Instrumentation
(-112°F) dewpoints however your intended control setpoint is at -50°C (-58°F), this is where your accuracy and performance is critical. For these dry, low dewpoint conditions, products using Vaisala DRYCAP polymer sensor type technology are recommended for optimum performance.
Refrigerant dryers In systems operating at higher levels, refrigerant dryers produce dewpoints between 2 and 4°C (35 and 40°F). For these mid-range measurements, sensors optimised for higher dewpoints are recommended, such as Vaisala HUMICAP sensor products as well as some of the DRYCAP sensor products. Since application conditions such as pressure and
temperature can affect the choice of sensor, it’s a good idea to consult a specialist before making your final decision. It is also important to know the pressure and temperature
at the measurement location. While the temperature of the compressed air will not have
any affect on the dewpoint value, it may negatively impact the performance of the dewpoint sensor. If the air is too hot at the intended measurement point, choosing a cooler downstream location or use of a sampling system may be required. Careful monitoring and management of pressure is critical
to making a good dewpoint measurement. Since changes in pressure have a dramatic affect on dewpoint, it’s important
to know the pressure at each measurement location in order to compare dewpoint readings and baseline them to a single pressure value.
Refrigerant dryers Will the measurement be made directly in the line or using a sample cell? Deciding how to install a dewpoint probe can be a difficult choice. Two options are usually considered – inserting the threaded probe directly into a T, or ball valve for an in-situ measurement or using a sample line and sample cell to isolate the measurement outside of the system. There are advantages and disadvantages associated with each method. For in-the-line measurements, the benefits of in-situ
measurements include a simplified and lower cost installation, and a potentially faster response time. However, drawbacks include fluctuating line pressures
(resulting in changing dewpoint values) and inability to isolate or remove the probe conveniently from the compressed air system when it’s time for service or calibration. Typically dewpoint probes come in two varieties, fixed and adjustable. Adjustable probes use a threaded compression fitting allowing for adjustable insertion depth into a line. Fixed probes have welded fittings that cannot be repositioned. The alternative approach involves the use of a sample cell
and stainless steel tubing connected to the main line at the point of interest. A small amount of compressed air is allowed to flow
past the sensor by opening a leak screw or valve. The flow rate through the cell must be carefully controlled in order to ensure minimal pressure drop from the process line which could cause erroneous readings. The advantages of this approach are better control of air
pressure minimising main line fluctuations, ability to isolate the sensor from the system, and option to cool and condition the air sample if necessary.
Contamination resistance Finally, good dewpoint sensors have particular features that ensure a clean and dry compressed air system. These include: fast wet-to-dry response time; high resistance to contamination - immunity to compressor oil and other chemicals; complete recovery from saturated conditions; and long calibration intervals. n
Fig. 2. Compressed air setups can vary considerably.
Steven Jiroutek is Regional Market Manager, Vaisala, Boston, USA.
www.vaisala.com
Moisture in compressed air
damage. Malfunctions in production plants may have serious consequences, particularly in demanding applications. Costly standstills and increased expenses for rejects and quality assurance quickly add up.
M 48
www.engineerlive.com
oisture in compressed air is a well-known problem and can lead to severe equipment
With the temperature, the relative humidity and the dew point (up to 50 bar), the mobile, cordless DP 207 dew point measuring device from Beko precisely measures the critical parameters in compressed air and other gases. With this, the company says the DP 207 dew point measuring device makes an excellent contribution to the monitoring of compressed-air quality at stationary and mobile
compressed-air generators, and in mobile applications. Beko’s Metpoint DPM range of portable and stationary dewpoint meters for compressed air are designed to offer high measurement accuracy, durability and reliability, flow optimised sensor geometry, optimum process reliability and suitability for various applications. n
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