Device and method for determining the dew or frost point temperature of a gas

Abstract

The invention relates to a device (1) and a method for determining the frost point temperature of a gas, wherein the device has an intermediate layer (10) having at least one opening (11). The device (1) further comprises a temperature measurement unit (12), a first pressure measurement unit (23), which is arranged upstream of the intermediate layer (10) in a gas flow direction, and a second pressure measurement unit (24), which is arranged downstream of the intermediate layer (10) in the gas flow direction.

Description

[0001] Device and method for determining the dew or frost point temperature of a gas

[0002] A special application of gas sensing and gas analysis is the determination of the moisture content in a gas. There are various technical approaches for this task.

[0003] Capacitive sensors exploit the change in the dielectric constant of a dielectric when the number of water molecules in the dielectric changes. Such sensors use a capacitor with a hydrophilic dielectric material, allowing water molecules from a surrounding medium to diffuse into the dielectric. Depending on the humidity of the surrounding medium, an equilibrium is established. The resulting dielectric constant can be measured by appropriate electrical control of the capacitor. The capacitance value thus allows conclusions to be drawn about the moisture content of the surrounding medium. The disadvantage of this method is that the diffusion process is relatively time-consuming.

[0004] In another technique, such as gas chromatography, the individual molecules of a gas mixture are separated by a carrier gas based on different boiling points and vapor pressures. This allows the release of the desired substance, such as water, at a specific time during the measurement to be observed at the outlet of a column. The intensity of the release then indicates the moisture content of the sample gas. Gas chromatography is very well suited for laboratory analysis of gases. However, it is complex and requires a carrier gas. Its use for the rapid and cost-effective determination of moisture content is therefore not very widespread.

[0005] Reflective dew-point sensors, which use a mirror to determine the dew point, exploit the fact that the saturation of moisture in the gas sample increases with decreasing temperature. When saturation reaches 100%, water molecules settle on a surface of the mirror and condense. They form dew or frost, which changes the mirror's reflective properties. This is detected accordingly. The resulting temperature and pressure then provide information about the moisture concentration in the gas being analyzed.

[0006] If the temperature of the beginning condensation is above 0°C, this is called the dew point temperature; if the temperature is below 0°C, the dew layer can turn into a layer of frost and this is called the frost point temperature.

[0007] In the context of this invention, dew point temperature and frost point temperature are used largely synonymously.

[0008] The reflector dew point measurement method offers high accuracy, but the sensor system is very complex and requires precise calibration and adjustment. Furthermore, the system requires a certain size. Furthermore, heating and cooling the mirror body requires a lot of energy due to its mass. This limits its use in mobile measuring devices.

[0009] The object of the invention is to overcome the aforementioned disadvantages and to provide a fast and inexpensive device and method for determining the frost point temperature.

[0010] This object is achieved according to the invention by the features of device claim 1 and method claim 9.

[0011] The device according to the invention for determining the frost point temperature of a gas with an intermediate layer with at least one opening, a temperature measuring device, a first pressure measuring device, arranged in a gas flow direction with a substantially constant gas flow upstream of the intermediate layer, and a second pressure measuring device, arranged in the gas flow direction downstream of the intermediate layer.

[0012] One advantage of this device is its simple and robust design. The at least one opening acts as a resistor, creating a pressure difference upstream and downstream of the intermediate layer in the direction of gas flow. If the temperature of the intermediate layer is above or below the frost point temperature, a stable pressure difference is established, with the pressure difference above the frost point temperature being smaller than below the frost point temperature. Only when the frost point temperature is reached from a higher temperature do water molecules from the gas flow settle at the at least one opening and narrow the at least one opening. This narrowing increases the resistance of the at least one opening, leading to an increase in the pressure upstream of the intermediate layer in the direction of gas flow without increasing the pressure downstream of the intermediate layer. The pressure difference thus increases.When the frost point temperature is reached from a lower temperature, the water molecules sublimate and release the gas flow further.

