KR0180725B1 - Fluidized bed for quenching steel wire - Google Patents

Abstract

None.

Description

Fluidized bed apparatus suitable for continuous quenching of steel wire and continuous quenching method of steel wire

1 is a side view of a fluidized bed apparatus in which one fluidized bed chamber is composed of a plurality of fluidized bed chambers directly connected to another fluidized bed chamber, and the first fluidized bed chamber is designed according to the present invention.

2 is a plan view of the first fluidized bed chamber of FIG.

The present invention relates to a fluidized bed apparatus suitable for intermittently quenching steel wires to temperatures up to 250 ° C. and a method of continuous quenching of steel wires. As is known, a fluidized bed device consists of a container filled to a certain height with particulates forming the fluidized bed. Particulates are unstable to high temperatures of 1500 ° C. or higher. At the bottom plate of the particulate layer, there is an inlet suitable for blowing the carrier gas upward into the particulate layer at an inflow flow evenly distributed over the bottom plate surface of the particulate layer as evenly as possible. Between the lowest and maximum blowing speeds, the particulates swirl up and down and the particulate layer is raised to act as a cooling fluid that can be traversed by the steel wire without interruption. Typical particulate materials are silica, alumina, or zirconiasand, silicon carbide or ferrosilicon, typical particulate sizes in the range of 0.03-0.5 mm and typical fluidized bed heights for steel wire application are about 0.3-0.6 m. The blowing rate into the fluidized bed for fluidization depends on the particulate type selected, with typical blowing rates in the range of 0.06-0.15 m / sec. In this way, the cooling medium accepts heat transfer coefficients for steel wires on the order of 200-600 W / m 2 ^O K, which is close to the heat transfer coefficient for cooling fluids. It is possible to quench the steel wire with such a cooling medium, ie cool it at a rate of 200 ° C./sec or more.

In order to be suitable for processing steel wires, the fluidized bed is also equipped with the necessary steel wire guidance and access means for guiding the steel wires in and out of the fluidized bed. In general, the fluidized bed is arranged to simultaneously process multiple steel wires (typically 10-50 pieces), which are passed side by side through the fluidized bed in the axial direction of the steel wire. Typical wire thicknesses are 1-9 mm and typical carbon contents range from 0.05-1%. Such fluidized beds must maintain their quenching temperatures. This means that the heat entering the fluidized bed through the hot steel wire and transferred to the cooling fluid must be released from the cooling fluid at the same rate. In a fluidized bed, this takes place via a carrier gas that is blown at a relatively low temperature, takes heat from the particulates, and leaves the fluidized bed top at a higher temperature. The temperature of the fluidized bed, which affects the inlet temperature of the carrier gas, is determined by a temperature controller, as described in EP 105,473 (Publication No.), with a certain value (no disturbance at the inlet temperature and the moving speed of the steel wire, and other Despite disturbances). It is further known from the above European patent to further cool the fluidized bed by means of an auxiliary device of a water-cooled pipe submerged in the fluidized bed or by a blower that blows cooling air above the fluidized bed.

However, such fluidized beds are limited in terms of the capacity of the fluidized bed per square meter of fluidized bed surface (ie the thickness of steel wire processed per second, kg / sec), and large production also requires a relatively large fluidized bed. Because the particles are blown out of the fluidized bed, the carrier gas and velocity through the fluidized bed cannot be greater than 0.15-0.20 m / sec, so primary cooling by the carrier gas is practically limited. Therefore, the input flow rate per cubic meter of fluidized bed surface (m 3 / sec) (same as velocity) has a limit, and the maximum possible difference between the inlet and outlet temperatures of the carrier gas is also limited by the quenching temperature imposed predominantly. Have Since subcooled pipes interfere with the fluidized bed, subcooling must be limited, and if there are too many water cooled pipes, the fluidized bed is rapidly blocked and collapses. When air blowing cooling is used on top of the fluidized bed, the heat release capacity of the air is too small, and when this air is mixed with the atomized water, this causes the top surface of the fluidized bed to cake together.

In addition, when the capacity per square meter of fluidized bed surface increases, another problem arises: the ability to adjust the fluidized bed temperature. Due to the fact that a large amount of steel wire has to be treated in a smaller fluidized bed, the larger irregularities in the inflow and outflow of heat are taken up by the smaller, more powerful and more rapidly acting regulators. There is also a large temperature change.

