Method for reducing raised stripes generated after paper sheet is discharged from calender

Abstract

The invention relates to the technical field of heat dissipation, in particular to a method for reducing raised stripes generated after paper leaves a calender, which comprises the following steps: arranging a nozzle array above the paper just leaving the transmission direction of the calender; an area corresponding to the nozzle array in the paper sheet conveying direction is a cooling working area; first, second and third sub-cooling working areas are sequentially arranged in the paper sheet conveying direction; detecting the initial temperature when the paper sheet just enters the cooling working area; calculating a target total temperature drop between the initial temperature and a set target temperature of the paper sheet, and controlling a nozzle array to perform graded cooling on the paper sheet according to the target total temperature drop; the first sub-cooling working area is primary cooling, and an impact cooling mode is adopted; the second sub-cooling working area adopts secondary cooling, and adopts a resonance cooling mode for cooling; and the third sub-cooling working area is three-stage cooling, and a laminar flow compensation mode is adopted for cooling. According to the invention, raised stripes generated after paper sheets are discharged from the calender are reduced, and the paper sheet quality is greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of heat dissipation, and specifically to a method for reducing the formation of convex stripes on the paper after it exits the calender. Background Art

[0002] In the process of papermaking production, the calender is a key device for improving the surface flatness, glossiness and other quality indicators of the paper. During the calendering process of the paper, the paper passes through multiple roller presses in the calender, and the temperature rises. The high temperature enhances the thermoplasticity of components such as cellulose in the paper, and the internal fibers of the paper have large thermal stress.

[0003] When the paper exits the calender, if the temperature is not properly controlled, the shrinkage or deformation of the thermoplastic material due to temperature changes is inconsistent, which easily causes convex stripes. Under the current technology, the problem of convex stripes frequently occurs after the paper is processed by the calender. This not only seriously affects the appearance quality of the paper, but also causes many problems in subsequent processing and use, such as interfering with the printing effect and weakening the paper strength.

[0004] Therefore, the present invention proposes a method for reducing the formation of convex stripes on the paper after it exits the calender. Summary of the Invention

[0005] The purpose of the present invention is to provide a method that can effectively reduce the formation of convex stripes on the paper after it exits the calender, greatly improve the quality of the paper, and reduce the convex stripes.

[0006] The method for reducing the formation of convex stripes on the paper after it exits the calender includes:

[0007] A nozzle array with a width greater than the width of the paper is arranged above the paper in the transmission direction just when the paper leaves the calender. The area corresponding to the nozzle array in the paper transmission direction is the cooling working area; the cooling working area is divided into three sub-cooling working areas, which are the first, second, and third sub-cooling working areas in sequence along the paper transmission direction; the first sub-cooling working area is close to the calender outlet; the third sub-cooling working area is located at the end of the cooling working area; the area other than the first sub-cooling working area and the third sub-cooling working area is the second sub-cooling working area;

[0008] According to the set temperature measurement frequency, the initial temperature of the paper at the position to be measured just when it enters the cooling working area is detected at the entrance of the cooling working area;

[0009] Calculate the target total temperature drop between the initial temperature and the set target temperature of the paper, and control the nozzle array to perform staged cooling on the paper according to the target total temperature drop;

[0010] Among them, the first sub-cooling working area performs primary cooling based on the impingement cooling mode, and the primary cooling is used for impingement cooling to rapidly reduce the temperature; the second sub-cooling working area performs secondary cooling based on the resonance cooling mode, and the secondary cooling is used for resonance cooling to uniformly control the temperature; the third sub-cooling working area performs tertiary cooling using the laminar flow compensation mode, and the tertiary cooling is used for laminar flow compensation to finely adjust the temperature.

[0011] Further, the temperature measured at the inlet of the cooling working area is denoted as T0; the target temperature when the paper sheet leaves the outlet of the cooling working area is set as T 目 ;

[0012] Infrared temperature sensors are provided at the junction of the first sub-cooling working area and the second sub-cooling working area, at the junction of the second sub-cooling working area and the third sub-cooling working area, and at the outlet of the cooling working area;

[0013] According to the set temperature measurement frequency, the temperatures of the position of the paper sheet to be measured passing through the junction of the first sub-cooling working area and the second sub-cooling working area, the junction of the second and third sub-cooling working areas, and the outlet of the cooling working area are obtained as T1, T2, and T3 in sequence;

[0014] Calculate the target total temperature drop ΔT 目 , ΔT 目 = T0 - T 目 ; According to the target total temperature drop ΔT 目 , allocate the first target temperature drop, the second target temperature drop, and the third target temperature drop ΔT 1目 , ΔT 2目 , ΔT 3目 .