[0013] The invention therefore exploits the fact that when condensation occurs in the opening, its free cross-section changes, resulting in a change in the pressure difference between a first pressure measured by the first pressure measuring device and a second pressure measured by the second pressure measuring device. The change in the pressure difference, in conjunction with a temperature measured by the temperature measuring device and the prevailing pressure, provides information about the frost point and thus the moisture content of the gas.

[0014] Advantageously, the pressure measuring devices are interconnected and configured to measure a pressure difference. Measuring the absolute pressure on both sides of the intermediate layer places high demands on the pressure measuring devices. Measuring a pressure difference with a measuring bridge, i.e., by interconnecting the pressure measuring devices, is simpler and more cost-effective.

[0015] It is preferred that the intermediate layer be thermally connected to a thermocouple. The thermocouple can be designed as a heating or cooling element to specifically increase or decrease the temperature of the intermediate layer until the frost point temperature is reached, resulting in a change in the pressure difference determined by the first and second pressure measuring devices. Direct control of the temperature of the intermediate layer enables precise and efficient temperature control of the intermediate layer with relatively low energy consumption.

[0016] The use of a Peltier element as a thermocouple is particularly advantageous. A Peltier element can be controlled relatively precisely and can provide both heating and cooling. It thus enables precise temperature adjustment and universal application.

[0017] In an advantageous embodiment, the intermediate layer divides a gas-flowing space into a first subspace and a second subspace. The first subspace is arranged upstream of the intermediate layer in the gas flow direction, and the second subspace is arranged downstream of the intermediate layer in the gas flow direction. The first pressure measuring device then determines the pressure in the first subspace, while the second pressure measuring device is assigned to the second subspace.

[0018] The at least one opening preferably has a diameter of less than 1 mm, particularly preferably less than 800 μm, and even more preferably less than 500 μm. The diameter of the openings determines the accuracy with which a frost or dew point can be determined.

[0019] Preferably, the intermediate layer can also have multiple openings. The number of openings is particularly tailored to the size of the gas flow, with a larger number of openings allowing a higher gas flow than a smaller number. This ensures that the diameter of the openings can be selected sufficiently small, even with a potentially required high gas flow, to ensure a change in the pressure difference when the frost point temperature is reached.

[0020] Preferably, the intermediate layer is disk-shaped, meaning it has a thickness that is significantly less than the height and width of the intermediate layer. This allows the intermediate layer to be manufactured with relatively little material and thus has a low thermal resistance.

[0021] In an advantageous embodiment, the intermediate layer is designed as a perforated diaphragm. A perforated diaphragm, especially a thin one, advantageously made of a material with high thermal conductivity, such as a metal, can be cooled or heated quickly and energy-efficiently. The thinner the perforated diaphragm or the less material the diaphragm contains, the less energy is required to cause a temperature change in the perforated diaphragm.

[0022] In a further advantageous embodiment, the intermediate layer consists of a porous material. A porous material is defined as a material with open porosity, i.e., cavities within the material that communicate with each other and with the environment. The individual pores of the porous material then represent the openings.

[0023] In an advantageous embodiment, the at least one opening is adjustable in size. With at least one variable opening, the device for determining the frost point can be configured for different gas flows with different expected frost point temperatures and different flow rates and velocities. In a further advantageous embodiment, an orifice plate is arranged upstream of the intermediate layer. This represents a relatively simple way of generating a substantially constant gas flow within the device. An orifice plate opening can then preferably be of the order of magnitude of the sum of the openings in the intermediate layer, but can also be larger or smaller depending on requirements.

[0024] In an alternative embodiment, an aperture is arranged downstream of the intermediate layer. This also allows a substantially constant gas flow to be generated in the device. The aperture can then preferably be of the order of magnitude of the sum of the apertures in the intermediate layer, but can also be larger or smaller depending on requirements.

