GB2266347A

GB2266347A - Dispensing fluids from containers

Info

Publication number: GB2266347A
Application number: GB9308169A
Authority: GB
Inventors: Arthur Hambrose Day
Original assignee: Planer Products Ltd
Current assignee: Planer Products Ltd
Priority date: 1992-04-21
Filing date: 1993-04-21
Publication date: 1993-10-27
Also published as: GB9308169D0; GB9208587D0

Abstract

Liquid nitrogen contained in an insulated vessel (12) is dispensed in gaseous and/or liquid form via an outlet pipe (50, 51). A sample of gas is taken from the headspace (30) and pumped through a closed loop (32) outside the container. The sample takes up heat in the loop and is re-introduced via an inlet (36) so as to impinge on the liquid surface (28), thus heating a surface layer and increasing the pressure in the container whereby fluid can be dispensed. In a modification the warmed gas sample is bubbled through the liquid pool (26) so as to heat a localised column of liquid. The invention is applicable to the controlled dispensing of fluids stored cryogenically. <IMAGE>

Description

DISPENSING FLUIDS FROM CONTAINERS This invention relates to methods and apparatus for dispensing fluids from containers.

In this application, the term "container" is to be understood to include anything that defines within it a space in which a fluid can be contained, and which is capable of defining, as part of that space or in communication therewith, a headspace above a free surface of the fluid contained in liquid form in the space. Typically the container will be a discrete, closed vessel; it may alternatively be a reservoir or receptacle that is part of a system of pipework or the like.

The invention is particularly, though not exclusively, applicable to the dispensing of liquified gas, i.e. a fluid which is a gas under ambient conditions, but which is stored in liquid form, at cryogenic temperatures, in a thermally-insulated closed vessel.

Such a vessel may typically be for use as a convenient source of the gas, which is dispensed from the vessel for use as and when required. A typical example of such a vessel is a dewar, thermally insulated by a vacuum jacket with coatings that reflect thermal radiation.

Conventional methods of dispensing a gas from a container in which it is stored in liquid form generally fall into two groups: use of mechanical pumps, and heating the fluid to drive it out by thermal expansion. Under these circumstances, mechanical pumping systems tend to suffer not only from sealing problems, but also from risk of mechanical failure, especially when the moving parts are maintained at low temperatures. In practice, the thermal expansion approach is more common.

In this type of dispensing system, the vessel is closed and the bulk of the liquefied gas is heated to a temperature only just above its normal boiling temperature, so that some of the liquid boils. This elevates the vapour pressure of the gas in the heads pace above the liquid, so elevating the boiling temperature until equilibrium is reached. Vapour pressure is thus progressively increased above the nearly boiling liquid, and this pressure is then used to expel either the liquid or the gas, or both, from the container through suitable outlet means.

In a conventional system of this type, it is known to use any one of three different heating techniques, namely (a) immersing an electric heater in the liquid, (b) submerging in the liquid a mass of material which is at a temperature well above that of the liquid itself, or (c) circulating liquid through suitable ducting that is not thermally insulated, so as to allow it to receive heat from outside. In an example of the second of these techniques, a mass of metal at room temperature is plunged into liquid nitrogen in the vessel as the latter is capped. Once the gauge pressure of the gas has built up to a suitable value, typically in the range of about 0.34 to 1.4 bar, the fluid can be dispensed in liquid or gaseous form simply by opening a valve in a pipe communicating with the interior of the vessel above or below the liquid surface as required.The gas pressure in the vessel then falls until no further fluid is dispensed, so that it is necessary then to repeat the process to obtain more fluid from the vessel.

These known heating methods all have various drawbacks.

For example, an electrical heater, if used, must be immersed in the liquid itself. Where uninsulated ducting or pipework is used in order to effect heat exchange from outside, access must be obtained to the bottom of the liquid pool. This gives rise to problems when the dispensing apparatus is to be used with vessels of different sizes, or with different depths of liquid in a given vessel.

