Introduction
[0001] The present invention generally relates to a desiccant unit for use in air conditioning
systems, and in particular in automotive air conditioning systems.
[0002] Desiccants are largely used in refrigeration systems, such as air conditioning systems,
to keep refrigerant liquids dry, because water is detrimental both to a correct operation
due to ice formation and to the properties of the refrigerant liquid itself due to
chemical degradation, such as hydrolysis, eventually leading to corrosion.
[0003] Desiccants generally used to this purpose are small granules or beads comprising
a major part of pure desiccant, such as molecular sieves, and a minor part of a mineral
binding component. As these desiccant granules are more or less subject to attrition,
especially in automotive air conditioning systems, a general requirement for these
systems is the presence of a filter capable of retaining particulate matter, namely
loose particles of desiccant and binding material. Hence, the desiccant is usually
enclosed in permeable containers, such as bags, pouches or cartridges made of mesh
or filter material through which the refrigerant liquid is passed to be separated
from moisture and filtrated.
[0004] US-4,013,566 to Adsorbex Inc. describes a flexible solid desiccant body comprising
finely divided particles of desiccant material encapsulated in a moisture transmissive
polymer solid matrix of a cured thermoset aliphatic epoxy resin. This patent further
states that certain epoxy copolymers, such as aromatic copolymers of epichlorhydrin-bisphenol
A are not suited due to their extremely low vapour transmission rates.
Object of the invention
[0005] The object of the present invention is therefore to provide a desiccant unit with
improved moisture adsorption capacity and which is not subject to physical attrition
and to the release of loose particles.
General description of the invention
[0006] In order to overcome at least some of the abovementioned problems, the present invention
provides for a desiccant unit as described in claim 1.
[0007] The present invention discloses a desiccant unit comprising a housing and a desiccant
body arranged inside said housing, said desiccant body including a desiccant agent
and a supporting matrix, wherein said supporting matrix comprises a thermoplastic
polymer material and a channel agent. According to the invention, the desiccant body
has a tubular shaped form with a first protruding spiral. This first spiral is arranged
on an inner and/or outer surface of the tubular shaped form, preferably along substantially
the whole length of the body.
[0008] A first role of this spiral is to guide the flow of refrigerant up or down the body
while lengthening the path taken by the fluid. A second function of the spiral is
to increase the contact or exchange surface of the desiccant body, thereby further
enhancing its moisture adsorbing capacity. An auxiliary advantage of this spiral is
to raise the dimensional stability of the desiccant body without undue increase in
weight.
[0009] In the case of an outer spiral, the clearance between the tip of the spiral and the
inner wall of the housing is generally chosen to substantially prevent the refrigerant
fluid from flowing over this tip and thereby 'shortcutting' the preset flow path.
[0010] Thermoplastic materials are known and largely used because of their advantageous
physical and chemical properties, such as high flexibility, resilience and resistance
to physical and thermal shocks, as well as their good chemical inertness. Since the
desiccant agent is integrated and strongly held inside such a thermoplastic supporting
matrix, desiccant particles are efficiently prevented from being released into the
refrigerant, even under heavy attrition conditions, such as in automotive air conditioning
systems. Additional filtration devices, such as bags, pouches or cartridges of mesh
material are therefore no longer required. Moreover, the risk of bag or filter leaking
is advantageously discarded.
[0011] Contrarily to the teaching in US-4,013,566, a polymer matrix with low vapour transmission
rates, such as thermoplastic polymers, may be used, especially in combination with
a channel agent. Furthermore, the present invention uses thermoplastic polymers rather
than thermoset epoxy polymers as described in the above patent. Thermoplastic polymers
are made of largely available and less expensive starting materials, they require
fewer ingredients and hence less complex preparation and mixing equipment and their
use does not involve subsequent curing steps.
[0012] Thermoplastic polymers that may be used in the present invention encompass polymers,
copolymers and block copolymers of one or more monomers, especially olefinic monomers.
According to a preferred embodiment of the invention, thermoplastic polymers, which
may advantageously be used in the supporting matrix, comprise one or more polymers
or copolymers of ethylene and/or propylene.
[0013] The channel agent may be any substance forming channels or passages inside the polymer
matrix, which, on one hand, increase the effectively exposed contact surface of the
desiccant particles to the refrigerant and which, on the other hand, allow the permeation
of the dried refrigerant fluid through the desiccant body. Examples of such channel
agents are polyethylene glycol, polypropylene glycol, etc.
[0014] The desiccant incorporated inside the polymer matrix may be any of the conventional
desiccant materials, such as molecular sieves, silica gel, etc.
[0015] In a more preferred design, the desiccant body further comprises one or more additional
spirals on at least part of its length arranged between two consecutive ribs of said
first spiral. Such additional spirals, preferably one or two, advantageously further
increase the contact surface between refrigerant and desiccant body without substantially
raising the pressure drop.
