FR2772898A1 - METHOD FOR DETERMINING THE DEVELOPMENT OF DRYING IN A DRYING PROCESS, AND IMPLEMENTING MACHINE - Google Patents

Abstract

The method of the invention, applied to a dryer, essentially consists in measuring the relative humidity rate for two different flow rates of the drying air, and in establishing variation curves of the difference between these two flow rates for different loads of laundry, to deduce the drying rate of the laundry, regardless of the temperature of the drying air.

Description

The present invention relates to a method for determining

  the evolution of drying in a drying process, as well as a

  machine for implementing this process.

  Currently, in the field of tumble dryers and washing machines, the end of the drying process is in fact determined, either by measuring the change in the temperature of the drying air and / or its derivative at using a thermistor placed in the drying air circuit (this temperature increases, at a substantially constant air flow rate, lo during the end of drying of the laundry), either by measurement, using electrodes in contact with the linen, the resistivity of the linen (this increases

as it dries).

  The result of the thermistor measurement process depends on parameters that cannot be controlled or controlled. The measurement result, at constant laundry load (type and quantity of laundry), depends on the mains supply voltage (in general, its nominal value can be between 187 v and 254 v, and may vary during the time) and the precision of the components, in particular due to manufacturing tolerances, these components being those which can influence the drying process (the nominal power of the air heating device can vary by + 10%, the temperature of the limitation can vary by + 5 C, and the tolerance on the value of the thermistor can influence + 3 C on the drying dynamics). The measurement result, at constant environment (voltage, heating power and temperature of the drying air) depends on the load to be dried (mass and type of laundry). In addition, the laundry drying phases (temperature rise and temperature stabilization phase, drying stage phase, during which the temperature is practically stable, and end of drying phase) are difficult to differentiate, which does not allow not to anticipate the end of drying sufficiently in advance, and even less to effectively regulate the power

heating air drying.

  In the case of the resistivity measurement, the result depends, randomly, on the load to be dried. It is easier to reliably dry a large load than a light load, since in the latter case the result depends on the average position of the load relative to the measuring electrodes. This results in large variations in the measurement in

  depending on the mass and type of textiles to be dried.

  In addition to their poor reproducibility, these known methods require numerous statistical tests, leading to long development times, and therefore a high cost of this development. It should also be noted that the results of the measurements made according to these known methods become even more uncertain when external events such as the power cut, the opening of the machine door for the addition of laundry occur during drying. during drying, the change of the set point (in general: drying time) by the user or if the

machine structures.

  The subject of the present invention is a method for determining the evolution of drying during a drying process for various products, for which the quality of the determination does not depend or practically does not depend on parameters that cannot be controlled or controlled, in particular particular: the mains voltage, the dispersion of the values of the components or parameters which can influence the drying process (nominal power of the heating element, temperature thresholds in the event of operation close to the admissible limits by the components, temperature such as measured by the thermistor or dispersion of the measurement results of the electrodes), mass and type of laundry to be dried, power supply cut off, door opened by the user,

  ambient temperature and humidity ...

  The present invention also relates to a machine (dryer or washer-dryer) implementing this process, the cost price of which is not appreciably increased by this implementation,

  and that is simple to use and maintain.

  The method of the invention is characterized in that the relative humidity of this air is predetermined at at least one point in the air circuit for different quantities and / or qualities of products to be dried, which are measured , at the start of the drying process, the relative humidity of the drying air when the drying air ventilation stops, only when this relative humidity drops below a threshold which is a function of I ' maximum relative humidity, the last phase of the drying process is started during which the drying air is circulated alternately in the drying air circuit of the machine with at least two different air flows, a specific relationship is established, such as a difference between relative humidity values or the relative humidities themselves, for the two different flow rates, for different quantities of products to be dried, that the laws of evolution are memorized of these relationships, and that in normal service, with the same alternations of the same air flow rates, we deduce from the relationships established from the measurements, the humidity values

  relative carried out in the same way, the state of drying of the products.

  l0 Advantageously, the relative humidity measurements are made at

  using components called "humidistors".

  The invention will be better understood on reading the description

  detailed of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which: - Figure 1 is a diagram showing the evolution of a relation of difference in humidity values relative for different loads of laundry, according to the invention, and - Figure 2 is a block diagram of an example of a

  determination of the degree of drying, in accordance with the invention.

  The invention is described below with reference to a washer-

  dryer or dryer, hereinafter referred to as a machine, but it is understood that it can be used in various processes of

  drying using an air flow, this air can be more or less hot.