Another, and serious, disadvantage is that all the known heating methods involve heating the whole of the liquid, which tends to result in very slow growth of the working pressure. This is unacceptable, especially where large quantities of liquid or gas are required to be delivered quickly. A further drawback is that it is difficult to achieve automatic constant pressure control, because of the relatively large thermal mass of the liquid.

Among objects of the present invention are to provide a method and apparatus for dispensing a fluid, and in particular a cryogenic fluid, from a container, which avoids the need to heat all of the liquid in the container before achieving working pressure.

Another object is to provide a method in which it is not necessary to bring any heating means into contact with the liquid in the vessel.

Another object is to provide a simple system which can be used regardless of the size or shape of the vessel, and regardless of the depth of liquid in the vessel.

Further objects include that of enabling a constant pressure to be maintained, with easy automatic control and without introducing any problems of sealing.

According to the invention in a first aspect, a method of dispensing a fluid from a container, the fluid being contained in the latter in mainly liquid form, comprises: passing some of the liquid as gas, from a heads pace above a free surface of the liquid, through a loop; causing the temperature of the gas to rise in the loop; re-introducing the resulting warmer gas from the loop to a localized zone of the liquid in the container, so as to heat the liquid in the localized zone and increase the pressure in the container; and allowing this increased pressure to drive a quantity of the fluid out of the container.

Thus the driving pressure for expelling the fluid from the container is obtained by causing liquid to evaporate from the surface of the liquid, by directing a quantity of warm gas to only a comparatively small part of the liquid in the container. The warm gas is itself obtained by causing the temperature of a sample of gas to rise, the sample being removed from the vessel for that purpose before being re-introduced as warm gas. The loop is essentially a closed loop, which reduces the risk of any sealing problems occurring.

The warmed gas may be directed on to the surface of the liquid itself, in which case the localized zone is a surface layer of the liquid. Alternatively, the heated gas may be introduced below the liquid surface, and allowed to bubble upwards through the liquid. In this latter case, the localized zone is a column of liquid, within the mass of liquid in the container in the immediate vicinity of the (or each) train of rising bubbles.

It will be noted that with the method according to the invention, the system is stable in that it always tends to achieve pressure equilibrium, but that there will be a temperature gradient through the liquid. For example, when the heated gas is applied on the liquid surface, the temperature reduces from the surface to the bottom of the liquid pool. Surface temperature and pressure at equilibrium will, in this case, be such as just to maintain the fluid in its liquid phase.

Because it is only a relatively small part of the mass of liquid in the vessel, rather than its bulk, that is heated, pressurising gas can be generated very rapidly, so that the fluid is available to be dispensed very rapidly.

According to the invention in a second aspect, dispensing apparatus for a fluid comprises: a container for storing the fluid in liquid form, defining a heads pace above a predetermined level in the container corresponding to the surface of liquid therein when the container is full; together with means for dispensing fluid therefrom and comprising a loop having an outlet from the headspace leading to an inlet into the container via driving means for urging fluid th-rough the loop, the loop being adapted to cause the temperature of a gas within it to rise; and main outlet means for removing the fluid from the container.

Where the heated gas is to be applied to the liquid surface, the inlet is open into the headspace. If on the other hand it is to be bubbled through the pool of liquid, the inlet extends down to a level below that corresponding to the surface of the liquid when the container is full.

The gas temperature in the loop may be caused to rise in any suitable way, for example by simple exposure of the gas to ambient temperature, e.g. by conduction through the wall of the loop. Alternatively, or in addition, the loop may be equipped with heating means such as at least one electric heater, a heating bath or the like.

One embodiment of the invention will now be described, by way of example only and with reference to the accompanying drawing, which shows diagrammatically a container in the form of a closed vessel containing liquid nitrogen at cryogenic temperature, in combination with a dispensing means according to the invention.

In the drawing, the container 10 consists of a vessel 12 having thermal insulation indicated diagrammatically at 14. The vessel is open at the top to define a mouth 16 in which a removable closure 18 fits sealingly.

Suitable means 20 are provided for retaining the closure 18 in position. In the drawing the means 20 are represented by a bar 22 held releasably on the container 10 by means of springs 24. It will of course be realised that the closure and any retaining means provided may be of any suitable form.