It is an object of the present invention to provide a fluidized bed with increased production capacity per square meter of fluidized bed surface by simple means, and to provide an effective temperature control device.

According to the present invention, the density of the pipe apparatus (indirect convection cooling) is greatly increased, the pipe apparatus using air instead of water, and the temperature control from the main cooling circuit to the auxiliary cooling (indirect convection cooling) furnace is transferred. Combine the three methods together.

It was actually found that the source of blockage and collapse of the fluidized bed in the presence of too many water-cooled pipes is the residual moisture of the carrier gas causing condensation on the cooling pipes. This results in a cake-shape around the cooling pipe which gives the cooling pipe a larger appearance diameter which results in the fluidized bed. From this fact, it is possible to greatly increase the density of the cooling pipes if care is taken to avoid such condensation. A possible method is to use a very dry carrier gas, but this requires special preparation of the carrier gas or otherwise limits the choice of carrier gas. For example, such a carrier gas may constitute an exhaust gas of a furnace having a large inherent moistness and is often undesirable to be limited in the choice of carrier gas.

Increasing the density of the cooling pipes wisely is the first method according to the invention, but not by sending the cooling water through the cooling pipes, even though the air has a smaller cooling capacity than the water, Is inhaled. However, due to the fact that it is air, not water, that passes through the cooling pipe, the outer surface of the cooling pipe does not reach the temperature of the cooling water (100 ° C. or lower, thus condensation), but at the outlet of the cooling air (at the outlet of the inlet blower). About 40 ° C.) and the fluidized bed temperature (above 200 ° C.). Thus, it is possible to pass through a pipe system with much higher density without any condensation of residual moisture, and to supply a very large flow rate of cheap surrounding space, which greatly compensates for the lower cooling capacity of the air.

Thus, the density of the pipe arrangement is such that at least the outer surface of the pipe is at least 0.4 m 2, preferably at least 0.8 m 2 per square meter of fluidized bed when cooling by convection of the fluidized bed takes place. The density of the pipe system is designed to send the nominal air flow through the pipe when in use, which is at least twice the cooling capacity of the main cooling by the carrier gas, preferably four times the cooling capacity of the convection cooler (KW / M 2 and m 2 give rise to the area of the fluidized bed surface. Auxiliary cooling devices (indirect convective cooling) do not necessarily have the form of a plurality of cooling pipes, but may have other forms as long as they are based on indirect convective cooling, i. have.

Also according to the invention, as a second method in combination with the above method, the temperature control of the fluidized bed is transferred from the main cooling circuit with the carrier gas to the auxiliary cooling circuit with indirect convective cooling of the air. This is easily feasible by adjusting the air flow rate that can be obtained at low temperatures without any limitation of the attentional air. The flow rate control of the water cooling system is even more difficult because it continues to be hampered by steam formation. Due to the fact that most of the cooling is transferred from the main cooling circuit to the subcooling circuit in accordance with the first method, the adjustment by subcooling (indirect convection cooling) at nominal cooling capacity at zero is a very powerful control device for temperature. To provide.

The cooling capacity of the convection chiller, which is supplied with the air drawn into the blower, can also be increased by injecting the atomized liquid, preferably water, into the air flow through the convection cooler of the cooler itself or the feed pipe. It is possible to adjust the temperature of the fluidized bed by changing the flow rate of the cooling air or by changing the flow rate of the liquid spray or both. In practice, based on the injection of the atomized liquid, the specific heat Cp of the cooling air is adjusted. This specific heat is at its lowest level when the air is completely dried, but by the injection of atomized liquid, very small droplets of evaporation heat per unit volume is added. In general terms, by changing the flow rate of cooling air and / or liquid spray, a change in the product of the flow rate with the specific heat of the air flow occurs. This product (H) is later called the specific heat flow rate, and thus is equal to the specific heat (Cp, Joule / ㎥ ℃) multiplied by the flow rate (㎥ / sec). Therefore, the specific heat flow rate H is expressed in units (Watt / ° C).

Accordingly, in more general terms, the convection cooler has an inlet connected to the air source, the specific heat flow rate H of the air flow through the convection cooler is changed, and the convection cooler maintains the fluidized bed temperature at a constant value by changing the specific heat flow rate. It includes a regulator for the.

