Molecular sieve abstract
The concentration of a constituent gas in a product gas delivered
by a molecular sieve type gas separation system 10 is controlled
by a flueric partial pressure sensor 15 connected to a spill-valve
16 which is conduitly connected to a product gas delivery duct 13.
The flueric partial pressure sensor 15 compares a reference bleed
of supply gas with a sample bleed of product gas and outputs signals
to control the spill-valve in spilling product gas from the delivery
duct 13 so that the rate of flow of gas through adsorption beds
11 of the system 10 is controlled to provide product gas having
a required concentration of the constituent gas.
Molecular sieve claims
What is claimed is:
1. Apparatus for controlling the concentration of a constituent
gas in a product gas delivered by a molecular sieve type gas separation
system including a concentrator having two or more adsorption beds
arranged for receiving pressurised supply gas and delivering product
gas by way of a delivery duct to an outlet, the apparatus comprising
a flueric partial pressure sensor, arranged to compare a reference
bleed of the supply gas with a sample bleed of the product gas,
having signal outlets connected to control means of an operably
associated spill-valve, the spill-valve being arranged for conduit
connection to the delivery duct so that the rate of flow of gas
through the adsorption beds may be controlled by spilling product
gas from the delivery duct, thereby controlling the concentration
of the constituent gas in the product gas.
2. Apparatus as claimed in claim 1 wherein the spill-valve control
means comprises a servo-valve.
3. Apparatus as claimed in claim 2 wherein the servo-valve is
controlled by a pneumatic actuator.
4. Apparatus as claimed in claim 2 wherein the servo-valve is
controlled by a solenoid.
5. Apparatus as claimed in claim 2 wherein the spill-valve control
means further comprises valve means responsive to ambient atmospheric
pressure or a pressure related thereto and arranged to negate operation
of the servo-valve.
6. Apparatus as claimed in claim 5 wherein the valve means includes
an aneroid.
7. Apparatus as claimed in claim 6 wherein the aneroid is arranged
so that in varying its length it allows movement of a magnetic valve
member between opening or closing a vent path controllable by the
servo-valve.
8. Apparatus as claimed in claim 2 wherein the spill-valve control
means includes valve means responsive to the pressure of product
gas being delivered by way of the spill-valve and arranged to negate
operation of the servo-valve.
9. Apparatus as claimed in claim 8 wherein the product gas pressure
responsive valve means comprises a piston operated valve-head arranged
for movement between opening or closing a vent path controllable
by the servo-valve.
10. A molecular sieve type gas separation system for supplying
oxygen-enriched product air comprising:
a concentrator having two or more molecular sieve adsorption beds
arranged for receiving pressurised supply air and delivering oxygen-enriched
product air by retention in the sieve beds of at least a percentage
of the nitrogen in the supply air;
a delivery duct connecting between the concentrator and an outlet
for delivery of oxygen-enriched product air from the concentrator
to the outlet;
a spill-valve having spill-valve control means and being conduitly
connected with said delivery duct for spilling oxygen-enriched product
air from said delivery duct;
a flueric partial pressure sensor having means for comparing a
reference bleed of supply air with a sample bleed of oxygen-enriched
product air and outputting signals from signal outlets of said flueric
partial pressure sensor; and
means connecting the signal outlets of said flueric partial pressure
sensor with said spill-valve control means whereby signals output
by said flueric partial pressure sensor cause said spill-valve to
spill oxygen-enriched product gas from said delivery duct so that
the rate of flow of air through said molecular sieve adsorption
beds is varied thereby controlling the concentration of oxygen in
the oxygen-enriched product air.
Molecular sieve description
DESCRIPTION OF INVENTION
This invention relates to molecular sieve type gas separation systems
and more particularly to apparatus for improving control of the
concentration value of a constituent gas in a product gas delivered
thereby.
Molecular sieve type gas separation systems are being introduced
into breathable gas supply systems, such as for life support of
medical patients and aviation aircrew members, where they provide
oxygen concentrators for the enrichment of air delivered to a user.
In such separation systems ambient air is supplied from a pressurising
source into two or more adsorber beds where nitrogen is removed
by use of a process generally known as a pressure swing adsorption
process. Such systems are often identified as molecular sieve oxygen
generator systems, MSOGS or, with respect to aircraft systems, frequently,
as on-board oxygen generator systems, OBOGS.