The vessel 12 contains liquid nitrogen 26 having a free surface 28, above which is a headspace 30 which of course contains nitrogen gas.

A closed control loop 32 comprises a pipe having an outlet section 34 joined to an inlet section 36 through a small mechanical pump 38 driven by an electric motor 40, the whole being sealed against both escape of gas and ingress of air from outside. The outlet and inlet sections 34, 36 both extend through the closure 18 into the headspace 30, the outlet end of the inlet section 36 being indicated at 37. One or more electric heaters 42 may, if desired, be provided in the loop, upstream or downstream of the pump 38 or both. A further pipe 44 carried by the closure 18 leads to a safety valve 46 and a pressure gauge 48. Delivery pipes 50 and 51 communicate with the interior of the vessel 12. In this example the pipe 50 leads from the headspace 30, while the pipe 51 is connected from the bottom of the vessel, so as to receive liquid from the bottom of the liquid nitrogen pool 26.As shown, the pipes 50 and 51 are carried by the closure 18, but they may extend through the wall of the vessel itself instead. Each pipe 50, 51 carries a stop valve 52. It will be understood that either of the pipes 50 or 51 may be omitted, if the nitrogen is only required for use as gas or liquid, as the case may be.

In operation, liquid nitrogen is poured into the container 10 up to the level 28, and the closure 18 is fitted so as to seal the vessel. Air originally present in the latter is of course displaced by gas evaporating from the liquid nitrogen. With the valves 52 closed, the container is now ready for nitrogen to be dispensed from it as and when required.

To dispense the nitrogen, the pump 38 is operated to draw gas from the space 30 through the pipe section 34, returning it into the space 30 via the pipe section 36 so that the gas impinges on the liquid surface 28. As it passes through the loop 32, this gas is compressed to some extent by the pump 38, and also receives heat, from outside by conduction through the walls of the loop and from the heater or heaters 42 if provided.

The temperature of the gas impinging on the surface 28 has thus been increased (typically to a value close to ambient), so that it heats a surface layer at the surface 28 and so causes more of the liquid to be vaporised. This raises the pressure in the space 30 and therefore in the vessel 12 generally. If either of the valves 52 is now opened, nitrogen is driven by this pressure out of the vessel via the corresponding pipe 50 or 51, being in the form of a gas in the former case and a liquid in the latter case.

In a modified embodiment, the inlet pipe section 36 is extended to a level close to the bottom of the container, as indicated in phantom lines in the drawing at 60. The warm gas then bubbles up in a narrow column of bubbles 62, defining a vertical localized zone of the liquid nitrogen which is thereby heated. The inlet extension tube 60 may be perforated, so that the warm gas is released at various levels in the nitrogen pool 26.

In a further modification, the extension tube 60 may terminate at any desired level between that of the surface 28 when the container is full and the bottom of the latter. Then, when the container is full, warm gas will bubble upwards, but when the liquid level falls below the end of the tube 60, warm gas simply impinges on the surface of the liquid. If the bubbling method is used, the capacity of the pump 38 will need to be increased.

So long as the pump 38 continues to operate, this process can be continued until the required amount of nitrogen has been dispensed. The valve or valves 52 can then be closed and the pump stopped.

It will be appreciated that the speed of the pump 38 can be varied, as can the rate at which heat is supplied by the heaters 42, in order to give very precise control of the rate at which the pressure within the vessel is increased. Thus not only is nitrogen made readily available via the pipes 50, 51, without involving a long wait while the working pressure builds up, but the rate at which it is dispensed can be adjusted as required with a very short response time.

If large volumes of gas are required to be delivered through the pump 38, it is preferable to provide at least one heater 42, so as to heat the gas impinging on the surface 28 at a sufficiently high rate, and/or to a sufficiently high temperature, which may then be above ambient. Control of the dispensing flow is then obtained mainly by adjusting the speed of the pump 38.

However, means may also be provided for reducing the output of the heaters 42 at very low pump speeds, to avoid over-heating.

Where the nitrogen is to be delivered in liquid form via the pipe 51, the volume of liquid required is generally small and the heaters 42 may then be unnecessary.

