EP0259413A1

EP0259413A1 - Method of and apparatus for measuring and controlling the humidity of drying air in a textile material drying machine

Info

Publication number: EP0259413A1
Application number: EP19870901495
Authority: EP
Inventors: Ralf ÖSTERLUND
Original assignee: Valtion Teknillinen Tutkimuskeskus
Current assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1986-02-24
Filing date: 1987-02-23
Publication date: 1988-03-16
Also published as: FI79198B; FI860784A; FI79198C; FI860784A0; WO1987004949A1

Abstract

Un procédé de mesure et de régulation de l'humidité de l'air de séchage dans une machine de séchage de matière textile (1) ayant la forme d'un voile se déplaçant consiste à mesurer d'abord à l'aide d'un thermomètre à bulbe sec puis à l'aide d'un thermomètre à bulbe mouillé la température de l'air de séchage, afin de déterminer l'humidité de l'air de séchage sur la base des valeurs mesurées et de commander, sur la base de l'humidité mesurée, un actuateur (7), tel que le ventilateur d'air de sortie ou la soupape d'air de sortie d'une machine de séchage, influant sur l'humidité de l'air de séchage. La température de l'air de séchage, prise à l'aide d'un thermomètre à bulbe mouillé, est mesurée en mesurant la température de surface de la matière textile (1) au moyen d'un appareil de mesure (8) de la température de surface, tel qu'un pyromètre de rayonnement, dans la machine à un point où la température de surface de ladite matière est égale à la température de l'air de séchage mesurée à l'aide d'un thermomètre à bulbe mouillé. La température prise à l'aide d'un thermomètre à bulbe sec est mesurée au moyen d'un capteur de mesure (9) monté en connexion avec un injecteur d'air de séchage (3). La mesure est de préférence effectuée à un point dont la distance calculée à partir de l'extrémité d'entrée d'une machine est le quart de la longueur totale de la machine.A method of measuring and regulating the humidity of the drying air in a textile material drying machine (1) in the form of a moving veil consists in first measuring with a dry bulb thermometer then using a wet bulb thermometer the temperature of the drying air, in order to determine the humidity of the drying air on the basis of the measured values and to control, on the basis of the measured humidity, an actuator (7), such as the outlet air fan or the outlet air valve of a drying machine, influencing the humidity of the drying air. The temperature of the drying air, taken using a wet bulb thermometer, is measured by measuring the surface temperature of the textile material (1) by means of a measuring device (8) of the surface temperature, such as a radiation pyrometer, in the machine at a point where the surface temperature of said material is equal to the temperature of the drying air measured using a wet bulb thermometer. The temperature taken using a dry bulb thermometer is measured by means of a measurement sensor (9) mounted in connection with a drying air injector (3). The measurement is preferably carried out at a point whose distance calculated from the entry end of a machine is a quarter of the total length of the machine.

Description

Method of and apparatus for measuring and controlling the humidity of drying air in a textile material drying machine.

The present invention relates to a method of measuring and controlling the humidity of drying air in a textile material drying machine, which method includes measuring the dry bulb temperature and wet bulb temperature of drying air, determining the humidity of drying air on the basis of the measured readings, and controlling on the basis of the obtained humidity reading an actuator having effect on the humidity of drying air.

The invention relates also to an apparatus for carry- ing out the above method.

In the textile industry drying processes, it is important to be capable of continuously measuring the humidity of drying air. The measuring result can be used to control the flow rate of drying air for obtaining a desired humidity reading. The proper humidity content of drying is significant for obtaining preferable drying conditions as well as in terms of both the drying rate and the energy costs of drying.

When using tentering and drying machines or so-called stenters in textile industry, the drying of a material therein is effected on a convenction principle by directing drying air from nozzles into contact with a textile material to be dried. This drying air attains inside the machine a certain humidity which is dependent e.g. upon the flow rate of drying air. This humidity can be measured by a variety of methods, based on measuring various humidity-related properties of drying air. In addition to measuring the humidity-related properties of drying air, a hygrometer can also be based on the dependence of one property of a sensor upon humidity, e.g. the electrical properties of an LiCl-crystal or the capacitive properties of a sensor. Sensors of the presently available machines are fitted either inside or outside the machine. In the former case, a problem is the contamination of sensors due to textile fibres loosening from a material to be dried and due to evaporating chemicals and, in the latter case, a gas sample must be delivered out from inside the machine, which makes the measuring complicated.

