The dew point of air as a function of decreasing pressure – for air with initial state Td 4°C and 7 bar

Accurate dew point measurements: Benefits for dryers
August 12th 2011

Dew point monitoring ensures that compressed air has been dried sufficiently, thereby helping to avoid the risk of corrosion and poor endproduct quality. It also helps to cut energy consumption, as Pekka Ravila, director, targeted industrial applications – Vaisala, explains

Operating compressed air systems comes with a high price tag; it takes seven to eight times more electricity to produce one horsepower with compressed air than with an electric motor. Fortunately, there are ways to boost efficiency. One option is to measure dew point.

Generally speaking, the two most common types of industrial dryers used in compressed air systems are desiccant and refrigerant. Refrigeration dryers, which are used in 80% of systems, 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) Modern desiccant dryer controls can base their cycling on dew point demand switching, easily saving up to 20% in energy costs when compared to constant time interval changing.

Refrigerant-type dryers can benefit from even greater savings. In particular, significant potential exists in systems with varying compressed air consumption. In these systems, energy consumption can be cut by 50% by controlling the refrigeration cycle based on actual dew point, as opposed to temperature measurement only.

Traditionally, refrigeration dryers have been equipped with only a temperature sensor – which are at times 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:

• Drain valves can fail

• Drain points can become blocked

• Condensate can overload the drain system

• Temperature measurement can be misleading in cases of high flowrates, as

the whole air mass is not cooled to the heat exchanger temperature.

One way to accurately measure moisture and monitor correct dryer operation is to use a dew point sensor. However, dew point is pressure dependent and because of that it is important to know where in the system dew point is being measured and what the conditions are at each location in order to draw correct conclusions about dryer performance. Figure 1 demonstrates how dew point changes as a function of decreasing pressure from its initial value of 4°C, a typical pressure dewpoint for refrigeration dryer. For 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 is that the best dew point measurement location is directly on the dryer outlet.

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 0.39ppm of water vapour; a system with the same pressure but a dew point of 10°C contains about 1800ppm water vapour; the concentration difference is more than three orders of magnitude.

The table (left) shows the moisture requirement for the different compressed air quality classes according to ISO 8753.1. It also shows typical Vaisala dewpoint instruments for each compressed air quality class.

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