[0015] Further, the nozzle array includes high-frequency pulsed air knife nozzles correspondingly arranged in the first sub-cooling working area, and the impingement cooling mode in the first sub-cooling working area uses the high-frequency pulsed air knife nozzles to cool the paper sheet just leaving the calender;

[0016] The high-frequency pulsed air knife nozzles are perpendicular to the paper sheet, and the distance between the high-frequency pulsed air knife nozzles and the paper sheet is within a preset distance; according to the paper sheet temperature T1, calculate the first actual temperature drop ΔT 1实 = T 0— T1, and use the temperature deviation between ΔT 1实 and ΔT 1目 as the input to the PID controller to obtain the control quantity u1(t); according to the control quantity u1(t), dynamically adjust the frequency, impinging wind speed, and pulse duty ratio of the high-frequency pulsed air knife nozzles.

[0017] Further, the dynamic adjustment of the frequency f of the high-frequency pulsed air knife nozzle is specifically as follows:

[0018] The initial frequency f0 = 2 kHz. The frequency f(t) is adjusted according to the PID control quantity u1(t) as f(t) = f0 + α×u1(t), where α is the frequency adjustment coefficient, and it is ensured that the frequency f ranges between 2 - 10 kHz; when f(t) < 2 kHz, f = 2 kHz; when f(t) > 10 kHz, f = 10 kHz.

[0019] Further, the dynamic adjustment of the impact wind speed v1 of the high-frequency pulsed air knife nozzle is specifically as follows:

[0020] The initial wind speed v1 = 15 m / s. The wind speed v1(t) is adjusted according to the PID control quantity u1(t) as v1(t) = v0 + β×u1(t), where β is the wind speed adjustment coefficient, and it is ensured that the impact wind speed v1 ranges between 15 - 30 m / s; when v1(t) < 15 m / s, v = 15 m / s; when v1(t) > 30 m / s, v1 = 30 m / s.

[0021] Further, the dynamic adjustment of the pulse duty cycle D of the high-frequency pulsed air knife nozzle is specifically as follows: The initial duty cycle D0 = 50%. The duty cycle D(t) is adjusted according to the PID control quantity u1(t) as D(t) = D0 + γ×u1(t), where γ is the duty cycle adjustment coefficient, and it is ensured that the pulse duty cycle D ranges between 30% - 70%; when D(t) < 30%, D = 30%; when D(t) > 70%, D = 70%.

[0022] Further, the nozzle array further includes an eddy current nozzle and a pulsed air knife corresponding to the second sub-cooling working area. The resonant cooling mode in the second sub-cooling working area is based on the combination of the eddy current nozzle and the pulsed air knife for cooling. The eddy current nozzle ejects a rotating air flow onto the paper by changing the inclination angle with respect to the paper width direction, reducing the transverse temperature difference of the paper; the resonant cooling mode realizes resonant cooling by synchronizing the paper vibration frequency with the pulsed air knife frequency and controlling the phase synchronization of the two.

[0023] Further, the transverse temperature difference of the paper in the second sub-cooling working area is monitored in real time by an infrared thermal imager. According to the transverse temperature difference in this area, the inclination angle between the eddy current nozzle and the paper transmission direction is dynamically adjusted. The eddy current nozzle ejects a rotating air flow at an inclination angle of 25° - 50°. By increasing the inclination angle of the eddy current nozzle, the rotating air flow is made closer to the high-temperature area, reducing the transverse temperature difference of the paper.

[0024] Further, when adjusting the inclination angle between the eddy current nozzle and the paper transmission direction, the vibration frequency of the paper in the second sub-cooling working area is simultaneously obtained by a laser vibrometer. The measured paper vibration frequency f 纸As a reference value, the switching frequency f of the pulsed air flow is adjusted according to the paper vibration frequency 气流 , so as to control the pulsed air flow frequency to match the paper vibration frequency and trigger the resonance effect; the triggering moment of the pulsed air flow lags behind the peak of the paper vibration wave by 30°-60°, so as to ensure the coordination between the air flow impact and the paper vibration direction.

