Dew Point Measurement How Accurate Dew Point Measurement Saves cost

Editor: Dominik Stephan

In a refrigerant-based compressed air dryer system, the dew point sensor installed at the dryer outlet helps monitoring the dryer’s performance most accurately.

Common cyclic and non-cyclic refrigerated air dryers (Picture: Vaisala)
Common cyclic and non-cyclic refrigerated air dryers (Picture: Vaisala)

Compressed air is vital for virtually any manufacturing industry, from operating pneumatic tools to spray painting, blowmoulding, and chemical mixing. Whatever be the application, the compressed air needs to be dry to avoid risk of corrosion, malfunctions and poor end-product quality, all o fwhich can result in unnecessary costs. Today, roughly 80 per cent of compressed air systems use refrigerant dryers to maintain dryness. Unfortunately, many of them lack accurate dew point measurement, leading to unnecessary operating costs and lower endproduct quality.

Depending on the desired pressure dew point, the choice in dryer selection will vary. Generally, the two most common types of industrial dryers used in compressed air Common cyclic and non-cyclic refrigerated air dryers systems are desiccant and refrigerant. Desiccant dryers use adsorbing materials – such as silica gel or activated aluminium to remove moisture from the air, whereas refrigeration dryers remove moisture by cooling it in a heat exchanger and purging the condensed water. Separate refrigerant compressor and heat exchanger are used to handle this cooling function. Refrigeration dryers can produce the dew point levels required for compressed air of class 4 quality or above, whereas desiccant dryers are needed for class 3 and below (based on ISO Standard 8570.1, see table 1).

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Measuring temperature – A Valid Equivalent for Dew Point Measurement

Traditionally, refrigeration dryers have been equipped with only a temperature sensor – which is mostly considered to be equivalent to a dew point measurement. Yet there are several potential reasons why the temperature measurement may not indicate the true dew point of the air. A few of such reasons are cited below:

● Drain valves can fail.

● Drain points can become blocked, leading to improper removal of water — and consequent contamination of the compressed air with micro droplets.

● The condensate can overload the drain system. This clearly means — that even a steady condensate flow is no guarantee of normal operation.

● Temperature measurement can also be misleading in cases of high flow rates, as the whole air mass is not cooled to the heat exchanger temperature.

As these factors indicate, the only way to accurately measure moisture and monitor correct dryer operation is to use a dew point sensor installed at the dryer outlet.

Energy Efficiency Comes with Accurate Dew Point Data

Refrigerant dryer control systems have developed considerably in recent years with modern controls utilizing accurate dew point data. For example, variable speed drives can now be used to adjust the refrigeration cycle – thereby optimizing dryer performance. Especially, in the case of varying load conditions, the savings in energy consumption can be as significant as up to 50 per cent.

Correct Sensor Placement

Since dew point is pressure dependent, in order to draw correct conclusions about the dryer performance, it is important to know — where in the system dew point is being measured and what the conditions are at each location.

Figure 1 demonstrates how dew point changes as a function of decreasing pressure from its initial value of +4°C, a typical pressure dew point for refrigeration dryer. To provide a practical example: the compressed air system is supposed to produce air with pressure of 7 bar and a dew point of +4°C.

If dew point is measured downstream at a position where the pressure is 6 bar, a dew point reading of +4°C might give the impression that the system is operating correctly. However, when pressure dependence is taken into account, the actual dew point at the dryer is only +6°C. The lesson learned from this example is that the best dew point measurement location is directly on the dryer outlet.

Choosing the Ideal Dew Point Measurement Instrument

Due to the wide variety of compressed air systems, no single product addresses all measurement needs. Even if factors such as tolerance, impurities, inherent stability, electrical and mechanical connections are disregarded, the dynamic measurement range required to accurately cover the full scale of compressed air quality classes is vast.

For example, a system operating at a pressure of 7 bar and a dew point of -70°C contains only 0.39 Parts Per Million (ppm) of water vapour, whereas a system with the same pressure but a dew point of +10°C contains about 1,800 ppm water vapour, i.e., the concentration difference is more than three orders of magnitude.

To satisfy these extremes, Vaisala has developed a range of specialized dew point measurement instruments optimized for various dewpoint levels. Table 1 shows the moisture requirement for the different compressed air quality classes according to the ISO 8753.1 standard. The table also shows the most typical Vaisala dew point instrument choices for each compressed air quality class.

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