A drying material disposed in the passage of a compressed air system
for removing moisture from air or gas includes a plurality of semi-rigid
porous substrates impregnated with lithium chloride. The drying
material is formed by preparing a saturated solution of lithium
chloride and a nonflammable solvent. Porous substrata are contacted
with the saturated solution to wet the substrata surface. The wet
substrata are heated at a sufficient temperature for a sufficient
duration to drive off excess liquid from the substrate. A desiccant
comprising lithium chloride impregnated substrata is produced.
1. A method of producing dried compressed air, comprising steps
compressing air, said compressing step to provide compressed air
that includes contaminant levels of oil therein; and
passing said compressed air through a desiccant comprising a porous
substrate impregnated with lithium chloride.
2. The method of producing dried compressed air of claim 1 further
comprising the step of periodically passing dried air through said
desiccant to regenerate the desiccant.
3. The method of producing dried compressed air of claim 1 wherein
the porous substrate comprises a plurality of semi-rigid porous
4. The method of producing dried compressed air of claim 3 wherein
said plurality of semi-rigid porous substrates are of a generally
5. The method of producing dried compressed air of claim wherein
the substrates have a diameter in a range of about 1/16" to
6. The method of producing dried compressed air of claim 3 wherein
said plurality of semi-rigid porous substrates are comprised of
7. The method of producing dried compressed air of claim 3 wherein
the plurality of lithium chloride impregnated semi-rigid porous
substrates arranged within a passage inside a housing to prevent
significant relative movement between them and to enable air to
flow past them.
8. The method of producing dried compressed air of claim 1 further
comprising the step of delivering said dried compressed air to an
air braking system.
9. The method of producing dried compressed air of claim 8 wherein
said air braking system is a trailer air braking system.
10. A compressed air supply system comprising:
a source of compressed air, said compressed air including contaminant
levels of oil therein; and
an air dryer comprising:
(a) a housing having a passage extending therethrough for receiving
air from said source and providing air to an outlet, said passage
adapted for holding a drying material for removing moisture from
the air; and
(b) a drying material disposed in the passage, said drying material
including a porous substrate impregnated with lithium chloride.
11. The compressed air supply system of claim 10 wherein the porous
substrate comprises a plurality of semi-rigid porous substrates.
12. The compressed air supply system of claim 11 wherein said plurality
of semi-rigid porous substrates are of a generally spherical configuration.
13. The compressed air supply system of claim 12 wherein the substrates
have a diameter in a range of about 1/16" to about 1/2.
14. The compressed air supply system of claim 11 wherein said plurality
of semi-rigid porous substrates are comprised of alumina.
15. The compressed air supply system of claim 11 wherein the plurality
of lithium chloride impregnated semi-rigid porous substrates arranged
within the passage to prevent significant relative movement between
them and to enable air to flow past them.
16. The compressed air supply system of claim 10 wherein said system
is a sub-system of an air braking system.
17. The compressed air supply system of claim 16 wherein said air
braking system is a trailer air braking system.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an air dryer system for use in a compressed
air system. More particularly, the invention relates to an air dryer
system comprising a lithium chloride desiccant.
2. Discussion of the Art
Compressed air systems are widely used to operate a number of devices,
and find particular use in braking systems for heavy vehicles. These
systems include an air compressor which compresses ambient air and
charges a storage reservoir. The compressor is engine operated and
a governor, responsive to the reservoir pressure, selectively enables
or disables the compressor as needed. Air dryers commonly employ
a drying material or desiccant through which air passes to remove
entrained moisture and limit problems related to moisture associated
with brake system components. Moisture is adsorbed by the desiccant
and removed from the compressed air before it is communicated to
the storage reservoir. In these known systems, periodic or cyclic
purging of the air dryers is required in order to purge or regenerate
Furthermore, in pneumatic systems such as those associated with
trailer air dryers, the moisture in the air is often mixed with
hydrocarbons. Low pH organic acids are formed which affect the various
rubber components associated with the system. In addition, the hydrocarbons
cause damage to the desiccants, and render them less productive.
For example, desiccants comprised of molecular sieves, zeolites
and silicas are oleophilic. As a result, air dryers associated with
these desiccants are less effective and function at reduced efficiency.
Entire desiccant cartridge assemblies are replaced on a periodic
basis, often between about one and five years. There is a need for
a desiccant useful in compressed air systems that tolerates oil
and efficiently absorbs the moisture present in the compressed air
that flows through a trailer air dryer.
SUMMARY OF THE INVENTION
The present invention solves the noted problems and others and
provides an air dryer useful in a trailer braking and suspension
According to the invention, an air dryer for a trailer braking
assembly comprises a housing having a passage extending therethrough
for receiving air from a supply end and providing air to a delivery
end. The passage is adopted for holding a drying material for removing
moisture from the air. A drying material disposed in the passage
includes a plurality of semi-rigid porous substrates impregnated
with lithium chloride.
