Dewpoint in Compressed Air
Frequently asked
questions:
2. What is the difference between dewpoint and “pressure
dewpoint?”
3. What is the effect of pressure on dewpoint?
4. Why is knowledge of dewpoint in compressed air important?
5. What is the typical range of dewpoint temperatures to be
found in compressed air?
6. What are the standards for quality of compressed air?
7. How is dewpoint in compressed air reliably measured?
8. What are the telltale signs of a malfunctioning dewpoint
sensor?
9. How
often should a dewpoint sensor be checked or calibrated?
Vaisala DRYCAP® Hand-held
Dewpoint Meter DM70
1. What is dewpoint?
Dewpoint
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 mixture of gases, the
total pressure of the gas is the sum of the partial pressures of the component
gases. This is Dalton’s law and it is represented as follows:
Ptotal = P1 + P2 + P3…
The quantity of
any gas in a mixture can be expressed as a pressure. The major components of
air are nitrogen, oxygen, and water vapor, so total atmospheric pressure is
composed of the
partial
pressures of these three gases. While nitrogen and oxygen exist in stable
concentrations, the concentration of water vapor is 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 60 °F (20 °C), the maximum
partial pressure of water vapor is 23.3 millibars (mb). The value of 23.3 mb is
said to be the “saturation vapor pressure” at 60 °F (20 °C). In a 60 °F (20
°C), “saturated” environment, the addition of more water vapor results in the
formation of condensation. This condensation phenomenon can be exploited to
measure water vapor content. Gas of unknown water vapor concentration is passed
over a temperature-controlled surface. The surface is cooled until condensation
forms. The temperature at which condensation forms is called the “dewpoint temperature.”
Because there is a unique correlation between temperature and saturation vapor
pressure (remember, the maximum partial pressure of water vapor, also known as saturation
vapor pressure, is strictly a function of temperature), measuring the dewpoint
temperature of a gas is a direct measurement of the partial pressure of water
vapor. Knowing the dewpoint temperature, the corresponding saturation vapor pressure
can be calculated or looked up. The following table
shows some
values for temperature and the corresponding saturation vapor pressure:
Temperature °F (° C) Saturation vapor pressure
(mb)
68 (20) 23.3
32 (0)
6.1
14 (-10)
2.8
-4 (-20)
1.3
-40 (-40)
0.2
2. What is the
difference between dewpoint and “pressure dewpoint?”
The term “pressure
dewpoint” is encountered when measuring the dewpoint temperature of gases at
pressures higher than atmospheric pressure. It refers to the dewpoint
temperature of a
gas under
pressure. This is important because changing the pressure of a gas changes the
dewpoint temperature of the gas.
Instruments with graphical displays are useful for
monitoring dewpoint over a longer period of
time.
3. What is the
effect of pressure on dewpoint?
Increasing the
pressure of a gas increases the dewpoint temperature of the gas. Consider an
example of air at atmospheric pressure of 1013.3 mb with a dewpoint temperature
of 14 °F (-10 °C). From the table above, the partial pressure of water vapor
(designated by the symbol “e”) is 2.8 mb. If this air is compressed and
the total pressure is doubled to 2026.6 mb, then according to Dalton’s law, the
partial pressure of water vapor, e, is also doubled to the value of 5.6
mb. The dewpoint temperature corresponding to 5.6 mb is approximately 30 °F (-1
°C,) so it is clear that increasing the pressure of the air has also increased
the dewpoint temperature of the air. Conversely, expanding a compressed gas to
atmospheric pressure decreases the partial pressures of all of the component
gases, including water vapor, and therefore decreases the dewpoint temperature
of the gas. The relationship of total pressure to the partial pressure of water
vapor, e, can be expressed as follows:
P1/P2 = e1/e2
By converting
dewpoint temperature to the corresponding saturation vapor pressure, it is easy
to calculate the effect of changing total pressure on the saturation vapor
pressure. The new saturation vapor pressure value can then be converted back to
the corresponding dewpoint temperature. These calculations can be done manually
using tables, or performed by various kinds of software.
A
variety of sample cell hardware, including quick disconnects,
cooling
coil and welded compression fitting, makes it easy to
install
a dewpoint sensor in any process.
4. Why is
knowledge of dewpoint in compressed air important?
The importance
of dewpoint temperature in compressed air depends on the intended use of the
air. In many cases dewpoint is not critical (portable compressors for pneumatic
tools, gas station tire filling systems, etc.). In some cases, dewpoint is important
only because the pipes that carry the air are exposed to freezing temperatures,
where a high dewpoint could result in freezing and blockage of the pipes. In
many modern factories, compressed air is used to operate a variety of
equipment, some of which may malfunction if condensation forms on internal
parts. Certain water sensitive processes (e.g. paint spraying) that require
compressed air may have specific dryness specifications. Finally, medical and
pharmaceutical processes may treat water vapor and other gases as contaminants,
requiring a very high level of purity.