[0025] The method according to the invention, which can be used in particular in conjunction with the device according to the invention, comprises the following steps: a. passing a substantially constant gas stream at a known operating pressure through an intermediate layer having at least one opening; b. measuring pressures before and after the intermediate layer and calculating a pressure difference therefrom; c. determining a change in the pressure difference; d. determining a temperature of the intermediate layer at the time of the change in the pressure difference as the frost point temperature.

[0026] The gas stream flows through at least one opening, wherein starting from a temperature above the dew or frost point temperature, a reduction in the temperature of the intermediate layer and thus of the gas stream flowing through the at least one opening results in condensation of water molecules from the gas stream when the dew point temperature is reached or in icing of condensed water when the frost point is reached. Starting from a temperature below the dew or frost point temperature, the temperature is increased until there is a reduction in condensate or a decrease in icing.

[0027] The condensate or icing forms particularly in the area of ​​the at least one opening, so that the at least one opening is narrowed by adhering dew or adhering ice crystals. As a result, the pressure upstream of the intermediate layer, i.e. at the first pressure measuring device, increases without the pressure downstream of the intermediate layer, i.e. at the second pressure measuring device, increasing. The temperature measuring device simultaneously measures the temperature of the intermediate layer. If the pressure at the first pressure measuring device increases at a given point in time without the pressure at the second pressure measuring device increasing, thus increasing the pressure difference, the opening of the intermediate layer is narrowed by dew or icing, and the temperature of the intermediate layer measured at this point in time corresponds to the dew point temperature or the frost point temperature of the gas.Conversely, when the temperature increases, a decrease in the phenomenon described above can be observed, i.e. a decrease in the pressure at the first pressure measuring device without a decrease in the pressure at the second pressure measuring device and thus a reduction in the pressure difference.

[0028] In an advantageous embodiment, the temperature of the intermediate layer is continuously or gradually reduced or increased in the direction of the presumed dew point or frost point of the gas.

[0029] For example, a temperature range can initially be set that lies above or below the presumed dew point or frost point temperature. If a constant pressure difference is established, the temperature can be continuously increased or decreased until the pressure difference changes significantly. If there is no or only a slight gas flow below the presumed frost point and the pressure difference is high, the temperature can be continuously increased until the pressure difference decreases. The temperature at the time of the increase or decrease in the pressure difference is the dew point or frost point temperature.

[0030] In a further advantageous method, a measuring bridge measures the pressure difference and is calibrated above the presumed frost point temperature. It emits a measurement signal when this pressure difference changes. The measuring bridge can be formed by appropriately interconnecting the first and second pressure measuring devices.

[0031] With a calibrated measuring bridge, changes in the pressure difference can be precisely determined. Various bridge circuits, such as full bridges or half bridges, can be used. The pressure measuring devices can include strain gauges or piezo elements, for example. Using common calibration methods, the measuring bridge is brought close to the dew or frost point temperature from below or above. A change in the pressure difference causes a change in the resistance of the measuring bridge, which in turn changes the measuring voltage.

[0032] Further features, details, and advantages of the invention will become apparent from the wording of the claims and from the following description of exemplary embodiments with reference to the drawings. Figure 1 shows a schematic structure of a device for determining the frost point.

[0033] Fig. 1 shows a device for determining the frost point temperature 1 with a gas that impinges on an intermediate layer 10 in a gas-flowing space 20 in a substantially constant gas flow 2 in a gas flow direction. This intermediate layer 10 has an opening 11 and is thermally connected to a temperature measuring device 12 and a thermocouple 13. The intermediate layer 10 divides the gas-flowing space 20 into a first sub-space 21, which is arranged upstream of the intermediate layer 10 in the gas flow direction, and a second sub-space 22, which is arranged downstream of the intermediate layer 10 in the gas flow direction. A first pressure measuring device 23 is arranged in the first sub-space 21. A pressure measuring device 24 is arranged in the second sub-space 22.

[0034] With the help of the thermocouple 13, the temperature of the intermediate layer 10 can be influenced until, upon reaching the dew or frost point temperature of the gas flowing through the opening 11, a pressure difference falling across the intermediate layer 10 changes. This is detected by the first and second pressure measuring devices 23, 24, with the temperature measuring device 12 determining the temperature of the intermediate layer 10. From this, and taking into account the prevailing pressure, the moisture content of the gas under investigation can then be determined relatively accurately.