In a psychrometric measuring method, the dry bulb temperature and wet bulb temperature of drying air is measured and the humidity of drying air can be calculated on the basis of these readings by means of an h,x-diagram. Because of the wet bulb temperature measuring arrangements this method requires sampling from inside the machine. A gas sample is delivered from the machine along a copper pipe into a measuring chamber which houses a wet bulb thermometer. Thus, the measuring is effected continuously and automatically in a manner that a moistened tubular wick passes continuously over the measuring head of a thermometer, whereby the wet bulb temperature reading can be read out of a scale. After it has passed by the measuring head, a blade cuts the wick open. Measuring of wet bulb temperature with this method is inconvenient since it requires complicated arrangements and the sampling pipe is easily blocked by the fibres.

The present invention is based on the discovery that, in a psychrometric measuring method, the wet bulb temperature of drying air can be measured by measuring the surface temperature of a textile material in a drying machine at a suitable spot. For example, in a continuous-action drying machine comprising 6 to 8 successive drying sections, the surface temperature of a textile material sets at the wet bulb temperature of drying in the second section of a machine and it can be measured by using a surface thermometer. The measurement can be effected in a non-contact fashion, e.g. in a manner that the meter is spaced from a material to be measured and this can be done by using a radiation pyrometer, whereby the entire measurement can be effected from outside the machine. In a method of the invention, the drying machine comprises in a way a large psychrometer with the actual textile material to be dried corresponding to a wick used in psychrometers.

A significant improvement gained by the method is that sampling is no longer needed and measurement of the wet bulb temperature becomes substantially more simple and cleaner since a wick conveyor mechanism, moistening and cutting open of a wick can now be eliminated. When using a radiation pyrometer, the fibres cannot contaminate the meter, either.

According to one preferred embodiment, the measurement and control of the humidity of drying air can be effected automatically by applying a method of the invention.

The invention will now be described in more detail with reference made to the accompanying drawings, in which figs. 1 and 2 illustrate the use of a method, and apparatus of the invention in a continuous-action textile material tentering and drying machine,

fig. 3 shows a typical temperature profile appearing in connection with a drying process in a continuous-action drying machine,

fig. 4 shows a mathematically obtainable drying action curve for selecting a suitable measuring spot lengthwise of the machine and

fig. 5 illustrates the operating principle of an apparatus of the invention.

Fig. 1 shows in a cross-section perpendicular to the travelling direction of a textile web a textile material tentering and drying machine, fitted with equipment for measuring the dry and wet bulb temperature of humid air. A textile material 1 to be dried is advanced and guided by chain conveyors 2. Drying air is directed into contact with the material from nozzles 3, aligned with each other above and below said material. A circulating fan 4 is used to circulate drying air in the machine as indicated by the arrows from the nozzle side through a nozzle 12 to the intake side of said circulating fan and through the fan to nozzles and air is heated directly by means of a burner 5. The air can enter the machine through slots or a replacement air opening in the intake side of a circulating fan and it leaves the machine through an outlet 6 and an air-flow controlling actuator 7, which can be an adjustable blower or an exhaust air valve whose position can be adjusted.

The machine is fitted with a meter 8 for the surface temperature of textile material 1, in the illustrated example a radiation pyrometer which measures IR-radiation and is mounted on top of the machine for unhindered measurement of the surface temperature of a textile advancing below. Contamination of the meter is prevented by means of a fan 17 creating an air lock. For measuring the dry bulb temperature of drying air, nozzle 3 is accompanied with a normal temperature measuring sensor 9, which can be any sensor used in this type of temperature measurements. In the process, the aim is to maintain the dry bulb temperature of drying air constant and this is achieved by means of prior known regulation systems, not described in detail in this context.

Fig. 2 shows schematically and in plan view a textile material tentering and drying machine as well as thewet bulb and dry bulb temperature measuring equipment mounted thereon. The travelling direction of a textile web is indicated in the figure by an arrow. The machine includes drying sections 10 located immediately in succession relative to the travelling direction of textile web 1, each section being provided with two pairs of nozzles, each comprising nozzles 3 aligned with each other and positioned above and below the web. Each drying section is fitted with its own burner and circulating air fan. Ventilation of drying air occurs lengthwise of the machine rather freely between the sections and, for example, a machine with six sections may have two outlets, one between the second and third section and the other between the fourth and fifth section, or each section is provided with its own outlet. A radiation pyrometer 8 is mounted on top of the machine above a textile material between the successive pairs of nozzles of the same section, whereby it is capable of unhindered measure ment of the surface temperature of textile material 1.