5. What is the
typical range of dewpoint temperatures to be found in compressed air?
Dewpoint
temperatures in compressed air range from ambient down to -112 °F (–80 °C),
sometimes lower in special cases. Compressor systems without air drying
capability tend to
produce
compressed air that is saturated at ambient temperature. Systems with
refrigerant dryers pass the compressed air through some sort of cooled heat
exchanger, causing water to condense out of the air stream. These systems typically
produce air with a dewpoint no lower than 23 °F (5 °C). Desiccant drying
systems absorb water vapor from the air stream and can produce air with a
dewpoint of -40 °F (–40 °C) 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 particle size for any remaining particles
•
Maximum allowable dewpoint temperature
•
Maximum remaining oil content
Each category is
given a quality class number between 1 and 6 according to the reference values
shown in the table below. As an example, a system that conforms to ISO8573.1
and is rated for class 1.1.1 will provide air with a dewpoint no higher than
-94 °F (–70 °C). All remaining particles in the air will be 0.1 um or smaller,
and the maximum oil content will be 0.01 mg/m3. There are other standards for
compressed air quality, such as ANSI/ISA- 7.0.01-1996 for instrument air.
ANSI/ISA-7.0.01-1996
for instrument air.
Quality Particle Dewpoint
Dewoint Oil Content
Class Size (um) °
C ° F (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 is
dewpoint in compressed air reliably
measured?
Some principles
of dewpoint measurement apply to all types of instruments, regardless of
manufacturer:
• Select an
instrument with the correct measuring range:
Some instruments are suitable for
measuring high dewpoints, but not low dewpoints. Similarly, some instruments
are suitable for very low dewpoints but are compromised when exposed to high
dewpoints.
• Understand the
pressure characteristics of the dewpoint instrument:
Some instruments are not suitable for
use at process pressure. They can be installed to measure compressed air after
it is expanded to atmospheric pressure, but the measured dewpoint value will
have to be corrected if pressure dewpoint is the desired measurement parameter.
• Install the
sensor correctly:
Follow
instructions from the manufacturer. Do not install dewpoint sensors at the end
of stubs or other “dead end” pieces of pipe where there is no airflow.
Vaisala
manufactures a family of instruments that are ideal for measuring dewpoint
temperature in compressed air. DRYCAP® sensor technology provides fast dewpoint
measurements from ambient temperature down to -76 °F (–60 °C) with an accuracy
of ±3.6 °F (±2 °C) over the entire range. In addition to the general principles
given above, consider the following when selecting and installing a Vaisala
dewpoint instrument:
A.
The best installation for a dewpoint
sensor isolates the sensor from the compressed air line. This is accomplished
by installing the sensor in a “sample cell” and connecting the cell to a “T” in
the compressed air line at the point of interest. A small amount of compressed
air is then bled past the sensor. The cell should be made of stainless steel
and connected to the “T” with tubing (1/4” or 6mm). It is useful to install an
isolation valve between the cell and the air line. This enables easy
installation and removal of the sensor.
B.
A flow-regulating device is necessary to
control to airflow past the sensor. The desired flow rate is only 1 slpm (2
scfh). The regulating device can be a leak screw or a valve. To measure
pressure dewpoint, the regulating device is installed downstream of the sensor,
so that when the isolation valve is opened, the sensor is at the process
pressure. To measure dewpoint at atmospheric pressure, the regulating device
should be installed upstream of the dewpoint sensor.
C.
Do not exceed the recommended flow rate.
When measuring pressure dewpoint, an excessive flow rate will create a local
pressure drop at the sensor. Because dewpoint temperature is pressure
sensitive, this will create an error in the measurement.
D.
The best tubing material is stainless
steel(SS). Nonmetallic tubing can absorb and desorb water vapor, creating a lag
in measurement response. If SS tubing is not available, consider using PTFE or
other materials that do not absorb water. Avoid the use of clear plastic tubing
or yellow rubber tubing.
E.
It is possible to reduce installation
costs for permanent dewpoint 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. What are the telltale signs of a
malfunctioning dewpoint sensor?
• 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 displays impossibly
dry or wet dewpoint values.
9. How often should a dewpoint sensor be
checked or calibrated?
It is best to
follow the manufacturer’s recommendation. Vaisala suggests a one or two year
calibration interval, depending on the instrument. Sometimes a simple field
check against a calibrated portable instrument is sufficient to verify correct
operation of other instruments. Vaisala provides detailed calibration information
in the User’s Manual that is shipped with each instrument. Any time that you
have doubts about the performance of your dewpoint instruments, it is wise to
check their calibration.
Tidak ada komentar:
Posting Komentar