Note: Descriptions are shown in the official language in which they were submitted.
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~3ACKGROUND OF T~[E INVENTION
This invention relates to gas recovery and is more
specifically concerned with th~ recovery of gases from en-
closures containing equipment which has to operate in a gas-
eous environment other than air.
There are many areas of technology in which fairly
large equipment is required to operate in an atmosphere of
a particular gas. One such area is the electrical supply
industry where, for example, electric circuit breakers are ~-
often required to operate in an atmosphere of sulphur
hexafluoride (SF6)gas.
Sulphur hexafluoride gas is relatively expensive and
it is normally recovered and stored if equipment which
normally works in the gas is to be serviced or repaired. As
the equipment may be sited in remote areas, a portable
assembly is used to recover the gas and to recharge the
equipment with the gas after the equipment has been serviced
or repaired. Such assembly comprises a large pump and a
storage vessel normally mounted on a vehicle. The amount
of gas to be recovered may be 200kg or more, and, as it is
normally at pressures between 4 and perhaps 20 atmospheres
in the enclosure, the pump used must have a high capacity
and the assembly is therefore a cumbersome and expensive
piece of equipment, which is used only intermittently.
SUMM~RY OF TEIE INVENTION
The present invention seeks to amend the need for high
capacity pumps by providing a gas recovery apparatus comprising
at least one vessel connectable to an enclosure or circuit
from which a gas is to be recovered and means for holding
the vessel at a temperature below that at which the gas is
transformed from its vapour phase to a liquid or solid phase.
The apparatus of the invention has the advantage that
the use of a high pressure pump is avoided altogether and
the gas storage vessels are reduced in size, perhaps four-
fold. The conversion of the recovered gas to its liquid
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phase produces a drop in pressure which induces its OWII
pumping action so that the gas can be rapidly withdrawn
from the enclosure or circuit by way of a coupling and, when
cold, stored at a pressure at or beneath atmospheric pressure.
The cooling arrangement conveniently comprises a bath
of liquid nitrogen. The vessel may take the form of a
stainless steel or aluminium cylinder mounted in the bath.
Liquid nitrogen has the advantage that air is not liquified
when contained in vessels at temperatures just above the
boiling point of the liquid nitrogen. The recovered gas
may therefore be separated from air which may have con-
taminated it through leakage, by applying a vacuum pump to
the vessel after the gas has been solidified in the vessel.
The invention is particularly well-suited to recovering
; sulphur hexafluoride from electrical circuit hreakers and
other such equipment. Sulphur hexafluoride is solid at the
temperature of liquid nitrogen (-196C) and liquid nitrogen
is readily available as a cheap safe, refrigerant. The
apparatus can be stored at a maintenance depot for servicing
a number of circuit breakers and temporarily transported
any breaker which requires servicing.
According to the invention there is provided a gas recovery
apparatus comprising at least one vessel connectable to an
enclosure or circuit from which a gas is to be recovered
such that the gas is enabled to flow from the enclosure or
circuit into the vessel and means for holding the vessel
at a temperature below that at which the gas is transformed
~; from its vapour phase to its liquid phase or to its solid
phase thereby storing the gas in the vessel.
According to the invention there is further provided
a method of recovering gas from an enclosure or circuit, the
method comprising connecting to the closure or circuit a gas
receiving vessel which is held at a temperature below that
at which the gas is transformed from its vapour phase to
its liquid phase or to its solid phase and storing the
gas in the receiving vessel.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail in
a preferred embodiment, by way of example, with reference
to the accompanying drawings, in which:-
FIGURE 1 is a graph showing the vapour pressure ofsulphur hexafluoride as a function of its temperature.
FIGURE 2 is a schematic circuit diagram of apparatus
for recovering sulphur hexafluoride from a circuit breaker,
the diagram showing also the sulphur hexafluoride gas
circuit for the breaker, and
FIGURE 3 is a code chart which identifies the legends
employed in Figure 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As is shown in the curve of Figure 1, between a
critical point 5 and a point 6, sulphur hexafluoride is
either at a liquid phase or a gaseous phase dependent on
its temperature and pressure. Between the points 6 and 7
sulphur he~afluoride exists either in its gaseous phase
or its solid phase in accordance with the temperature and
pressure. Point 8 on the curve of Figure 1 shows that at
temperatures beneath -60C the sulphur hexafluoride exists
in its solid state at atmospheric pressure.
It will be seen from the curve that at a temperature of
-196C, which is the temperature of liquid nitrogen, sulphur
hexafluoride exists in a solid phase unless its pressure is
reduced below about 10 2-5 Torr. Bearing in mind that one
Torr is equal to 1/760th of an atmosphere, this is almost
a pure vacuum condition. As with most gases, the conversion
of sulphur hexafluor~de from its gaseous to its solid phase
is accompanied by a very great reduction in volume, exceeding
100:1 at one atmosphere pressure.
Referring now to the apparatus as shown schematically
in Figure 2 and having the elements identified by the legends
encoded in Figure 3:
Figure 2 shows, within the broken line 10 equipment
associated with a gas-filled circuit breaker using sulphur
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hexafluoride gas. This equipment comprises an arc chamber
11 from which sulphur hexafluoride gas travels through a
fast opening valve 12 to a dump tank 13. Typically, the
gas is at a pressure of four atmospheres and the arc chamber
and its surrounding enclosure together contain about 8
litres of gas, although this vaxies with design. Gas from
the dump tank is cleaned by passage through filters 14
before being recycled back to the arc chamber with additional
gas provided for topping-up purposes from a gas bot-tle 15.
