsed as it allows a better optimization of the blowing. The gas nozzles 9 farther from the recess 8 move the scale under the nozzles 9 that are closer to the recess 8 and so on. The pulsed gas prevents the use of gas when there is no scale to move thus preventing waste of gas. Each gas nozzle 9 is linked to the secondary gas circuit 10 and is controlled by secondary valves 13. The blowing of gas should push the scale particles 5 towards the recess 8 while not perturbating the fluidization of the solid particles 2.

[0025] Another embodiment of the means for blowing gas is a plurality of nozzles 14 having two openings, a first opening facing the recess 8 and a second opening facing the bottom of the chamber 1 . All the nozzles 14 are linked to the main gas circuit 7 and have means to alternate between the first opening to blow gas to move the scale and the second opening to fluidize the solid particles 2. Preferably, some fluidization nozzles 6 are replaced by nozzles 14 with two openings. This configuration is represented in Fig 3.

[0026] To facilitate the gathering of scale particles 5, the bottom of the chamber 1 preferably has a slope shape towards the recess 8 as represented in Fig 2. This allows to use gravity to facilitate the blowing of the scale particles 5 towards the recess 8.

[0027] Fig 5 represents a preferred embodiment for the bottom of the chamber 1 . It forms a plurality of channels 15 directed towards the recess 8. The channels 15 allow a first gathering of the scale particles 5 as they fall naturally into the channels 15. The means for blowing gas, here the gas nozzles 9, are placed above the channels 15. This configuration allows a better overall gathering as it assure that most of the scale particles 5 are gathered into the recess 8 and that there is no permanent accumulation elsewhere.

[0028] Another embodiment is that the bottom of the chamber 1 forms a tiled roof and the means for blowing gas are located under the tiles. This is represented in Fig 4.

[0029] After the scale particles 5 are gathered, they need to be removed from the chamber 1 . In a preferred embodiment, an endless screw 1 1 is used to continuously remove the scale particles 5 from the recess 8 as they fall into it, as represented in Fig 2. However, any means for removing the scale particles 5 from the chamber 1 can be used.

[0030] As the means for removing scale particles 5 from the chamber1 can also remove solid particles 2 from the fluidized bed, it happens with the endless screw 11 for example, these solid particles 2 removed with the scale particles 5 are recycled back into the fluidized bed in order not to decrease the volume of the fluidized bed that would result in a loss of efficiency of the device for heat transfer. In a preferred embodiment, a magnetic separator is used to separate solid particles 2 and scale particles 5 removed from the chamber.

[0031] The means for gathering scale particles and the means for removing scale particles can also be used to remove any undesirable particle from the fluidized bed as long as this undesirable particle is denser than the solid particles 2 of the fluidized bed.

[0032] In Fig 6 is illustrated a slab 16, which is an example of a semiproduct. Said slab 16 has a parallelepipedal shape and comprises a top 16a and a bottom broad face, two small faces 16b and two edges 16c. The broad faces define the width W and the length L of the slab, said width W being usually comprised between 700 and 2 500 mm, the length L between 5 000 and 15 000 mm and the thickness T of the slab is usually comprised between 150 and 350 mm. More generally, a flat product can be defined as a parallelepiped wherein the smallest dimension (e.g. the thickness T) is negligible compared to the others (e.g. the length L), for example the smallest dimension being at least smaller than the biggest dimension of a factor 15. The broad faces of the parallelepiped are the faces which do not include the smallest dimension. Other examples of semiproducts are billets, bloom, beam blanks or coils.

[0033] The solid particles 2 preferentially have a heat capacity comprised between 500 and 2000 J / kg / K. Their density is preferentially comprised between 1400 and 4000 kg / m3. They may be ceramic particles such as SiC, alumina, olivine or steel slag. They may be made of glass, or any other solid materials stable up to 1400° C. They preferably have a size comprised between 30 and 500 pm. These particles are preferably inert to prevent any reaction with the slab 16.

[0034] The heat exchanger 4 may be composed of a first pipe wherein a transfer medium is circulating so as to bring it to the heat exchanger 4, a second pipe wherein the transfer medium is recovered and third pipes connecting the first pipe and the second pipe and going through the chamber 1 and the fluidized bed of solid particles 2.