In accordance with the general principles of control engineering, such regulators are devices that detect fluidized bed temperatures that generate a signal indicative of temperature, when a fluidized bed temperature is compared with a predetermined temperature to be adjusted and a calibration signal is generated that indicates observed deviations. In this case, a comparator capable of adding an integral and / or derivative over this time of difference (known as P, Pl, PD or PID regulator), and the calibration signal where the temperature is adjusted (in this case It is composed of a calibration device that is transformed into a change in size by the air flow and / or liquid spray).

Although it is not always necessary to avoid oxidation during quenching, it is often desirable to avoid oxidation, and sometimes it is absolutely necessary to keep the fluidized bed in a non-oxydising atmosphere. In such a case, conventional non-oxidizing carrier gases are used, where the atmosphere above the fluidized bed and the fluidized bed is, for example, an external atmosphere by means of a closed fluidized bed casing (but with the required passages for the carrier gas and steel wire) as far as possible. To be separated as much as possible. It is possible to have a carrier gas supplied from the furnace in a cheap and simple way, in which combustion occurs with a slight lack of oxygen in the furnace, the exhaust gas before being blown as a carrier gas in the combustion gas is first passed through the cooling system, In the chiller the exhaust singer is cooled to a temperature other than 120 ° C. to avoid condensation of water in the exhaust gas. In such a case, the apparatus of the present invention, on the one hand, does not need to be controlled by the inlet temperature of the exhaust gas any more as an adjustment factor for the temperature and on the other hand, an auxiliary cooling device (indirect convection cooler) which makes such a temperature change. It is very suitable because the change of temperature of exhaust gas as carrier gas does not cause much change any more because there is a strong adjusting device.

The apparatus according to the invention, in which the fluidized bed is maintained in a non-oxidizing atmosphere and the carrier gas is from the furnace of incomplete combustion, is particularly suitable for quenching when continuously parting the steel wire. In this way, the steel wire is first passed continuously through the austenitization furnace so that the wire is heated to a temperature displacement between 900-1050 ° C and then from 530 ° C to 579 at the exit of the austenitization furnace. Quench immediately to ℃.

In this case, the maximum heat release capacity of the carrier gas per square meter of the fluidized bed surface is limited to about 25 KW. Since there is a strong secondary convection cooling, there is no need to design the fluidized bed to maximum cooling, so it can be designed more freely and the fluidized bed can be designed for 10-15 KW heat dissipation per square meter of fluidized bed surface. The nominal flow rate of auxiliary air cooling is designed to be at least four times the above value, for example at least five times, in some cases at least 50 KW / m 2, for example 75 KW / m 2.

The invention will be described with reference to the drawings.

FIG. 1 shows a fluidized bed apparatus used to continuously pattern a series of steel wires 1, which are moved side by side in the axial direction of the wire, ie in the direction of the arrow 2. As a series of steel wires are located in one plane perpendicular to the plane of the drawing, only one steel wire is visible. However, in FIG. 2, which is a partial plan view, all parallel steel wires 1 are visible. The entire fluidized bed apparatus consists of four fluidized bed chambers 3, 4 and 5, which are separated from each other by partitions 7 and 8, followed by one chamber in the flow direction of the steel wire, followed by another chamber.

The first chamber depends on the temperature of the austenitization range (depending on the steel wire and the final properties required for the steel wire, usually in the range 900-1050 ° C.) and the formation of the pitting temperature, i.e. the formation of fine sorbite structures. It is suitable for quenching steel wires that can be introduced up to the temperature that can be started (steel wires and the final properties required for steel wires, usually in the range 530-570 ° C). Quenching should take place in the first chamber, so that the implementation of the present invention proceeds in the first chamber. The second, third and fourth chambers are suitable for maintaining the steel wire at the parting temperature for the time required to transform it into sorbite. No heat dissipation takes place here, but when the device is used for other types of metallization, in this case more than one chamber can be used to quench the steel wire. When this device is used for parting steel wires, if the second, third and fourth chambers are used only to keep the steel wires at a fixed temperature, the temperature of each chamber must be the same temperature for the four chambers. It can be adjusted to a temperature that does not need to be. For rapid quenching in the first chamber, a rather large temperature difference between the steel wire and the fluidized bed is required, but in order to maintain the steel wire at the temperature of the next chamber, the theoretical temperature difference may be zero or supplement the radiant heat. In order to achieve this, the fluidized bed temperature may be slightly higher. The temperature of the latter three chambers does not necessarily need to be the parting temperature when the steel wire is quenched in the first chamber, but the parting temperature (530-570 ° C) depends on the metallographic structure in order to form a sorbite structure. Can be varied up and down 30 ° C. Finally, the length of each chamber can be different and the number of chambers can also vary from 2-8 or more.