[0025] Furthermore, the laminar flow compensation mode in the third sub-cooling working area adopts a long-strip porous diffusion laminar flow nozzle; the wind direction is horizontal and the wind speed ≤ 5 m / s; according to the lateral temperature distribution of the paper detected by the infrared thermal imager in the third sub-cooling working area, the air volume of each nozzle is dynamically adjusted to make the lateral temperature difference ≤ 1.5 °C; at the same time, according to the comparison result between ΔT 3实 and ΔT 3目 in the third actual temperature drop in the third sub-cooling working area, ΔT 3实 = T2 - T3, the opening degree and frequency of the porous diffusion laminar flow nozzle are adjusted to control the output air volume.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] In the present invention, primary cooling is performed in the first sub-cooling working area based on the impact cooling mode, and the primary cooling is used for impact cooling to quickly reduce the temperature; secondary cooling is performed in the second sub-cooling working area based on the resonance cooling mode, and the secondary cooling is used for resonance cooling to uniformly control the temperature; tertiary cooling is performed in the third sub-cooling working area using the laminar flow compensation mode, and the tertiary cooling is used for laminar flow compensation to finely adjust the temperature. When the paper passes through the cooling working area, the temperature is adjusted in stages, realizing the rapid reduction of the paper temperature and the uniformity of the lateral paper temperature, avoiding the stress non-uniformity caused by too high or non-uniform temperature, reducing the raised stripes, and greatly improving the paper quality. Description of the Drawings

[0028] Figure 1 It is the method logic diagram of the present invention. Detailed Embodiments

[0029] The technical solution of the present invention will be clearly described below in conjunction with the description of the drawings. Obviously, the described embodiments are not all embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] This embodiment provides a method for reducing the raised stripes generated after the paper exits the calender, including:

[0031] S1. A nozzle array is arranged above the paper in the direction of the paper just leaving the calender. The width of the nozzle array is greater than the width of the paper. The area corresponding to the nozzle array in the paper transport direction is the cooling working area. The cooling working area is divided into three sub-cooling working areas, which are the first, second, and third sub-cooling working areas in sequence along the paper transport direction. The first sub-cooling working area is close to the calender outlet. The third sub-cooling working area is located at the end of the cooling working area. The area other than the first and third sub-cooling working areas is the second sub-cooling working area.

[0032] S2. According to the set temperature measurement frequency, the initial temperature is detected at the entrance of the cooling working area when the position to be measured on the paper just enters the cooling working area.

[0033] S3. Calculate the target total temperature drop between the initial temperature and the set target temperature of the paper, and control the nozzle array to perform staged cooling on the paper according to this target total temperature drop.

[0034] Infrared temperature sensors are set at the entrance of the cooling working area, at the junction of the first and second sub-cooling working areas, at the junction of the second and third sub-cooling working areas, and at the outlet of the cooling working area.

[0035] According to the set temperature measurement frequency, when the paper is in the process of being transported, the temperatures in the longitudinal direction of a certain position to be measured on the paper at the entrance of the cooling working area, at the junction of the first and second sub-cooling working areas, at the junction of the second and third sub-cooling working areas, and at the outlet of the cooling working area are measured as T0 (initial temperature), T1, T2, and T3 in sequence.

[0036] Set the target temperature when the paper leaves the outlet of the cooling working area as T 目 ; According to the initial temperature T0 and the target temperature T 目 , calculate the target total temperature drop ΔT 目 , ΔT 目 = T0 - T 目 .

[0037] Set the paper transport speed as V; set the length of the total cooling working area corresponding to the first, second, and third sub-cooling working areas as L, and the length ratios of the first, second, and third sub-cooling working areas are k1:k2:k3. The lengths of the first, second, and third sub-cooling working areas are L1:L2:L3 = (k1:k2:k3)×L. According to the characteristics and thickness of the paper, adjust the ratio between k1:k2:k3. Specifically, the length ratios k1, k2, and k3 of the first, second, and third sub-cooling working areas are 40% - 60%, 20% - 35%, and 10% - 25% respectively.