One form of contemporary MSOGS comprises a concentrator having
two adsorber beds adapted for alternately and sequentially adsorbing/desorbing
nitrogen and arranged to deliver oxygen-enriched air as product
gas to a delivery regulator by way of a volumetric capacity which
provides a buffer between the delivery characteristics of the MSOGS
and the breathing requirements of the user.
The concentration value of the enriched gas delivered to the user
may require to be sensed to maintain a predetermined level such
as in a medical application or to vary the level appropriate to
a changing environmental condition such as during variations in
cabin pressure (altitude) of an aircraft in an aviation application.
In a prior art form of OBOGS, shown schematically in FIG. 1 where
the level of concentration, or partial pressure of oxygen, PPO.sub.2
of the product gas delivered to an aircrew member is required to
be appropriate to the cabin pressure (altitude), it is the practice
to achieve this requirement by utilising, in combination with a
cabin pressure sensor A, a polarographic oxygen sensor means B and
an operably associated spill-valve C positioned in the delivery
line D from the concentrator E at or intermediate a volumetric capacity
F and a delivery regulator G to which the aircrew member conduitly
connects his mask.
Such an arrangement provides control of the PPO.sub.2 of the oxygen-enriched
air delivered to the mask by enabling regulation of the rate of
flow through the adsorber beds of the concentrator, in which a greater
or lesser flow reduces or increases, respectively, the amount of
nitrogen adsorbed from a unit volume of the supplied air. When the
polarographic sensor senses a PPO.sub.2 value above its set point
value a signal is passed to open the associated spill-valve which
spills enriched air from the delivery conduit system so that a greater
flow rate of supply air then passes through the beds of the concentrator
and less adsorption of nitrogen occurs, so that the PPO.sub.2 value
of the product gas decreases. When the desired lower PPO.sub.2 value
is sensed by the polarographic sensor then the signal is cancelled
and the spill-valve closed so that the flow rate of supplied air
passing through the beds of the concentrator reduces into accord
with the demand flow made of the delivery regulator.
However such control of the prior art type OBOGS does not provide
an ideal response to changing cabin pressure, because of long time
constants arising from the slow reaction of polarographic sensors
and the volume of product gas in the delivery conduit system between
the beds of the concentrator and the sensor, which result in a considerable
destabilising influence in the control of the PPO.sub.2 value of
the delivered product gas, so that this value is often less appropriate
to pertaining cabin pressure than desired. In order not to add to
the considerable time delay, a fast-acting solenoid type bleed valve
is usually utilised; however, this results in high pressure swings
in the product gas with each operation of the solenoid.
It might be possible to compensate electronically for the destabilising
influence of the system time constants by suitable modification
of the operating circuitry of the polarographic oxygen sensor means
and its operably associated spill-valve. However, it is considered
that a sufficiently inexpensive and compact electronic unit cannot
be achieved if the long time constants, delays and tuning of all
the variations in the performance of the concentrator and the polarographic
sensor are all to be accommodated.
Accordingly, the present invention provides apparatus for controlling
the concentration of a constituent gas in a product gas delivered
by a molecular sieve type gas separation system including a concentrator
having two or more adsorption beds arranged for receiving pressurised
supply gas and delivering the product gas towards an outlet, the
apparatus comprising a flueric partial pressure sensor, arranged
to compare a reference bleed of the supply gas with a sample bleed
of the product gas, having signal outlets connected to control means
of an operably associated spill-valve, the spill-valve being arranged
for conduit connection to a product gas delivery duct so that the
rate of flow of gas through the adsorption beds may be controlled
by spilling product gas from the delivery duct thereby controlling
the concentration of the constituent gas in the product gas.
The spill-valve control means preferably includes a servo-valve
which whilst it may be controlled by a solenoid is, preferably,
controlled by a pneumatic actuator.
When, however, the servo-valve is controlled by a solenoid, translation
of the output signal of the flueric partial pressure sensor from
a pneumatic to an electrical form may be obtained by inclusion of
a pressure/electrical transducer, such as a proximity switch.
The flueric partial pressure sensor is, preferably, one which automatically
maintains a constant set-point value in changing ambient pressure
as disclosed in EP-A-O 036 285 so that a separate altitude sensor
and signal integrator means is not required.
The spill-valve control means may further comprise valve means
responsive to ambient atmospheric pressure or a pressure related
thereto and arranged to negate operation of the servo-valve.
This pressure responsive valve means may include an aneroid which
is, preferably, arranged so that in varying its length it allows
movement of a magnetic valve member between opening and closing
a vent path controllable by the servo-valve.