[0035] In order to minimize the influence of the gas flow, the intermediate layer 10 is preferably flowed through with a gas flow 2 that is as constant as possible. This can be easily achieved by means of an upstream or downstream aperture (not shown).

[0036] The invention is not limited to one of the previously described embodiments, but can be modified in a variety of ways. In particular, the shape of the intermediate layer as well as the number and size of the openings can vary depending on the requirements for measurement accuracy and the volume flow rate.

[0037] All features and advantages arising from the claims, the description, and the drawings, including design details, spatial arrangements, and method steps, may be essential to the invention both individually and in a wide variety of combinations.

[0038] 1 device for determining the frost point temperature

[0039] 2 Constant gas flow

[0040] 10 Intermediate layer

[0041] 11 Opening

[0042] 12 Temperature measuring device

[0043] 13 Thermocouple

[0044] 20 Gas-flow room

[0045] 21 first subspace

[0046] 22 second subspace

[0047] 23 first pressure measuring device

[0048] 24 second pressure measuring device

Claims

Patent claims 1. Device (1) for determining the frost point temperature of a gas, comprising an intermediate layer (10) with at least one opening (11), a temperature measuring device (12), a first pressure measuring device (23) arranged upstream of the intermediate layer (10) in a gas flow direction, and a second pressure measuring device (24) arranged downstream of the intermediate layer (10) in the gas flow direction.

2. Device according to claim 1, wherein the intermediate layer (10) divides a gas-flown space (20) into a first sub-space (21) and a second sub-space (22), and the first sub-space (21) is arranged upstream of the intermediate layer (10) in the gas flow direction, and the second sub-space (22) is arranged downstream of the intermediate layer (10) in the gas flow direction.

3. Device according to claim 1 or 2, wherein the intermediate layer (10) is designed as a pinhole.

4. Device according to claim 1 or 2, wherein the intermediate layer (10) consists of a porous material.

5. Device according to one of the preceding claims, wherein the intermediate layer (10) is thermally connected to a thermocouple (13).

6. Device according to claim 5, wherein the thermocouple (13) is a Peltier element.

7. Device according to one of the preceding claims, wherein the two pressure measuring devices (23, 24) are arranged to detect a change in a pressure difference falling across the intermediate layer (10).

8. Device according to claim 7, wherein the pressure measuring devices are designed such that they serve as resistors of a measuring bridge and are arranged such that a change in the pressure difference causes a measuring signal.

9. Device according to one of the preceding claims, wherein the at least one opening (11) has a diameter which is less than 1 mm, in particular less than 800 μm or less than 500 μm.

10. Device according to one of the preceding claims, wherein an orifice plate is arranged in front of the first pressure measuring device (23) in the gas flow direction.

11. Device according to one of the preceding claims, wherein an orifice plate is arranged downstream of the second pressure measuring device (24) in the gas flow direction.

12. A method for determining the frost point temperature of a gas, comprising the following steps: a. passing a substantially constant gas stream (2) at an operating pressure through an intermediate layer (10) having at least one opening (11); b. measuring pressures before and after the intermediate layer (10) and calculating a pressure difference therefrom; c. determining a change in the pressure difference; d. determining a temperature of the intermediate layer at the time of the change in the pressure difference as the frost point temperature.

13. The method according to claim 10, wherein between steps b. and c. the following step is carried out: e. Continuously reducing or increasing the temperature of the intermediate layer towards a presumed frost point temperature of the gas.

14. Method according to one of claims 12 or 13, wherein a measuring bridge is calibrated above the presumed frost point temperature and outputs a measuring signal when the pressure difference changes.

15. Method according to one of claims 12 or 13, wherein a measuring bridge is calibrated below the presumed frost point temperature and outputs a measuring signal in the event of a change in the pressure difference.

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