According to another aspect of the invention, an adsorbent desiccant
for dehydrating moist gas by adsorption comprises a plurality of
semi-rigid porous substrata carriers. Each carrier is impregnated
with lithium chloride for removing moisture from the compressed
According to another aspect of the invention, a desiccant for removing
moisture from air in a compressed air system is formed by preparing
a saturated solution of lithium chloride and a polar solvent. Porous
substrata are contacted with the saturated solution to wet the substrata
surface. The wet substrata are heated at a sufficient temperature
for a sufficient duration to drive off excess solvent from the substrate.
A desiccant comprising lithium chloride impregnated substrata is
According to a still further aspect of the invention, there is
a method of drying compressed air in a truck brake system. A desiccant
comprising a plurality of lithium chloride impregnated semi-rigid
porous substrata is packed in an air passage defined by an air dryer
housing. Moist air is passed through the desiccant. Water vapor
is removed from the moist air, and liquefied lithium chloride is
retained on the porous substrate. Low humidity air is passed through
the desiccant to remove water from the liquefied lithium chloride.
According to a yet still further aspect of this invention, there
is a method for dehydrating a gas. A desiccant comprising lithium
chloride on rigid porous substrata is placed into a container strong
enough to withstand inlet gas pressure. Saturated inlet gas flows
through the desiccant and water is adsorbed by the lithium chloride.
Dried gas is passed through the desiccant to remove water from and
regenerate the desiccant.
A primary benefit of the present invention is the ability for the
desiccant in the trailer air dryer to remove moisture from the compressed
air at a high capacity.
Still another benefit resides in the oleophobic nature of the desiccant
employed in the trailer braking system which results in a longer
lasting desiccant over molecular sieve, zeolite based silica or
alumina based substrata.
Yet another benefit of the invention is realized by the ease with
which the lithium chloride impregnated substrata are made.
Still other advantages and benefits of the invention will become
apparent to those skilled in the art upon a reading and understanding
of the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal, cross-sectional view of a preferred form
of trailer air dryer assembly; and
FIG. 2 is cross-sectional view taken generally along lines 2.2
of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 and 2 are cross-sectional views of a preferred embodiment
of the trailer air dryer 10. The air dryer includes a housing 12
which is preferably an aluminum extrusion that is simple and cost
effective to manufacture. Of course, alternative constructions are
also contemplated without departing from the scope and intent of
the present invention. The housing includes a series of generally
parallel passages extending therethrough. A first passage, or supply
passage 14 has the largest cross-sectional dimension of all the
passages and is adapted to supply air used for trailer braking.
A second, or control line, passage 16 is of slightly smaller cross-sectional
dimension and is intended to carry a control signal or pneumatic
pressure signal to actuate the valve in response to operator demand.
The supply and control line passages each include a drying material
such as a desiccant material 18 or other drying agent. The drying
material fills the entire cross-section of the passage and extends
for a majority of the longitudinal extent of each of the passages.
The drying material is preferably contained at opposite ends by
filter members 30. For example, a preferred filter member is a perforated
cloth and filter media that removes particles from the air flow
through the supply and control line passages. The drying materials
and filter members are axially held in place in the passages by
end covers 32 34. End cover seals 36 38 are interposed between
the housing and the end covers to provide a sealed arrangement.
As will be appreciated, any conventional type of seal assembly can
In addition, a biasing assembly is provided to maintain the desired
preload and compaction of the drying material. In the preferred
arrangement, the biasing assembly is defined by a pair of springs
40 42 received at one end of the passages 14 16 respectively.
The springs are illustrated as coil springs that engage the end
cover 32 at one end and the associated filter member at the other
end. The springs impose a slight compressive force or compaction
on the drying material so that it operates in its intended manner.
Each end cover includes passages that communicate with the supply
line and the control line (not shown), respectively. For purposes
of identification, these passages are identified as 50 52 in the
supply side of the end cover and 54 56 in the delivery side end
cover. During normal operation of the trailer air dryer, air flow
proceeds into the opening 50 through the filters and drying material
in passage 14 and exits via the passage 54 in the second end cover.
Likewise, the new pneumatic control signal is sent through opening
52 through the filter members and drying material of passage 16
and outlets through opening 56 in the second end cover.
The desiccant drying material 18 disposed in the first and second
passages is comprised of a plurality of semi-rigid porous substrates
impregnated with lithium chloride. A variety of materials can be
used for the substrata including ceramics, plastics, pebbles, paper
(e.g. honeycomb) or other porous semi-rigid materials. Preferably,
the substrate is comprised of alumina. The material chosen for the
substrata should be significantly strong enough to withstand vibration
of trucks. It should also be able to withstand the temperature swings
associated with outdoor environments. Alumina meets these qualifications.
Furthermore, alumina is desirable because it adsorbs 5% to 200%
of its weight in water. No specific substrata configuration is required,
although a generally spherical configuration is preferred. The spherical
configuration is desirable because the shape is self packing in
the air dryer unit passage and can be compressed satisfactorily.
