Method and apparatus for indirectly determining the dew point of compressed air

Abstract

According to one embodiment, the invention comprises an apparatus for indirectly determining the dew point (103) of compressed air at a particular operating pressure, the apparatus comprising a capacitive sensor (508) for measuring relative humidity (104), a heating element (510) for heating and cooling a fraction of the compressed air, a controller for controlling the heating element (510) based on the measured relative humidity, and a temperature sensor (509) for determining the temperature of the fraction, the controller being further configured to control the heating element (510) such that the fraction is maintained at a predetermined constant relative humidity such that the dew point can be determined based on the temperature of the fraction.

Description

[Technical Field]

[0001] The present invention is in the field of determining the dew point of air, and more particularly of determining the dew point of compressed air produced by a compressor. [Background technology]

[0002] Compressed air is air that is produced by compressing air with a compressor. Compressed air is used for a variety of purposes, such as powering machines and tools, cleaning parts by spraying, inflating tires, etc. It is also used for medical purposes, such as breathing, or in the food industry.

[0003] Some applications impose strict quality requirements on the moisture content of compressed air (expressed as a function of the dew point). The dew point is the temperature at which the air can no longer hold water vapor and water droplets begin to form. In other words, the dew point of (unsaturated) air is the temperature to which it must be cooled, at the same vapor pressure, so that the air becomes saturated with water vapor and dew forms.

[0004] Dew point is measured to ensure that compressed air meets certain quality requirements. There are two methods known in the art for measuring dew point.

[0005] On the one hand, there is the so-called mirror method, in which a fraction of compressed air is cooled by a mirror, causing it to condense. The temperature of the mirror at the moment the condensation occurs corresponds to the dew point of the compressed air. This method is described in particular in US Pat. No. 19480035275.

[0006] Although this method is highly accurate, its first drawback is that impurities in the mirrors can adversely affect this accuracy. As a result, devices equipped with such mirrors must be periodically recalibrated by warming the mirrors, thoroughly cleaning them, or a combination of both. Another drawback is that the moment of condensation must be determined visually. This can be done automatically, for example, using laser technology, but this increases the complexity of the device. As a result, this method is not necessarily efficient from an economic point of view.

[0007] Alternatively, the dew point can be determined by measuring the relative humidity of the compressed air using a capacitance hygrometer, as disclosed in WO 2001 / 042776. Such a hygrometer includes a capacitance sensor with electrodes, and the impedance between the two electrodes is proportional to the relative humidity. The dew point can be determined based on the relative humidity and the temperature. This determination can be made based on a psychrometric chart, using a conversion formula, and / or using a look-up table.

[0008] Dew point can also be determined by a capacitive sensor based on the charge and discharge frequency of a capacitor, which is a measure of relative humidity.

[0009] This capacitive dew point measurement is relatively simple and inexpensive, but has the disadvantage of low accuracy. In addition, the accuracy decreases as the relative humidity decreases, in other words, the measurement error increases. However, since compressed air users often impose low relative humidity as a quality requirement, this method is not necessarily efficient from a technical point of view. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US No. 19480035275 [Patent Document 2] International Publication No. 2001 / 042776 Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, there is a need for an improved method for measuring the dew point of compressed air at low relative humidity. [Means for solving the problem]

[0012] SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for accurately measuring the dew point of compressed air and which overcomes certain of the above mentioned drawbacks.

[0013] This object is achieved according to a first aspect of the present invention by providing a method as set forth in claim 1.

[0014] The present invention includes a method for indirectly determining the dew point of compressed air at a particular operating pressure using a capacitive sensor configured to measure relative humidity, the method comprising iteratively repeating the steps of separating fractions of the compressed air, measuring the relative humidity of the fractions using a capacitive sensor, varying the temperature of the fractions to maintain a predetermined constant relative humidity, and measuring the temperature of the fractions, and the dew point is determined based on the temperature.

[0015] The separated air fraction is a representative fraction of the compressed air whose dew point needs to be determined. For example, the compressed air fraction is separated in a cavity or hollow space that is periodically passively replenished. In other words, by repeatedly repeating the following method steps, the fraction remains representative of the characteristics of the compressed air.

[0016] The next step is to measure the relative humidity of this fraction, which is therefore representative of the relative humidity of the compressed air itself. This measurement is performed by a capacitive sensor, as is known in the art. This measurement can therefore be performed in a fast and simple manner.