[0016] In a further embodiment, the clearance between the tip of the ribs of the first spiral
and the inner wall of said housing is smaller than the clearance between the tip of
the ribs of said additional spirals and the inner wall. Hence, in such an embodiment
the tip of the first spiral may extend toward and even touch the inner wall, whereas
the height of the additional spirals is chosen to leave a sufficient clearance to
allow for flow balancing and mixing inside two consecutive ribs of the first spiral.
[0017] Although the general shape of the desiccant body largely depends on the shape of
the housing or vice versa, as mentioned above, the desiccant body preferably displays
a generally tubular shape with one or more spirals on its outer and/or its inner surface.
In a particularly preferred embodiment, said tubular shaped form is cylindrical, which
is especially easy to manufacture.
[0018] In an alternative embodiment, this tubular shaped form preferably is biconical or
hourglass shaped, thereby varying the cross sectional area of the flow path between
to consecutive ribs of the first spiral. There are two major beneficial effects with
this design. First, due to this varying cross section, the flow speed of the refrigerant
is slowed down toward the centre of the desiccant body, thereby increasing the contact
or exchange time between fluid and desiccant. Second, the varying flow speed is favourable
to a better mixing of the refrigerant, thus further enhancing the moisture extraction
from the fluid.
[0019] In a further embodiment, wherein said first and optional additional spirals are arranged
on the outer side of said desiccant body, the latter advantageously further comprises
an inner support structure. This support structure increases the dimensional stability
thereof and thus allows reducing the wall thickness and the weight of the desiccant
body.
[0020] Preferably, such an inner support structure comprises protrusions radially extending
from the inner wall of the desiccant body. The height of these protrusions may only
represent part of the inner radius, thereby forming longitudinal ribs inside the body.
These protrusions may also connect in the centre of the body dividing the inside of
the body into separate channels or ducts around a central longitudinal solid or hollow
axis. An additional beneficial effect of such an inner support structure is that the
overall exchange surface and hence the overall efficiency is further increased.
[0021] In order to install and maintain the desiccant body in a correct position inside
the housing, it preferably further comprises fixing means on at least one end of said
desiccant body to secure it inside said housing. These fixing means may be attached
to or preferably be part of the desiccant body, for example an extension of the tubular
shaped body or an extension of the inner support structure.
[0022] In a further aspect, the invention also encompasses the use of a desiccant unit as
described above in an air conditioning system, especially in automotive air conditioning
systems.
[0023] In a particularly preferred aspect, said desiccant unit is part of an integrated
receiver-dryer unit, i.e. its housing is produced as one part of or attached to the
condenser housing, e.g. by welding, brazing, etc.
[0024] In a still further aspect of the invention, the desiccant unit may be used optionally
or additionally as part of pipes, compressor inlet, condenser inlet, evaporator inlet
or outlet. It is of particular advantage to gain an additional dehydration function
out of other parts usually made of plastic material, such as trumpet tubes, T-shape
tubes, etc., without added weight. Furthermore such additional dehydration will take
place all along the flow path of the refrigerant fluid.
Detailed description with respect to the figures
[0025] The present invention will be more apparent from the following description of two
non limiting embodiments with reference to the attached drawings, wherein
Fig. 1 is a schematic view of a condenser with integral receiver dryer.
Fig. 2a and 2b are cross sections (section W-W in Fig. 1 and X-X in Fig. 2a, respectively)
of an embodiment of a desiccant unit of the invention.
Fig. 3a shows a view and Fig. 3b and 3c are cross sections (section Y-Y and Z-Z, respectively)
of another embodiment of the invention.
[0026] Referring first to Fig. 1, a typical condenser 10 of the cross flow, headered type
has an inlet/outlet header tank 12 on one side, and a return header tank 14 on the
other, each of which is divided into upper U and lower L sections by separators 16
and 18, respectively. Heated, compressed refrigerant vapour enters the upper section
U of header tank 12, above separator 16, and flows across and through the flow tubes
in the main pass section (not illustrated in detail). In the main pass, refrigerant
is condensed to liquid form and flows into the upper section U of return tank 14,
above the separator 18. From there, all liquid refrigerant is forced, by the separator
18, to flow through an upper inlet 20 and into an attached reservoir canister housing
22 comprising a prior art desiccant bag or a desiccant body 30 according to the invention
(not illustrated within canister housing 22 in Figure 1, see Figures 2, 3a, 3b and
3c for preferred embodiments). After its passage through the desiccant body 30, liquid
refrigerant can flow down and through a lower outlet 21, into lower section L of return
tank 14 and ultimately into a sub cooler section of condenser 10, comprised of those
flow tubes located below the two separators 16 and 18. In the subcooler section, liquid
refrigerant is further cooled, below the temperature necessary to simply condense
it, and flows finally back into the lower section L of header tank 12.