  In the machine implementing the method of the invention, the humidity levels of the drying air are measured using a component called "humidity". This component is in the form of a capacitor, the dielectric constant of which is modified as a function of the humidity of the ambient air. The capacity of this capacitor varies according to the relative humidity of the ambient air present between its electrodes. More generally, it is possible to use in place of the humidity any electrical component whose electrical response varies as a function of the humidity of the ambient air, such as for example resistive components whose resistance varies as a function of this humidity . It is understood that if a component other than a humidifier is used, its connection circuit to the processing circuit (microcontroller described below) controlling the drying machine is modified accordingly. To determine this relative humidity, insert the capacitor into the tuning circuit of an oscillator, and measure the frequency of this oscillator. This frequency is therefore a function of the capacitance of the capacitor, which itself is a function of the relative humidity of the air in which this capacitor is immersed. It is therefore easy to determine the relative humidity at all times by measuring

a frequency value.

  In the case of a machine such as a dryer, the relative humidity of the drying air can vary between 0 and 100%. The accuracy of 1 O relative humidity measurement is generally only necessary at the end of drying, when the gradient of decrease in relative humidity begins to increase slightly, after having remained substantially constant during the central part of the drying process, and that we want to avoid overdrying of the laundry and save energy in heating the drying air. In addition, the relative humidity measuring device must be able to withstand condensation without damaging or deviating from its characteristics. The humidity, used as described below,

meets these requirements.

  There is at least one such humidity in the drying air circuit of the machine, and it is connected to an oscillator followed by an appropriate operating circuit making it possible to control the drying circuit and

  offering the user the possibility of choosing a determined degree of drying.

  The realization of such an oscillator and such an operating circuit is obvious

  for the skilled person on reading this description, and will not

not described in detail.

  The invention is particularly intended for machines in which the drying air flow can either be interrupted periodically or take at least two different values. There are, for example, machines in which the ventilation turbine is asymmetrical and rotates alternately in each of the two directions. In one direction of rotation, the ventilation flow is high (approximately 150 m3 / h), and in the other direction of rotation, the flow is lower (approximately 40 m3 / h). At the beginning of the drying process, when the air flow is high, the water contained in the laundry is vaporized. However, when the air flow

  is weak or null, the vaporization of water is weaker.

  As the drying process proceeds, the difference in relative humidity for the two air flows decreases and tends to zero. In fact, at the start of drying a damp cloth, the water molecules permeate both the "skin" (peripheral part) of the fibers of the cloth and their core, and those of the "skin" are easily vaporized. Then, the water molecules permeating the heart migrate towards the periphery, but this movement of migration is relatively slow (its speed depends in particular on the nature of the fibers and the arrangement of the linen in the drum of the machine). It was found that at the end of drying, this i0 migration speed is low and substantially constant, whatever the drying air flow rate. The method of the invention takes advantage of this phenomenon by monitoring at least at the end of drying said difference in humidity for

  determine the end of the drying process.

  The diagram in FIG. 1 represents the evolution of the relative humidity rate RH for two different masses of wet linen as a function of time. The curves of FIG. 1 are recorded on a prototype or pre-series machine, then their remarkable values are memorized in memories such as ROMs installed on serial machines, to be exploited in the manner described below. These masses are for example 1 and 4 kg. The drying process comprises three phases

  noted respectively P0, P1 and P2.

  For all these phases of the laundry drying process, the RH relative humidity measurements are carried out with the stabilized stop of the drying air flow. Also, for all these phases, the air flow varies periodically. Each period T comprises a succession of four elementary times: [Dmax, A, Dmin, A], the meanings of which are as follows: Dmax, Dmin: maximum and minimum flow rate respectively, A: stopping the drying air ventilation. The values of Dmax and Dmin are different depending on the phases of the drying process. Thus, according to an example, for the phases P0 and P1, the values of Dmax and Dmin are respectively 140 m3 / h and 40 m3 / h and their respective durations within a period T are 31 seconds and 6 seconds, the downtime A being 2 seconds. For this same example, during phase P2, the values of the flow rates are the same, but their durations are 12 seconds for Dmax and 25 seconds for Dmin, the downtime A always being 2 seconds. Of course, the values cited for this example may all be different in other examples without departing from the scope of the invention.

  The phases P0, P1 and P2 are determined as follows.