The equipment also includes manual valves, one-way
valves, meters, regulators and additional filters, as
identified by the legend encoding of Figure 3. These
elements form a normal part of most fluid circuits and are
not therefore detailed herein.
The apparatus for recovering sulphur hexafluoride from
the circuit shown in Figure 2 is contained within the broken
outline 20. It comprises a dewar flask 21 containing a
stainless steel vessel 22 connected by a valve 23 and a
quick-connector coupling 24 to the gas circuit of the
equipment and apparatus 10. The vessel 22 is carried at
one end of a yoke 25 which carries at its other end a second
similar vessel 26 also connected by a valve 27 and a quick-
release coupling 28 to the gas circuit of the breaker. The
yoke 25 can be raised and lowered by a double-acting piston
30 operating in a cylinder 31 in which the piston can rotate
when raised.
The vessel 26 is partially immersed in a heated water
tank 33 which contains a further vessel 34 containing
sulphur hexafluoride and connected through a valve 35 which
can be opened to provide a rapid pressure build up of sulphur
hexafluoride gas to the circuit breaker enclosure as it by-
passes the regulators which control the gas flow into the
circuit from the vessel 26.
A pressure switch 40 and associated ~-part, 3-position
; hand operated valve controlling a compressed air supply is
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`used to provide compressed air as the working f1uid to the
cylinder 31.
A large dewar flask of liquid nitrogen (not illustrated)
may also be provided as a liquid nitrogen reservoir.
When the sulphur hexafluoride gas is to be recovered
from the circuit breaker enclosure the vessel 22 is connected
to the gas circuit by means of the quick connector couplings
and operation of the non-return valve. The sulphur hexa-
fluoride gas from the gas circuit of the breaker flows quickly
into the stainless steel vessel 22 where it liquifies and
quickly solidifies. A pumping rate of approximately 400
grms per square meter of cooled cylinder area is maintained
provided the connecting pipe-work can deliver this quantity
of gas. The gas in the circuit is rapidly converted into
liquid and then ~lid sulphur hexafluoride in the cooled
vessel 22. It is important that the neck of the vessel is
not held at temperatures at which it could be choked by SF6
snow formation as it enters the vessel 22. Also the SF6
should preferably pass from a gaseous phase to a liquid
phase in the vessel 22, before solidifying, so that it
collects in the base of the vessel 22 and leaves the vessel
walls free of any insulation caused by solid SF6 build up.
The pressure within the vessel falls to very low values
and the consumption of nitrogen is approximately one litre
per kg of sulphur hexafluoride gas recovered. Air which
may have seeped into the gas circuit is not liquified at the
temperature of liquid nitrogen, and can be evacuated by a
vacuum pump normally provided but not shown.
When the sulphur hexafluoride has been recovered, valve
23 is closed and the circuit breaker enclosure can ~hen be
opened up for maintenance or other work to be carried out.
When necessary work has been completed, the enclosure
is evacuated by means of the vacuum pump 50 provided and the
system is recharged with sulphur hexafluoride gas. This
gas may be obtained by raising, turning and then lowering
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^the yoke 25 so that the vessel 22 is located in the heated
bath 33 in the position previously occupied by vessel 26.
Vessel 22 thus becomes vessel 26, as shown in Figure 2, and
as such it is heated to cause the previously recovered
sulphur hexafluoride to be discharged into the gas circuit
enclosed by the dotted line lO. Vessel 34 also may be used
to recharge the gas circuit quickly, if required, as its
connections by-pass the regulators.
The vessel 22 and 26 can be positionally interchanged
after raising the piston, by rotating the yoke, as above
described. Alternatively cylinder 22 can be used alone if
heated by electric heaters when it is required to recharge
the equipment with the previous recovered gas.
In such case the apparatus would comprise a single
liquid nitrogen flask 21 containing an electrical heater
and the vessel 22. Connections would allow the liquid
nitrogen to be fed between the flask and a reservoir of
liquid nitrogen. Control circuitry would enable the vessel
22 to be cooled when sulphur hexafluoride was to be recovered
and heated when it was to be fed back into a breaker con-
nected temporarily to the apparatus. The apparatus could
be portably mounted.
From the above description it will be appreciated that
the gas recovery apparatus of the invention is cheap to make,
has low running costs, is very much more compact than
currently available gas recovery apparatus, and is perhaps
one quarter of the size. secause there are few moving
parts, there is also a lesser risk of damage. Although care
has to be taken when handling liquid nitrogen, it is a cheap
commodity and requires no special precautions to be taken
other than those associated with low temperature liquids
and which are fairly well known at this time.
Although the invention has been above described in the
context of removing hexafluoride gas from an electric circuit
breaker, it is to be understood that this is only one
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application of the invention and that it has broader sig-
nificance. Thus, the invention may be applied to the
recovery of any gas from any device, using the concept of
holding the vessel which is used in recovery of the gas at
a temperature beneath that at which the recovered gas is
transformed from its vapour phase to a liquid or solid phase.
It has been found that the apparatus for carrying out
the invention may be even more economically provided by
using aluminium vessels instead of stainless steel vessels.
Aluminium vessels are commercially available and are cheaper
than stainless steel vessels. Additionally, if the invention
is being applied to the recovery of sulphur hexafluoride
gas from a high voltage circuit breaker and the servicing
time on the breaker is likely to be extensive, the liquid
or solid phase gas stored in the vessel may be allowed to
slowly return to room or ambient tempexature without the
aid of the aforementioned heating bath. Under such cir-
cumstances the natural heat transfer from the ambient
; to the liquified or solidified gas may be sufficient over
the seryice time to bring the stored gas back to the ambient
temperature and hence at appropriate pressure for that
temperature.