[0038] According to the target total temperature drop ΔT 目 , the first, second, and third target temperature drops ΔT 1目 , ΔT 2目 , ΔT 3目 borne by the first, second, and third sub-cooling working areas are proportionally allocated. For example, the primary cooling bears 60%-70% of the target total temperature drop and rapidly reduces the temperature through impingement cooling; the secondary cooling bears 20%-30% of the target total temperature drop, and the tertiary cooling bears 5%-10% of the target total temperature drop. The overall temperature drop target is achieved through hierarchical control. In a specific implementation, the allocation is as follows:

[0039] The first target temperature drop ΔT 1目 borne by the first sub-cooling working area = 0.7×ΔT 目 ;

[0040] The second target temperature drop ΔT 2目 borne by the second sub-cooling working area = 0.2×ΔT 目 ;

[0041] The third target temperature drop ΔT 3目 borne by the third sub-cooling working area = 0.1×ΔT 目 ;

[0042] S4. The first sub-cooling working area is the primary cooling, and the impingement cooling mode is used to perform primary cooling on the paper sheet, which can rapidly reduce the temperature of the paper sheet; the second sub-cooling working area is the secondary cooling, and the resonance cooling mode is used to perform secondary cooling on the paper sheet to further uniformly cool; the third sub-cooling working area is the tertiary cooling, and the laminar flow compensation mode is used to perform tertiary cooling on the paper sheet to ensure uniform temperature across the width of the paper sheet.

[0043] The nozzle array includes a plurality of high-frequency pulsed air knife nozzles correspondingly arranged above the first sub-cooling working area, a combination of a plurality of eddy current nozzles and air knives correspondingly arranged above the second sub-cooling working area, and a porous diffusive laminar flow nozzle correspondingly arranged above the third sub-cooling working area.

[0044] The length of the nozzle array in each sub-cooling working area is equal to the length (L1, L2, L3) of the sub-cooling working area in that region. The width of the nozzle array needs to cover the width of the paper sheet and is usually the same as the cross-machine width of the paper sheet.

[0045] The distance between the high-frequency pulsed air knife nozzles, the eddy current nozzles and the air knives and the paper sheet is within 15 - 25 mm; the distance between the porous diffusive laminar flow nozzle and the paper sheet is within 5 mm.

[0046] Determine the number of nozzles in each sub-cooling working area according to the width of the paper sheet, the length of each sub-cooling working area, and the nozzle spacing corresponding to each sub-cooling working area.

[0047] Multiple high-frequency pulsed air knife nozzles are arranged in a staggered manner. In a specific embodiment, the diameter of a single high-frequency pulsed air knife nozzle is 1-3 mm, and the spacing is 1.5 times the diameter of a single high-frequency pulsed air knife nozzle.

[0048] In the impact cooling mode of primary cooling, high-frequency pulsed air knife nozzles are used to cool the paper sheet that has just left the calender. The high-frequency pulsed air knife nozzles are perpendicular to the paper sheet. By impacting the surface of the paper sheet with strong air flow, heat is dissipated quickly through convection. For the high-frequency pulsed air knife nozzles, the adjustable range of the pulse frequency is 2-10 kHz, and the range of the impact air speed is 15-30 m / s. Through high-frequency pulses, an "air knife effect" is generated to form an instantaneous high-pressure air flow layer on the surface of the paper sheet, and the heat of the paper sheet is quickly carried away by the high-frequency pulsed air flow.

[0049] According to the calculated first target temperature drop ΔT 1目 , and the measured temperatures T0 (initial temperature) and T1 at a certain temperature position to be measured on the paper sheet, calculate the temperature deviation of the first sub-cooling working area:

[0050] e1(t) = ΔT 1目 -ΔT 1实(t) ; Dynamically adjust the frequency, impact air speed, and pulse duty cycle of the high-frequency pulsed air knife nozzles, where the pulse frequency range is 2-10 kHz, the impact air speed range is 15-30 m / s, and the pulse duty cycle is 30-70%.

[0051] ΔT 1实(t) = T0 - T 目 is the actual temperature drop at a certain moment t in the first sub-cooling working area, and ΔT 1实 is the first actual temperature drop; According to the PID control algorithm, input the temperature deviation e1(t) to obtain the control quantity u1(t); According to the control quantity u1(t), dynamically adjust the frequency, impact air speed, and pulse duty cycle of the high-frequency pulsed air knife nozzles.

[0052] Within the first duration t1 = (k1×L) / V corresponding to the first sub-cooling working area at a certain temperature position to be measured on the paper sheet, after the temperature position to be measured on the paper sheet passes through the first sub-cooling working area, the first actual temperature drop ΔT of the temperature position to be measured 1实 is close to the first target temperature drop ΔT 1目 .

[0053] The dynamic adjustment of the frequency f of the high-frequency pulsed air knife nozzles is specifically as follows:

[0054] The initial frequency f0 = 2 kHz, and the frequency f(t) = f0 + α×u1(t) is adjusted according to the PID control quantity u1(t), where α is the frequency adjustment coefficient, and it is ensured that the frequency f ranges between 2 - 10 kHz; when f(t) < 2 kHz, f = f min = 2 kHz; when f(t) > 10 kHz, f = f max = 10 kHz.