[0017] The next step of the present invention is to continuously change the temperature of the fraction, in other words, the fraction is heated or cooled depending on the desired result, as will be explained below.

[0018] The change in temperature, and therefore the desired result, is controlled by the relative humidity measurement. The purpose of changing the temperature is to maintain the relative humidity of the compartment at a constant value. In other words, the set point of the control device that controls the temperature change is a preset constant relative humidity, and the measurement comes from a capacitive sensor.

[0019] When the compressor operates in the steady state region, the quality requirements and therefore the properties that describe the compressed air in terms of, among other things, the dew point, converge to a constant value. The temperature at which the fraction is maintained will consequently converge to a constant value in terms of the controlled dynamic system.

[0020] Furthermore, it should be understood that this constant value may exhibit variations, as known to those skilled in the control arts, for managing dynamic systems based on measurements.

[0021] The temperature of the fraction is then measured, which serves as the basis for determining the dew point.

[0022] Determining the dew point based on temperature can be based on a conversion formula, a look-up table, or a psychrometric chart. In this way, the dew point is determined indirectly.

[0023] Note that cooling or heating a fraction changes the temperature but not the dew point. This can be illustrated by a psychrometric chart: if there is no addition or subtraction of moisture and the pressure is constant, the heating or cooling can be represented by a horizontal line on the chart, which corresponds to air with the same dew point.

[0024] The advantage of this method is that the dew point can be determined economically and efficiently while avoiding drawbacks such as poor measurement accuracy at low relative humidity levels due to the use of a capacitive sensor. For example, the preset constant relative humidity can be set to a value that corresponds to the sensor's minimum measurement error, i.e., maximum measurement accuracy. According to the capacitive sensor's technical documentation, this value is preferably equal to a relative humidity of 15% or higher.

[0025] The predetermined constant relative humidity can also be set according to the compressor's technical specifications, in other words, the minimum change in the temperature of the compartment that can be expected to be required to maintain the relative humidity at a constant value.

[0026] Changing the temperature of the compartment can be achieved using a Peltier element. A Peltier element, also known as an active thermoelectric element or Peltier-Seebeck element, is a heating element or electrical component that transfers heat from a colder area to a warmer area and / or vice versa. Thus, the temperature of the compartment can be changed, in other words, heated or cooled.

[0027] The Peltier element is then controlled by a controller whose setpoint is a predetermined constant relative humidity, such as a PID controller or other controller suitable for controlling the Peltier element and further suitable for maintaining the fraction at a constant relative humidity.

[0028] According to a second aspect of the present invention, there is provided an apparatus as claimed in claim 6.

[0029] The present invention further comprises an apparatus for indirectly determining the dew point of compressed air at a predetermined operating pressure, the apparatus comprising a capacitive sensor configured to measure relative humidity, a heating element configured to heat and cool a fraction of the compressed air, a controller configured to control the heating element based on the measured relative humidity, and a temperature sensor for determining the temperature of the fraction, the controller further configured to control the heating element such that the fraction is maintained at a predetermined constant relative humidity so that the dew point can be determined based on the temperature of the fraction.

[0030] According to one embodiment, the device further comprises a small cavity or hollow located on the outside of the sensor housing, through which the compressed air can contact the active part of the capacitive sensor.

[0031] Furthermore, according to one embodiment, the heating element can be in direct contact with the capacitive sensor. In other words, the heating element and the capacitive sensor form thermal contact such that not only is the fraction cooled and / or heated, but the sensor itself is also directly cooled and / or heated. As a result, thermal equilibrium between the fraction and the sensor can be achieved more quickly, and high measurement accuracy can be achieved more quickly.

[0032] The dew point can be determined by reading the temperature from the device and then processing this reading in an external computer. According to one embodiment, the device can further comprise a processing unit configured to calculate the dew point itself based on the temperature. In this way, a user can read the dew point directly from the device, or this direct calculation can further be used to control other devices, such as the compressor itself, to meet imposed quality requirements.

[0033] Additionally, the central processing unit may be configured to determine the dew point by a look-up table, which reduces the complexity of the central processing unit and, consequently, the energy required to perform the calculations.

