|Compressed air has applications
in virtually all industries. Its uses range from being an efficient,
reliable source of pneumatic power in manufacturing plants to operating
critical instrumentation in process industries. In every application, air
quality can be adversely affected by contaminants like water, oil and dirt.
The result is lower productivity, increased maintenance, and higher
operating costs. For these reasons, air system designers must recognize and
understand the types of contaminants they face.
The most prevalent contaminant is
water. Water, in the form of vapor, enters the air system at the compressor
intake, and is concentrated to the saturation point by compression. As
cooling occurs downstream of the compressor, the moisture in this saturated
air condenses into harmful liquid water.
Still another contaminant, oil, is
injected into air systems by lubricated compressors. Many gallons of oil can
enter an air system over the course of a year in this way.
Dirt takes many forms in air
systems since it enters from several different sources. Small particles of
atmospheric dust not removed by intake filters are concentrated by
compression. Pipescale forms over time by the gradual rusting process.
Hot-running, lubricated compressors create fine particles of carbon dust
when lubricants break down.
Today's air system designer is
faced with the challenge of improving air quality through the selection of
drying and filtering equipment that is best suited to removing these
contaminants. Judgments must be made based on the particular application,
the degree of dryness and cleanliness required, and the types of equipment
How much water is in compressed air?
For example, a 25 HP Air Compressor running continuously for 24 hours at
full load, with inlet conditions of 75 degrees and 75% relative humidity can
generate up to 18 gallons per day. There will be proportionately more or
less water depending on the compressor size, inlet conditions and duty
How much water can be removed by drying
Actually, nearly two-thirds of the water in compressed air (or 12 of 18
gallons in the answer to question one can be removed with an efficient and
clean aftercooler. An aftercooler is a heat exchanger located at or
downstream of the air compressor which is either cooled by a fan or can be
water cooled. By reducing the compressed air temperature every twenty
degrees, half of the water vapor is condensed to a liquid and can be removed
with a moisture separator and automatic drain trap. Most aftercoolers reduce
the temperature to about 100 degrees F.
However, this is still not "dry enough" for many uses of compressed air,
such as painting, instrumentation, robotics, etc.
The next step in drying is to install a refrigerated air dryer. This
continues to cool the air to just under 40 degrees F., which reduces the
water left to only 5 % of the total (using the example in question one, of
the 18 gallons per day total, less than one gallon of water is left). This
is adequate drying for 90% of the compressed air systems.
If the compressed air lines are not exposed to temperatures below 40 degrees
F. then the remaining water will stay in a vapor or gas form and will not
condense or liquefy.
If there are air lines exposed to freezing temperatures or additional
dryness is required due to the critical use of the compressed air, then a
desiccant dryer would be required. Desiccant dryers remove over 99% of the
water in compressed air and are usually rated with pressure dew points of
minus 40 or minus 100 degrees F.
A membrane dryer could also be used downstream of a refrigerated dryer for
point-of-use/ critical applications.
What's the best type of dryer?
It depends. A desiccant dryer provides more efficient water removal, but
it's also more expensive initially and most require about 15% of the rated
inlet capacity (SCFM) for purging, so you lose compressed air.
Therefore, the "best" type of dryer in many cases is a refrigerated dryer
due to its lower initial and operating cost.
Many newer compressed air systems may use a combination of dryers. A
refrigerated dryer sized for the air compressor will remove 95% of the water
and provide adequate dry air for most plant applications. Additional
desiccant or membrane dryers sized for the specific application (i.e. dust
collector exposed freezing temperature;
CNC plastic molding equipment; food processing
application, etc.) can be installed downstream of the refrigerated dryer and
prior to where the critical dry air is required.
What type of filters are recommended for
We recommend a 5-micron pre-filter for a refrigerated dryer. This removes
the larger particles and droplets and helps keep the internal heat exchanger
on the dryer cleaner. A high efficiency coalescing filter rated at .01
microns can be placed downstream of the refrigerated dryer for additional
On a desiccant air dryer we recommend the 5 micron pre-filter followed
immediately by the coalescing .01 micron filter. This helps prevent any oil
contamination of the desiccant material. A 1 micron particulate filter
should be placed downstream of the desiccant dryer to capture any desiccant
dust from migrating downstream.
A vapor filter which removes oil vapor, odors and taste can be used as a
final filter in either case.
In all filter selections, the rated pressure drop should be as low as
possible (1-3 psi) to ensure longer element life.
I have a refrigerated
installed and it had been working fine, but suddenly I'm finding water in
my compressed air,
what's going on?
There could be several explanations for this problem. The most common is the
auto drain on the moisture separator of the dryer is fouled up and not
working. "Float-type" drain traps require regular maintenance. To help
eliminate this problem, we recommend electric solenoid valve drain traps.
The new Wilkerson WRD-series dryer
has this as a standard. Older or smaller dryers can be retrofitted to use an
electric drain trap.
Other maintenance-related reasons could be excessive high inlet temperature,
high ambient temperature, a dirty condenser, heat exchanger, or a
malfunctioning fan motor.
The most serious explanation could be a leak in the refrigerant system or a
bad refrigeration air compressor.
The new Wilkerson WRD-series
refrigerated dryers have a visual dew point indicator on its control panel
which can forewarn you of a problem before you get water downstream. Also
available as an option is a set of dry contacts which will signal a high dew
point alarm to a separate horn, light or master building computer. This
option can be added later to any WRD dryer.
Harmon Distributing wishes to
thank Wilkerson Air for the preceding information.
More information may be found
Richland, Michigan 49083 USA