FACTS ABOUT COALESCING FILTERS
COALESCING PROCESS
EFFICIENCY
- Coalescing media have a random fiber network, unlike a conventional filter.
- Conventional filters act as a sieve, particles larger than the space in the filter are retained and smaller particles
penetrate the filter. - Conventional filters operate most efficiently above the 5-micron size range.
- Oil droplets that reach the secondary section of the oil separator are smaller than 2 microns.
- Conventional filters cannot remove sufficient amounts of oil from compressed gases.
- The fiber diameter in coalescing filters is in the same range as the diameter of the particles they are designed to
capture, 0.1 to 2 microns. - Oil particles strike the fibers of the coalescing filter and stick to the fiber.
- The random fiber matrix increases probabilities of collision between the fiber and oil particles.
- The sub-micron size oil particles grow into droplets as additional particles impinge on the fibers.
- Droplets coalesce more droplets and grow into large drops.
- The droplets move due to a pressure differential caused by gas flow and gravitational migration.
- Drops of oil appear as a liquid film on the downstream side of the filter, which drains into a sump from where it is
returned by a liquid seal. - This is coalescing - the agglomeration of submicron oil particles into droplets and into relatively larger drops.
- Filter media used in coalescing elements is composed of a random network of extremely fine borosilicate glass
fibers laid in layers of thin felt bonded with phenolic resin. - The fiber diameter, resin content, density and thickness are closely controlled to meet gas flow resistance and
filtration efficiency specification. - High collection efficiencies are obtained by the following mechanisms:
- 1 - Direct Interception
- 2 - Inertial Impaction
- 3 - Brownian Motion (Diffusion)
COALESCING PROCESS
- Direct interception occurs when a particle passes a fiber at a distance less than particle’s radius. The particle
makes contact with the fiber and is collected. - Inertial impaction occurs when oil particles 3 microns and larger collide with the fibers because their inertia or
momentum as gas flows through the random fiber network. - The particles are collected on the surface of the fibers and become a part of the liquid film which wets the fibers.
- Brownian Motion (Diffusion) -- Oil mist particles smaller than 0.6 microns are collected by Brownian diffusion.
These small sub-micron particles are recovered because they exhibit a random movement generally transverse to
the flow path. This results in a behavior of the sub-micron particles similar to droplets with an apparent diameter
equal to the amplitude of the motion. This rapid random motion or Brownian diffusion causes the capture of sub-
micron size particles that collide with fibers. Brownian displacement of particles increases as their size gets smaller,
hence the collection efficiency of coalescing filters is 100% below 0.3-micron size. - Larger oil-mist particles are collected by direct interception and inertial impaction while smaller particles are
collected by Brownian diffusion.
EFFICIENCY
- Oil is not absorbed or trapped in coalescing filters but drained as quickly as it enters and a coalescing filter
operates at optimum separation efficiency when it is saturated with oil. - If the coalescing filter is sized correctly the quantity of oil in the compressed gas will not affect the life if the filters.
- In most gas compression systems, coalescing filters are located downstream of the compressor where the oil and
gas streams are essentially free of dirt and solid particles due to prior filtration and scrubbing action of oil. - Dirt holding capacity and retention efficiency is not an important factor in coalescing filters.
- In gas compression applications initial high separation efficiency of the coalescing filter is maintained through out
the life of the filter if the system has stayed clean. - Dirt, foreign particles, weld slag, metallic chips from pipe fabrication and oil deterioration byproducts etc., end up
in coalescing filters resulting in high differential pressure across the coalescing elements. - The oil separator has to be serviced and coalescing filters replaced. Whenever the entire oil charge is replaced, it
is important to change the filters. If the pressure drop across the coalescing filters reach or exceed 3 PSID, the
coalescers need to be changed
Section
[Code of Federal Regulations]
[Title 40, Volume 31]
[Revised as of January 1, 2008]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR1065.590]
TITLE 40--PROTECTION OF ENVIRONMENT
CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
PART 1065_ENGINE-TESTING PROCEDURES--Table of Contents
Subpart F_Performing an Emission Test in the Laboratory
Sec. 1065.590 PM sampling media (e.g., filters) preconditioning and
tare weighing.
Before an emission test, take the following steps to prepare PM
sampling media (e.g., filters) and equipment for PM measurements:
(a) Make sure the balance and PM-stabilization environments meet the
periodic verifications in Sec. 1065.390.
(b) Visually inspect unused sample media (e.g., filters) for defects
and discard defective media.
(c) To handle PM sampling media (e.g., filters), use electrically
grounded tweezers or a grounding strap, as described in Sec. 1065.190.
(d) Place unused sample media (e.g., filters) in one or more
containers that are open to the PM-stabilization environment. If you are
using filters, you may place them in the bottom half of a filter
cassette.
(e) Stabilize sample media (e.g., filters) in the PM-stabilization
environment. Consider an unused sample medium stabilized as long as it
has been in the PM-stabilization environment for a minimum of 30 min,
during which the PM-stabilization environment has been within the
specifications of Sec. 1065.190.
(f) Weigh the sample media (e.g., filters) automatically or
manually, as follows:
(1) For automatic weighing, follow the automation system
manufacturer's instructions to prepare samples for weighing. This may
include placing the samples in a special container.
(2) For manual weighing, use good engineering judgment to determine
if substitution weighing is necessary to show that an engine meets the
applicable standard. You may follow the substitution weighing procedure
in paragraph (j) of this section, or you may develop your own procedure.
(g) Correct the measured mass of each sample medium (e.g., filter)
for buoyancy as described in Sec. 1065.690. These buoyancy-corrected
values are subsequently subtracted from the post-test mass of the
corresponding sample media (e.g., filters) and collected PM to determine
the mass of PM emitted during the test.