[0027] Fig. 2a and 2b show an embodiment of a desiccant unit comprising a housing 22 with
a cylindrical desiccant body 30 arranged inside. A single (first) spiral 32 on the
outer side of the desiccant body 30 with a sealing separator 34 between upper U and
lower L sections forces the refrigerant entering the housing through inlet 20 (partially
hidden) up along the flow path defined by said first spiral 32. While in contact with
the desiccant body 30, the moisture contained in the refrigerant is withdrawn and
the liquid permeates through the channels formed by the channel agent to the inner
ducts 36 (as shown in Fig. 2b) formed by joining inner radial protrusions 42 of the
inner support structure 37. The dehydrated liquid drops fall down through the inner
ducts 36 to the lower section L where the liquid passes through outlet 21 into the
subcooler section of the condenser 10 (see Fig. 1). The desiccant body 30 further
comprises upper and lower fixing means 38' and 38", such as a plastic foot, to secure
the structure inside said housing 22. The bottom of the latter is closed by a cover
40, which may be removable for serviceable devices or welded or brazed for non-serviceable
applications.
[0028] In Fig. 3a, 3b and 3c an alternative embodiment (without housing 22) is shown, wherein
an hourglass shaped desiccant body 30 comprises two additional spirals 33 on its outer
side between two consecutive ribs of a first spiral 32. The additional spirals 33
do not extend to the bottom sealing separator 34 to facilitate the passage of the
refrigerant entering through upper inlet 20 (not shown). The heights of the tip of
the first spiral 32 and of the additional spirals 33 are chosen to get a greater clearance
with respect to the inner wall of housing 22 (not shown). Upper and lower fixing means
38' and 38" are provided to lock the desiccant body 30 inside the housing. The six
radial protrusions 42 forming an inner supporting structure 37 shown in Fig. 3c do
not join in the centre and leave an essentially hollow core representing a single
inner duct 36 wherethrough dried liquid flows to a lower outlet 21 (not shown) in
the lower section L.
Example
[0029] A classic desiccant composite is made of 80% pure desiccant and 20 % of natural mineral
component. 60 grams of molecular desiccant beads of 2 mm in average with a density
of 0.85 kg/litre are enclosed in a bag of mesh material. This amount represents around
3000 beads with an exchange surface of about 60,000 mm
2.
[0030] For a desiccant body according to the invention, the proportion of native product
is 70% desiccant to 30% thermoplastic polymer matrix. Hence, 40 grams of pure desiccant
are mixed with 20 grams of plastic polypropylene and channel agent to reach a total
of 60 grams. The volume of this polypropylene-desiccant mixture is about 53 cm
3 with a density of 0.88 kg/litre. This mixture is then moulded to form a tube with
an outer diameter of 20 mm, an inner diameter of 14 mm, a length of 240 mm reinforced
with an internal cross of 2 mm thick and 3 spirals on the whole length with a 2 mm
gage, 5 mm wide with a pitch of 120 mm. The exchange surface thus obtained is about
70,000 mm
2.
[0031] The moisture adsorption results achieved are at least as good as those obtained with
classic desiccant beads, without the need of tedious assembly steps and the risk of
leaking. Furthermore, the weight can easily be adjusted by decreasing or increasing
the wall thickness of the desiccant body.
1. A desiccant unit comprising
• a housing (22) and
• a desiccant body (30) arranged inside said housing (22), said desiccant body including
a desiccant agent and a supporting matrix and said supporting matrix comprising a
thermoplastic polymer material and a channel agent,
characterized in that said desiccant body (30) has a tubular shaped form with a first spiral (32) arranged
on an inner and/or outer surface of the tubular shaped form.
2. The desiccant unit according to claim 1, wherein said thermoplastic polymer material
comprises one or more polymers or copolymers of ethylene and/or propylene.
3. The desiccant unit according to claim 1 or 2, further comprising one or more additional
spirals (33) on at least part of the length of said desiccant body between two consecutive
ribs of said first spiral (32).
4. The desiccant unit according to claim 3, wherein the clearance between the tip of
the ribs of said first spiral (32) and the inner wall of said housing (22) is smaller
than the clearance between the tip of the ribs of said additional spirals (33) and
the inner wall of said housing (22).
5. The desiccant unit according to any of claims 1 to 4, wherein said tubular shaped
form is cylindrical.
6. The desiccant unit according to any of claims 1 to 4, wherein said tubular shaped
form is biconical (hourglass shaped).
7. The desiccant unit according to any of claims 1 to 6, wherein said first and optional
additional spirals (33) are arranged on the outer side of said desiccant body, further
comprising an inner support structure (37).
8. The desiccant unit according to claim 7, wherein said inner support structure (37)
comprises radially extending protrusions (42).
9. The desiccant unit according to any of claims 1 to 8, further comprising fixing means
(38' and/or 38") on at least one end of said desiccant body to secure it inside said
housing (22).
10. Use of a desiccant unit according to any of the preceding claims in an air conditioning
system.
11. The use according to claim 10, wherein said desiccant unit is part of an integrated
receiver-dryer unit.
12. The use according to claim 10, wherein said desiccant unit is part of pipes, compressor
inlet, condenser inlet, evaporator inlet and/or evaporator outlet.