Kinetics of contact ultrasonic drying of thick woolen fabric

. The article is devoted to an experimental research of the effectiveness of continuous contact ultrasonic drying of dense woolen fabrics. For drying, an experimental stand was created in the form of a small-sized continuous-action drying machine. In the process, the dried fabric is rewound from reel to reel, through the zone of ultrasonic (150 W) and thermal (at temperatures of 50 °C and 80 °C) exposure. It has been established that ultrasonic action (together with thermal) provides a reduction in drying time from 18% to 28%. Moreover, at a lower temperature, the contribution of ultrasonic vibrations to the reduction of drying time is higher. The drying schedules were approximated by a formula expressing the dependence of the moisture content of the material on temperature and the presence of ultrasonic exposure. The resulting approximation formula can be used to analyze the curves of convective drying of dense woolen fabrics, taking into account the temperature of the drying agent and the presence of ultrasonic exposure, including predicting the duration of such types of drying.


Introduction
Drying of flat fibrous textile materials, including dense woolen fabrics, is carried out mainly by the convective method in continuous dryers. Intensification by a reasonably chosen method and calculation of the drying process of fibrous materials is one of the urgent tasks in the chemical technology of finishing production of textile enterprises [1][2][3]. It is known that physical fields, including an ultrasonic field, are widely used to intensify the drying process of various materials [4][5][6][7][8]. The analysis of domestic and foreign literature sources shows that there have been practically no studies of the process of continuous drying of flat textile materials, including dense woolen fabrics with ultrasonic exposure to intensify the process.

Experimental results
The purpose of this work is an experimental study of the kinetics of the continuous convective drying process of a typical dense woolen fabric, intensified by ultrasonic action, and the approximation of the obtained kinetic curves.
A dense woolen fabric of special purpose, consisting of 90% wool and 10% polyamide fiber, was chosen as the drying object. The surface density of the fabric is M = 760 g/m 2 , the thickness of the fabric is l = 2.0 mm, the porosity is ε = 50%, the normalized conditioned humidity is 12.2-13.0%. Drying of dense woolen fabrics in industrial conditions is carried out by convective method in drying-shirol machines (DSM) and drying-shirol stabilization machines (DSSM) of continuous actions. During drying, it is not allowed to extract the fabric and shrink it in width.
The diagram of the laboratory installation is shown in Fig. 1. The stand consists of a frame 1, which is equipped with a fabric broaching system, a heating system, an oscillatory system for the effect of ultrasonic (ultrasonic) frequency vibrations on the fabric, a tissue mass measurement system and a tissue irrigation system.
The broaching system provides rewinding of the fabric from one bobbin to another with the possibility of adjusting the tension force of the fabric. Adjusting the pulling force allows you to change the force of pressing the fabric to the oscillating working tool 6 ultrasonic oscillatory system (USOS) 5. The fabric broaching system consists of two bobbins for winding the fabric 2, two broaching rollers 3 driven by gear motors, two spring-loaded rollers 4 designed to press the stretched fabric to the broaching rollers. To regulate the tension of the fabric, friction brakes are installed on the shaft of the broaching rollers. The bobbins are driven by a belt drive from the pulleys' broaching rollers mounted on the axes of the broaching rollers. Each electric drive in the form of a gear motor mounted on the shaft of each broaching roller has an electronic system for regulating and maintaining a constant broaching speed.
The heating system of the supplied air consists of a fan unit and an electric heater installed in series. The heated air is supplied through the flue to the area formed by the USOS bracket 9, frame and drying cloth. The outgoing humidified air exits down through the gaps between the frame and the dried fabric.
For the contact effect of the ultrasonic frequency oscillation on the fabric, the ultrasonic oscillating system is installed on the frame using a bracket. The working tool is positioned in such a way that the fabric passing between the blocks of the broaching/clamping rollers wraps around the end of the working tool. The stabilization system ensures the constancy of the oscillation amplitude of the working tool when external influences change (changes in the clamping force or the area of contact of the material with the ultrasonic tool).
To determine the moisture content in the fabric, the stand includes a system for measuring the mass of the drying object based on strain gauges mounted on the bearing units of the bobbins. The mass of the fabric can be measured only immediately before the start of the movement (broaching) of the dried material from one bobbin to another and back.
To moisten the tissue during the research, a tissue irrigation system consisting of two fan nozzles is provided 10, pumps for pumping water, water tank and power supply.
The experimental stand has the following characteristics: The main technical characteristics of the ultrasonic device are presented in Table 1 [4]. At the first stage of experimental studies, dense woolen fabric was dried under ultrasonic (ultrasonic), thermal (T) and combined (ultrasonic + T) influences. The air temperature under thermal and combined effects was 50 оС. The air flow rate at the inlet is 8.5 m/s. At the second stage of the study, the fabric was dried under thermal (T) and combined (US + T) influences, the temperature of the drying agent was increased to 80 оС. The flow rate of the drying agent at the inlet was 6 m/s. When conducting experimental studies, the accuracy and reproducibility of measurement results were ensured by the use of verified measuring instruments with a low instrumental error. During the drying process, continuous measurement of the mass of the dried material was provided by weighing the bobbins using strain gauges equipped with an analog-to-digital converter (ADC) and a data transmission system to computer. The error of mass measurement for the "load cell + ADC" node, declared by the manufacturer, is 0.2%. Sensitivity -0.5 g.
To increase accuracy and reduce methodological and instrumental errors, the data obtained were averaged on a computer using the sliding window method.
The temperature of the drying agent was maintained by controlling the heater power. For this purpose, a Proportional-integral-differential regulator with a temperature sensor installed at the outlet of the heating installation duct was used. The error stated by the manufacturer is 0.5%.
The flow rate of the drying agent was controlled by measuring the flow rate at the outlet of the duct of the heating unit using an anemometer UNI-T UT363S. The flow rate of the drying agent was manually regulated by means of a slide valve installed at the inlet of the heating unit.
Pre-conducted trial (3-fold drying of the material for 20 minutes) experiments showed high reproducibility and convergence of the results obtained. The discrepancy between the results of weighing the dried material in the trial experiments did not exceed 5%. This made it possible to conduct targeted experiments on drying the material to equilibrium humidity once. Figure 2 shows experimentally obtained kinetics curves of drying dense woolen fabric at different temperatures, with air blowing at different speeds V of the drying agent, with ultrasonic action and without intensifying action. Analysis of experimental kinetics curves of drying dense woolen fabric with various types of energy exposure shows that the most effective for this fabric is a combined effect (ultrasonic and thermal) at the temperature of the drying agent (air) 80 o C and the speed of the drying agent is 6 m/s. It is also established that the effect of ultrasound on the kinetics of the process is more effective at lower air temperatures.

