Summary
Presented here is a protocol that guarantees uniform distribution of initial moisture inside of a fabric and investigates the effects of hot-air thermodynamic parameters (velocity, temperature, and direction) and thickness on the fabric's drying characteristics (e.g., temperature variation) under the condition of air impingement.
Abstract
Impinging dryness is now a widely used and effective way for fabric drying due to its high heat and mass transfer coefficient. Previous studies on fabric drying have neglected the contributions of moisture uniformity and diffusion coefficient to the drying process; though, they have recently been shown to have a significant influence on drying characteristics. This report outlines a step-by-step procedure to investigate the effects of air impingement parameters on a fabric's drying characteristics by controlling the uniformity of its area moisture distribution. A hot air blower unit equipped with an angle adjustable nozzle is used to generate air flow with different velocities and temperatures while the drying process is recorded and analyzed using an infrared thermograph. In addition, a uniform padder is adapted to ensure the fabric's moisture uniformity. Impinging drying is studied under different initial conditions by changing the air flow temperature, velocity, and direction, then the applicability and suitability of the protocol are evaluated.
Introduction
Impinging drying is a very effective drying method due to its high heat, mass transfer coefficient, and short drying time. It has attracted extensive attention due to its numerous applications including chemical industry, food1, textile, dyeing2, paper making3,4, etc. Now, impinging drying is widely used for its enhanced transport characteristics, especially for the drying of textiles in the heat setting process5.
Fabric is impinging dried by the nozzle array for the heat setting. Nozzle layout affects the uniformity of drying temperature, which has a significant influence on the fabric properties, drying efficiency, and on the fabric surface directly. Thus, it is necessary to understand temperature distribution on the textile surface to design a better nozzle array. There has been little investigation in this field at the present, though there has been plenty of research on heat and moisture transfer performance of the fabric drying process so far. Some research has mainly focused on the natural evaporation of a textile under a specified heat source, in which the impinging drying process was not involved in these studies6,7. Some have focused on heat and moisture transfer of the textile with hot air drying, but the textile moisture and temperature were assumed to be uniform in these studies8,9,10,11. Furthermore, a few of these studies attempted to obtain the temperature distribution variation with time for studying the heat and moisture transfer of the textile under impinging drying.
Etemoglu et al.2 developed an experimental set-up for obtaining temperature variation with the time of the fabric and total drying time, but this set-up is limited to single-point temperature measurements. The initial moisture content distribution in the fabric is also neglected in this type of research. Wang et al.12 intended to obtain temperature distribution on the fabric by pasting thermocouples on the textile surface at various points, but surface temperature distribution was not able to be accurately obtained with their method. Obtaining temperature distribution at the air impingement area on a fabric with even humidity distribution is important for industrial printing and dyeing production, and it will provide better guidance on the distribution and arrangement strategy for object drying with a multi-nozzle13. The following procedure provides details to study the heat and moisture transfer of a fabric during the impinging drying process. The initial moisture content is well-controlled to be evenly distributed, while the surface temperature at every point of the fabric is obtained via the experimental set-up.
The experimental set-up consists of a hot air blower unit, infrared thermograph unit, uniform padder system, and other auxiliary devices. The hot air blower unit supplies the hot air with a specified temperature and velocity in an adjustable direction according to the experimental requirements. The infrared thermograph unit records the temperature history of each impinging drying process; thus, the temperature at each pixel point of the recorded video can be extracted with a supporting post-processing tool. The uniform padder system controls the even distribution of moisture content at every point of the fabric. Finally, the influence of air impingement parameters on fabric drying characteristic with fabric moisture uniform control method are investigated. The process can be carried out in a reproducible fashion following the standard protocol described below.
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Protocol
1. Experimental rig set-up
NOTE: See Figure 1.
- Hot air blower unit
- Ensure that the hot air blower is connected to the air nozzle through a high temperature resistant silicone pipeline that is heat-insulated with asbestos material. Gradually adjust the air nozzle to the desirable incline angle to control the air flow direction. For this work, the incline angle, α, varied between 60° and 90°.
- Switch on the air blower fan and the resistance wire.
NOTE: The order in which the fan and resistance wire are turned on cannot be reversed. - Set the outlet temperature of the hot air blower by gradually adjusting the current through the resistance wire with the controller of the air blower, and measure the air flow temperature using the digital temperature sensor. For this work, the hot air temperature, T, varied between 70 °C and 130 °C.
- Measure the air flow velocity at the outlet of the air nozzle using the handheld multifunctional anemometer at room temperature (RT). For this work, the hot air velocity, Va, varied between 8-20 m/s. For accurate measurement of the air velocity, the probe should be perpendicular to the direction of the airflow.
