METHOD AND APPARATUS FOR INDIRECT DETERMINATION OF THE DEW POINT OF COMPRESSED AIR

Abstract

An apparatus for indirectly determining the dew point of compressed air at a particulate operating pressure including a capacitive sensor for measuring a relative humidity, a heating element for both heating and cooling a fraction of the compressed air, a controller for controlling the heating element based on a measured relative humidity, a temperature sensor for determining the temperature of the fraction. The controller is further configured to control the heating element 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.

Claims

1-11. (canceled)

12. A method for indirectly determining the dew point of compressed air at a particulate operating pressure using a capacitive sensor configured to measure a relative humidity, the method comprising iteratively repeating the steps of: separating a fraction of the compressed air; measuring the relative humidity of the fraction using the capacitive sensor; changing the temperature of the fraction such that it is maintained at a predetermined constant relative humidity; measuring the temperature of the fraction; and wherein the dew point is determined based on the temperature.

13. The method according to claim 12, wherein changing of the temperature is done by means of a Peltier element.

14. The method according to claim 13, wherein the Peltier element is controlled by means of a PID controller.

15. The method according to claim 12, wherein the predetermined constant relative humidity comprises a value corresponding to a lowest measurement error of the capacitive sensor.

16. The method according to claim 12, wherein the predetermined constant relative humidity comprises at least a value of 15%, preferably 15%.

17. An apparatus for indirectly determining the dew point of compressed air at a particulate operating pressure, comprising: a capacitive sensor configured to measure a relative humidity; a heating element configured to both heat and cool a fraction of the compressed air; a controller configured for controlling the heating element based on a measured relative humidity; a temperature sensor for determining the temperature of the fraction; and wherein the controller is further configured to control the heating element 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.

18. The apparatus according to claim 17, wherein the capacitive sensor comprises a cavity for isolating the fraction.

19. The apparatus according to claim 17, wherein the heating element is in direct contact with the capacitive sensor.

20. The apparatus according to claim 17, wherein the heating element comprises a Peltier element.

21. The apparatus according to claim 17, further comprising a central processing unit configured to calculate the dew point based on the temperature.

22. The apparatus according to claim 21, wherein the central processing unit is further configured to determine the dew point by means of a look-up table.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will now be further described with reference to the drawings in which:

[0034] FIG. 1 illustrates a psychrometric diagram;

[0035] FIG. 2 illustrates the dew point as function of the relative humidity with different air temperatures as a parameter;

[0036] FIG. 3 illustrates technical specifications comprising the measurement accuracy of two capacitive sensors;

[0037] FIG. 4 illustrates measurement results of measurements performed by the apparatus of the invention versus a capacitive sensor; and

[0038] FIG. 5 illustrates an embodiment of the apparatus of the invention.

DESCRIPTION OF EMBODIMENTS

[0039] FIG. 1 illustrates a psychrometric diagram at a particular pressure. The state of air at a certain pressure can be read on a psychrometric diagram. This condition comprises, in addition to pressure, wet bulb temperature, dry bulb temperature, dew point, relative humidity, humidity ratio, specific enthalpy, and specific volume.

[0040] 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 curves 104 illustrate relative humidity. Furthermore, the lines 107 illustrate the specific enthalpy. The wet bulb temperature can be read on the oblique lines 112.

[0041] Furthermore, the state of a particular fraction of air corresponds to a unique point in the psychrometric diagram.

[0042] Furthermore, in FIG. 1, an illustration is made of a fraction of air whose state is to be determined in terms of the above characteristics. It is assumed that the fraction of air has a humidity ratio corresponding to the value indicated by the arrow 102. It is from this fraction of air with humidity ratio 102 that the dew point is to be determined. Note that the dew point can be read on the saturation curve 103.

[0043] If there is a relatively high measurement error for both the measurement of the relative humidity and the temperature, a zone 106 exists that corresponds to a possible measurement zone. For the fraction of air 102, it is then possible to measure a dew point using the points 105 and 109 as margin. If the temperature of the fraction is lowered, thus simultaneously reducing the measurement error of the sensor because the relative humidity increases, the measurement zone is 108. The limits of determining the dew point then correspond to points 110 and 111, which is a smaller margin than the one determined by points 105 and 109.

[0044] Furthermore, FIG. 2 illustrates the dew point as function of the relative humidity with different air temperatures as a parameter. 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 illustrated with temperatures of thirty, twenty, ten, and zero degrees Celsius, respectively. It can be noted from this figure that in the area 201, hence with a low relative humidity, the dew points converge towards each other for the different temperatures. It can therefore be concluded that low relative humidity levels are difficult to measure.