-US
In industrial conditions, at textile fine-cloth factories, drying of dense woolen fabric is usually carried out in DSM and DSSM at a temperature of t = 110-145 о С, however, there are examples of the process at t = 50 о С with problems with heating the coolant.
The use of continuous ultrasonic intensification on these dryers will reduce the temperature of the drying process to 80 °C, shorten its duration, which will lead to more economical energy consumption, reduce thermal emissions in the work area and into the environment, which will increase environmental and industrial safety.

Processing of experimental data
As a result of approximation of experimental data on the kinetics of drying of a typical dense woolen fabric, the equation is obtained: where: U0 -is the initial moisture content of the fabric, kg/kg;  -time in minutes; b -is the coefficient characterizing the speed of the drying process: For all curves of drying kinetics, the sum of squares of deviations of experimental and calculated data does not exceed 5.5%. Figure 3 shows the curves of drying kinetics calculated using equation (1) at different temperatures and in the presence or absence of ultrasonic effects. Experimental values are also shown here.  The results shown in Fig.3 illustrate a fairly good approximation quality of the family of kinetic curves obtained in this study.
The obtained results complement the data of other studies related to the study, generalization and calculation of drying kinetics, including convective drying of woven materials [9][10].

Conclusions
The analysis and kinetic calculation of convective drying processes of flat textile materials requires the accumulation and systematization of data on the kinetics of the process under various conditions, this is especially important for continuous drying, intensified by physical fields.
Experimental studies of the kinetics of the convective drying process of a typical dense woolen fabric on a specially created installation under various temperature conditions, including with ultrasonic intensification, have been carried out.
It is established that the most effective for the studied object is a combined effect. The effect of ultrasonic intensification on the kinetics of the process is more effective at lower temperatures of the drying agent.
Experimental data on the kinetics of drying a typical dense woolen fabric are analyzed and approximated. The obtained results can be used to analyze and calculate the kinetics of convective drying of dense woolen fabrics, taking into account the experimentally substantiated dependence of the moisture content of the material on the temperature of the drying agent and the presence of intensifying ultrasonic action, to predict the kinetics of convective drying of dense woolen fabrics.