[0055] The calculation formula for the frequency adjustment coefficient α is: where is the positive maximum value of the PID output when the frequency of driving the high - frequency pulsed air knife nozzle reaches f max . By increasing the frequency of the high - frequency pulsed air knife nozzle, the impact density can be enhanced and the cooling rate can be accelerated.

[0056] The impact wind speed v1 of the high - frequency pulsed air knife nozzle is dynamically adjusted as follows:

[0057] The initial impact wind speed v1 = 15 m / s, and the wind speed v1(t) = v0 + β×u1(t) is adjusted according to the PID control quantity u1(t), where β is the wind speed adjustment coefficient, and it is ensured that the impact wind speed v1 ranges between 15 - 30 m / s; when v1(t) < 15 m / s, v1 = v1 min = 15 m / s; when v1(t) > 30 m / s, v1 = v1 max = 30 m / s.

[0058] When u1(t) = u max , the impact wind speed v1 reaches the maximum value v1 max ,

[0059] β = (v1 max - v1 min ) / u max = (30 - 15) / u max , where u max is the positive maximum control quantity of the PID output when the impact wind speed v1 reaches v1 max .

[0060] Increasing the wind speed can enable more cooling gas to quickly carry away the heat of the paper sheet, ensuring that the paper sheet can achieve the expected cooling effect in the first sub - cooling working area. In order to achieve the same cooling effect in a shorter time, it is necessary to increase the impact wind speed so that the cooling gas can more effectively exchange heat with the paper sheet. Regardless of how the paper sheet transmission speed changes, the cooling intensity can be maintained stable by adjusting the wind speed, avoiding the generation of wavy stripes due to uneven cooling caused by the change in transmission speed.

[0061] By comprehensively considering the temperature drop and the paper sheet transmission speed to dynamically adjust the impact air velocity, the paper sheet can be cooled relatively uniformly at different positions in the first sub-cooling working area, reducing the uneven internal stress distribution of the paper sheet and generating convex stripes.

[0062] The dynamic adjustment of the pulse duty cycle D of the high-frequency pulse air knife nozzle is specifically as follows: the initial duty cycle D0 = 50%, and the duty cycle D(t) = D0 + γ × u1(t) is adjusted according to the PID control quantity u1(t), where γ is the duty cycle adjustment coefficient, and it is ensured that the pulse duty cycle D ranges between 30% and 70%; when D(t) < 30%, D = D min = 30%; when D(t) > 70%, D = D max = 70%.

[0063] The calculation formula for the duty cycle adjustment coefficient γ is:

[0064] γ = unilateral maximum duty cycle change amount / unilateral maximum control quantity = 20% / u max ; u max is the positive maximum control quantity of the PID output that makes D(t) reach D max . Among them, the unilateral maximum duty cycle change amount = 20% = 50% - 30%. Or 20% = 70% - 50%.

[0065] The pulse duty cycle D refers to the proportion of the high level in a pulse cycle. In the paper sheet cooling, by adjusting the pulse duty cycle D, the time proportion of the air flow ejected from the air knife nozzle can be controlled. A larger duty cycle means that the air knife ejects air flow for a longer time in one cycle, and the cooling effect on the paper sheet surface is stronger.

[0066] Along the width direction of the paper sheet, a set of eddy current nozzles and pulse air knives are arranged at certain intervals. The eddy current nozzles and pulse air knives need to be evenly spaced. The eddy current nozzles are arranged on the upstream side close to the paper sheet transmission, and the pulse air knives are on the downstream side. The eddy current nozzles first use the rotating air flow to destroy the boundary layer on the paper sheet surface, enhancing the heat exchange between the air and the paper sheet, creating favorable conditions for the subsequent resonance effect of the pulse air knives, and strengthening the cooling effect.

[0067] The inclination angle θ of the eddy current nozzle with respect to the width direction of the paper sheet ranges from 25° to 50°, and the initial value is set to 35°. An infrared thermal imager is arranged in the second sub-cooling working area. The infrared thermal imager is used to collect the transverse temperature distribution of the paper sheet in real time, form a temperature cloud map, and obtain the transverse temperature difference (the difference between the highest temperature and the lowest temperature) of the paper sheet in the second sub-cooling working area. According to the transverse temperature difference ΔT 横向 of the paper sheet in the second sub-cooling working area, θ is dynamically adjusted,

[0068] θ = 35° + 0.5° × ΔTtransverse.