- Gradually adjust the rotation speed of the air blower fan with the frequency converter to obtain the desired air flow velocity. Cover the air nozzle with a high thermal resistance board to disperse the heat flow to avoid thermal damage to people or devices.
- Infrared thermograph unit
- Fix the infrared thermograph onto the support frame directly above the air nozzle with about a 1 m distance. Connect the infrared thermograph with the computer using the net cable. Power on the infrared thermograph and open the operation software of the infrared thermograph on the computer. Select the connection mode as Ethernet so an IP address is assigned automatically to the infrared thermograph by the computer, and the object temperature can be read in real-time with the infrared thermograph.
- Fix a piece of standard fabric sample over the air nozzle with the needle plate fixture and adjust the distance between the air nozzle and sample to the desired value. In this work, 30 mm is used, which is 3x the diameter of the nozzle.
- Adjust the focus of the camera and set basic parameters via the computer. Open the "parameters" dialog box, set the temperature unit to °C, set the thermal radiance to 0.95, set the ambient relative humidity to 50%, set the ambient temperature to 25 °C, and set the distance between the measured object and camera to 1.5 m. This will ensure that the measured temperature is correct.
- As the tested fabric sample is prepared, fix it at the same location as the standard fabric sample, then record the temperature of the fabric with the computer as a video.
- Uniform padder system
- Make sure the uniform padder is connected to the air compressor through the pipeline. Power on the air compressor and set its maximum output pressure to 0.8 MPa.
- Manually adjust the pressure regulators to control the air pressure to the couple of the clamp cylinders, which are connected to the upper roller of the padder, so that residual moisture in the fabric can be controlled. Ensure that the pressure on both sides of the roller are equal so that the moisture content of the fabric distributed over each area is even.
- Power on the padder, make sure the roller can rotation freely, then put the saturated fabric sample with sufficient moisture absorption onto the upper roller of the uniform padder, so that the tested fabric can be squeezed through the roller couple and moisture distribution inside the fabric can be controlled to be uniform.
- Power off the padder.
- Weight and thickness measuring unit
- Place an electronic scale on a horizontal platform and tare it. Place standard weights on the balance for calibration so that the specimen can be accurately weighted.
- Cut out a rectangular fabric sample with a width and length of 10 cm and 31 cm, respectively. Power on the thickness testing instrument (see Table of Materials) and connect it to the computer. Place these fabric samples on the test platform of the FTT. Open the operation software of the FTT, click "start" on the operation interface, then automatically test the thickness of the fabric by the FTT and record it on the operation interface.
- Drying stove unit
- Power on the drying stove and make sure no samples are in the drying room. Set the drying stove to a high temperature (120 °C is used in this work) for 30 min to evaporate the moisture absorbed by the inner wall of the stove.
- Preheat the drying stove to the desired temperature (45 °C is used in this work) so that the stove can be used directly for drying fabric samples.
2. Testing specimen and the manufacturing process
- Cut out square fabrics of 250 mm x 250 mm from the same fabric used for the rectangular fabric sample with scissors and a triangle ruler for the manufacturing of test specimens (area of the fabric = 6.25 × 104 mm2). Put the fabric specimens into the drying stove to evaporate the resident moisture absorbed from environment so that their net weight can be obtained.
- Take out one piece of fabric specimen from the drying stove, then measure the initial weight, W0, of the sample with the electronic balance.
- Immerse the fabric sample in water for 5 min to ensure that the fabric absorbs the moisture until saturation. Tile the saturated fabric sample onto the upper roller of the uniform padder to obtain the desired even initial moisture content.
- Power on the padder and set an initial pressure with the pressure regulators. As the tiled sample passes through the roller couple, power off the padder and remove the sample from the padder.
- Measure the weight of the wet fabric sample, W1, of the sample with the electronic balance. The residual moisture of the fabric can be calculated as C = (W1-W0)/W0, and the area average moisture content in the fabric can be calculated as Wa = (W1-W0)/A.
- If the desired moisture content, Cd, is not obtained, dry the rollers with a towel or paper towel first, and then repeat steps 2.4-2.5 until Cd is set.
- If necessary, cut out a sample strip from the same fabric used for preparing the testing specimen, then measure and record its thickness.
3. Data acquisition, post-processing, and analysis
- As done in step 1.1, set the outlet temperature and velocity of the air blower to the desired values and cover the nozzle with the high thermal resistance board. Once the tested fabric sample is prepared (section 2), fix it with the needle plate fixture for sequence testing and power on the infrared thermograph. Start to record the sample temperature.
