Industrial Utility Efficiency

# Dew Point Measurement in Compressed Air Systems

Wherever compressed air is used, accurate and continuous monitoring of the dew point temperature is advisable. The dew point provides information about the absolute humidity content of the compressed air. A too high humidity content can have negative effects on the quality of the final product, lead to problems during the manufacturing process, or even result in complete system shutdown. Therefore, operators of compressed air systems should address this issue before it causes serious and costly issues. The following explains the basics of dew point measurement and what is important in practice.

### Dew Point and Dalton’s Law

The dew point, or dew point temperature (Td), is the temperature to which the air must be cooled for the water contained in the air to start condensing. The dew point measurement relates to the absolute amount of water vapor contained in the air. In contrast, the measurement of relative humidity (RH) indicates how close the air is to the saturation point.

Dalton's Law states the total pressure of an (ideal) gas mixture is the sum of the partial pressures of the individual gases. The main constituents of air are nitrogen (N2), oxygen (O2), argon (Ar) and carbon dioxide (CO2). Other components of air can be ignored as their share is negligible. Therefore, for fully dry air at sea level (1013.25 hPa) the following applies:

PTotal = pN2 + pO2 + pAr + pCO2

Another common component of air is water vapor (= water in its gaseous phase). For the total pressure of humid air, the following applies:

PTotal = pN2 + pO2 + pAr + pCO2 + e‘ or pTotal  = pa + e‘

• e‘ = Water vapor partial pressure
• pa = Partial pressure of dry air
• p = Total pressure of the air

The maximum possible water vapor partial pressure in the air is called saturation vapor pressure and is determined by the temperature. As of this point, the air cannot hold any more water vapor, resulting in condensation.

### The Effect of Temperature on Dew Point

As the dew point temperature relates to the absolute amount of water vapor in the air, it is not affected by changes of the air temperature. When the air temperature approaches the dew point temperature, the relative humidity increases and, with it, the risk of condensation. These relationships, including the calculation of measurement uncertainties, can be calculated very easily with an online calculator, such as the E+E Elektronik’s humidity calculator.

Here's a practical example of this relationship in a compressed air network:

The air at the compressor outlet is at 50°C (122°F) and is fully saturated. Thus, the air has a dew point temperature Td = 50°C (122°F) and the relative humidity RH = 100%. The air is then dried to a dew point of Td = 10°C (50°F) and thereby cooled to 30°C (86°F). The RH is now = 28.9%, and consequently there no risk of condensation.

In a secondary pipeline, the same air is cooled to T = 5°C (41°F) by cold weather. By this, the air temperature T drops under the dew point temperature of Td = 10°C (50°F), which causes condensation in the pipe.

In order to prevent condensation in the secondary pipe, the air must be dried in advance so its dew point temperature is lower than the minimum possible air temperature, so Td < 5°C (< 41°F).

### The Effect of Pressure on Dew Point

If the dew point temperature of compressed air is measured, this is called the pressure dew point. A change in the pressure, e.g. through compression in a compressed air line, has a great effect on the dew point. If air is compressed, the dew point temperature rises. Here’s an example:

In the case of air with a dew point temperature Td = 10°C (50°F) and pressure p = 1 bar (14.5 psi), the water vapor partial pressure is 12.33 mbar (1233 Pa). If this air is now compressed sevenfold to 7 bar (101.5 psi), then the water vapor partial pressure (see Dalton's Law) also increases sevenfold to 86.31 mbar (8631 Pa). This water vapor partial pressure corresponds to Td = 42.9°C (109.22°F). In this case, this means that at T < 42.9°C (< 109.22°F), water from the air will condense. In practice, this effect can be observed with an air compressor. If the compressed air is fully saturated and the ambient temperature falls below the dew point, then condensed water will be present in the compressed air line.

### Measurement Using a Dew Point Hygrometer

The dew point can be measured very accurately and directly with a dew point hygrometer (dew point mirror).  This involves cooling down a temperature-controlled mirror until condensation arises on the surface. The condensation changes the reflectivity of the mirror's surface, which is detected by the integrated measurement optics. The corresponding mirror temperature is the dew point temperature of the air or gas. Modern dew point mirrors can measure in the range -100°C Td (-14°F Td) to 100°C Td (212°F Td) with an accuracy of ±0.1°C Td (±0.18°F Td).

Dew point mirrors are high-precision, expensive measuring devices primarily employed as reference devices in calibration laboratories. Due to their design, dew point mirrors are unsuitable for installation in compressed air lines.

### Measurement Using a Hygrometric Dew Point Transmitter

In practice, compact dew point transmitters with hygrometric measuring principle are used for dew point measurement in compressed air lines. Such a dew point meter features a capacitive humidity sensor and a temperature sensor. From the measured relative humidity and temperature the device calculates the dew point temperature with an accuracy of ±2°C Td (±3.6°F Td) across the measuring range relevant for compressed air applications, -60°C Td (-76°F Td) to 60°C Td (140°F Td).

The prerequisite for this is a high quality capacitive sensor together with an auto-calibration procedure, such as the E+E’ Elktronik’s HMC monolithic dew point sensor. The humidity and the temperature sensor of the dew point transmitter are built on the same substrate. The reproducible auto-calibration procedure is only made possible by the thermal coupling between the two sensors.

During the periodical auto-calibration the sensor is heated up, which reduces the relative humidity at the sensor to virtually 0% RH. This automatic zero point adjustment enables measuring accuracy of very low relative humidity and of the calculated low dew point temperature.

### Why Does Compressed Air Need to be Dried?

As the temperature rises when air is compressed, the condensation does not arise directly in the air compressor, but in the pipeline after the air compressor, when the air temperature decreases. Although the water is removed from the compressed air by a condensate drain and filter, the air compressor output air is still very close to the saturation vapor pressure, its RH is near to 100%.

Importantly, the problems caused by humid air in compressed air lines have been ignored for many years. All the while the development of increasingly demanding manufacturing processes and products has significantly increased the need for clean, dry compressed air. Compressed air dryers and reliable measuring technology are indispensable for high quality compressed air supply.

Modern compressed air dryers are becoming standard, and are used for the following reasons:

• In production operations and in the process industry, where many processes depend on the smooth functioning of the systems, moisture in the compressed air causes problems and breakdowns in the operation of pneumatics, solenoid valves and nozzles. Air compressor motors are damaged by rust and the increased wear of moving parts, since the lubrication is washed off.
• Moisture has a negative effect on the color, adhesion and the finishing of paint applied with compressed air.
• In cold weather, freezing moisture can lead to malfunctions in pneumatic control lines.
• Corrosion caused by moisture on pneumatic components can result in malfunction, interruption or breakdown of process and machinery.
• In the food and pharmaceutical industry, moisture can have a strong negative impact on the required sterile manufacturing conditions.

### Accurate Dew Point Monitoring Invaluable

Those who cut corners when it comes to compressed air drying risk poor product quality, malfunctions in the process, and even system downtime and loss of production. Accurate dew point monitoring leads to a high, stable quality of the compressed air supply and to relevant savings. It is worth investing in tailor-made solutions for dew point measurement.

Martin Raab is a Product Manager with E+E Elektronik Ges.m.b.H.