Gas condensation

A pressure-liquefiable gas or vapor fed into the interior of a container which may constitute the fuel tank of a power unit for a model aircraft or other working model, is condensed within the container by introducing either a liquid derived from said gas or vapor or a liquid refrigerant from an external source into the interior of a hollow condensing element mounted in the container with its outer surface in contact with said gas or vapor inside the container, and then exhausting the contents of the condensing element to cool the outer surface thereof to a temperature below the condensation temperature of said gas or vapor.

The present invention relates to the condensation of gases. In general, the 
invention is concerned with the condensation of gases that may be 
liquefied at normal climatic temperatures under the effect of pressure 
alone, such as carbon dioxide, the FREONs (Registered Trade Mark), butane, 
propane and ammonia. In particular, the present invention is concerned 
with the condensation of such gases in the tanks that supply gas to 
gas-operated motors but it may also be applied to other containers, 
vessels, condensers or other equipment in which the easy condensation of a 
pressure-liquefiable gas is desired. 
Motors driven by a supply of pressurised carbon dioxide are known and have 
been used for example as the power plants of model aircraft. Such motors 
generally employ a tank to contain the pressurised carbon dioxide and the 
normal practice is to charge this tank by connecting it to another 
container holding liquid carbon dioxide. A major disadvantage of this 
practice is that vaporised carbon dioxide within the tank or entering the 
tank during the charging process does not condense and so limits the 
amount of liquid carbon dioxide which can be charged into the tank. In 
consequence the tank must be greatly increased in size and weight in order 
to carry the desired quantity of carbon dioxide. 
A further problem relating to such motors and their supply tanks is that, 
if the level of liquid carbon dioxide within the tank is higher than the 
centre of volume for example, there is a risk of liquid carbon dioxide 
being supplied to the motor which may be damaged in consequence. 
Very much larger motors, for example of the turbine type driven by vapour 
that after expansion in the turbine is condensed and then vaporised again 
for re-use in a closed cycle, are also known and also suffer from 
difficulties in condensing the vapour. British Pat. No. 1,102,464 
describes a closed cycle turbine power plant wherein a liquid coolant is 
used to promote condensation of the expanded vapour, but at the cost of 
considerable complexity. 
The main object of the present invention is to promote the easy 
condensation of a vapour within a tank or other apparatus associated with 
a motor or a device supplied by a gas or a vapour. However, the present 
invention may equally be applied to promote the easy condensation of a 
vapour in any container, for example for the filling of containers with 
carbon dioxide or like gas for use in a variety of applications. 
To this end, according to a first aspect of the invention, there is 
provided a method of condensing a pressure-liquefiable gas or vapour, said 
method comprising feeding said gas or vapour into the interior of a closed 
container, introducing either a liquid derived from said gas or vapour or 
a liquid refrigerant from an external source into the interior of a hollow 
condensing element mounted in said container with its outer surface in 
contact with the gas or vapour inside the container, and exhausting the 
contents of said condensing element to cool the outer surfaces thereof to 
a temperature below the condensation temperature of said gas or vapour. 
According to a second aspect of the invention, there is provided apparatus 
for promoting the condensation of a pressure-liquefiable gas or vapour, 
said apparatus comprising a closed outer container having an inlet for 
said gas or vapour, an inner condensing element mounted within said 
container and comprising a hollow receptacle the exterior of which is 
exposed to the contents of the container and the interior of which 
communicates either with the interior of said container or with an 
external source of liquid refrigerant, and means for exhausting the 
contents of said receptacle in order to cool the outer surface of said 
condensing element to a temperature below the condensation temperature of 
said gas or vapour. 
A further object of the present invention is to reduce the likelihood of 
liquid phase being supplied by a tank or other apparatus containing liquid 
carbon dioxide or like gas, for example to prevent liquid carbon dioxide 
from being supplied to a motor (where it may cause damage) or to a drinks 
carbonator, or to prevent liquid FREON (Registered Trade Mark) from being 
expelled from an aerosol dispenser. 
To this end, according to a third aspect of the present invention, the gas 
or vapour is withdrawn from the interior of said container at a point 
which normally lies above the level of the liquid therein and is given a 
swirling motion at a velocity sufficient to separate any entrained liquid 
therefrom by centrifugal force before leaving the container. 