[0035] The transfer medium circulating in the heat exchanger 4 is preferably pressurized water which, once heated by the heat released by the fluidized solid particles 2, is turned into steam. Pressurized water may have an absolute pressure between 1 and 30 Bar. Pressurized water may then be turned into steam by a flash drum or any other suitable steam production equipment. Preferentially the water remains liquid inside the heat exchanger 4. The produced steam may then be reused within the metal production plant by injection within the plant steam network, for hydrogen production for example or for RH vacuum degassers or CO2 gas separation units in the case of a steel plant. Having both steam reuse plant and metal product manufacturing plant within the same network of plant allows to improve the overall energy efficiency of said network.

[0036] The transfer medium circulating in the heat exchanger 4 may also be air or molten salts having preferably a phase change between 400 and 800° C. which allow to store the capture heat. The transfer medium may comprise nanoparticles to promote heat transfer.

[0037] The heat stored during the step of cooling can be reused during the step of reheating. Two or more devices according to the invention can be used together, for the step of cooling a reheating.

[0038] The device for heat transfer according to the invention allows good scale management that allows continuous operation without decrease of its efficiency.

Claims

CLAIMS1 . A device for heat transfer comprising: a chamber (1 ) including a fluidized bed of solid particles (2), the solid particles exchanging heat with a metal semi-product (16), the solid particles (2) circulating along a circulation direction, a gas injector (3) to inject gas within the chamber (1 ), a heat exchanger (4) having a circulating transfer medium, the heat exchanger (4) being in contact with the fluidized bed so that the solid particles (2) exchange heat with the transfer medium, means for gathering scale particles (5) released from said metal semi-product into said chamber (1 ), means for removing scale particles (5) from said chamber (1 ).

2. A device for heat transfer according to claim 1 , wherein said gas injector (3) is composed of a plurality of fluidization nozzles (6) comprising an opening towards the bottom of the chamber (1 ).

3. A device for heat transfer according to claim 1 or 2, wherein said means for gathering scale particles (5) include means for blowing gas towards said scale particles (5) to move them.

4. A device for heat transfer according to anyone of claims 1 to 3, wherein said means for gathering scale particles (5) include a recess (8) located at the bottom of said chamber (1 ).

5. A device for heat transfer according to claim 4, wherein said means for blowing gas is composed of a plurality of gas nozzles (9) comprising an opening facing said recess (8).

6. A device for heat transfer according to claim 4, wherein said means for blowing gas is composed of a plurality of nozzles (14) having two openings, a first opening facing said recess (8) and a second opening facing the bottom of the chamber (1 ).

7. A device for heat transfer according to any of claim 4 to 6, wherein the bottom of the chamber (1 ) has a slope shape to further facilitate the accumulation of said scale particles (5).

8. A device for heat transfer according to claim 7, wherein the bottom of the chamber (1 ) comprises channels (15) to further facilitate the accumulation of said scale particles (5).

9. A device for heat transfer according to claim 7, wherein the bottom of the chamber (1 ) forms a tiled roof to facilitate the accumulation of said scale particles (5) and wherein the means for blowing gas is located under the tiles.

10. A device for heat transfer according to any of the preceding claims, wherein the means for removing said scale particles is a tube with an endless screw (1 1 ).1 1. A method for removing scale from a device for heat transfer according to any of the preceding claims, wherein scale particles (5) are gathered then removed from the chamber (1 ) of said device using means for accumulating scale particles and means for removing scale particles and wherein the solid particles (2) from the fluidized bed removed with the scale particles are recycled back into said fluidized bed.

12. A method for removing scale according to claim 11 , wherein scale particles (5) are accumulated into a recess (8) at the bottom of the chamber (1 ) by blowing gas onto said scale particles (5) towards said recess (8) through nozzles having at least one opening facing said recess (8).

13. A method for removing scale according to claim 12 wherein the blowing of gas is pulsed.

14. A method for removing scale according to claim 13, wherein the blowing of gas by the nozzles is alternated between a first opening facing said recess (8) and a second opening facing the bottom of the chamber (1 ).

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