Dew Point in Compressed Air –Frequently Asked Questions
1. What is dew point?
Dew point temperature is a measure of how much water vapor there is in a gas. Water has the property of being able to exist as a liquid, solid, or gas under a wide range of conditions. To understand the behavior of water vapor, it is first useful to consider the general behavior of gases.
In any gas mixture, the gas total pressure is the component gases’ sum of the partial pressures. This is Dalton’s law and it’s represented as shown below:
Ptotal = P1 + P2 + P3 …
The quantity of any gas in a mixture can be expressed as pressure. The main air components are water vapor, oxygen, and nitrogen, so overall atmospheric pressure’s consisting of the partial pressures of these three gases. While oxygen and nitrogen both exist in stable concentrations, the water vapor concentration’s highly variable and must be measured to be determined.
The maximum partial pressure of water vapor is strictly a function of temperature. For example, at 20 °C (68 °F), the maximum partial pressure of water vapor is 23.5 mbar. The value of 23.5 mbar is said to be the “saturation vapor pressure” at 20 °C (68 °F). In a 20°C (68°F), “saturated” environment, the addition of more water vapor results in condensation being formed. This condensation phenomenon can be used for water vapor content measurement.
Gas of unfamiliar water vapor concentration’s passed through a temperature-controlled surface. The surface’s cooled until the formation of condensation. The temperature to the formation of condensation’s referred to as the “dew point temperature.” Because there’s a unique correlation between saturation and temperature vapor pressure (remember, the water vapor’s maximum partial pressure, also referred to as saturation vapor pressure, is a temperature function), measuring the dew point temperature of a gas is a direct measurement of the partial pressure of water vapor. Knowing the temperature of the dew point, the pressure of the corresponding saturation vapor can be looked up or calculated. The table in the following indicates some temperature values and the corresponding saturation vapor pressure:
| Temperature
°C (°F) |
Saturation vapor
pressure (mbar) |
| 20(68) | 23.3 |
| 0(32) | 6.1 |
| -10(14) | 2.8 |
| -20(-4) | 1.3 |
| -40(-40) | 0.2 |
2. What is the difference between dew point and “pressure dew point?”
“Pressure dew point” is a term that’s encountered when you measure a gas’ dew point temperature at pressures higher than atmospheric pressure. It refers to the dew point temperature of a gas under pressure. This is important because changing the pressure of a gas changes the dew point temperature of the gas.
3. What is the effect of pressure on dew point?
Increasing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point temperature of -10 °C (14 °F). From the table above, the water vapor’s partial pressure (designated by the symbol “e”) is 2.8 mbar. In case this air is compressed as well as the overall pressure increased twofold to 2026.6 mbar, then in accordance with Dalton’s law, the partial pressure of water vapor, e, is also increased twofold to the 5.6 mbar value. The dew point temperature corresponds to 5.6 mbar’s about -1 °C (30 °F), so it’s clear that increasing the air pressure has also increased the dew point temperature of the air.On the contrary, expanding a compressed gas to atmospheric pressure reduces the partial pressures of every component gas, which includes water vapor, and thus reduces the gas’ dew point temperature. The total pressure’s relationship to the partial pressure of water vapor, e, can be indicated below:
P1/P2 = e1/e2
By having the dew point temperature converted to the corresponding pressure of saturation vapor, it’s simple to calculate the changing total pressure effects on the saturation vapor pressure. Then, the value of the new saturation vapor pressure can be reverted back to the temperature of the corresponding dew point. These calculations can manually be conducted with the use of tables, or done with different types of software.
4. Why is knowledge of dew point in compressed air important?
The significance of dew point temperature in compressed air is depending on the air’s intended use. In numerous cases, the dew point isn’t critical (portable compressors for gas station tire filling systems, pneumatic tools, etc.).
In certain cases, the dew point is only important due to the pipes that carry the air are exposed to freezing temperatures, where a high dew point could result in freezing and blockage of the pipes. In most modern factories, compressed air is utilized to function various equipment, some that could malfunction if there’s a formation of condensation on internal parts. Certain water-sensitive processes (paint spraying for example) that need compressed air may have particular dryness specifications.
Finally, pharmaceutical and medical processes could treat water vapor, especially other contaminated gases, needing a purity that’s of an extremely high level.
A variety of sample cell hardware, including quick disconnects, cooling coil, and welded compression fitting, makes it easy to install a dew point sensor in any process
5. What is the typical range of dew point temperatures to be found in compressed air?
Dew point temperatures in compressed air range from ambient down to -80 °C (-112 °F), sometimes lower in special cases. Compressor systems with no air drying capability are inclined to produce compressed air that’s saturated at an ambient temperature. Systems with refrigerant dryers pass through the compressed air via some type of cool heat exchanger that causes water to be condensed out of the air stream. These systems typically produce air with a dew point no lower than 5 °C (23 °F). Desiccant drying systems have water vapor absorbed from the air stream and can create air with a dew point of -40 °C (-40 °F) and drier if required.