[0069] Based on the cross - machine temperature difference of the paper in the second sub - cooling working area, determine whether the temperature on one side of the paper is too high. If it is too high, appropriately increase the inclination angle of the eddy current nozzle on that side to make the rotating air flow more inclined towards the high - temperature area. At the same time, the rotating air flow can better disturb the air boundary layer in the high - temperature area, destroy the stability of the thermal boundary layer, promote heat dissipation, and reduce the temperature difference with other areas. If the cross - machine temperature difference is uniform, keep the initial inclination angle to ensure uniform cooling of the paper in the entire cross - machine direction.

[0070] The air jet direction of the pulsed air knife should be perpendicular to the paper surface to effectively stimulate the vibration of the paper, facilitate matching with the natural vibration frequency of the paper, and achieve resonance. The pulse frequency of the pulsed air knife is reduced to 1 - 3 kHz, and this frequency band is more likely to couple with the natural vibration frequency of the paper; the impact wind speed of the pulsed air knife is 15 - 20 m / s.

[0071] After the paper enters the second sub - cooling working area, when adjusting the inclination angle between the eddy current nozzle and the paper transmission direction, simultaneously obtain the vibration frequency of the paper in the second sub - cooling working area through a laser vibrometer. Take the measured vibration frequency f of the paper 纸 as the reference value, and adjust the pulsed air knife f according to the vibration frequency of the paper 气流 , control the matching of the pulsed air flow frequency and the paper vibration frequency, and control the matching error within ±3%. f 气流 - f 纸 ≤±3%; set ΔT 2实 as the second actual temperature drop, ΔT 2实 = T1 - T2; according to the second actual temperature drop ΔT of the paper 2实 and the second target temperature drop ΔT 2目 , as well as the vibration frequency f of the paper 纸 , adjust the pulsed air knife f 气流 .

[0072]

[0073] When the actual temperature drop ΔT 2实 <ΔT 2目 , increase f 气流 to enhance the cooling intensity.

[0074] The laser vibrometer synchronously records the vibration waveform of the paper and identifies the peak position. According to the peak phase, adjust the trigger time of the pulsed air knife. Control the pulsed air flow phase to lag behind the paper vibration peak by 30° - 60°.

[0075] According to the second actual temperature drop and the second target temperature drop of the paper, adjust the impact wind speed v2 (unit: m / s) of the pulsed air knife: The basic wind speed v 基础 = 15 m / s, and k takes 0.01.

[0076] The eddy current nozzle first weakens the boundary layer on the paper surface through rotational airflow to reduce the thermal resistance; the pulsed air knife then excites the paper vibration at a matching frequency to further accelerate heat transfer and ensure the temperature uniformity of the paper.

[0077] In the third sub-cooling working area, the porous diffusion laminar flow nozzle undertakes fine temperature regulation and lateral temperature difference compensation. The nozzle body of this part is distributed in a long strip shape along the width direction of the paper, and small circular holes (the aperture of the spray holes is 0.5 - 2 mm) are evenly arranged on the surface. The spray hole density is adjusted according to the paper width to ensure that the airflow coverage has no dead corners. A honeycomb-shaped flow guide plate is integrated inside to guide the gas to diffuse evenly and form a laminar flow pattern. After the gas diffuses through the pores, a stable and parallel laminar airflow is formed, gently acting on the paper surface. The laminar airflow can avoid mechanical disturbance to the paper, and at the same time achieve gentle cooling through thin-layer heat conduction.

[0078] After the gas enters the nozzle, under the guidance of the honeycomb-shaped flow guide plate, it diffuses orderly and sprays out from the spray holes, forming a stable, parallel and low-speed laminar airflow that covers the entire width of the paper. The airflow direction is horizontal, parallel to the paper transmission direction, and the wind speed is controlled at ≤5 m / s to avoid disturbing the paper.

[0079] According to the third actual temperature drop ΔT in the third sub-cooling working area 3实 and ΔT 3目 the comparison result, ΔT 3实 = T2 - T3,, adjust the opening degree and frequency of the porous diffusion laminar flow nozzle to control the output air volume.