The closed loop 32 does of course act as a heat exchanger for conduction of heat to the nitrogen gas within it from outside, and may be provided with a suitable extended external heating surface for this purpose.

It will be realised that the various parameters may be controlled in order to ensure delivery of gas and/or liquid at a constant pressure, by providing a pressure sensor (not shown), in association with the pipe 50 or 51 or both, linked through a suitable microprocessor or other control circuit with means for controlling the speed of the pump 38, the output of the heaters 42, and degree of opening of the valves 52 if the latter are made variable.

The loop, corresponding to the loop 32, need not be outside the vessel in which the pool of liquid is stored, but may be inside it, e.g. in the headspace or incorporated within the structure of the container itself. In this connection the latter can be any type of suitable container, of any required size, and can be of any special design suitable for the requirements of the user. Thus it can for example be a vessel having a complex wall structure, with or without a removable closure member such as a pressure-tight sealed cover.

Where such a cover is provided, the complex wall structure can be part of the cover instead of the vessel.

This structure can for example be hollow to define the ductwork of the loop, with elements such as the pump, heater or heaters if provided, and any associated valves and/or instrumentation, being arranged within it. In general, wherever the loop is located, it is preferably thermally insulated in any suitable way from the interior of the vessel, so that the heat to be applied to the gas sample is not dissipated into the pool of liquid or the headspace above it.

In place of a heater or heaters, or in addition, the loop may include an extended heat exchange surface, e.g. finned ducting, whereby ambient air can yield heat to the gas sample; such an extended surface may be part of a heat exchanger which may be fan-assisted.

Claims

1. A method of dispensing a fluid from a container, the fluid being contained in the latter in mainly liquid form, comprising the steps of: passing some of the liquid as gas, from a headspace above a free surface of the liquid, through a loop; causing the temperature of the gas to rise in the loop; reintroducing the resulting warmer gas from the loop to a localized zone of the liquid in the container, so as to heat the liquid in the localized zone and increase the pressure in the container; and allowing this increased pressure to drive a quantity of the fluid out of the container.

2. A method according to Claim 1, wherein the warmed gas is introduced above the free liquid surface so as to impinge on the latter.

3. A method according to Claim 1 or Claim 2, wherein the warmed gas is introduced below the free liquid surface so as to bubble up through the liquid, at least when the container is substantially full.

4 A method according to any one of the preceding Claims, wherein the fluid held at a cryogenic temperature is in liquid form in the container.

5. Dispensing apparatus for a fluid, comprising: a container for storing the fluid in liquid form, defining a headspace above a predetermined level in the container corresponding to the surface of liquid therein when the container is full; together with means for dispensing fluid therefrom and comprising a loop having an outlet from the headspace leading to an inlet into the container via driving means for urging fluid through the loop, the loop being adapted to cause the temperature of a gas within it to rise; and main outlet means for removing the fluid from the container.

6. Apparatus according to Claim 5, wherein the means for urging fluid through the loop comprises a pump arranged in the loop.

7. Apparatus according to Claim 5 or Claim 6, including heating means in the loop.

8. Apparatus according to any one of Claims 5 to 7, wherein the said inlet terminates in the headspace.

9. Apparatus according to any one of Claims 5 to 7, wherein the said inlet terminates at a level between the said predetermined level and the bottom of the container.

10. Apparatus according to any one of Claims 5 to 9, wherein the loop extends outside the container, the said outlet and inlet of the loop passing through a wall portion, or respective wall portions, of the container.

11. Apparatus according to any one of Claims 5 to 9, wherein at least part of the loop, including the said driving means, is arranged or incorporated in the container.

12. Apparatus according to any one of Claims 5 to 11, further including heat exchange means in the loop for transfer of heat from the atmosphere outside the container to fluid in the loop

13. A method of dispensing a fluid from a container holding the fluid in liquid form, substantially as described in the foregoing description with reference to the accompanying drawing.

14. Dispensing apparatus for storing a gas in liquid form and dispensing it therefrom, substantially as described in the foregoing description with reference to the accompanying drawing.