Dew Point Measurement
How Accurate Dew Point Measurement Saves Cost
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.