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During the process of compressing
air, atmospheric air along with water vapor and atmospheric
contaminants (hydrocarbon or chemical vapors), are drawn into the
compressor intake. Additionally, the compression chambers of most
compressors require oil for lubrication, sealing and cooling. Once
compressed, the air flows into an aftercooler to remove the heat of
compression. As the air cools in the aftercooler, water and
hydrocarbon vapors will condense. Additional condensation takes
place as the air is further cooled in the piping and air dryers.
This condensed moisture must be removed from the compressed air
system to prevent damage to downstream components and processes.
Drain valves are installed on moisture separators, coalescing
filters, air receivers, air dryers and drip legs to remove this
condensate from the compressed air system. The condensate should be
piped from automatic drain valves to oil/water separators to remove
the oil from the condensate prior to discharge to a drain.
Environmental regulations strictly
prohibit the discharge of oily wastes and chemicals, including the
condensate drained from a compressed air system. Because of these
requirements, municipalities regulate the discharge of compressor
condensate to surface water, wastewater treatment facilities, and
Compressor condensate must therefore
be either collected or treated prior to disposal. An oil/water
separator can be used here to remove the oil from the
Collection involves the drainage of
the condensate into drums or storage tanks. The drums or tanks are
then hauled away to an approved disposal facility.
Transportation, storage, and disposal
costs (TSD) can exceed $500 for a single 55-gallon (208 liter)
drum. A typical 25 hp (18 kW) compressed air system will generate
approximately 20 gallons (76 liters) of condensate in one (1) day.
Eleven 55-gallon (208 liter) drums are required to dispose the
condensate produced in only one (1) month of operation resulting in
a cost of $5,500. This is a significant disposal cost.
Since the condensate is approximately
95% water and 5% oil, oil/water separators have been developed to
reduce or eliminate the amount of oil in the condensate.
An oil/water separator system,
installed on the 25 hp (18 kW) compressed air system example above,
can reduce the number of 55-gallon (208 liter) drums from eleven
(11) to less than one (1). Condensate disposal costs will therefore
be reduced by $5,000 per month. Payback on an oil water separator
is typically much less than one year.
Oil Water Separators
Oil/water separators are available in
four basic technologies:
- chemical adsorption
- gravitational separation
- mechanical separation
- vaporization separators
Chemical Absorption Oil/Water
Chemical absorption oil/water separators (fig.
OWS1-1) are filled with a chemical media developed to attract the
oil while repelling water molecules. Depressurized oily condensate
drains into the chemical absorption separator were the oil is
bonded to the media. Clean water then flows to drain.
The life of the chemical separator
depends on the quantity of oil in the condensate. The oil
concentration in the condensate may vary from 40 ppm to more than
600 ppm. Actual oil concentration is dependant on the type and
condition of the compressor, the type of oil and ambient relative
Since absorption capacity is
approximately 50% of the media bed weight, a 15 gallon (57 liter)
absorption separator will capture 7 - 8 gallons (26 - 30 liters) of
Gravitational Separation Oil
Gravity separation is accomplished by flowing the condensate into
a settling tank. Oil is skimmed off the top and water is pumped off
the bottom of the oil/water separator tank. The water removed from
the bottom of the tank still contains oil. The percent depends on
the demulsibility of the oil. Gravitational separation devices are
simple and will separate free oils that have migrated to the top of
the settling tank. Gravitational oil water separators are not
effective on oils that have emulsified in the water since the oil
does not naturally separate from the water.
A coalescer performs mechanical
separation through pressure drop, torturous path, and oleophobic
attraction. The pressure drop across the coalescer causes some of
the oil to drop out of phase and separate. Pores create a torturous
path through the coalescer and oil droplets adhere to the coalescer
and combine coalescing. To enhance the oil's collecting and forming
droplets and draining off the coalescer element an oleophobic (oil
resistant) filter media is used. The combination of these factors
make coalescers more than 99% effective oil separators. The actual
efficiency of the coalescer will depend on the type of oil being
separated. Some synthetic lubricants will not be removed by a
coalescing element. Coalescer element's foul over time and require
replacement based on pressure drop.
Activated charcoal adsorbs oil and
most synthetic lubricants. The condensate is passed through a
charcoal chamber. The activated charcoal adsorbs the oil reducing
the amount of oil in the condensate. Activated charcoal chambers
foul over time and require scheduled replacement.
Mechanical oil/water separators are
usually a combination of a gravitational settling tank with a
coalescer element. Charcoal is also typically used as a polishing
bed to remove oils not separated by the coalescer.
(Note: Since the condensate will
contain a mixture of water and lubricating oil, be sure to drain
all condensate in a manner approved by all federal, state and local
regulatory agencies. Oil/water separators are available to assist
in this function.)
Vaporization Oil Water
Vaporization oil water separators utilize an external heat source,
such as electric or steam heaters, to boil-off the water. The
remaining oil is then drained into a container for proper disposal.
Vaporization separators will efficiently separate almost all oils,
however attention must be given to the materials of construction of
the oil/water separator since the oily waste may be corrosive.
Installation and Maintenance
Installation of the oil/water is typically quite simple. The oily
condensate from drain valves is piped to a depressurization
chamber/vessel to reduce the pressure to atmospheric pressure. The
oily condensate then flows into the oil/water separator.Oil water
separators are normally installed indoors to prevent freeze-up of
the water during wintertime operation. If outdoor installation
cannot be avoided, contact the manufacturer for approval and
- Drain collected oil as necessary. Properly dispose all
collected oil as required by law.
- Clean the settling tank or boiling tank with water to remove
particulate build-up once a year or as necessary. Dispose of water
- Change the activated carbon adsorber before it is fully
saturated with oil.
- Replace the coalescer element based on Delta P.
Follow the manufacturer's maintenance
Disposal of Condensate
Since the condensate will contain a mixture of water and
lubricating oil, be sure to drain all condensate in a manner
approved by all federal, state and local regulatory agencies.
Oil/water separators are available to assist in this function.