Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B21/00—Arrangements or duct systems, e.g. in combination with pallet boxes, for supplying and controlling air or gases for drying solid materials or objects
  • F26B21/06—Controlling, e.g. regulating, parameters of gas supply

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the fibres cannot contaminate the meter, either.
• the measurement and control of the humidity of drying air can be effected automatically by applying a method of the invention.
• 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
• 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.
• 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.
• 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.
• 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
• 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.
• a suitable control element 16 The above-described apparatus can be designed by using normal instrumentation and automatization techniques.
• the exemplified drying machine is an 18 m long Artos convection drying machine with six sections and the material to be dried is a
• the amount of water evaporated by the machine can be calculated with, a formula (2).
• m H 2 O w N P Lev (AK -JK) (2)
• JK residual moisture of cloth, %/100
• 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.
• the measurements can be used to calculate the peak output of a machine.
• the running speeds can be used to calculate in which time the extra cloth moisture evaporates.
• Drying time in case a (t a ) is:
• the machine length (1 LK ) required for the evaporation of extra moisture is:
• measuring of the wet bulb temperature is effected by placing a surface temperature meter in this area.
• the machine length (l L ) needed for heating is:
• the theoretically calculated heating length is 1.2 m.
• a cloth warms up more slowly because of the simultaneous evaporation.
• (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 dr ⁇ 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.
• the drying effect is known (28.3 kg/m 2 h)
• the position of point B can be calculated.
• point B is thus located at the outset of third section (the length of each section 3 m).
• 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.
• 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.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Treatment Of Fiber Materials (AREA)

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• 1986
  • 1986-02-24 FI FI860784A patent/FI79198C/fi not_active IP Right Cessation
• 1987
  • 1987-02-23 WO PCT/FI1987/000028 patent/WO1987004949A1/en unknown
  • 1987-02-23 EP EP19870901495 patent/EP0259413A1/en not_active Withdrawn

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