The entire fluidized bed device is surrounded by a casing (9) to separate the fluidized bed chambers (3, 4, 5, 6) from the outside atmosphere as much as possible, but to allow a series of steel wires (1) to enter and exit the inner part of the fluidized bed device. The inlet and outlet slender holes 10, and the carrier gas inlet hole 11 and outlet hole 12 of each fluidized bed chamber are exceptions.

The four fluidized bed chambers 3, 4, 5 and 6 consist of respective fluidized beds 13, 14, 15 and 16 filled with aluminum oxide particles having a particle size of 0.03-0.5 mm, which are in fluidized state. Depending on the time required to keep the carrier in contact with the fluidized bed particles, a general low reaches a height of 0.3-0.6 m. The temperature at which the fluidized bed of the first chamber should be adjusted is the required cooling rate of the steel wire, that is, the diameter of the steel wire and the speed of movement of the steel wire, so that the temperature of the fluidized bed of the first chamber can be cooled to the center of the steel wire during the short stopping time of the steel wire in the first chamber. Depends on. For the moving speed used in this example, a temperature of about 500 ° C.-40d was taken (d is the diameter of the steel wire in mm).

The fluidized bed of the first chamber according to this embodiment has a length of 101. m and a width of 1 m in the length direction of the wire, the maximum number of steel wires that can be guided through the fluidized bed is the maximum heat dissipation capacity of the fluidized bed and the steel wire Depends on its diameter. In this example, the maximum total heat dissipation capacity was designed to be 105 Kw, which corresponds to the ability to quench up to 1500 kg of steel wire per hour during the parting process, which selects the number of steel wires of a given diameter. Should be considered when In such a selection, it is also necessary to take into account the required downtime of the steel wire in the first chamber, which is proportional to the diameter of the steel wire inversely. Thus, in the case of steel wire with a diameter of 2 mm, the apparatus can process up to 30 parallel steel wires with a maximum heat dissipation capacity of 105 Kw with a moving speed of about 0.475 m / sec. In this embodiment, the device for guiding the steel wires through the fluidized bed is designed to guide 30 steel wires with a diameter of 1-6 mm. In the case of larger diameters, less than 30 steel wires should be treated in parallel so as not to exceed the maximum designed capacity.

For the carrier gas for the fluidized bed 13 of the first chamber, the exhaust gas is taken from a furnace (not shown) and is located just above the fluidized bed apparatus of FIG. 1 with respect to the movement of the furnace steel wire, The furnace is traversed with the same steel wire to achieve austenitization temperatures (900-1050 ° C). In this furnace, combustion deficient in oxygen occurs, and this carrier gas can cause any oxidation of the steel wire. The exhaust gas is sucked by the blower via the heat exchanger 18, and is further blown up to the first fluidized bed 3. In the heat exchanger 18, the exhaust gas is cooled to about 150 ° C., which is plenum chamber 19 below the fluidized bed 13 via the inlet hole 11 of the fluidized bed 3. Is sent out). The plenum chamber 19 is separated from the fluidized bed 13 by the bottom plate 20 of the first fluidized bed chamber 3, and a plurality of blow orifices 21 are provided on the bottom plate so that the carrier gas is discharged from the plenum chamber. Into the fluidized bed chamber, it is blown through the blower orifice at a temperature of about 120 ° C., with a somewhat even distribution over the base plate surface. It is used in a base plate with a blow orifice as described in US Pat. No. 4,813,653.

In the fluidized bed, a uniformly distributed carrier gas flow occurs in an upward direction, whereby the fluidized bed is fluidized, and the carrier gas coming out of the upper part is discharged from the fluidized bed chamber via the outlet hole 12. For steel wires with a diameter of 2 mm, the outlet temperature is adjusted to about 420 ° C., which corresponds to a heat release of about 12 kW. Since much of the heat dissipation is removed via secondary cooling (indirect convection cooling), this relatively small portion of the main cooling by carrier gas (up to 15 kilowatts per square meter of fluidized bed surface) of the overall cooling capacity is responsible for the parting process. In this quenching step is possible.