  Phase P0 begins with the start of the drying process and ends when the mass of laundry and the parts of the machine heated by the flow of hot drying air have reached the maximum temperature that this hot air can give them. This end of P0 coincides with the instant when the relative humidity RH of the drying air reaches its maximum (HRo), RH being measured when the ventilation is stopped (for example after 25 seconds off, in order to be able to obtain a stabilized value of the relative humidity) between two successive periods T, T being as defined above. To simplify the implementation, one can, in series machines, take P0 of fixed duration equal to the largest value measured in the laboratory on pre-series machines. The frequency of measurement of HR can be variable according to the machines and according to the masses of linen. It usually takes just a few measurements to determine the end of P0, which occurs fairly quickly after the start of drying (usually after a few minutes). We can see that the HRO values are clearly different for the two loads of laundry

  taken as an example (these HRO values are denoted HRO - 1 and HRO-4.

  Then begins phase P1, during which we measure relative humidity values called HR. These values are measured between two periods T, also when the ventilation is stopped, for example after 25 seconds of stop. We note that HR is constant, then decreases appreciably linearly over time, and faster for

low laundry loads.

  This is due in particular to the fact that the drying air is more difficult to pass through a large quantity of laundry (occupying almost the entire interior volume of the machine drum) than in

  a small amount of laundry, and therefore takes less moisture.

  The end of phase P1 is determined as follows. We can for example take as a reference the instant when the drying rate of the laundry reaches 20% (the drying rate being defined as the ratio between the mass of water remaining in the laundry and the mass of laundry). Since this drying rate cannot be measured directly in a machine in service with the user, the corresponding relative humidity rate HR1 is determined in the laboratory, in an identical machine. This rate is noted x. HRO, with o <x <l, and this, for different quantities of wet laundry, HR1 being always measured as described above. For reasons of clarity, only two curves have been shown in FIG. 1, corresponding to wet laundry masses of 1 and 4 kg, but in reality, we proceed with

different other masses.

  Phase P2 begins as soon as the relative humidity drops below x. Corresponding HRO. During this entire phase, which ends theoretically when the laundry is completely dry, two different measurements are made for each load of laundry: a measurement of HR1 as during phase P1, and a measurement of HR2 during elementary times where the drying air flow is equal to Dmin. Measurements of HR1 and HR2 during phase P2 are carried out more often than measurements of HR1 i.5 during phase P1, because their values decrease very quickly. We note for different values of the difference (HR1 - HR2) the corresponding drying rate (or an equivalent indication such as laundry ready to be ironed, laundry completely dry, ...), because, as seen from after FIG. 1, this difference decreases as a function of time, and therefore as a function of the rate of drying of the laundry, and vanishes when the laundry is completely dry. It will be noted in this FIG. 1 that the curves relating to different loads of laundry have phases P1 and P2 of different durations and that the ends of the phases P2 are sufficiently distant from each other over time so that a

  processor can easily distinguish them.

  Consequently, it suffices to memorize in the memory of each serial machine the shapes of the different curves HR1 and HR2 (OR of the characteristic points of these curves, between which an interpolation can take place) for different loads of laundry, these different curves having have been recorded on a prototype or pre-series machine, in order to very easily determine the end of drying of the laundry. To this end, it is not necessary for the user to know the mass of linen, since the curves relating to different masses and / or qualities of linen are sufficiently differentiated from one another. It is therefore sufficient for the machine control device to carry out a few humidity rate measurements (by means of frequency measurements) at "characteristic" instants, that is to say at the start of drying, to determine HRO (when the humidity is at its maximum), then a little before the foreseeable end of P1 (ie shortly before going down to x. HRo), and then periodically during P2 , by regularly alternating the measurements of HR1 and HR2 until the difference (HR1 - HR2) reaches the desired value, which is a function of the drying rate (or a similar indication) displayed by the user. Note that the curves relating to a small load of laundry (a few pieces of laundry) and a maximum load of laundry (about 4 to 5 kg, for a standard machine) are sufficiently distant from each other so that the control circuit of the machine can, even before the end of PO, quickly determine what is the curve relating to any load of laundry to be dried, even if this load

  differs markedly in composition from that used to establish the curves.

  It is important to note that these measurements are practically independent of the temperature of the drying air (in particular during P2) and of the various aforementioned phenomena intervening in the drying process (mains voltage, reloading of laundry, drifts of the components, etc.). .). The humidity of an air flow being a diffuse phenomenon, and not concentrated in a small zone of the section of this flow, the positioning of the humidity does not need to be carried out in a precise place. It is therefore easy to place it in a place where it least risks

  fouling with fluff from the dried laundry.