[0045] FIG. 3 illustrates technical specifications comprising the measurement accuracy of two capacitive sensors 300 and 301. The dotted lines 303 and 305 illustrate the maximum measurement error and the solid lines 302 and 304 the standard measurement error, both as function of the relative humidity. In other words, on the horizontal axis, the relative humidity RH (%) is illustrated and on the vertical axis, the measurement deviation RH (% RH) as function of the relative humidity is illustrated.

[0046] In the illustration of FIG. 3, and more particularly that of sensor 300, it can be noted that at very low values of relative humidity, the measurement error is large. Below the value of 10% RH, the standard measurement error increases from 2% to 4%, and the maximum measurement error increases from 4% to 8%. It should also be noted that for high values of the relative humidity, the measurement error also increases. At a relative humidity from a value of 90% at sensor 300, the standard and maximum measurement error also increase from 2% to 4%, and from 4% to 8%, respectively. At sensor 301, the maximum measurement error increases from 2.5% to 4% at a relative humidity greater than 90%.

[0047] The value of the predetermined constant relative humidity will thus depend on the type of sensor, and more particularly on the technical specifications comprising the measurement accuracy. The imposed quality requirements of the compressed air, the specifications of the compressor, and the measuring accuracy of the sensor shall be taken into account in order to set this value of constant relative humidity.

[0048] Furthermore, FIG. 4 illustrates measurement results of measurements performed by the apparatus of the invention versus measurements performed by a mirror dew point sensor that directly measures relative humidity. The measurements were carried out over a period of several days. Graph 401 illustrates these measurements, in which the solid black line illustrates the measurements from the apparatus of the invention, and the grey line illustrates measurements from a mirror dew point sensor with direct measurement of the dew point.

[0049] Furthermore, graph 400 illustrates the difference between both measurements. It should be noted that the average is located between a deviation of zero and minus two degrees Celsius.

[0050] FIG. 5 illustrates an embodiment of the apparatus of the invention. The apparatus comprises a CAN interface with power supply 500 for external communication. The CAN interface 500 is threaded and further comprises a hexagonal nut 501. Furthermore, the thread and nut 501 are suitable for externally connecting the apparatus to another apparatus via the CAN interface 500.

[0051] Furthermore, the apparatus comprises a control board 502 and a transformer 503 to convert an electrical voltage from the CAN interface 500 into a suitable voltage for the sensor and the control thereof, as well as the Peltier element.

[0052] The portion 513 of the apparatus according to this embodiment comprises a capacitive 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 the sensors 508, 509 and the Peltier element 510, and a cooling fin 506 to cool the power supply that controls the Peltier element 510 when a lot of power is to be directed.

[0053] Furthermore, the apparatus comprises an airtight connection 504 between the control board 502 and the portion 513 of the apparatus. This allows the measurements to be performed without ambient air influencing them. Furthermore, the apparatus comprises threads 505 to durably attach the portion 513.

[0054] Furthermore, the sensors 508, 509 are mounted on a holder 514 which is in direct contact with the Peltier element 510 via the ribs 511, which form part of the Peltier element 510. The material of the holder 514 and the ribs 511 then preferably have a high thermal conductivity, such that the Peltier element 510 can efficiently and quickly cool or heat the sensors 508, 509.

[0055] Furthermore, the apparatus may comprise a processing unit 515 to calculate the dew point. This can be done on the basis of a pre-programmed look-up table, in which case the temperature, as the only variable, uniquely corresponds to a dew point, such that little computing power is required.

[0056] To increase the accuracy, the processing unit 515 may be further configured to calculate the dew point via the following conversion formula,

[00001]$X=1-\left(0,01\mathrm{RH}\right)$$\begin{array}{c}K=-\left(14,55+0,114T_c\right)X-{\left(\left(2,5+0,007T_c\right)X\right)}^3\\ -\left(15,9+0,117T_c\right)X^{14}\end{array}$$T_d=\left(K1,8\right)+32$

where RH is the relative humidity, T.sub.c the measured temperature, and T.sub.d the calculated dew point. For the value of RH, the predetermined constant relative humidity can then be chosen as the value, or a measured value from the sensor can be used in order to increase the accuracy.

[0057] The present invention is by no means limited to the embodiments described by way of example and shown in the figures, but a method and apparatus according to the invention can be realized in all kinds of shapes and dimensions without departing from the scope of the invention.