Basics of Dew Point Temperature Measurement

Currently, compressed air, a universal and reliable energy source, is used almost everywhere in production processes.

Depending on the tasks at hand, various requirements may be applied to the quality of compressed air. The key to the constant and uninterrupted operation of the compressor includes specific parameters of humidity and dew point temperature of the compressed air.

Usually, compressed air is produced from ambient air by piston or screw compressors and then undergoes a drying process. The goal is to produce dry air, free from oil and dust. Oil and dust particles can be removed through a complex filter system. Humidity levels must be reduced using dryers (refrigeration, membrane, adsorption, etc.).

How does water get into compressed air?

Air can contain more water vapor the higher the temperature and the larger the volume of air. When air is compressed, its capacity to hold vapor decreases. At a certain stage of compression, the air becomes fully saturated, and the water contained in the air precipitates as condensate. Lowering the temperature increases the volume of condensing water. Thus, the relative humidity of the air at the compressor outlet is always 100%. The amount of water that can form during air compression can be quite large. For example, at 60% humidity and an ambient temperature of 20°C, a 30 kW compressor releases up to 20 liters of water into the pneumatic system. For larger compressors, this value will be much higher.

Consequences of moisture content

Depending on the tasks at hand, various requirements may be applied to the quality of compressed air. However, in all cases, the key to long-term uninterrupted operation of the entire system is monitoring air humidity parameters. The pipeline in the pneumatic system is usually made of non-galvanized steel. Since the corrosion rate significantly increases when humidity exceeds 50%, this must never be allowed. In the case of non-galvanized pipelines, high humidity eventually leads to corrosion. Rust gradually spreads and reaches intake points, causing blockage of outlet holes, failure of control elements, and production downtime.

Besides corrosion-related problems, moisture content directly affects the quality of the produced products. Here are the most common problems that can be caused by increased humidity:

• Water-absorbing products (spices, sugar) may clump together during transportation through the pneumatic transport system.
• Bubbles may form during varnishing and applying other coatings.
• Drilled holes may become clogged with dust.
• In winter, in unheated rooms, control valves may freeze.

Functions of the dryer

To solve problems caused by excessively high humidity, dryers of various types are used. When working with pneumatic systems, the dew point temperature is used as a parameter defining air dryness. The dew point temperature of compressed air is the temperature at which the moisture contained in the air condenses into water. The less water contained in the compressed air, the lower the dew point temperature value.

There are different types of dryers. Adsorption and refrigeration dryers are the most common.

Refrigeration dryers

Refrigeration dryers cool compressed air to approximately 2-5°C. Since the dew point temperature is about the same, excess water vapor condenses as water. After that, the air is reheated to room temperature.

In most cases, when using refrigeration dryers, only temperature monitoring is performed, and humidity and dew point sensors are installed only at large enterprises or in particularly critical production cases. Nevertheless, temperature information is usually insufficient, as the following dryer malfunctions can cause humidity to exceed the permissible limit even at normal temperature:

• Condensate is not drained from the dryer.
• Compressed air enters the dryer (heat exchanger pipes are worn out).
• Compressed air enters the bypass pipeline (wet compressed air enters the bypass pipeline instead of the dryer).
• Excess condensate due to a faulty pre-separation system.

If the refrigeration dryer fails, this inevitably leads to moisture condensation in the compressed air system. This creates additional problems (besides those already mentioned) if condensate accumulates in dead-end branches and is not automatically drained. Removing water in this case requires considerable effort or drying and blowing it out with a large amount of compressed air. This often leads to increased dew point values without any visible damage to the dryer. In such cases, it can be extremely difficult to find the cause of the dew point temperature increase or even condensate formation.

Adsorption dryers

The operation of adsorption dryers is based on the principle of mass attraction. Water vapor condenses (is adsorbed) on the surface of the drying substance.

Effective adsorption dryers are capable of drying air to a state where the dew point is -40°Ctd and below.

Regenerative adsorption dryers consist of two tanks filled with drying substance. At different stages of operation, one tank undergoes regeneration, while the other dries the air. Depending on operating conditions, the drying substance should be replaced every 3-5 years. The following conditions may shorten the lifespan of the substance:

• Excessively large volumes of air passing through the dryer
• Unsatisfactory preliminary separation
• Presence of oil in the air
• Extended regeneration time of one of the tanks

For production safety, it is necessary to monitor the dew point temperature at all times and have an alarm that triggers when permissible values are exceeded.

Modern dew point temperature measuring instruments

The DS400 set, equipped with a dew point sensor for both refrigerated and adsorption/membrane dryers with a measurement limit up to -80°Ctd, allows easy and safe monitoring in production.

The DS400 system comes with all necessary accessories for connection. Therefore, there is no need to study the manual in detail to connect the device.

Threshold exceedance indication can be acoustic as well as optical. The 2 signal relays are freely configurable. The alarm trigger delay can be set for each of the two relays. This allows recording only long-term threshold exceedances. There is also the possibility to reset the alarm.

The DS400 set consists of the multifunctional measuring device DS400 and the dew point sensor FA410, which includes a measuring chamber for pressure measurements up to 16/50/350 bar. For pressures above 16 bar, the use of a special measuring chamber is necessary.

The dew point sensor is based on a proven humidity sensor manufactured by the German company CS Instruments. For fast and accurate measurement, the sensor must be constantly flushed with compressed air. To achieve this, the measuring chamber is equipped with a capillary tube that continuously passes the pressurized air. The sensor, equipped with the measuring chamber, can be connected to the compressed air system via a quick-release coupling.

The advantage of the DS400 over other paperless data recorders is the ability to easily and quickly evaluate the obtained information. The intuitive 3.5-inch display with zoom function and a save button for printing is unique in this price range. The display can show current humidity, temperature, and dew point values. All values are saved in the data logger. The user can view flow graphs directly on the device without downloading data to a computer. This allows quick analysis of the compressed air drying process. With a special button, screenshots can be saved as graphic files on the built-in memory card or exported to USB and then printed without any additional software.

The device is ideal for documenting measured values and graphs on-site. Color graphs can be saved in graphic format and sent by email or included in a report.

The built-in data logger allows saving information for several years. Recorded data can be exported via USB drive or Ethernet network and processed on a PC using CS Soft Basic software.

Advantages of the DS400:

• 3.5-inch color touchscreen
• Zoom function for precise analysis of measured values
• Drying analysis with generation of daily/weekly/monthly reports
• Construction of color graphs with naming of each variable
• Mathematical calculation functions
• Save for printing: saving data in graphic format
• Data saving to USB drive and sending by email
• Ability to operate without software
• Two signal relays for threshold exceedance signaling
• Easily adjustable alarm trigger delay with reset function
• Up to 4 recording channels: connection of flow meters, dew point sensors, pressure, temperature, current meters, optional third-party sensors: Pt100/1000, 0/4..20 mA, 0-1/10 V
• Modbus interface, pulse input
• Built-in data logger with 2 GB memory
• Interfaces USB, Ethernet, RS485
• Web server