In this manner, the receptacle within the container may be exhausted prior 
to the desired condensation and in consequence of such exhaustion will 
fall in temperature as liquid phase vaporises and expands from within the 
receptacle to a lower pressure. Vapour then admitted to the container will 
contact the cooled receptacle and thereby be induced to condense. Liquid 
phase thereby condensed within (or otherwise supplied to) the receptacle 
substantially remains therein until the contents of the container are 
consumed or carried away, whereupon the receptacle may again be exhausted, 
cooled and again used to promote a further condensation. This process may 
be repeated indefinitely. 
However, because this process does induce condensation so effectively, it 
follows that the liquid level within the container may thereby rise to a 
high level. Consequently, the third aspect of the present invention is, in 
many cases, necessary to ensure that, in subsequent employment of the 
container so as to supply vapour or gas, substantially no liquid phase is 
supplied. Therefore, the third aspect of the present invention 
(hereinafter referred to as "centripetal gas offtake") is in many cases a 
necessary part of the overall invention. 
Nevertheless the present invention does not exclude those applications 
wherein it is acceptable to supply liquid phase from the container or 
wherein other means are employed to vaporise such liquid phase, and hence 
is not restricted to applications wherein all aspects of the present 
invention are employed together. 
However, in those many applications wherein the high liquid level achieved 
by the first and second aspects of the present invention may result in the 
supply of undesired liquid phase, the centripetal gas offtake herein 
disclosed as the third aspect of the present invention provides a simple 
means of substantially preventing such liquid phase supply. Accordingly, 
the centripetal gas offtake comprises firstly an aperture positioned above 
the level of the liquid surface so as to reduce the likelihood of liquid 
phase passing into the aperture (though it is to be noted that in many 
applications such as in the tanks of model aircraft, power plants and 
other devices it is possible that the motion of the model aircraft or the 
handling of the device will cause the aperture to become occasionally 
submerged beneath the liquid surface level and thus to pass liquid phase), 
secondly a cavity communicating with the aperture and designed to 
encourage the gas or gas/liquid mixture entering it from the aperture to 
flow with a swirling or circular motion so that any liquid phase (being of 
higher density than the gas phase) tends to remain at the outer wall of 
the cavity owing to its centripetal acceleration and consequent 
centrifugal force, and thirdly an outlet positioned substantially at the 
central axis of the circular motion so that gas phase, being less dense 
than liquid phase, tends to move towards the central axis i.e. tends to 
move centripetally and thence into the outlet. From the known densities of 
the gas and liquid phases, the rate at which gas is required to flow from 
the outlet and the known thermodynamic properties of the 
pressure-liquefiable gas, it is then possible to design a centripetal gas 
offtake according to the present invention whereby the angular velocity of 
the circular motion within the cavity will produce sufficient centripetal 
acceleration to ensure that only the less-dense gas phase will be 
available at the central outlet, and to induce vaporisation of the liquid 
phase at the outer wall by means of the falling pressure gradient ocurring 
towards the central axis and thereby encouraging the escape of gas phase 
from the outer liquid phase.

FIG. 1 illustrates, in vertical cross-section and approximately twice full 
size, a gas-supply tank of 3 cubic centimeters capacity and adapted 
according to the present invention to carry upto 21/2 grams of 
largely-liquid carbon dioxide and to supply carbon dioxide vapour suitable 
for use, inter alia, in driving the motor of a model aircraft. FIG. 1 is a 
view in elevation. 
FIG. 2 is an end view, simplified and in vertical cross-section, of the 
gas-supply tank illustrated in FIG. 1 and to the same scale, showing a 
cross-section of the container and receptacle of the present invention and 
indicating the surface levels of the liquid carbon dioxide typically 
obtained in the said container and said receptacle. 
FIG. 3 refers to another embodiment of the present invention as applied to 
a steam condenser and illustrates, schematically and to a much reduced 
scale, components of the steam condenser and of apparatus according to the 
present invention adapted to promote condensation of steam entering the 
steam condenser. 
Referring to FIG. 1, a substantially cylindrical container 1 advantageously 
made of injection-moulded high-strength plastic material such as acetal is 
sealed at one end by the tank `O` ring to a body member 3 and is sealed at 
its other end by the screw `O` ring 4 to the screw head 5. The screw head 
5 engages by threaded connection with a hollow screw 6 and these two 
components are preferably made of high-strength thermally-conducting 
material such as aluminium alloy, although in some applications the screw 
head 5 may advantageously be of low thermal conductivity material such as 
TUFNOL (Registered Trade Mark) for reasons given later herein. 