[0034] The invention will now be further explained with reference to the drawings. [Brief explanation of the drawings]

[0035] [Figure 1] A psychrometric chart is shown. [Figure 2] 1 shows the dew point as a function of relative humidity for different air temperatures. [Figure 3] The technical specifications, including the measurement accuracy, of the two capacitive sensors are shown. [Figure 4] 4 shows the results of measurements performed by the device of the present invention versus measurements of a capacitive sensor. [Figure 5] 1 shows an embodiment of an apparatus of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] Figure 1 shows a psychrometric chart for a particular pressure. The state of the air at a particular pressure can be read on the psychrometric chart. In addition to pressure, this state includes the wet-bulb temperature, dry-bulb temperature, dew point, relative humidity, humidity ratio, specific enthalpy, and specific volume.

[0037] The dry bulb temperature can be read on the horizontal axis 101. The humidity ratio can be read on the vertical axis 100. The leftmost curve 103 represents the saturation curve. On this saturation curve, the wet bulb temperature and dew point always correspond to the dry bulb temperature. The other curve 104 represents the relative humidity. Additionally, line 107 represents the specific enthalpy. The wet bulb temperature can be read on the diagonal line 112.

[0038] Furthermore, the condition of a particular air fraction corresponds to a unique point on the psychrometric chart.

[0039] 1 further shows the air fraction whose condition is determined in terms of the above characteristics. The air fraction is assumed to have a humidity ratio corresponding to the value indicated by the arrow 102. The dew point is determined from this air fraction with humidity ratio 102. Note that the dew point can be read off from the saturation curve 103.

[0040] When there is a relatively high measurement error for both the relative humidity measurement and the temperature measurement, there is a zone 106 which corresponds to the measurable zone. For air fraction 102, it is possible to measure the dew point using points 105 and 109 as a margin. If the temperature of the air fraction is reduced, the relative humidity increases at the same time, so the measurement error of the sensor decreases, and the measurement zone becomes 108. As a result, the limits for determining the dew point correspond to points 110 and 111, which are smaller than the margin determined by points 105 and 109.

[0041] Furthermore, Figure 2 shows the dew point as a function of relative humidity for different air temperatures. The relative humidity RH is expressed as a percentage on the horizontal axis, and the dew point is expressed in degrees Celsius on the vertical axis. Furthermore, four curves are shown, one for 30°C, one for 20°C, one for 10°C, and one for 0°C. From this figure, it can be noted that in the low relative humidity region 201, the dew points converge with each other for different temperatures. Therefore, it can be concluded that low relative humidity levels are difficult to measure.

[0042] 3 shows the technical specifications, including the measurement accuracy, of two capacitive sensors 300 and 301. The dotted lines 303 and 305 show the maximum measurement error, and the solid lines 302 and 304 show the typical measurement error, both as a function of relative humidity. In other words, the horizontal axis shows the relative humidity RH (%), and the vertical axis shows the measurement deviation ΔRH (%RH) as a function of relative humidity.

[0043] 3, and more particularly for sensor 300, it can be seen that measurement error is large at very low relative humidity values. Below 10% RH, the typical measurement error increases from 2% to 4%, and the maximum measurement error increases from 4% to 8%. Note also that at higher relative humidity values, the measurement error also increases. For sensor 300, the typical measurement error increases from 2% to 4%, and the maximum measurement error increases from 4% to 8% at relative humidity levels above 90%. For sensor 301, the maximum measurement error increases from 2.5% to 4% at relative humidity levels above 90%.

[0044] The predetermined constant relative humidity value will therefore depend on the type of sensor and, more particularly, its technical specifications, including its measurement accuracy. The imposed compressed air quality requirements, compressor specifications, and sensor measurement accuracy shall be taken into account in setting this constant relative humidity value.

[0045] 4 shows the results of measurements taken with the device of the present invention and a mirror-based dew point sensor that directly measures relative humidity. Measurements were taken over several days. Graph 401 shows these measurements, with the solid black line representing the measurements taken with the device of the present invention and the gray line representing the measurements taken with a mirror-based dew point sensor that directly measures dew point.

[0046] Additionally, graph 400 shows the difference between both measurements. Note that the average lies between zero deviation and minus 2°C.