Auxiliary cooling takes place by air, which is sucked from the ambient air by the blower 22 via the inlet 36 and passes through the pipe device 24 towards the outlet 25 via the flow regulator 23. It is blown. In this diesel passage, the pipe arrangement consists of eight U-shaped pipes 26, which are obliquely immersed in the fluidized bed and connected together in parallel. In FIG. 1, the U-shaped pipe is not visible because the plane of each U-shaped pipe, and or both legs of the U-shaped pipe, are perpendicular to the plane of the drawing. However, in the top view of FIG. 2, the U shaped pipe can be seen even if it is not located in the (horizontal) plane of the drawing. Each of the eight U-shaped pipes comprises a straight and horizontally continuous inlet leg 27 and outlet leg 28 and are connected together in a U shape by an elbow 29. All inlet legs 27 are in the same horizontal plane 30 (FIG. 1) and all outlet legs 28 are in the other lower plane. The diameter of the pipe is not so large that it is impossible to penetrate the pipe device by the vertical transparency (figure 2), and the pipe device is not full. The gap 32 with the other leg is always visible in the vertical projection. In this way, the flow through the relatively tightly packed pipework is not dangerous. In order to obtain good flow, care must be taken not to concentrate the cooling elements in one horizontal plane in the case of convection devices, but the cooling elements must be distributed over two or more horizontal planes. It can also be seen that the gap between the cooling elements can be reached as well as possible by the vertical gas flow and the resistance to the vertical gas flow should be distributed as evenly as possible over the fluidized bed surface. On the one hand, when viewed in vertical projection, it is preferable that the cooling member of one plane covers as little or as little as possible the cooling member of the other plane, and on the other hand, the vertical projection of all cooling members of the cooling device It does not cover the entire fluidized bed, but covers only 50-80%. In other words, it can be seen that the vertical projection chiller still shows a gap in vertical transparency and is still transparent.

In the figure, the inlet and outlet legs 27, 28 are connected in parallel to the inlet and outlet tubes 3, 34 via a plurality of vertically continuous connecting tubes 35 on the casing side. The inlet and outlet legs preferably cross perpendicular to the direction of movement of the steel wire, but need not necessarily be vertical, but may cross in a non-vertical direction.

In order to maintain the fluidized bed temperature at a constant value despite disturbances such as heat release by the carrier gas or changes in heat inflow through the steel wires (mainly speed changes), the flow regulator 23 is used to control the fluidized bed temperature of the two steel wires. It is adjusted by the adjusting device 37 for adjustment. Usually such a control device includes a temperature sensor (not shown) located in the fluidized bed near the steel wire, which sends the output signal of the host vehicle to a comparator that measures the error of the measured value from a predetermined value. This error, analog or digital, is transformed into a calibration signal (usually with proportional, differential and integral) and the calibration signal is used to increase or decrease the cooling airflow in the desired range. It acts on (23).

The cooling pipe is made of steel and has an outer diameter of 4.8 cm. This provides about 2 m 2 of cooling surface per square meter of fluidized bed surface. In the normal treatment of steel wire with a diameter of 2 mm moving through the fluidized bed at a rate of 0.475 m / sec, the outlet temperature of the air is about 200 ° C. at a steady flow of 2000 Nm 3 / h, which is This corresponds to a normal heat release of about 93 Kw, taking into account heating. This is 7.75 times the heat dissipation capacity of the main cooling system. However, the advantage of the present invention is that it can be sufficiently used when the cooling surface of the auxiliary cooling (indirect convection cooling) circuit is more than 0.4 m2 per square meter of the fluidized bed surface and when the heat emission by the cooling circuit is more than three times the heat emission of the main cooling circuit. have.

In the embodiment of the present invention, the second, third, and fourth fluidized bed chambers 4, 5, 6 each have an inlet for a carrier gas. Since these chambers are suitable for maintaining steel wire at a temperature of sorbite transformation, the carrier gas must be blown at this temperature (530-570 ° C). This temperature may be different from the chamber. This carrier gas is preferably taken from the same arsenate furnace, but must be cooled to a lower range.