- Remove the covered board that the hot air can impinge on the lower surface of tested sample directly. Observe changes in the drying temperature of the fabric on the computer during the drying process. When the drying temperature increases to a steady value and lasts for approximately 30 s, which means the sampled fabric is dry to the target status, stop recording. Take the sample away from the fixture and cover up the nozzle with the high thermal resistance board again.
- If necessary, set the target analysis area with a supporting post-processing tool for the infrared thermograph (for data plotting, saving, etc.) so that the drying features (normally how the temperature varies with time) of that point of the tested fabric can be obtained.
- If necessary, navigate the video to the portion of different drying stages and save the video frame as a colorful image. Then, the area of the region dried by hot air can be calculated by the image processing method according to the following steps14. First, gray the colorful image with the weighted average method to grayscale the image, then binarize the obtained grayscale image with the OSTU method by setting the threshold to the grayscale value in which the temperature in the image is close to the hot air temperature. Thus, the area of the dried region can be calculated on the binarization image.
- Repeat steps 3.1-3.4 and record the drying characteristics of each fabric sample by adjusting the air flow velocity, temperature, direction, as well as fabric material, physical parameters, etc.
- Observe all differences under varying air temperature, air velocity, airflow direction, and fabric thickness.
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Representative Results
The data presented in Figure 2 are typical temperature contours for cotton fabric at different drying stages under the condition that air velocity and temperature at the nozzle outlet are 20.0 m/s and 120 °C, respectively. It can be figured from Figure 2A,B,C,D that under the air impingement drying, temperature decays from the center to the periphery and forms sets of concentric circles. Meanwhile, temperature decays dramatically at the edge of the direct impingement area. The temperature distribution along an arbitrary trajectory can be drawn with the special supporting post-processing tool for the infrared thermograph. Figure 2E shows temperature along the fabric's horizontal center line at different stages in a typical drying process. This is caused by the fabric's high diffusion coefficient or thermal resistance in a horizontal direction, and even by extension of the drying time to 50 s; as shown, the temperature near the edge of the impingement area increases very little compared to that of the steady state (see Figure 2C; the drying process reaches steady state at approximately 20 s).
The historical data at each point of the video can also be plotted out with the post-processing tool. Figure 3 illustrates some typical results measured at the center point of the impingement area under different initial conditions. Figure 3A,B shows the influence of air temperature and velocity on the drying process. Normally, the higher the temperature or velocity, the faster the fabric to be dried; however, air temperature influenced the temperature both at the constant-rate state and steady state, while air velocity only influenced the steady state temperature. Figure 3C shows the drying process for fabrics with the same initial area average moisture content when thickness is different. The uniform padder is important for controlling the moisture distribution in every corner of the fabric to be uniform. As the saturated moisture content of a thin fabric is apparently lower than that of a thicker one, then the desirable moisture content, Cd, of the thicker fabric in this situation is very difficult to set. Thus, the specimen should be processed with the padder two or more times.
Figure 3C reveals that the higher diffusion coefficient of thicker samples slows down the drying process. This is important for a multi-nozzle drying process because a designed system is always used to dry fabric with the same material but with different thickness. Figure 3D shows the drying process under different airflow directions, while Figure 3E shows the temperature contour under a steady state at 60 s. As revealed in Figure 2, the fabric temperature changes little after reaching the steady state, and the dried area can be calculated with the image processing method based on the temperature contour. The binarization results are shown as Figure 3F, in which the area in white represents the dried area and the ratio of these five states from 65° to 90° is 0.61:0.81:1.07:1.02:1.01:1. This is also caused by the fabric's high diffusion coefficient and fluid thermodynamic parameters in a horizontal direction, which is important in strategies for setting the drying time.
Figure 1: Experimental rig. Shown is a schematic representation of the experimental rig, consisting of the hot air blower unit for supplying impingement air with different temperatures, velocities, and directions. Also represented is the uniform padder system used for controlling the even distribution of moisture content in every area of the fabric, infrared thermograph unit for recording the temperature history of each impinging drying process, and some auxiliary devices for measuring fabric weight, fabric thickness, and so on. The obtained results are then analyzed on the computer system. Please click here to view a larger version of this figure.
Figure 2: The temperature contour of cotton fabric at different drying stages. Temperature contours are shown under the conditions Va = 20.0 m/s, T = 120 °C, and Cd = 70%. Figure 2A shows the temperature contour at t = 0 s, while Figure 2B,C,D shows those at t = 5 s, 20 s, and 50 s. Legends P01, P02, P03, and P04 in each image show the temperature variation at different sampling points on the fabric in digital form. Figure 2E illustrates the temperature distribution along the horizontal fabric center line at different times. Please click here to view a larger version of this figure.
