| 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
available.
- How
much water is in compressed air?
Answer 1: 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
cycle.
- How
much water can be removed by drying compressed air?
Answer 2: 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° 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° 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° 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° 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?
Answer 3: 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 dryers?
Answer 4: 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
cleaning.
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 air dryer installed and it had been working
fine, but suddenly I'm finding water in my compressed air, what's
going on?
Answer 5: 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 at:
Wilkerson
Operations
Pneumatic Division
v
Richland, Michigan 49083 USA
Phone: 269.629.2550
Fax: 269.629.2475
|
Harmon
Distributing