[0047] 5 shows one embodiment of the device of the present invention. The device comprises a powered CAN interface 500 for external communication. The CAN interface 500 is threaded and further comprises a hex nut 501. Furthermore, the threads and nut 501 are suitable for externally connecting the device to other devices via the CAN interface 500.

[0048] The device further comprises a control board 502 and a transformer 503 for converting the voltage from the CAN interface 500 into a voltage suitable for the sensor and its control unit, as well as the Peltier element.

[0049] Portion 513 of the device according to this embodiment includes a capacitance sensor 508, a temperature sensor 509, a Peltier element 510, a controller 512 for the Peltier element 510, electrical connections 507 between the controller 512 and sensors 508, 509 and the Peltier element 510, and cooling fins 506 for cooling the power supply that controls the Peltier element 510 when large power is indicated.

[0050] Furthermore, the device comprises an airtight connection 504 between the control board 502 and part 513 of the device, which allows measurements to be made without being affected by the ambient air. Furthermore, the device comprises a screw thread 505 for firmly attaching part 513.

[0051] Additionally, the sensors 508, 509 are mounted on a holder 514, which is in direct contact with the Peltier element 510 via ribs 511 that form part of the Peltier element 510. The material of the holder 514 and ribs 511 preferably has high thermal conductivity, allowing the Peltier element 510 to efficiently and quickly cool or heat the sensors 508, 509.

[0052] Additionally, the device may include a processing unit 515 that calculates the dew point, which may be based on a pre-programmed look-up table, in which case very little computing power is required since the only variable, temperature, uniquely corresponds to the dew point.

[0053] For increased accuracy, the processing unit 515 may be further configured to calculate the dew point by the following conversion formula: X = 1 - (0.01 x RH) K=-(14,55+0,114×T c )×X-((2.5+0.007×T c )×X) 3 -(15,9+0,117×T c )×X 14 T d =(K×1,8)+32 where RH is the relative humidity, T c is the measured temperature, T d is the calculated dew point. For the RH value, a constant preset relative humidity can be selected, or measurements from a sensor can be used for greater accuracy.

[0054] The invention is not limited to the exemplary embodiments described and shown, and the method and device according to the invention can be realized in any kind of shape and size without departing from the scope of the invention. [Explanation of symbols]

[0055] 103 Dew point 104 Relative Humidity 508 Capacitive Sensor 509 Temperature Sensor 510 heating element

Claims

1. 1. A method for indirectly determining the dew point (103) of compressed air at a particular operating pressure using a capacitive sensor (508, 509) configured to measure relative humidity (104), the method comprising: separating a fraction of the compressed air; measuring the relative humidity (104) of the fraction using the capacitance sensor; Varying the temperature (101) of said fraction so as to maintain a predetermined constant relative humidity; measuring the temperature (101) of the fraction; Repeatedly repeat The method of claim 1, wherein the dew point (103) is determined based on the temperature (101).

2. The method of claim 1, wherein the step of varying the temperature (101) is performed by a Peltier element (510).

3. The method of claim 2, wherein the Peltier element (510) is controlled by a PID controller.

4. The method of any of claims 1 to 3, wherein the predetermined constant relative humidity comprises a value corresponding to a standard measurement error (302, 304) of the capacitive sensor (508).

5. The method of claim 1 , wherein the predetermined constant relative humidity comprises a value of at least 15%.

6. 1. An apparatus for indirectly determining the dew point (103) of compressed air at a specific operating pressure, comprising: a capacitive sensor (508) configured to measure relative humidity (104); a heating element (510) configured to heat and cool a fraction of said compressed air; a controller configured to control the heating element (510) based on the measured relative humidity; a temperature sensor (509) for determining the temperature of said fraction; Equipped with The controller is further configured to control the heating element (510) such that the fraction is maintained at a predetermined constant relative humidity such that the dew point can be determined based on the temperature of the fraction.

7. The apparatus of claim 6 , wherein the capacitive sensor (508) comprises a cavity for separating the fractions.

8. 8. The apparatus of claim 6 or 7, wherein the heating element (510) is in direct contact with the capacitive sensor (508).

9. The apparatus of claim 6 , wherein the heating element (510) comprises a Peltier element.

10. The apparatus of claim 6, further comprising a central processing unit (515) configured to calculate the dew point based on the temperature.

11. The apparatus of claim 10, wherein the central processing unit (515) is further configured to determine the dew point by a look-up table.

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