Figure 3: Typical results measured at the center point of the impingement area under different initial conditions. Figure 3A shows the influence of air temperature at Va = 20.0 m/s and Cd = 70%. Figure 3B shows the influence of air velocity at T = 120 °C and Cd = 70%. Figure 3C shows the influence of fabric with the same initial area average moisture content, Wa, of 48 g/m2; however, their thickness was different at Va = 20.0 m/s and T = 120 °C. Figure 3D,E,F show the influence of airflow direction at Va = 20.0 m/s, T = 120 °C, and Cd = 70%. Please click here to view a larger version of this figure.
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Discussion
This section provides a few tips necessary to ensure reliable quantitative results. First, the fabric specimens must be kept completely dry to ensure the initial weights are correct. This is achievable through the drying process (i.e., using a suitable drying stove). If possible, an environment humidity that is kept constant benefits the experiment.
Secondly, the fabric specimens must be well-processed to ensure that the moisture at each region of the fabric is uniform. This can be done by manually processing with a uniform padder or similar process. The key for operating the uniform padder is to make sure that the air pressure supplied to the clamping cylinders on both sides of the upper roller are equal, which prevents a press force difference to the fabric.
Appropriate calibration of the infrared thermograph must be ensured to obtain an accurate temperature. Meanwhile, the temperature recording process is launched manually and several seconds ahead of removal of the high thermal resistance board, so users are also required to estimate how many frames should be skipped. This may vary among individuals, so several trial tests for practicing are recommended before taking actual measurements.
One limitation of the technique is that the fabric specimens are dried under an open environment, and the desired surrounding temperature and humidity cannot be set; thus, the experimental results do not directly reflect the drying processes under actual working conditions of a heat setting. The test rig is to be further improved for future work.
The reported procedure provides details to study the heat and moisture transfer of the fabric during the impinging drying process. The initial moisture content is well-controlled to be uniform, while the surface temperature at every point of the fabric is obtained via the developed set-up.
In summary, the procedure outlined in this report can be used to study the effects of air impingement parameters on a fabric's drying characteristics by controlling the fabric moisture to a uniform status. It should be noted that the moisture distribution is normally ignored in current research of different fields, but it significantly influences the drying process and drying results. It is recommended that all steps of this protocol are performed in an environment without air convection to avoid any ambient-related degradation.
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Disclosures
The authors have nothing to disclose.
Acknowledgments
This work was supported by the NSFC-Zhejiang Joint Fund for the Integration of Industrialization and Informatization (grant number U1609205) and National Natural Science Foundation of China (grant number 51605443), the Key Research and Development Project of Zhejiang Province (grant number 2018C01027), the 521 Talent Project of Zhejiang Sci-Tech University, and the Young Researchers Foundation of Zhejiang Provincial Top Key Academic Discipline of Mechanical Engineering of Zhejiang Sci-tech University (grant number ZSTUME02B13).
Materials
| Name | Company | Catalog Number | Comments |
| Air Blower | Zhejiang jiaxing hanglin electromechanical equipment co., Ltd. | HLJT-3380-TX10A-0.55 | Air Volume: 900 m3/s; |
| Anemometer | KIMO | MP210 | Measurement range: 0-40 m/s; Accuracy: ±0.1 m/s |
| Drying stove | Shanghai Shangyi Instrument Equipment Co., Ltd. | DHG 101-0A | precision: 1 °C; Temperature control range:10-300 °C |
| Electronic Balance | Hangzhou Wante Weighing Instrument Co., Ltd. | WT1002 | Precision: 1 °C; Range: 100 g |
| Fabric Style Measuring Instrument | SDL Atlas | M293 | |
| Fabric Touch Tester | SDLATLAS Ltd | Fabric thickness tester | |
| High thermal resistance board | Baiqiang | Flame resistance, Heat resistance is greater than 200 °C | |
| High-temperature resistant silicon pipeline | Kamoer | 18# | Temperature range: -60-200 °C |
| Infrared Thermogragh | Hangzhou Meisheng Infrared Optoelectronic Technology Co., Ltd. |
R60-1009 | Temperature measuring range: -20-410 °C; Maximum measuring error: ±2 °C |
| Padder | Yabo textile machinery co., Ltd. | Roller pressure: 0.03-0.8 MPa; Stable pressure; Easy adjustment | |
| Personal Computer | Lenovo Group. | L460 | |
| Temperature Sensor | Taiwan TES electronic industry co., Ltd. | 1311A | resolution: 1 °C; Temperature measuring range: -50-1350 °C |
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