(h) You may repeat measurements to determine the mean mass of each
sample medium (e.g., filter). Use good engineering judgment to exclude
outliers from the calculation of mean mass values.
(i) If you use filters as sample media, load unused filters that
have been tare-weighed into clean filter cassettes and place the loaded
cassettes in a clean, covered or sealed container before removing them
from the stabilization environment for transport to the test site for
sampling. We recommend that you keep filter cassettes clean by
periodically washing or wiping them with a compatible solvent applied
using a lint-free cloth. Depending upon your cassette material, ethanol
(C2H5OH) might be an acceptable solvent. Your cleaning frequency will
depend on your engine's level of PM and HC emissions.
[Code of Federal Regulations]
[Title 40, Volume 31]
[Revised as of January 1, 2008]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR1065.590]
TITLE 40--PROTECTION OF ENVIRONMENT
CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
PART 1065_ENGINE-TESTING PROCEDURES--Table of Contents
Subpart F_Performing an Emission Test in the Laboratory
Sec. 1065.590 PM sampling media (e.g., filters) preconditioning and
tare weighing.
Before an emission test, take the following steps to prepare PM
sampling media (e.g., filters) and equipment for PM measurements:
(a) Make sure the balance and PM-stabilization environments meet the
periodic verifications in Sec. 1065.390.
(b) Visually inspect unused sample media (e.g., filters) for defects
and discard defective media.
(c) To handle PM sampling media (e.g., filters), use electrically
grounded tweezers or a grounding strap, as described in Sec. 1065.190.
(d) Place unused sample media (e.g., filters) in one or more
containers that are open to the PM-stabilization environment. If you are
using filters, you may place them in the bottom half of a filter
cassette.
(e) Stabilize sample media (e.g., filters) in the PM-stabilization
environment. Consider an unused sample medium stabilized as long as it
has been in the PM-stabilization environment for a minimum of 30 min,
during which the PM-stabilization environment has been within the
specifications of Sec. 1065.190.
(f) Weigh the sample media (e.g., filters) automatically or
manually, as follows:
(1) For automatic weighing, follow the automation system
manufacturer's instructions to prepare samples for weighing. This may
include placing the samples in a special container.
(2) For manual weighing, use good engineering judgment to determine
if substitution weighing is necessary to show that an engine meets the
applicable standard. You may follow the substitution weighing procedure
in paragraph (j) of this section, or you may develop your own procedure.
(g) Correct the measured mass of each sample medium (e.g., filter)
for buoyancy as described in Sec. 1065.690. These buoyancy-corrected
values are subsequently subtracted from the post-test mass of the
corresponding sample media (e.g., filters) and collected PM to determine
the mass of PM emitted during the test.
(h) You may repeat measurements to determine the mean mass of each
sample medium (e.g., filter). Use good engineering judgment to exclude
outliers from the calculation of mean mass values.
(i) If you use filters as sample media, load unused filters that
have been tare-weighed into clean filter cassettes and place the loaded
cassettes in a clean, covered or sealed container before removing them
from the stabilization environment for transport to the test site for
sampling. We recommend that you keep filter cassettes clean by
periodically washing or wiping them with a compatible solvent applied
using a lint-free cloth. Depending upon your cassette material, ethanol
(C2H5OH) might be an acceptable solvent. Your cleaning frequency will
depend on your engine's level of PM and HC emissions.
|
(j) Substitution weighing involves measurement of a reference weight
before and after each weighing of PM sampling media (e.g., filters).
While substitution weighing requires more measurements, it corrects for
a balance's zero-drift and it relies on balance linearity only over a
small range. This is most advantageous when quantifying net PM masses
that are less than 0.1% of the sample medium's mass. However, it may not
be advantageous when net PM masses exceed 1% of the sample medium's
mass. If you utilize substitution weighing, it must be used for both
pre-test and post-test weighing. The same substitution weight must be
used for both pre-test and post-test weighing. Correct the mass of the
substitution weight for buoyancy if the density of the substitution
weight is less than 2.0 g/cm\3\. The following steps are an example of
substitution weighing:
(1) Use electrically grounded tweezers or a grounding strap, as
described in Sec. 1065.190.
(2) Use a static neutralizer as described in Sec. 1065.190 to
minimize static electric charge on any object before it is placed on the
balance pan.
(3) Select a substitution weight that meets the requirements for
calibration weights found in Sec. 1065.790. The substitution weight
must also have the same density as the weight you use to span the
microbalance, and be similar in mass to an unused sample medium (e.g.,
filter). A 47 mm PTFE membrane filter will typically have a mass in the
range of 80 to 100 mg.
(4) Record the stable balance reading, then remove the calibration
weight.
(5) Weigh an unused sample medium (e.g., a new filter), record the
stable balance reading and record the balance environment's dewpoint,
ambient temperature, and atmospheric pressure.
(6) Reweigh the calibration weight and record the stable balance
reading.
(7) Calculate the arithmetic mean of the two calibration-weight
readings that you recorded immediately before and after weighing the
unused sample. Subtract that mean value from the unused sample reading,
then add the true mass of the calibration weight as stated on the
calibration-weight certificate. Record this result. This is the unused
sample's tare weight without correcting for buoyancy.
(8) Repeat these substitution-weighing steps for the remainder of
your unused sample media.
(9) Once weighing is completed, follow the instructions given in
paragraphs (g) through (i) of this section.
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Quality American Manufacturer & distributor of Industrial Air and Liquid filters and filter
fabrics for an unlimited variety of industries and applications, including standard OEM and
custom specifications.
fabrics for an unlimited variety of industries and applications, including standard OEM and
custom specifications.
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- Standard OEM specifications or custom-designed, engineered
and manufactured for your application..
