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Dirt,
water and oils within compressed air systems are a source of corrosion and
contamination. Air contains water vapor which condenses into liquid droplets
due to temperature and pressure changes in compressed air systems. Trace
amounts of oil, pipe scale and other particulates from the local
environment, air compressor, receiver tanks and distribution plumbing
contribute to coating the pipeline and equipment in the system and it all
coalesces to low points in the plumbing. While coalesced liquid at low
sections of piping can be removed with float drain traps without any loss of
compressed air, centrifugal separators will separate both entrained droplets
and slugs of water that accumulate in those low areas of piping when drain
traps are not installed.
Thus, centrifugal separators serve an
important function within a compressed air system, to prevent entrained
liquid and particles moving at high velocity through the system from
clogging, wearing or fouling the other components of the air system.
Centrifugal air/liquid separators achieve separation of entrained droplets
and particles larger than 10 microns in diameter with 99% efficiency through
impingement, change in velocity and centrifugal inertia.
Impingement and Velocity Reduction
The pathway for moist air
within the separator is non-direct; it enters the separator and is deflected
away from the outlet nozzle. Due to the difference in density of air and
droplets/particles, the trajectory of the heavier droplets/particles causes
impingement, especially at the separator entrance. This impingement
coalesces the droplets and particles into a larger mass and via
gravitational force, drain out of the bottom of the separator.
Simultaneously, since the separator body has a larger cross section than the
inlet and outlet piping, the air velocity slows down as it passes through
the separator. The gravitational force acting upon droplets and particles
larger than 10 microns is strong enough to affect their trajectory downwards
and away from the outlet nozzle. Thus, impingement and air velocity
reduction combined is why separation efficiency is maintained at low flow
conditions – the droplets are simply too heavy to exit the vessel. This is
why we describe air/liquid separators as having an “infinite” turndown
ratio, meaning that as long as the separator is sized for the high flow rate
condition, it will maintain efficiency all the way down to a zero-flow
condition because at lower flow rates the effects of gravity are more
pronounced.
Centrifugal Vortex
At higher flow
rate conditions, a centrifugal vortex is formed in the center of the
separator, creating an “air barrier” in which only the finest droplets
(smaller than 10 microns in diameter) get carried though the outlet nozzle
of the separator; all of the larger droplets and particles are propelled
towards the inner circumference of the vessel where they coalesce into a
liquid film and traverse to a common drain port.
Drainage
The separated liquid and particles collect in the lower section of the
separator body, thus periodic drainage is required to prevent the liquid
level from rising too high and exiting the separator. While actuated valves
and pumps can be used to drain the separated liquid, for air compressor
systems it is most common to use a mechanical valve referred to as a float
drain trap.
The separated liquid and particles fill the trap body and an internal float rises, lifting a valve off its seat. The resulting differential pressure between the system and the drainage location results in purging the liquid from the trap at a rate dictated by both the differential pressure and the valve seat orifice diameter. Non-floating particles sink to the bottom of the trap which has a blow-down plug for periodic clean-out. It is also common to install a Y strainer between the separator drain and float drain trap to protect the trap from fouling.
We
offer cast iron gas/liquid separators having an integral float drain trap
which is easier to install. We also offer stand-alone cast iron, ductile
iron, cast steel and stainless-steel float drain traps which can be
installed at any air/liquid separator drain port. The most common air/liquid
separator materials are cast iron and carbon steel with 304L and 316L SS
sometimes used for food/beverage/pharmaceutical type applications.
Air/Liquid Separators Locations
It depends upon the size
of the air system, typically air/liquid separators are used at the outlet of
the air compressor and often times prior to point of use or critical
pipeline components in which entrained liquid and particles would either be
detrimental to those components or the process itself. Large compressor
systems may require several separators.
Aftercooler
Compressed air systems larger than 10HP often
include an aftercooler, which is a section of water-cooled piping used to
reduce the specific volume of air to increase the efficiency of the
compressor. An aftercooler also condenses up to 2/3 of the liquid within the
air, thus a separator installed after the aftercooler stage removes both
condensate and oil carry-over from the compressor.
Intercooler
Similar to an Aftercooler except installed
prior to the compressor or between compression stages to improve air
compressor efficiency and protect the compressor. The removal of heat from
the air results in the condensation of the water vapor suspended in the air.
Separator Styles
There are several different styles
of separators; the most common versions used for compressed air applications
are the type T and L series. The L series has a horizontal oriented
cylindrical shaped body, making it very compact, however its liquid
separation capacity is limited to the equivalent of 5% of its maximum flow
rate capacity. Probably more importantly, the L series cannot handle liquid
slugs, which is how we describe the phenomenon of liquid which builds up in
low sections of piping and periodically swept downstream as a large liquid
droplet whose diameter matches the pipeline ID.
The most common style
of air liquid separator for compressed air applications is referred to as a
type T because its profile looks like the letter “T”, having an inlet and
outlet 180° apart and in-line with each other. The T style has a liquid
removal capacity up to 40% of its maximum flow capacity and separates liquid
slugs.
We do offer additional separator designs to support a variety of
flow pathways including vertical and combinations of vertical and
horizontal.
Separator Sizing
There are four basic
design criteria to properly size the separator body for a compressed air
application:
1. Design temperature
2. Minimum operating pressure and
design pressure
3. Maximum flow rate (volumetric or mass units)
4.
Estimated flow of liquid to separate
If you have a reciprocating
style air compressor, your maximum flow rate should be input at twice the
rated value (whether you use sizing charts or our web-based sizing
calculator).
It is often difficult to estimate the amount of
entrained liquid to separate; our sizing calculator provides the maximum
liquid removal rates in both volumetric and mass units in hours, minutes and
seconds to assist with estimating the proper sized separator.
If the
separator body size required is larger than the pipeline it is to be
installed into, we can provide the separator with smaller connections to
match your piping. If the separator size required is smaller than your
existing piping, we typically recommend using a larger size separator to
match your piping.
Our web based sizing calculator enables you to
estimate your separator size very quickly and analyze “what if” scenarios
using larger separators or performance with different design criteria; if
you have many separators to size for a system, our calculator will simplify
that aspect of system design immensely:
https://fd-separators.com/sizing/FDPP-WA%20CALCULATOR-WEB/gas-separator-sizing-tool.html