[0080] When the deviation between ΔT 3实 and ΔT 3目 is within 1 °C, maintain the current air volume; when the deviation between ΔT 3实 and ΔT 3目 is within 2 °C, slightly adjust the opening degree of the orifice diffusion laminar flow nozzle by ±5%; when the deviation between ΔT 3实 and ΔT 3目 is within 2 °C, adjust the fan frequency of the orifice diffusion laminar flow nozzle by 2 Hz; when the deviation between ΔT 3实 and ΔT 3目 is 5 °C or more, adjust the opening degree of the orifice diffusion laminar flow nozzle by 10% and the fan frequency of the orifice diffusion laminar flow nozzle by 5 Hz simultaneously.

[0081] Based on the closed-loop control of laminar flow compensation for the lateral temperature difference of the paper in the third sub-cooling working area monitored by an infrared thermal imager, when it is detected that the temperature on the right side of the paper is 1.5 °C higher than that on the left side, increase the gas flow rate of the right-side nozzle until the temperature difference ≤1.5 °C. Finally, achieve the control target of the lateral temperature difference of the paper ≤1.5 °C, effectively suppressing the generation of convex stripes.

[0082] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than limiting the protection scope of the present invention. Any simple modification or equivalent replacement made by those of ordinary skill in the art to the technical solution of the present invention shall not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for reducing the formation of camber stripes after the paper sheet exits the calender, characterized in that, Including: A nozzle array with a width greater than the width of the paper sheet is arranged above the conveying direction of the paper sheet just leaving the calender. The area corresponding to the nozzle array in the paper sheet conveying direction is the cooling working area. The cooling working area is divided into three sub-cooling working areas, which are the first, second, and third sub-cooling working areas in sequence along the paper sheet conveying direction. The first sub-cooling working area is close to the calender outlet. The third sub-cooling working area is located at the end of the cooling working area. The area other than the first sub-cooling working area and the third sub-cooling working area is the second sub-cooling working area. According to the set temperature measurement frequency, when the position of the paper sheet to be measured for temperature just enters the cooling working area at the entrance of the cooling working area, measure the initial temperature. Calculate the target total temperature drop between the initial temperature and the set target temperature of the paper sheet, and control the nozzle array to perform staged cooling on the paper sheet according to the target total temperature drop. Among them, the first sub-cooling working area performs primary cooling based on the impingement cooling mode. The primary cooling is used for impingement cooling to rapidly reduce the temperature. The second sub-cooling working area performs secondary cooling based on the resonant cooling mode. The secondary cooling is used for resonant cooling to uniformly control the temperature. The third sub-cooling working area performs tertiary cooling using the laminar flow compensation mode. The tertiary cooling is used for laminar flow compensation to finely adjust the temperature.

2. The method for reducing the occurrence of camber streaks after the paper sheet exits the calender according to claim 1, wherein, The temperature measured at the entrance of the cooling working area is denoted as T0; the target temperature when the paper sheet leaves the exit of the cooling working area is set as T 目 ; Infrared temperature sensors are set at the junction of the first sub-cooling working area and the second sub-cooling working area, the junction of the second sub-cooling working area and the third sub-cooling working area, and the outlet of the cooling working area. According to the set temperature measurement frequency, obtain the temperatures of the position of the paper sheet to be measured for temperature passing through the junction of the first sub-cooling working area and the second sub-cooling working area, the junction of the second and third sub-cooling working areas, and the outlet of the cooling working area as T1, T2, and T3 in sequence. Calculate the target total temperature drop ΔT 目 , ΔT 目 = T0 - T 目 ; According to the target total temperature drop ΔT 目 , allocate the first target temperature drop, the second target temperature drop, and the third target temperature drop ΔT 1目 , ΔT 2目 , ΔT 3目 borne by the first sub-cooling working area, the second sub-cooling working area, and the third sub-cooling working area respectively according to a preset ratio.

3. The method for reducing the occurrence of camber streaks after the paper sheet exits the calender according to claim 2, characterized in that, The nozzle array includes high-frequency pulsed air knife nozzles corresponding to the first sub-cooling working area. The impingement cooling mode of the first sub-cooling working area uses high-frequency pulsed air knife nozzles to cool the paper sheet just leaving the calender. The high-frequency pulsed air knife nozzle is perpendicular to the paper sheet, and the distance between the high-frequency pulsed air knife nozzle and the paper sheet is within a preset distance; according to the paper sheet temperature T1, calculate the first actual temperature drop ΔT 1实 = T 0－ T1, take the temperature deviation between ΔT 1实 and ΔT 1目 as the input to the PID controller to obtain the control quantity u1(t); according to the control quantity u1(t), dynamically adjust the frequency, impact wind speed, and pulse duty cycle of the high-frequency pulsed air knife nozzle.