  The method of the invention can be implemented both in machines for evacuating drying air and in condensing machines. Advantageously, the accuracy of determining the drying rate can be improved by determining the average type of the product to be dried and its drying characteristics (textile in the present case) for example by measuring the slope of the curves in FIG. 1. In Indeed, the nature of the textile fibers influences the speed of migration of the water molecules from their core towards the surface, therefore the speed of drying. In the extreme case of fibers absorbing practically no water, the evaporation of the surface retained water is rapid, and therefore the drying is rapid, which increases the slope of the corresponding curve. It then suffices to modulate the speed of

drying of these products.

  Another possibility of refining this precision is to associate with the humidity a temperature probe, which makes it possible to take into account the influence of the variations in temperature on the variations in relative humidity, and therefore to correct the latter. In addition, it is thus possible to

  determine the absolute humidity of the drying air.

  The circuit 1 shown diagrammatically in FIG. 2 comprises, as a device for processing sensor signals and a computing device, a digital microcontroller 2. A main capacitive sensor 3, of the "humidity" type, is connected to an oscillator 4 followed by a shaping circuit 5 delivering rectangular signals at variable frequency depending on the capacity of the sensor 3. The output of circuit 5 is connected to a digital input 6 of the microcontroller 2. The microcontroller 2 is also connected to a memory 8 in which values of drying rate are stored as a function of different values of relative humidity and / or values of differences in relative humidity for the air flows of the machine. If no humidity is available, a resistive humidity sensor (7) can be used instead. This alternative solution has been shown in broken lines in FIG. 3. This sensor 7 is connected to an analog input 6 ′ of the microcontroller 2. This input 6 ′ is connected, inside the

  microcontroller to an analog / digital converter (not shown).

  A temperature sensor 9, for example a thermistor, is connected to another analog input 10 of the microcontroller 2. The microcontroller 2 is programmed in an obvious manner for the person skilled in the art on reading the

  present description. The temperature sensor 9 makes it possible to correct, if

  if necessary, the information provided by the sensor 3. A drying air flow sensor 11 is connected to another input 12 of the microcontroller. This input 12 is an analog or digital input, depending on the type of sensor 11. In the simplest case, this sensor 11 is a tachometer integral with

  the drying air circuit turbine.

Claims (9)

  1. Method for determining the evolution of drying during a drying process implemented in a machine in which a stream of heated air is circulated, characterized in that it is predetermined at at least one point of the air circuit the relative humidity (RH) of this air for different quantities and / or qualities of products to be dried, which is measured, at the start of the drying process, the relative humidity of the air (RH ) drying when the drying air ventilation stops, that when this relative humidity drops below a threshold (x. HRO) which is a function of the maximum relative humidity (HRO), the last phase (P2) of the drying process during which there is alternately circulated, in the drying air circuit of the machine, drying air with at least two different flow rates, which is established at least a predetermined relationship between relative humidity values which are a function of the measurements es for the two different flow rates, for different quantities of product to be dried, that the laws of evolution of these relationships are memorized, and that in normal service, with the same alternations of the same air flow rates, we deduce relationships established from measurements of humidity values   relative, carried out in the same way, the state of drying of the products.   2. Method according to claim 1, characterized in that said determined relationship is a difference between relative humidity values for the two different bit rates.   3. Method according to claim 1 or 2, characterized in that the temperature of the drying air is further measured at at least one point of its circuit, in order to correct, if necessary, the humidity measurements relative.   4. Method according to one of the preceding claims,   characterized in that the slope of the law of evolution of the relative humidity is measured, in order to take account of the type of product to be dried and its drying characteristics.   l1 5. Machine for drying products by a current of drying air, characterized in that it comprises a microcontroller (2) to which are connected: at least one relative humidity sensor (3, 7), one sensor (1 ) drying air flow rate, and a memory (8) in which drying rate values are stored as a function of different relative humidity values and / or relative humidity difference values for the flow rates machine air (D1, D2).   6. Machine according to claim 5, characterized in that one at   o less of the relative humidity sensors (3) is of the "humidity" type.   7. Machine according to one of claims 5 or 6, characterized in   that at least one of the relative humidity sensors (7) is a resistive sensor. 8. Machine according to claim 5 to 7, characterized in that it   further comprises at least one drying air temperature sensor (9).   9. Machine according to one of claims 5 to 8, characterized in that   whether it is a dryer or washer-dryer.