Fig. 3 illustrates a typical temperature profile appearing lengthwise of a machine in connection with a textile material drying process. Temperature Ts represents the surface temperature of a textile material in different sections of a continuous-action machine. The representation indicates that temperature first rises steeply at the forward end of a machine until the rising levels itself in a section where the material surface temperature Ts is equal to the drying air wet bulb temperature Tw. There is a slight increase in the surface temperature even in this section since the humidity, and thus the wet bulb temperature, of drying air increases slightly lengthwise of the machine. The length of a wet bulb temperature section in the machine depends e.g. on drying effect, the properties and velocity of a material in the machine, and the determination of this section will be explained in the example following hereinbelow. At the trailing end of a drying machine, following the wet bulb temperature section, temperature begins to rise towards the dry bulb temperature Td of drying air. The measurement of a textile material surface temperature must be effected in that section of a machine where the material surface temperature Ts is equal to the drying air wet bulb temperature Tw. In addition to calculative methods, this section can be determined by various measurements, e.g. by measuring the cooling of drying air between nozzle 3 and outlet 6 by means of temperature measuring sensors mounted thereon. It can be calculatively indicated that the best spot for measuring is at 0,25 x machine length, starting from the forward end of a machine. In a machine comprising, six to eight sections, the second drying section from the forward end of a machine is located within this area when running the process at values generally used in drying and the measurement is most preferably performed by installing the meter in this section. A more accurate measuring spot is determined on the basis of the construction of a machine.

Adjustment of the flow rate of exhaust air on the basis of the measuring results can be effected in a variety of ways. If each individual section of a drying machine is provided with its own drying air outlet, these can be compiled to form a single common outlet fitted with an actuator controlling the flow rate of exhaust air and, thus, the humidity of drying air. On the basis of the humidity reading obtained by means of the measuring results, this actuator is regulated and the flow rate of exhaust air can be set at a desired value. If, on the other hand, a machine is provided with two outlets, the measuring results can be used to control the actuator of each outlet at a desired mutual relationship if, for example, it is desired to have a different exhaust air flow rate in the forward and trailing end sections of a machine.

Fig. 5 shows diagrammatically the operation of an apparatus, mounted in connection with a drying machine and measuring the humidity of drying air with a method of the invention. The apparatus comprises a measuring sensor 9 for measuring the dry bulb temperature and a surface temperature meter 8 for measuring the wet bulb temperature. Measuring the dry and. wet bulb temperature is effected alternately and the messages from both measuring units are converted in a digitizer 11 into the form suitable for a computer, whereafter the central processing unit 13 of a computer calculates the humidity reading of drying air on the basis of the h,x-diagram data fed into it. The diagram can also be presented as a formula (1) that is highly suitable for computer processing

x = f(Tw, Td) = 7.571 e^0-053Tw -0,2 e^0.016Tw Td -0,523 (1)

where x = humidity of drying air, g/kg of dry air Tw = wet bulb temperature, ºC Td = dry bulb temperature, ºC

The value obtained in central processing unit 13 is compared in a reference unit 14 with a set value fed into it and, on the basis of the reference data, an actuator 7 acting on the humidity of drying air is controlled by means of a suitable control element 16. The above-described apparatus can be designed by using normal instrumentation and automatization techniques.

Example of determination of a measuring area in the longitudinal direction of a machine

Described hereinbelow is the determination of a suitable measuring area by calculating the drying effect curve lengthwise of a machine. The exemplified drying machine is an 18 m long Artos convection drying machine with six sections and the material to be dried is a

1,5 m wide cotton cloth. The total surface area of cloth in the machine is thus 27 m². A similar study can also be performed on other machines and textile materials. The humidity readings are reported as a water/cloth ratio.