The hollow screw 6 fits tightly within a sleeve portion 7 of the body 
member 3 so as to prevent any significant flow of liquid carbon dioxide 
through the sleeve portion 7, although a passageway 8 in the sleeve 
portion and a groove 9 and hole 10 in the hollow screw permit carbon 
dioxide to flow into and out of the hollow screw. The leftward end (as in 
FIG. 1) of the hollow screw is plugged sealingly by the screw plug 11 and 
the same end of the hollow screw engages by threaded connection 12 with 
the outlet adaptor 13. Both male and female threads of the threaded 
connection 12 are truncated so as to permit carbon dioxide to flow 
therethrough along the two helical pathways formed between the truncated 
threads. 
The outlet adaptor 13 is provided with a substantially cylindrical cavity 
14 terminating in a cup-shaped `O` ring retainer 15 which retains a tube 
`O` ring 16 and squeezes it to seal on an outlet tube 17. The outlet 
adaptor is sealed by the adaptor `O` ring 18 to the body member 3 which is 
joined in a gas-tight manner (preferably by ultrasonic welding) to a 
filling nozzle 19 provided with an interior frustro-conical seat 20 and a 
ball valve 21 so as to allow carbon dioxide to be injected into the 
container 1 from an external refilling bottle (not shown, being of known 
type) though preventing escape of carbon dioxide from the filling nozzle 
19. 
A gas offtake aperture 22 is provided at the highest point of the shoulder 
23 of the body member so that carbon dioxide may pass therethrough from 
within the container 1 to the threaded connection 12 and thence to the 
outlet tube 17 which is positioned substantially on the central axis of 
the cavity 14, the cup-shaped `O` ring retainer 15 and the screw plug 11. 
Outlet tube 17 is adapted for connection to the cylinder of a gas 
expansion engine 56 which, in combination with container 1, forms a power 
unit, such as for a model aircraft or other model. 
In some applications of the present invention it may be advantageous to 
fill the hollow screw 6 at least partly with an auxiliary material 24 for 
a variety of reasons. By way of example the auxiliary material 24 may 
comprise numerous small capsules of a buffer substance as defined and more 
fully explained in UK Pat. No. 1,561,831 which is incorporated herein for 
reference, whereby the heat released by a change of state of the buffer 
substance at a certain falling temperature may be usefully employed to 
assist vaporisation and/or superheating of the carbon dioxide. 
Alternatively or in addition, the auxiliary material 24 may comprise 
granules of fused alumina which will promote vaporisation of carbon 
dioxide and suchlike gases. Alternatively or in addition, the auxiliary 
material may comprise charcoal, silica or alumina or the like which, in 
their activated forms, have the property of adsorbing large amounts of 
carbon dioxide and the like and simultaneously releasing heat (which may 
then assist the vaporisation and/or superheating of the carbon dioxide or 
the like) and, conversely when the container 1 of the present invention is 
discharged, have the property of desorbing the carbon dioxide or the like 
whilst simultaneously absorbing heat and thereby becoming very cold. Such 
coldness, generated within the hollow screw 6 and transferred to its outer 
wall, is the primary process whereby the main object of the present 
invention is achieved, namely, the condensation of gas or vapour into 
liquid phase within the container 1, as is now explained. 
In operation of the FIG. 1 embodiment, the outlet tube 17 is first 
connected for example to a motor or other device requiring vaporised 
carbon dioxide or the like in such manner that vapour does not flow out of 
the outlet tube until the motor or other device is put into operation. The 
container is then given a first charge of carbon dioxide by application of 
a refilling bottle (not shown) to the filling nozzle 19, whereupon 
approximately 1 gram of carbon dioxide will flow in. The first charge of 
carbon dioxide will normally be largely of vapour phase, with only a small 
puddle of liquid phase forming on the base of the container 1 and another 
small puddle forming within the hollow screw 6. Indeed, such a charge of 
only some 1.0 gram in a 3.0 cubic centimeter tank (i.e. a "Filling Ratio" 
of 0.333 gm/cc) is all that can be normally achieved in conventional 
simple tanks used heretofore; the main object of the present invention is 
to achieve a Filling Ratio considerably higher than 0.333 gm/cc. 
