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什么是露点,如何测量它?

露点和压力的关系
工业制造和工艺
工业测量

什么是露点?
露点是使空气中的水蒸汽冷凝为露水或霜而必须将空气冷却到的温度。在任何温度下,都有空气可容纳的最大水蒸汽量。这个最大量被称为饱和水蒸气压力。增加更多水蒸汽将导致冷凝。

为什么水分带来问题?
加压空气中的冷凝会带来问题,因为它会导致管道堵塞、机械故障、污染和冰冻。

压力对露点有什么影响?
空气压缩会增大水蒸气压力,因而导致出现露点。如果您在测量前正在将空气排放到大气中,必须要考虑这一点。测量点处的露点将与工艺过程中的露点不同。

露点的一般范围是多少?
在特殊情况下,压缩空气中的露点温度可能从环境温度降低到 -80°C (-112 °F)。没有气体干燥功能的压缩机系统更易于产生在环境温度下饱和的压缩空气。压缩空气通过冷干机系统中的热交换器冷却压缩空气,导致气体中的水冷凝析出。这些系统通常产生露点不低于 5 °C (41°F) 的空气。吸干机系统中的干燥剂从压缩空气中吸收水蒸气,可产生露点为 -40 °C (-40 °F) 的空气,必要时可进一步干燥。

如何可靠地测量露点?

  1. 选择具有正确测量范围的仪表。
  2. 了解露点仪表的压力特征:维萨拉的仪表适用于在工艺压力过程中使用,但是市场上的一些仪表就不尽然。这些仪表可以在压缩空气膨胀到大气压后对压缩空气进行测量,但如果压力露点为必需的测量参数,就必须对测量的露点值进行校正。
  3. 正确安装传感器:按制造商的说明操作。不要在短管末端或其他无气流的管道的“死区”部分安装露点传感器

了解有关压缩空气的更多信息或向下滚动以查看维萨拉的露点仪表。

Comment

nikhil

十二 月 3, 2019
good info

Prasanta Pal

九 月 24, 2021
Need the dew point measurement device for drying tower outlet at Sulphuric acid manufacturing process

Juhani Lehto

十一 月 9, 2021
Hello Pal
Thank you for contacting us. Sulphur acid is a harmful chemical (it corrodes the sensor easily) that must be kept away from the dew point transmitter. If the exposure of the Sulphur acid can be minimized, then you can choose the right product depending on the dewpoint measurement range you are looking for. One useful chart to choose the right product can be found here: https://www.vaisala.com/sites/default/files/documents/CEN-G-CompAir-Product-Brochure-B211208EN.pdf.

Best regards
Juhani, Product Manager

Tony

五 月 31, 2022
What is the equation and/or conversion tables that takes the electric signal from the capacitor to the values of degrees for dew point? Trying to understand how the variables pressure (absolute and relative), along with temperature, and flow impact the dew point reading. Thank you

Juhani Lehto

六 月 7, 2022
Hello Tony
Thanks for contacting us and showing your interest in dew point measurements. The capacitance signal is measured by converting it first to frequency, where the sensor is a variable capacitance component in the oscillator circuit. This frequency is then counted and converted to relative humidity based on the sensor calibration and linearization (this part is based on the sensor model and other proprietary algorithms that are not public, unfortunately). Please note we always measure relative humidity, and therefore, we need also precise temperature reading that is measured on the humidity sensor surface. This is a normal resistive (pt100) measurement.

With a help of this document, you can derive the dew point calculation based on the measured RH and T: https://www.vaisala.com/system/files/documents/Humidity-conversion-formulas-Technical-eBook-B210973EN-L.pdf
I hope you will also find answers to other humidity-related calculations like absolute humidity, pressure-dependency, and other humidity formulas under the same document.

Best regards
Juhani

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