The spill-valve control means may also include valve means responsive
to the pressure of product gas being delivered by way of the spill-valve
and arranged to negate operation of the servo-valve.
The valve means responsive to the product gas pressure is preferably
a piston-operated valve-head arranged for movement between opening
and closing the vent path controllable by the servo-valve. Preferably
the piston and the valve-head comprise an integral element.
The invention will now be described, by way of example, with reference
to the accompanying drawings in which:
FIG. 1 schematically represents a prior art molecular sieve type
gas separation system as discussed above;
FIG. 2 schematically represents a molecular sieve type gas separation
system including apparatus in accordance with one embodiment of
the present invention;
FIG. 3 is a schematic representation of a flueric partial pressure
sensor suitable for use in the apparatus of the system illustrated
in FIG. 2;
FIG. 4 is a schematic representation of a spill-valve suitable
for use in the apparatus of the system illustrated in FIG. 2;
FIG. 5 is a schematic representation of an alternative spill-valve
suitable for use in the apparatus of the system illustrated in FIG.
2; and
FIG. 6 illustrates an additional valve arrangement applicable to
the spill-valve of FIGS. 4 or 5.
In the embodiment of the invention illustrated in FIG. 2 a molecular
sieve oxygen-enriched air generator system 10 (MSOGS or OBOGS) for
aircraft use includes a concentrator 11 having two or more nitrogen
adsorption beds (not shown) arranged to receive pressurised air
supplied from an engine of the aircraft (not shown) by way of a
pressure regulating valve 12. A product gas delivery duct 13 connects
the concentrator 11 with an outlet comprised by a gas delivery regulator
14 to which an aircrew member couples his breathing mask (not shown).
Apparatus for controlling the concentration of oxygen in the delivered
product gas comprises a flueric partial pressure sensor 15 arranged
to provide operation of a spill-valve 16 conduitly interconnected
with the delivery duct 13 intermediate the concentrator 11 and the
gas delivery regulator 14.
The flueric partial pressure sensor 15 is connected so as to receive
supply air from downstream of the pressure regulating valve 12 by
way of a conduit 17 which includes a pressure reducer 18 of any
suitable known form and, further, to receive product gas through
ducting 19 which connects with a transfer duct in spill-valve 16.
As shown in FIG. 3 the supply air conduit 17 connects with an
aspirator 20 of the flueric partial pressure sensor 15 and also
with one leg 21 of a bridge therein for passing air as a reference
gas therethrough. The second leg 22 of the bridge is connected with
the ducting 19 for receiving product gas as a sample gas. The two
bridge legs 21 22 each, respectively, include a linear resistor
23 24 and an orifice resistor 25 26 arranged in series.
A bridge signal is generated from between each pair of linear and
orifice restrictors 23 25; 24 26 and connected to two control
ports 27 28 of the first stage of a two-stage flueric amplifier
29 which forms an integral part of the partial pressure sensor 15.
Operating signals from the second stage of the amplifier 29 are
conveyed to the spill-valve 16 by way of two ducts 30 31. A flueric
partial pressure sensor of this form is disclosed in EP-A-O 036
285.
A preferred form of spill-valve 16 shown in FIG. 4 comprises
a body 35 having a main duct 36 for the passage of product gas passing
from the concentrator 11 to the gas delivery regulator 14. A spill
flow duct 37 connects the main duct 36 with a region external to
the body 35 preferably the interior of the aircraft cabin, by way
of a diaphragm-operated outlet valve 38. The diaphragm 39 of the
outlet valve 38 forms one wall of a control pressure chamber 40
which is connected to the main duct 36 by way of a restricted duct
41. The control pressure chamber 40 is arranged for venting into
the region external to the body 35 by way of a duct 42 controlled
by a diaphragm-operated servo-valve 43 and an aneroid-operated valve
44 positioned downstream of valve 43.
The diaphragm 45 of the servo-valve 43 divides the servo pressure
chamber into two sub-chambers 46 47 which are connected to the
outside of the body 35 by ducts 48 49 respectively for connection
to the amplifier ducts 30 31 of the flueric partial pressure sensor
15. The transfer duct for conveying sample product gas to the flueric
partial pressure sensor 15 is provided by a restricted duct 50 connecting
the main duct 36 to the outside of the body 35. The aneroid-operated
valve 44 comprises an aneroid capsule 51 arranged within a chamber
52 of the vent duct 42 and exposed to pressure external of the body
35 e.g. ambient atmospheric pressure or a pressure related thereto,
in a manner such that in expanding it loads a compression spring
to overcome the magnetic attraction between a valve member 53 and
magnetic structure 54 forming a part of the capsule chamber 52.
