A desiccant system is for maintaining an enclosure at atmospheric
pressure and in a gas controlled condition. The system includes
desiccant material contained in two gas permeable packets each having
a flattened side. A perforated box has a cover plate and is adapted
for retaining the two packets with the respective flattened sides
juxtaposed. An open tube has a length to internal diameter ratio
of at least about ten and an outer end open to atmosphere through
the cover plate. An inner end of the tube is positioned centrally
between the flattened sides of the packets. The box is insertable
into a port in an enclosure with the cover plate adapted to seal
What is claimed is:
1. A desiccant system for an enclosure, comprising desiccant material
contained in two gas permeable packets each having a flattened side,
a perforated box having a cover plate and being adapted for retaining
the two packets with the respective flattened sides juxtaposed,
and an open tube having a length to internal diameter ratio of at
least about ten and further having an outer end open to atmosphere
through the cover plate and an inner end positioned centrally between
the flattened sides of the packets, the box being insertable into
a port in an enclosure with the cover plate adapted to seal the
port, whereby the enclosure is maintained at atmospheric pressure
and in a gas controlled condition.
2. A desiccant system according to claim 1 wherein the tube is
coiled between the packets.
3. A desiccant system according to claim 2 wherein the length to
internal diameter ratio is at least about 100.
The present invention pertains to a desiccant system and particularly
to a desiccant system for maintaining an enclosure at atmospheric
pressure and with a controlled gas environment.
BACKGROUND OF THE INVENTION
There often is a requirement to maintain an enclosure with a controlled
gas environment such as to be free of moisture, carbon dioxide and/or
oxygen. For example in a precision optical instrument such as a
spectrometer, or in certain electronic equipment, it is necessary
to purge and maintain the enclosure with a gas such as nitrogen.
However such an enclosure is susceptible to changes in atmospheric
pressure which can result in leakage. A well sealed enclosure either
may buckle under pressure changes or must be designed with thick
walls and specially sealed joints and is, therefore, relatively
Desiccant materials are commonly used in enclosures for adsorbing
water vapor, carbon dioxide and/or oxygen that may leak in. However,
such detrimental gases may reach a critical component in the instrument
before being adsorbed and removed by the desiccant. Therefore such
systems have been less than satisfactory.
There also is known to be a Fick's law whereby a tube, with a length
to internal diameter ratio of at least ten, minimizes diffusion
of gas such as water vapor from one end of the tube to the other
at constant pressure. Functioning of such a tube, being statistical,
does not ensure blockage of moisture from an enclosure and generally
has not replaced desiccants.
SUMMARY OF THE INVENTION
Therefore objects of the present invention are to provide an improved
desiccant system for an enclosure, and particularly to provide a
desiccant system that allows an enclosure to be maintained at atmospheric
pressure while also maintaining a controlled gas environment in
The foregoing and other objects are achieved by a desiccant system
for an enclosure, comprising a desiccant, retainer means for retaining
the desiccant in an enclosure, and a tube having a length to internal
diameter ratio of at least about ten and preferably at least about
100. The tube has an outer end open to atmosphere and an inner end
positioned proximate the desiccant.
According to a preferable embodiment the desiccant system comprises
desiccant material contained in two gas permeable packets each having
a flattened side, and a perforated box having a cover plate and
being adapted for retaining the two packets with the respective
flattened sides juxtaposed. An open tube with a length to internal
diameter ratio of at least about ten has an outer end open to atmosphere
through the cover plate. An open inner end of the tube is positioned
centrally between the flattened sides of the packets. The box is
insertable into a port in an enclosure with the cover plate adapted
to seal the port.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is an exploded view of an embodiment of the present
DETAILED DESCRIPTION OF THE INVENTION
With reference to the drawing, a retaining box 10 has at least
one perforated side, two shown at 12 and 14 and a cover plate 16.
First and second packets of desiccant material 1820 have a size
and shape to fit somewhat tightly into the box. For example as shown
the packets are in the form of flattened rectangles. One side 12
(which itself need not be perforated) has a pair of tabs 24 which
fit slotted tabs 2628 in end plates 3031 of the box. This or any
other fastener may be provided as desired for convenient replacement
of the desiccant.
The desiccant is any conventional, desired type, for example Molecular
Sieve.TM. Type 4A of Union Carbide. The packet material is porous
to gas, such as Tyvek.TM. sold by Dupont, so that the surrounding
gas may circulate freely from the perforated sides through the desiccant
in the packets.
A tube 32 is provided with a length of at least 10 and preferably
100 times its internal diameter, for example 25 cm long and 1.56
mm diameter for a ratio of 160. This tube has an open outer end
34 which is connected to an orifice fitting 36 in cover plate 16.
The tube is coiled and fitted so that its inner end 38 is positioned
centrally between the flattened sides of the packets, effectively
imbedded in the desiccant. More generally, the inner end should
be at least proximate the desiccant material.
The assembled box assembly 10 with desiccant 1820 and tube 32
is mounted in a port 40 in an enclosure 42 containing the desired
sensitive optical and/or electrical components or the like (not
shown). Screws 44 thread into tapped holes 46 or retained nuts next
to the port on the enclosure to retain the box in port 40. A gasket
48 seals the cover plate to the enclosure. The outer end of tube
32 is open to the atmosphere via fitting 36 in cover plate 16.
Other variations are possible within the spirit of the invention.
For example, a desiccant box may be mounted on the enclosure with
a perforated bottom plate sealed over the port, the box thus being
essentially an extension of the enclosure.
In use the tube of at least 10 to one length to diameter ratio
minimizes diffusion of air and moisture from the surrounding atmosphere
into the enclosure, while allowing pressure inside to equalize to
atmospheric. A ratio of 100 increases the reliability especially
with some expected air flow when there are small pressure differentials
from varying atmospheric pressure. Furthermore, with the inner end
38 of the tube being proximate and preferably imbedded in the desiccant
according to the present invention, any minute amounts of moisture
and other undesirable constituents of air that do find their way
through the tube are immediately adsorbed by the desiccant.
It has been found that with 192 gms of desiccant in an assembly
as described herein mounted in an enclosure having a volume of 17
liters, moisture and carbon dioxide are maintained satisfactorily
low at approximately 1% of atmospheric for at least 6 months before
it becomes necessary to replace the desiccant and repurge the enclosure.
While the invention has been described above in detail with reference
to specific embodiments, various changes and modifications which
fall within the spirit of the invention and scope of the appended
claims will become apparent to those skilled in the art. The invention
is therefore only intended to be limited by the appended claims
or their equivalents. PG,6