The preferred size range of the substrate spheres is such that
there is not a great pressure differential across the desiccant.
If the substrata are too small they will tend to flow about as the
compressed air passes or flows therethrough. If they are too large
the air will not be dried effectively. Hence, it has been determined
that preferred size range of the spherical substrata is in the range
of about 1/16" to 1/2" in diameter, preferably about 1/4".+-.1/16".
The desiccant of the present invention is useful in trailer air
dryers. It is also useful as an adsorbent desiccant in pressure
swing dehydration systems. It functions to dehydrate compressed
air or gas. The desiccant prevents freeze-ups or the accumulation
of liquid water in air passageways. These events can be particularly
detrimental, particularly in making brakes inoperable. The porous
substrates impregnated with lithium chloride remove water vapor
from compressed air. Air exiting the desiccant has a lower dew point
and a relative humidity below 13%. This prevents or reduces the
likelihood that water vapor will condense downstream.
In preparing the desiccant of the present invention, lithium chloride
is dissolved in a polar solvent in an amount up to its saturation
point. Preferably, the polar solvent is water, but other polar solvents
such as low molecular weight alcohols are contemplated by this invention.
The resulting liquid is applied to the substrata in a manner that
distributes the lithium chloride throughout the porous substrates.
For example, the saturated liquid can be sprayed onto porous substrata,
or the substrata can be immersed in it. Preferably, the saturated
liquid is poured over the substrata and allowed to sit until the
bubbling diminishes, thus ensuring that all air is forced out. The
substrata are completely wet.
The wet substrata are heated or baked at a temperature and for
a duration sufficient to drive off the moisture therefrom. For example,
it has been determined that the wet substrata can be heated at about
220-230 F. for about two hours to drive off any remaining water.
With all the water removed, substrata impregnated with solid lithium
chloride remain. These coated substrata are then stored in an airtight
container until used in an air dryer system.
The substrata act as carriers for the lithium chloride desiccant.
Dried lithium chloride impregnated porous substrata are packed into
compressed air passageways through which moist air is passed. The
lithium chloride desiccant removes moisture from the air, generally
down to a safe 13% or lower level of relative humidity. The dryer
removes moisture generally at high pressure, i.e., greater than
about 50 psig, generally about 100-200 psig. Drier air passes out.
A portion is captured or retained for regeneration of the desiccant.
The remaining air is expanded to atmospheric pressure. As air flows
through the desiccant, the lithium chloride begins to liquefy as
water vapor is removed. Water vapor will continue to be removed
as long as air or gas passing through the desiccant has a relative
humidity greater than 13%. The dew point of air exiting the desiccant
is reduced sufficiently so that water vapor will not condense downstream.
Moisture removal will continue until the solid lithium chloride
on the porous substrata is completely diluted with removed water.
The porosity of the substrata or spheres allows the lithium chloride
and water to be retained on the spheres. This prevents liquefied
lithium chloride from migrating downstream.
Once the lithium chloride becomes saturated, the process is reversed.
Dry or low humidity air, such as the air that was captured or retained,
is introduced and passed over the desiccant to regenerate or remove
water from the liquefied lithium chloride. Water is evaporated from
the desiccant, thus leaving the solid lithium chloride to remove
moisture from the next wave of moist air.
The capacity of lithium chloride to adsorb water vapor is much
greater than solid desiccants such as molecular sieve and silica
gel. For example, lithium chloride offers four to six times greater
adsorption capacity than the molecular sieve of the prior art. In
general, molecular sieves absorb roughly 10% of their weight and
silicates about 30% of their weight. Lithium chloride has 100% capacity.
It is capable of doubling its weight when water is adsorbed.
The desiccant of the present invention is useful in removing moisture
from gases other than air. Gas dehydration is important in improving
the operation of equipment and processes where the gas is used.
It is contemplated that the desiccant is useful in dehydrating gases
by using the pressure swing adsorption method. Here, lithium chloride,
solution or solid, is placed on substrata. The substrata are rigid,
porous and preferably spherical, although other materials and configurations
will serve the purpose as well. The desiccant is placed in a container
that can safely handle the inlet gas pressure. Inlet saturated gas
passes through the substrata at high pressures (i.e., over 50 psig)
where water is removed. Dried or moisture-reduced gas re-expands
through an orifice to low or atmospheric pressure and passes downstream.
A small amount of the dried gas is captured and diverted. Once the
desiccant becomes saturated, it is dehydrated or regenerated by
flowing the dried gas through it.
The cycle can be contorted electrically, electronically, or pneumatically
depending on the application. Various check valves, control valves
and process valves are used to control the process. The timing depends
on system sizing and other factors.
The invention has been described with reference to the preferred
embodiments. Obviously, modifications and alterations will occur
to others upon a reading and understanding of the detailed description.
The invention is intended to include such modifications and alterations
in so far as with they come within the scope of the appended claims
and equivalents thereof.