An alternative form of spill-valve 61 as shown in FIG. 5 comprises
a body 62 having a main duct 63 and a spill flow duct 64 which connects
a region external of the body with the main duct 63 by way of an
outlet valve 65 of diaphragm type. The diaphragm 66 of this valve
65 forms one wall of a control chamber 67 and also, on its side
remote therefrom, is engageable with a valve seat 68 in the spill
flow duct 64. The control chamber 67 is connected with the main
duct 63 by way of a restricted passage 69 and also with the exterior
of the body 62 by way of a vent duct 70 that is obturated by a solenoid
valve 71. The vent duct 70 includes a valve chamber 72 housing an
aneroid-operated valve 73 arranged to close upon a valve seat 74
in the duct 70. The aneroid 75 of the valve 73 is exposed to ambient
atmospheric pressure or a pressure related thereto so that in expanding
it loads a compression spring to overcome the magnetic attraction
between a valve member 76 and magnetic structure 77 forming part
of the valve chamber 72. A relay 78 is incorporated into the valve
body 62 for control of the solenoid valve 71 and is arranged to
receive electrical power from a power tapping which is interruptable
by interconnection with a diaphragm-type proximity switch (not shown)
operable by connection to the amplifier output ducts 30 31 of the
flueric partial pressure sensor 15. A transfer duct for conveying
sample gas to the flueric partial pressure sensor 15 is provided
by a restricted duct 79 connecting the main duct 63 with the outside
of the body 62.
In operation of this embodiment, pressurised air from a compressor
stage of an aircraft gas turbine engine is supplied to the system
via pressure regulating valve 12 from whence it is delivered at
a steady pressure of, say, 172.35 kPa (25 psi) to the concentrator
11 and by conduit 17 to the pressure reducer 18 in which it is reduced
to 31.02 kPa (4.5 psi) relative to cabin pressure. This lower pressure
is fed to the flueric partial pressure sensor 15 to provide a reference
sample gas (air) and also a power supply to the two stages of the
amplifier 29 and to the aspirator 20 for inducing flow through the
reference and sample ducts of the sensor. Air is thus made available
to the reference bridge leg 21 while product gas taken from the
delivery duct 13 by way of the transfer duct 50 in the spill-valve
16 and ducting 19 is made available to the sample bridge leg 22
in which legs the respective gases pass through the serially arranged
linear and orifice resistors 23 25 and 24 26. A small differential
pressure (bridge output) obtains at the control ports 27 28 of
the amplifier 29 and applies a greater pressure either to duct 30
or duct 31 according to the concentration level of oxygen in the
product gas relative to a predetermined datum oxygen partial pressure
level, say 250 mm Hg. The ducts 30 31 being connected to ducts
48 49 respectively, and thus sub-chambers 46 47 of the servo-valve
43 diaphragm control chamber, allow the output pressures from the
amplifier 29 to obtain in these two sub-chambers. Connection of
these ducts is so arranged that when the oxygen concentration datum
level is exceeded the greater pressure appears in sub-chamber 46
whereby the servo-valve 43 is urged to open and allow product gas
to vent from the main duct 36 and spill flow duct 37 of the spill-valve
16; whereby flow of air through the concentrator is increased and
the amount of nitrogen adsorbed per unit volume of air is reduced
and thus the oxygen-enrichment process abated. As soon as the oxygen
concentration level in the sample gas falls below the datum level
the bridge output pressure difference reverses and the sub-chamber
47 obtains the greater pressure so that the servo-valve 43 is urged
into closing and spillage of product gas from the main duct 36 prevented.
The aneroid-operated magnetic valve 44 operates automatically at,
say, approximately 6400 meters (21000 ft) by expanding to a point
where it increases the spring load on the valve member 53 such that
the magnetic attraction between this and the magnetic structure
54 is overcome and the member then snaps onto its associated valve
seat under the influence of the spring. This action closes the vent
duct 42 so that pressure in the control chamber 40 cannot be relieved
(and the outlet valve opened to spill product gas) whereby, above
the chosen altitude at which valve 44 operates, the concentration
level of oxygen in product gas passed to the delivery regulator
14 remains at maximum value.