The amount of water evaporated by the machine can be calculated with, a formula (2). ^mH₂O = w N_P Lev (AK -JK) (2)

where ^mH₂O = evaporated amount of water, kg/h w = rate, m/h

N_P = quadratic weight of cloth, kg/m² AK = initial moisture of cloth, %/100

JK = residual moisture of cloth, %/100

Lev = width of cloth, m

On the basis of runs performed with the drying machine on various parameters there is obtained a formula (3) to represent the dependence of the machine drying effect upon various factors

R_f = 16.8 - 18.2 ∆ _p 0.6 + 0.67 JK + 0.085 AK + 0.025 P (3)

where R_f = drying effect, kg H₂O/m² h ∆p = pressure loss in filter, %

JK = residual moisture, % AK = initial moisture, % P = position of outlet valve (0 - 100)

Drying effect varies lengthwise of a machine. Evaporation is slower in the first section as long as the cloth is warming up. Thereafter, the drying continues at a uniform rate as long as there is water to be evaporated at the surface of the cloth. Towards the end of drying, the surface temperature begins to rise again and the rate of drying decreases. Drying effect lengthwise of a machine is illustrated in fig. 4.

Measurements have indicated that the mean drying effect is dependent on initial moisture. By way of an example, the measurements can be used to calculate the peak output of a machine.

Example a b

Cloth width 1.5 m 1.5 m initial moisture 80 % 85 % residual moisture 5 % 5 % quadratic weight 0.12 kg/m² 0.12 kg/m² rate 57 m/min drying effect 17.1 kg/m² h

= 461.7 kg/h

On the basis of the measuring results, it is possible to calculate the drying effect and rate in example b. Since only the initial moisture is different, the drying effect is obtained according to formula (3):

R_f = [ 17.1 + (85 - 80) × 0.085] kg/m ² h

= 17.53 kg/m² h (3)

The running speed will be

The running speeds can be used to calculate in which time the extra cloth moisture evaporates.

Drying time in case a (t_a) is:

Drying time in case b (t_b) is:

The machine length (1_LK) required for the evaporation of extra moisture is:

The extra moisture in cloth in case b is entirely evaporated at maximum machine output (R_{f , MAX}).

When residual moisture in case b has dropped down to 80 %, the outset of case a has been reached. Thus, the maximum machine output R_f is obtained by a formula:

17.53 x 18 = 0.69 × R_{f, MAX} + (18 - 0.69) × 17.1

R_f, _MAX = 28.3 kg/m² h

Now, it is also necessary to calculate the position of points A and B in fig. 4. The area between said points is where the cloth surface temperature is at the wet bulb temperature, the drying thus proceeding with maximum output or effect. According to the invention, measuring of the wet bulb temperature is effected by placing a surface temperature meter in this area.

For this purpose it is necessary to calculate the amount of energy required for heating the cloth and water present in the cloth. If the cloth temperature in the first section rises from 20 ºC to 60 ºC, the energy consumption, will be: Heating of water:

^QH₂O = 492 kg/h x 4.2 kJ/kg ºC x 40 ºC = 82.6 MJ/h

Heating of cloth: Q_K = 616 kg/h × 1.3 kJ/kg ºC x 40ºC

= 32.0 MJ/h QH₂O + Q_K = 114.6 MJ/h

From water evaporation rate (28.3 kg/m² h) it is possible to calculate energy transfer from drying air to cloth. The evaporation energy of water at 60 ºC is 2.25 MJ/kg. Energy transfer (q) is thus

q = 28.3 kg/m² h × 2.25 MJ/kg = 63.7 MJ/m² h

The machine length (l_L) needed for heating is:

Thus, the theoretically calculated heating length is 1.2 m. In practice, a cloth warms up more slowly because of the simultaneous evaporation.

A point at which the drying effect begins to drop again

(B) depends on textile material and especially on the residual moisture at which the textile material is still at the wet bulb temperature . Comapred to artificial fibres, the drτ ing effect with natural fibres begins to drop at higher residual moisture, in this case, for example, the limit value of cotton is 40 % of the residual moisture. As the drying effect is known (28.3 kg/m² h), the position of point B can be calculated.

Thus, with initial moisture of 80 % and quadratic weight of 0.12 kg/m², it is necessary to evaporate 0.12 x

0..4 kg/m² = 0.048 kg/m² water from the cloth.

Thus, drying to residual moisture of 40 % (t₄₀) takes;

During this time, the cloth travels

0.102 x 57 m = 5.8 m

Since the cloth heating distance was 1.2 m, point B is thus located at the outset of third section (the length of each section 3 m).