To achieve this main object, the first charge of about 1 gram of carbon 
dioxide is discharged by operating the motor or other device connected to 
the outlet tube, or by depressing the ball valve 21 to unseat it from the 
frustro-conical seat 20 (by use of a suitable probe pressed into the 
filling nozzle 19), or by other means not shown in FIG. 1. The effect of 
discharging the FIG. 1 embodiment is to cause the hollow screw 6 to cool 
down significantly, as the carbon dioxide therewithin vaporises and 
expands through the hole 10 and thence through the outlet tube. Moreover 
the hollow screw stays cool for a long time (of the order of 1 to 10 
minutes, or more with suitable design) after such discharging, because it 
is thermally insulated from ambient sources of heat by the surrounding 
container 1, outlet adaptor 13 (which for this reason should be of 
thermally-insulating material such as plastics) and body member 3. Indeed 
the screw head 5 provides the only significant path along which heat may 
flow into the hollow screw and accordingly is designed to provide a 
desired delay before the hollow screw loses its coldness: if made of 
aluminium alloy to the FIG. 1 embodiment the delay will be approximately 
1-2 minutes; if made of steel, 2-4 minutes; if made of TUFNOL (Registered 
Trade Mark), 4-12 minutes; and so on with other materials and suitable 
variations in design and sizing of relevant components. 
A second charge is then applied whereupon the entering carbon dioxide 
(which will normally be partly vaporised) comes into contact with the cold 
hollow screw which has the effect of promoting rapid condensation. Indeed, 
without this central feature of the present invention, entering vapour 
normally experiences the phenomenon of .cent.supercooling" whereby it 
fails to condense despite being several degrees K below its saturation 
temperature. In contrast, contact with the cold hollow screw (which should 
be a few degrees colder still than the coldest temperature that the vapour 
can remain as supercooled vapour) has the effect of de-stabilising any 
supercooled vapour and causing its immediate condensation. This effect 
propagates to any surrounding vapour and causes it likewise largely to 
condense, though the latent heat of vaporisation released by such 
condensation will normally allow a relatively small amount of vapour to 
remain at the top of the container and of the hollow screw. 
Referring now to FIG. 2 which shows a simplified cross-section of the 
container 1 and the hollow screw 6, the liquid levels 25 and 26 in the 
container and hollow screw respectively are depicted as they will normally 
result after charging the FIG. 1 embodiment on a second or subsequent 
occasion following a previous discharging and within the delay period 
before the hollow screw warms up as hereinbefore described. These depicted 
liquid levels correspond to a carbon dioxide Filling Ratio of 0.75 to 0.85 
gm/cc compared with the normal maximum of about 0.333 gm/cc achieved 
heretofore. Thus the present invention increases the useful mass of carbon 
dioxide or the like charged into a container of a given capacity--and 
increases the useful work or other service performed thereby--by a factor 
of approximately 2.5. 
However as seen in FIG. 2 the liquid level in the container is much higher 
than the central axis, which is a preferred position for the outlet tube 
in many applications as shown in FIG. 1, and hence poses the problem of 
undesired liquid phase rather than vapour being delivered. Therefore, in 
its second aspect, the present invention discloses means to overcome this 
problem as now described. 
The problem is first minimised by selecting a gas offtake level 27 higher 
than the highest expected liquid level 25, as seen in FIG. 2, when the 
container is in its normal operating attitude e.g. horizontal as in the 
FIG. 1 embodiment which is intended for horizontal mounting in model 
aircraft or for use in other applications with the leftward end (as in 
FIG. 1) inclined upwards. Referring to FIG. 1, the gas offtake aperture 22 
is then designed to be at a level at least as high as the selected gas 
offtake level 27. 