The present invention is not limited to quenching of the parting process, but may be applied to an apparatus having one or more fluidized bed chambers, in which case each chamber has its own function in the entire heat treatment program in which the steel wire is treated, among which One chamber serves to quench the steel wire from high temperature to low temperature, but not below about 250 ° C. to avoid condensation of moisture in the carrier gas.

Claims (13)

In a fluidized bed unit equipped with an indirect convection cooler, suitable for continuous quenching of steel wire with at least 250 ° C of ondol, the indirect convection cooler has a cooling surface of at least 0.4 m2 per square meter of fluidized bed surface and is also an air source. And means for varying the specific heat flux (H) (H = air flow rate x specific heat) of the air flow through the indirect convection cooler, wherein the indirect convection cooler also changes the fluidized bed temperature by changing the specific heat flow rate. Fluidized bed device characterized in that it has a predetermined device for maintaining a constant. The fluidized bed apparatus according to claim 1, wherein the fluidized bed apparatus includes means for adjusting the flow rate of the air supply by the output signal of the adjusting device. 2. The liquid injector of claim 1, wherein the inlet of the indirect convection cooler comprises a liquid atomizer which is a device for injecting a spray of liquid into the passage of the air flow from the air source towards the indirect convection cooler, wherein the liquid dispenser is capable of varying the effluent flow rate. And controlled by an output signal of the adjusting device. 4. An indirect convection cooler according to any one of the preceding claims, wherein the indirect convection cooler consists of a plurality of cooling elements, the members being distributed over one or more horizontal planes, the vertical projection of one plane cooling element being perpendicular to the other plane cooling element. A fluidized bed apparatus wherein the projection of all the cooling members occupies up to 80% of the total fluidized bed surface without any projection. 5. The U-shaped cooling pipe of claim 4, wherein the indirect convection cooler has legs that horizontally penetrate the fluidized bed in a direction transverse to the axial direction of the wire, and a plurality of U-shaped cooling pipes with one leg at the top and the other leg at the bottom horizontal plane. Wherein the pipe shows an interstice between the legs in the vertical projection, the legs being connected in parallel between the inlet and outlet of the indirect convective cooler. The fluidized bed apparatus according to claim 1, wherein a casing is provided for separating the atmosphere in and above the fluidized bed from the external atmosphere, and the inlet for carrier gas of the casing is connected to the outlet of the furnace via a cooling device. A plurality of adjacent fluidized-bed chambers immediately followed by another chamber in one chamber to quench and sorbitic transformation of the steel wire, including a furnace for austenizing the steel wire and including the shaft of the steel wire. A fluidized bed apparatus for continuous parting of a series of steel wires in series and parallel to each other in parallel, wherein the first chamber is a chamber for performing continuous quenching of the steel wires according to any one of claims 1 to 6. Fluidized bed device for continuous parting. In a method in which steel wires are continuously guided through a fluidized bed of a fluidized bed device equipped with an indirect convection cooler and continuously quenched at a temperature of at least 250 ° C., the air flow is sent through the convection cooler, whereby the carrier gas of the fluidized bed ( A method of continuously quenching steel wires, characterized by releasing at least three times the heat carried by a carrying gas and adjusting the temperature of the fluidized bed by adjusting the specific heat flow rate (H) of the cooling air. 9. The method of claim 8, wherein the temperature of the fluidized bed is adjusted by varying the flow rate of air sent through the indirect convection chiller. The method of claim 8, wherein the atomized liquid is injected into the air stream, but the temperature of the fluidized bed is adjusted by varying the amount of atomized liquid injected. The method according to any one of claims 8 to 10, wherein the fluidized bed is maintained in a non-oxidizing atmosphere and a carrier gas is supplied to the fluidized bed from a combustion device in which combustion is performed while lacking oxygen. 12. A method of continuous quenching of steel wires, through which austenitic furnaces are first passed through and sorbite transformation through quenching in a fluidized bed to continually tent the steel wires. A method of continuous quenching of steel wires, characterized by applying the method according to the method and obtaining a carrier gas from the exhaust gas from the austenitic furnace. 13. The method of claim 12, wherein the carrier gas emits less than 15 Kw of heat per cubic meter of fluidized bed surface and the indirect convection cooler emits at least 50 Kw of heat per cubic meter.

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