Friday, August 23, 2013

Talking Dewpoint

  Please define dewpoint.
Dewpoint is defined as the temperature to which a gas (e.g. air) must be cooled, at constant pressure, for water vapor to begin to condense to liquid water. In other words, when the dewpoint temperature has been reached, the gas is fully saturated with water vapor. The term “pressure dewpoint” refers to the dewpoint temperature of a gas at pressures higher than atmospheric pressure. When addressing dewpoint in pressurized compressed air, the correct terminology is actually “pressure dewpoint,” but this is often shortened to “dewpoint” in common usage.

Why is dewpoint so important in pharmaceutical applications?
Compressed air may be used for a number of applications in the pharmaceutical industry, such as raw material transport, processing equipment, pneumatic power sources, and cleaning. The importance of knowing the dewpoint in a compressed air line may be critical for some applications but less relevant for others. For example, bulk solid and powder conveyers used for moving product rely on sufficiently dried and filtered air in order to perform their function properly and prevent product contamination. Continuous monitoring and control of dewpoint is often a requirement for instrument air, drying processes, packaging, and actuating process control valves. The risks associated with letting dewpoint levels go unchecked can include equipment failure, condensation in process lines and on finished product, and the potential for bacterial formation.

Why is dewpoint so important in laboratory environments?
Laboratory environments are often designed to maintain a controlled atmosphere in order to eliminate airborne contaminants and any sources of error that may interfere with testing. Dewpoint can be an important parameter to control. This is usually accomplished through the environmental control system and has little to do with compressed air. Some lab equipment, such as glove boxes, may require their feed gas to meet an established dewpoint level in order to maintain the inert atmosphere of the chamber.

How is dewpoint measured and monitored in most facilities?
When discussing a typical facility’s compressed air system, it’s helpful to divide the entire network into two separate subsystems: the supply side and demand side. The supply side consists of the compressors and air treatment equipment up to the flow/pressure controller. The demand side consists of the distribution and storage systems or everything after the flow/pressure controller. On the supply side, dewpoint transmitters providing analog signals can be built into the dryer control system or can be installed in-line before or after the receiver tank. On the demand side, fixed mount instruments providing a local display, alarm relays and datalogging capability are quite common throughout the distribution network and before critical end-use applications to give operators and plant personnel a quick assessment of dewpoint conditions at specific points in the system. This helps ensure that the dewpoint level of the air being produced at the dryers is maintained through the entire facility and to the end use points. Portable devices are an excellent tool for verifying dryer performance, conducting quality audits, and checking the calibration of fixed mount instruments.

vaisala monitor

How is dewpoint measured by refrigerated air dryers?
Refrigerated dryers operate by using a refrigerant to cool the supply air with heat exchangers (usually to between 35ºF to 40ºF) and condense out water vapor for removal by a moisture separator and drain. Due to their relatively low initial cost, long term reliability and minimal maintenance requirements, refrigerated dryers often do not integrate a dewpoint transmitter into their design for monitoring or control purposes.

How is dewpoint measured by desiccant air dryers?
Desiccant air dryers can benefit from a dewpoint sensor for monitoring dryer performance, controlling desiccant tower regeneration, or both. Most regenerative desiccant type dryers (heated or heatless) produce a dewpoint of around -40ºC/ºF. Installing a dewpoint instrument with a display or with built in alarm relays to measure the exit air from the dryer is a smart way to ensure good dryer performance. However dryer efficiency can be significantly improved by using a dewpoint device to control the regeneration cycle – known as Dewpoint Demand Switching (DDS). Desiccant dryers operate using two separate towers containing desiccant – one tower is always in operation while the other tower is being regenerated or purged using a portion of the dried exiting air. Some towers switch based on a timer, regardless of whether the desiccant has been fully saturated. By integrating a dewpoint sensor with the dryer control system, the towers will not switch until the dewpoint transmitter senses a degrading dewpoint temperature, thus ensuring full utilization of each desiccant tower and minimizing wasted purge air.

How is dewpoint measured in point-of-use applications?
For point-of-use dewpoint measurements, generally there are two options available, direct in-line insertion or sample extraction. Each method offers advantages and disadvantages that should be considered carefully. Direct insertion involves installing the probe through a threaded connection or “T” in the line. The benefits of this approach are ease of installation with no accessories required and no venting or loss of the compressed air. Line pressure fluctuations and sensor removal however can present drawbacks. The best installation for a dewpoint instrument isolates the sensor from the main line using a stainless steel sample line and sample cell. This setup allows for “valving off” from the main line and the ability to regulate the pressure, which has a considerable affect on the dewpoint reading. Easy installation and removal of the sensor can also be an important advantage.

What are the different prevalent technologies used to measure dewpoint?
With the vast number of different hygrometer technologies currently available on the market for measuring a wide range of dewpoints suited to different applications and industries, it would be difficult to cover all of them here in any detail. I’ll limit the scope of the discussion to briefly address only the most common sensor types used in compressed air measurement.
Condensation hygrometers, often called “chilled mirrors” operate by cooling a surface in a controlled manner until condensation begins to form; this temperature is recorded as the dewpoint temperature of the air. The most common detection method for determining when liquid water has begun to form is optical reflectance, which uses a light source to measure the amount of reflected light from the surface. These devices are well known for their high accuracy (usually +/-0.2ºC dewpoint) but generally require more maintenance to keep their reflective surface clean. They become prohibitively expensive for measuring very low dewpoint temperatures.
Aluminum oxide, silicon oxide, and various other capacitive sensors share some common traits. In all cases, a capacitor is formed between two electrodes with a hygroscopic material serving as the dielectric of the capacitor. The hygroscopic material adsorbs or desorbs water vapor in proportion to the amount of water vapor surrounding the sensor. This changes the dielectric constant of the material and therefore the capacitance of the sensor. The choice of dielectric materials is critical to the performance of the sensor. Aluminum oxide is sensitive to very low dewpoints, but peforms less well in atmospheric humidity levels. These sensors can be economical for low dewpoints when compared to chilled mirrors.
Thin film polymer capacitive sensors operate on the same principle as aluminum oxide sensors but use polymers instead of metallic oxides as the dielectric material. Many polymer sensors are optimized for use in atmospheric levels of humidity and are not suitable for the measurement of gases with dewpoint temperatures lower than -20°C. However, some polymer sensors are designed for low dewpoint measurement, and they typically distinguish themselves by implementing active, automatic self-calibration schemes to monitor and adjust the performance of the sensor. These sensors are cost competitive with aluminum oxide devices and offer the benefit of enhanced long term stability.

What does Vaisala recommend for measurement and monitoring of dewpoint?
When selecting a dewpoint instrument for a particular application, it’s important to consider the following about the installation:
• What is the expected dewpoint level at the intended measurement location?
• What is the pressure range?
• What is the temperature range?
• Will the probe be installed directly in the line or will a sample line be used for external measurement?
• Should the instrument be portable or fixed mount?
• What type of signal output is desired – local display, analog, serial communication?
• What other functionality is of interest – power supply, datalogging, alarm relays?
With this information specified, the field of dewpoint instruments that fit these criteria will be significantly reduced.

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