However, situations such as the aerobatic manoeuvring of a model aircraft 
etc. may still cause the gas offtake aperture to become occasionally 
flooded by liquid carbon dioxide or the like and therefore the present 
invention proposes means such as the helical pathways formed by the 
truncated threads of the threaded connection 12 whereby any liquid phase 
entering therein will issue into the cavity 14 with a high-speed swirling 
motion. The cross-sectional area of the flow paths formed by the said 
truncated threads are chosen so that, at the known rate of gas or vapour 
consumption by the e.g. motor, the angular velocity of any liquid phase 
issuing into the cavity will be sufficient to throw such liquid phase 
firmly outwards against the bounding cylindrical wall of the cavity. For 
example in the FIG. 1 embodiment the total cross-sectional flow area of 
the two helical pathways is not more than 0.1 square millimeters so that, 
at the lowest designed flow rate of 2 grams per minute, the angular 
velocity in the cavity will be at least 800 radians/second with vapour 
flow and generally over 200 radians/second when liquid phase is present. 
With a cavity diameter of 0.6 centimeters an angular velocity of 200 
radians/second provides a centripetal acceleration at the cavity wall of 
12,000 cm/sec.sup.2 which is generally sufficient to prevent liquid phase 
from reaching the outlet tube 17; instead, vapour phase escapes from the 
layer of liquid phase flung outwards against the cavity wall and migrates 
towards the central axis and hence into the outlet tube. To assist such 
vaporisation in the cavity, the cavity wall should advantageously be of 
thermally-conducting material in thermal connection with ambient 
surroundings or provided with an enclosing jacket containing a buffer 
substance as described hereinbefore and in UK Pat. No. 1,561,831; suchlike 
components are not shown in FIG. 1, being of known art and unnecessary at 
the low power output level provided by the FIG. 1 embodiment. 
Referring to FIG. 3, a condenser 30 of the shell-and-tube type is supplied 
with steam by an inlet pipe 31 in order to produce water condensate from 
an outlet pipe 32. The great majority of latent heat of vaporisation 
extracted from the steam in this process is removed by a stream of coolant 
(which may be water or other suitable medium) which enters a header 33, 
flows through a plurality of tubes 34 and leaves the condenser 30 by a 
coolant outlet pipe 35. 
A receptacle 36, advantageously provided with numerous fins 37 which extend 
into the steam flowing into the condenser and constructed of high thermal 
conductivity material such as aluminium alloy, is supported within the 
condenser and linked by suitable pipework 38 to an external circuit of 
apparatus including a venting valve 39, a vapour receiver 40, a compressor 
41, a cooler 42 supplied with a suitable coolant 43 such as cold water, a 
liquid receiver 44, a feed pump 45 and a stop valve 46. The liquid 
receiver 44 is charged with a refrigerant liquid 47 which may suitably be 
the refrigerant commonly known as R12 and having the chemical formula 
CCl.sub.2 F.sub.2 or which may be another of the FREONs (Registered Trade 
Mark), butane, propane, ammonia, carbon dioxide or the like, according to 
known refrigeration art and the desired steam condensing temperature in 
the condenser. The choice of this refrigerant liquid 47 should be such 
that, at the pressure within the vapour receiver 40, the refrigerant 
liquid will vaporise at a temperature at least 10 degrees K and preferably 
20 to 30 degrees K lower than the steam condensing temperature. 
In operation of the FIG. 3 embodiment, stop valve 46 is opened and venting 
valve 39 is closed and the feed pump 45 is used to charge the receptacle 
36 with refrigerant liquid 47. Stop valve 46 is then closed and venting 
valve 39 opened, whereupon the refrigerant liquid within the receptacle 
vaporises and rapidly cools the receptacle and its fins 37 to a 
temperature several degrees K below the lowest supercooled temperature 
that can be exhibited by the steam entering the condenser 30; this has the 
effect of de-stabilising any supercooled steam in the proximity of the 
receptacle and causing rapid condensation thereof, the latent heat of 
vaporisation so released being carried away largely by the coolant leaving 
by the coolant outlet pipe 35. 
Vaporised refrigerant produced from the receptacle flows to the vapour 
receiver 40, is compressed by the compressor 41, condensed by the cooler 
42 and stored in the liquid receiver 44. 
The receptacle-charging and liquid-vaporising operation hereinbefore 
described may then be repeated to induce a further surge of condensation 
within the condenser, and so on in a cyclical manner so as to achieve a 
desired enhancement and speeding-up of the steam condensation. 
Of course the FIG. 3 embodiment may be adapted to promote and enhance the 
condensation of vapours other than steam, for example the vapours of the 
FREONs (Registered Trade Mark), butane, propane, ammonia and the like.