4. The method for reducing the formation of camber streaks after the paper sheet exits the calender according to claim 3, characterized in that, The dynamic adjustment of the frequency f of the high-frequency pulsed air knife nozzle is specifically as follows: The initial frequency f0 = 2 kHz. Adjust the frequency f(t) = f0 + α×u1(t) according to the PID control quantity u1(t), where α is the frequency adjustment coefficient, and ensure that the frequency f ranges between 2 - 10 kHz. When f(t) < 2 kHz, f = 2 kHz. When f(t) > 10 kHz, f = 10 kHz.

5. The method for reducing the formation of camber stripes after the paper sheet exits the calender according to claim 3, characterized in that, The dynamic adjustment of the impingement wind speed v1 of the high-frequency pulsed air knife nozzle is specifically as follows: The initial wind speed v1 = 15 m / s. Adjust the wind speed v1(t) = v0 + β×u1(t) according to the PID control quantity u1(t), where β is the wind speed adjustment coefficient, and ensure that the impingement wind speed v1 ranges between 15 - 30 m / s. When v1(t) < 15 m / s, v1 = 15 m / s. When v1(t) > 30 m / s, v1 = 30 m / s.

6. The method for reducing the formation of camber stripes after the paper sheet exits the calender according to claim 3, characterized in that, The pulse duty cycle D of the dynamic adjustment high-frequency pulse air knife nozzle is specifically as follows: the initial duty cycle D0 = 50%, and the duty cycle D(t) = D0 + γ × u1(t) is adjusted according to the PID control quantity u1(t), where γ is the duty cycle adjustment coefficient, and it is ensured that the pulse duty cycle D ranges between 30% and 70%; when D(t) < 30%, D = 30%; when D(t) > 70%, D = 70%.

7. The method for reducing the formation of wavy streaks after the paper sheet exits the calender according to claim 2, characterized in that, The nozzle array further includes an eddy current nozzle and a pulse air knife correspondingly arranged in the second sub-cooling working area. The resonance cooling mode in the second sub-cooling working area is based on the combination of the eddy current nozzle and the pulse air knife for cooling. The eddy current nozzle sprays rotating air flow onto the paper by changing the inclination angle with the paper width direction, reducing the transverse temperature difference of the paper. The resonance cooling mode realizes resonance cooling by synchronizing the paper vibration frequency with the pulse air knife frequency and controlling the phase synchronization of the two.

8. The method for reducing the occurrence of camber streaks after the paper sheet exits the calender according to claim 7, characterized in that, The transverse temperature difference of the paper in the second sub-cooling working area is monitored in real time by an infrared thermal imager. According to the transverse temperature difference in this area, the inclination angle between the eddy current nozzle and the paper transmission direction is dynamically adjusted. The eddy current nozzle sprays rotating air flow at an inclination angle of 25° - 50°. By increasing the inclination angle of the eddy current nozzle, the rotating air flow is made closer to the high-temperature area, reducing the transverse temperature difference of the paper.

9. The method for reducing the formation of wavy streaks after the paper sheet exits the calender according to claim 8, characterized in that, When adjusting the inclination angle between the eddy current nozzle and the paper conveying direction, synchronously obtain the vibration frequency of the paper in the second sub-cooling working area through a laser vibrometer, and use the measured paper vibration frequency f 纸 as the reference value, and adjust the switching frequency f of the pulsed air flow according to the paper vibration frequency 气流 , control the pulsed air flow frequency to match the paper vibration frequency to trigger the resonance effect; the triggering moment of the pulsed air flow lags behind the paper vibration peak by 30°-60° to ensure that the air flow impact is coordinated with the paper vibration direction.

10. The method for reducing the formation of camber streaks after the paper sheet exits the calender according to claim 2, characterized in that, The laminar compensation mode in the third sub-cooling working area adopts a strip-shaped porous diffusion laminar nozzle; the wind direction is horizontal, and the wind speed ≤ 5 m / s; according to the transverse temperature distribution of the paper sheet detected by the infrared thermal imager in the third sub-cooling working area, the air volume of each nozzle is dynamically adjusted to make the transverse temperature difference ≤ 1.5 °C; at the same time, according to the comparison result between ΔT 3实 and ΔT 3目 , where ΔT 3实 = T2 - T3, the opening degree and frequency of the porous diffusion laminar nozzle are adjusted to control the output air volume.

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