The dependence of the position of points (A and B) upon the initial moisture is illustrated in table 1.

TABLE 1

60 % 80 % 90 %

To be evaporated 0.024 0.048 0.06 kg/m² ^t40 0.051 0.102 0.127 min Rf 15.4 17.1 17.95 kg/m² h w 70 57 52.8 m/min

A-B 3.56 5.8 6.7 m

^QH₂O 72.6 82.6 86.2 MJ/h Q_K 39.3. 32 29.6 MJ/h

C-A 1.17 1.2 1.21 m

C-B 4.7 7 7.9 m

Thus, when initial moisture is 60 %, the cloth dries down to residual moisture of 40 % over a distance of 4.7 m. It can be summarized from the tabulated results that a suitable distance of a measuring point from the forward end of a machine is 0.25 x total length.

The above description is by no means intended to limit the invention, but it can be modified within the scope of an inventive idea set forth in the annexed claims. The method can be carried out by using several generally known surface temperature meters, operating on both contact and non-contact principle. The invention can be applied in several drying processes in textile industry, e.g. in rotary driers in addition to tentering and drying machines. A textile material to dried, which is in the form of a moving web, refers in this context to a material in the form of both cloth and loose fibres, said material moving in a drying machine and following a certain pre-calculable temperature profile in the direction of movement. In all these processes, a surface temperature measuring point can be selected in a manner that, at the measuring point, a textile material is at the wet bulb temperature of drying air and the determination of a measuring point can be performed by using the calculative method set out in the preceding example or by using methods generally known to a skilled person. Neither is the invention limited to its application in drying processes only, but it can also be applied in connection with a combined drying and fixing process. A process can also involve the use of several measuring points for measuring humidity or moisture with a method of the invention.

Claims

1. A method of measuring and controlling the humidity of drying air in a drying machine for textile material

(1) in the form of a moving web, said method including the steps of measuring the dry bulb temperature and the wet bulb temperature of drying air, determining the humidity of drying air on the basis of measured values, and controlling on the basis of the obtained humidity reading an actuator (7), such as the outlet air fan or outlet air valve of a drying machine, acting on the humidity of drying air, c h a r a c t e r i z e d in that the wet bulb temperature of drying air is measured by measuring the surface temperature of textile material (1) in the machine at a point where the surface temperature of said material is equal to the wet bulb temperature of drying air.

2. A method as set forth in claim 1, c h a r a c t e r ¬i z e d in that the surface temperature is measured by means of a contact thermometer which is in contact with the surface of textile material.

3. A method as set forth in claim 1, c h a r a c t e r i z e d in that the surface temperature is measured by means of a non-contact surface temperature meter.

4. A method as set forth in claim 3, c h a r a c t e r i z e d in that said surface temperature meter (8) is an IR-radiation measuring device, such as a radiation pyrometer.

5. A method as set forth in claim 3, c h a r a c t e r i z e d in that said, surface temperature meter (8) is based on the measurement of heat transfer between the surface of textile material and the. sensor head of a meter at a constant temperature.

6. A method as. set forth in claim 1, 2, 3, 4 or 5, c h a r a c t e r i z e d in that the measurement of surface temperature is effected at a point whose distance from the forward end of a machine in the longitudinal direction of a machine is circa 0.25 x the machine length.

7. An apparatus fitted in connection with a drying machine for carrying out the method of claim 1, c h a r a c t e r i z e d in that said apparatus comprises

- a dry bulb temperature measuring sensor (9),

a surface temperature meter (8) for measuring the wet bulb temperature,

- a digitizer (11) for converting the measuring data into digital form,

a unit (13) for calculating a humidity reading on the basis of the measuring data,

- a reference unit (14) for comparing the obtained humidity reading with a set value (15), - actuating means (16) for controlling an actuator (7) acting on the humidity of drying air on the basis of reference data.

8. An apparatus as set forth in claim 7, c h a r a c t e r i z e d in that said surface temperature meter (8) is a non-contact, surface temperature meter, such as an IR-radiation measuring radiation pyrometer.

9. An apparatus, as. set forth in claim 7 or 8, c h a r a c t e r i z e d in that, said dry bulb temperature measuring sensor (9) is mounted in connection with a drying air inlet fan (3).