In operation of the alternative spill-valve 61 typically, a 28
volt D.C. electric current is made available to the relay 78 by
way of the diaphragm-type proximity switch (not shown) which is
connected to the amplifier outlet ducts 30 31 of the flueric partial
pressure sensor 15 in a manner for deflection of the diaphragm
of the switch to control of operation of the solenoid valve 71.
Product gas passes through the main duct 63 and also feeds to the
flueric partial pressure sensor 15 via duct 79 and into control
pressure chamber 67 through restricted duct 69. While the solenoid
valve 71 remains seated, pressure in the control pressure chamber
67 urges the diaphragm 66 into seating and so prevents the spillage
of product gas through the spill flow duct 64. When the flueric
partial pressure sensor 15 responds to a higher than datum reference
value, the signal output from the amplifier 29 causes deflection
of the diaphragm of the proximity switch (not shown) so as to complete
the electrical circuit through the relay 78 to energise the solenoid
valve 71 into lifting and so enable release of pressure from the
control pressure chamber 67 and thereby allow the diaphragm 66 to
be unseated and the spill flow duct 64 to be opened, which consequently
causes a greater flow of supply air to pass through the concentrator
11. This reduces enrichment of the air delivered as product gas
until the level of oxygen concentration therein falls to the datum
reference level, at which the sensor 15 alters its output signal
to cause opposite deflection of the diaphragm of the proximity switch
with opposite result. The aneroid-operated valve 73 functions in
like manner to that of the aneroid-operated valve 44 described above
in respect of the preferred spill-valve 16 when its closure above
6400 meters (21000 feet) serves to block the vent duct 70 so that
pressure in control chamber 67 cannot be relieved (and the outlet
valve opened to spill product gas from the main duct 63) thereby
maintaining the delivered product gas at a maximum enrichment level.
In use of some forms of concentrator on aircraft having a particular
flight envelope such that the supply pressure to the system can
become abnormally low, a combination of conditions is able to occur
which results in low product gas pressure at the regulator delivering
to the aviator's mask and high pressure swings within the mask during
breathing cycles. This result can be aggravated further should the
spill-valve spill product gas when such conditions occur. As so
far described, the spill-valves of the illustrated embodiment do
not allow spilling of product gas at pressures below the predetermined
operating pressure of the aneroid valve 44 or 73 i.e. above a given
cabin altitude; however, in order to prevent spillage when supply
pressure becomes low at cabin altitudes below that at which the
aneroid valve closes, it is desirable to include a further valve
which operates at a predetermined low product gas delivery pressure.
One such valve comprises a valve assembly 80 as shown in FIG.
6 arranged to obturate the control pressure vent duct 70. The valve
assembly 80 includes a valve-head 81 that is urged by a compression
spring 82 towards closing onto a valve seat formed in the duct 70
and is integral with a piston 83 but distanced therefrom by a stem.
The piston 83 is responsive to product gas pressure in the main
duct 63 of the spill-valve, so that this pressure and the spring
load on the valve-head 81 are in opposition. Pressure external to
the spill-valve (cabin pressure) is also effective on the valve-head
81 when the aneroid valve 73 is not closing the vent duct 70.
In operation of the valve assembly 80 assuming that the pressure
of the product gas in the main duct 63 is above a predetermined
minimum value of, say, 34.5 kPa (5 psi) and the aneroid valve 73
is in the open condition, then the product gas pressure on the piston
83 and the spring load combined with the cabin gas pressure on the
valve-head 81 balance and hold the valve-head in the open position.
This allows control pressure to vent from the spill-valve via duct
70 when the solenoid valve is energised into the open position and
so enable the outlet valve 65 to open for product gas to be spilled
from the main duct 63. When, however, the product gas pressure in
the main duct 63 falls below the predetermined minimum value the
spring 82 then becomes dominant and moves the valve-head 81 into
closing the vent duct 70 which disenables the outlet valve 65 to
open and thus keeps the pressure of the product gas at the maximum
value available.
Whilst the alternative spill-valve 63 utilising a solenoid valve
71 in its operation provides a very satisfactory control arrangement
in regulation of the level of oxygen-enrichment of air delivered
from a molecular sieve oxygen generator or concentrator 11 the
preferred spill-valve 16 which is totally of pneumatic operation
provides lower amplitudes of pressure fluctuations in the delivery
of product gas to the regulator 14 during its operation. |