6. What are the standards for the quality of compressed air?
ISO8573.1 is an international standard that specifies the quality of compressed air. The standard defines limits for three categories of air quality:
• Maximum oil content that remains
•Maximum allowable dew point temperature
• Maximum particle size for any particles that remain
Each category is given a quality class number between 1 and 6 according to the reference values shown in the table below. For instance, a system that adapts to ISO8573.1 and will be rated for class 1.1.1 will give air with a dew point that’s not higher than -70 °C (-94 °F). Every air particle will be 0.1 um or smaller, and the oil content will be 0.01 mg/m3 at its maximum. There are other standards for compressed air quality, like ANSI/ISA 7.0.01-1996 for instrument air.
ANSI/ISA-7.0.01-1996 for instrument air.
| Quality
Class |
Particle
Size (um) |
Dew
point °C |
Dew
point °F |
Oil
Content (mg/m3) |
| 1 | 0.1 | -70 | -94 | 0.01 |
| 2 | 1 | -40 | -40 | 0.1 |
| 3 | 5 | -20 | -4 | 1 |
| 4 | 15 | 3 | 37 | 5 |
| 5 | 40 | 7 | 45 | 25 |
| 6 | – | 10 | 50 | – |
7. How do you reliably measure the dew point in compressed air?
A few principles of dew point measurement apply to every type of instrument, no matter the manufacturer:
- Select an instrument with the correct measuring range:
Some instruments are suitable for measuring high dew points, but not low dew points. Likewise, some instruments are appropriate for extremely low dew points but when exposed to high dew points, they are compromised.
- Understand the pressure characteristics of the dew point instrument: Some instruments aren’t suitable to be used at process pressure. They could be installed to have the compressed air measured after it expanded to atmospheric pressure, however, the dew point value that was measured will need to be corrected in case the pressure dew point is at the measurement parameter desired.
- Install the sensor correctly: Follow instructions from the manufacturer. Don’t install dew point sensors at the end of stubs as well as other “dead end” pieces of pipe where there’s no airflow.
Vaisala manufactures a family of instruments that are ideal for measuring dew point temperature in compressed air. DRYCAP® sensor technology provides quick dew point measurements from temperatures that are ambient down to -60 °C (-76 °F) with an accuracy of ±2 °C (±3.6 °F) over the entire range.
In addition to the general principles given above, think about the following when selecting and then having the Vaisala dew point instrument installed:
A. The perfect dew point sensor installation separates the compressed air line’s sensor. This is done by having the sensor installed in a “sample cell” and linking the cell to a “T” in the air line that’s compressed at the point of interest. Then, a little amount of compressed air’s bled beyond the sensor. The cell must be made of stainless steel and with tubing,linked to the “T” (1/4” or 6 mm). It’s beneficial to have an isolation valve installed between the air line and the cell. This allows easy removal and installation of the sensor.
B. A flow-regulating device is essential to control airflow beyond the sensor. The wanted flow rate is only 1 slpm (2 scfh). The regulating device could be a valve or a leak screw. To make measurements for the pressure dew point, the regulating device’s installed to the sensor’s downstream, so that when the isolation valve’s opened, the sensor is at the process pressure. To make measurements to the dew point at atmospheric pressure,you need to have the regulating device installed at dew point sensor’s upstream.
C. Don’t go beyond the flow rate recommended. When you measure pressure dew point, an overwhelming flow rate will lead to a local pressure drop at the sensor. Due to the fact that dew point temperature’s pressure sensitive, this will lead to an error during the measurement.
D. Stainless steel(SS) is the perfect tubing material. Nonmetallic tubing can desorb and absorb water vapor, which creates a lag in the response of measurement. If SS tubing isn’t available, consider using PTFE or other materials that do not absorb water. Avoid using yellow rubber tubing or clear plastic tubing.
E. It’s possible to reduce the cost of installation for permanent dew point instruments by installing the sensor directly in the compressed air line. In these cases, it is important to choose a location where the sensor has adequate airflow and where the temperature of the compressed air is at or near ambient.
8. How often must a dew point sensor be calibrated or checked?
- An instrument that displays one value all of the time, as if the output or display were locked.
- An instrument that is “bottomed out,” always reading its lowest possible value.
- An instrument that is erratic, changing rapidly or randomly over a wide range of values.
- An instrument that shows impossibly wet or dry dew point values.
9. How often should a dew point sensor be checked or calibrated?
It’s best to go for the recommendation of the manufacturer. Vaisala suggests a 1 or 2 year calibration interval, which depends on the instrument. At times, a simplistic field check towards a calibrated portable instrument’s enough to verify the correct instrument’s (and other) operation.
Vaisala provides detailed calibration information in the User’s Guide that is shipped with each instrument. Any time that you have doubts about the performance of your dew point instruments, it is wise to check their calibration.
Source: Vaisala