GB2064082A

GB2064082A - Improvements in or relating to cryogenic liquid transfer systems

Info

Publication number: GB2064082A
Application number: GB8026688A
Authority: GB
Original assignee: SECR SOCIAL SERVICE BRIT; UK Secretary of State for Social Services
Current assignee: SECR SOCIAL SERVICE BRIT; UK Secretary of State for Social Services
Priority date: 1979-09-07
Filing date: 1980-08-15
Publication date: 1981-06-10
Also published as: GB2064082B

Abstract

When transferring cryogenic liquids at low flow rates the fluid effusing from the transfer pipe can have an unacceptably low liquid fraction. Usually a pre-cool of the transfer system is employed, however, this is time consuming and wastes liquid cryogen. Above a critical wall temperature the heat flux is sufficient to produce a Leidenfrost flow which substantially insulates the liquid from the wall or the transfer pipe. In the invention a transfer pipe (1) with a cryogenic liquid flowing within it is a heated by a flow of warm gas (4) passing over it. The temperature of the gas is maintained by a heating coil (3) and the gas is contained within an envelope (5). The principal application of the invention is in the supply of liquid cryogen to and through a surgical cryoprobe which may also contain an endoscope. <IMAGE>

Description

SPECIFICATION Improvements in or relating to cryogenic liquid transfer systems This invention relates to cryogenic liquid transfer systems.

A problem encountered when transferring cryogenic liquids at low flow rates is that unless the boiling rate within the transfer system is also low, the fluid effusing from the transfer pipe will have an unacceptably small liquid fraction.

Traditionally a pre-cool of the transfer system to a temperature close to the boiling point of the cryogen has been employed. However, when the cryogen flow rate is low this is very time consuming and uses a substantial quantity of the liquid cryogen.

A critical pipe wall temperature exists for a liquid pipe flow system. Above this temperature, hereinafter referred to as the critical heat flux temperature, the heat flux is sufficient to produce a film boiling flow which provides a gaseous barrier between the liquid droplets of the cryogen flow and the internal wall of the transfer pipe, consequently reducing the boiling rate by substantially thermally insulating the liquid droplets from the transfer pipe wall, this condition is hereinafter referred to as Leidenfrost flow.

When a cryogenic liquid is passed along a transfer pipe which is at ambient temperature the temperature of the internal transfer pipe wall is initially well above the critical heat flux temperature and Leidenfrost flow occurs. The temperature of the pipe wall is reduced by the heat used to boil the liquid cryogen, when the pipe wall temperature falls below the critical heat flux temperature. Leidenfrost flow breaks down with a consequent rapid increase in boiling rate.

Leidenfrost flow breaks down with a consequent rapid increase in boiling rate. Leidenfrost flow also breaks down if the liquid cryogen droplets are forced into intimate contact with the pipe wall as a result of having to negotiate abrupt changes in flow direction or obstacles such as a step change in pipe internal diameter.

The present invention seeks to provide a method and apparatus in which a stable Leidenfrost flow is maintained and utilized to ensure a substantially immediate supply of liquid cryogen from a warm transfer pipe especially when low liquid cryogen flow rates are being used.

According to the invention a liquid cryogen transfer system includes: a liquid cryogen transfer pipe, hereinafter referred to as the transfer pipe, having a heating means adapted to maintain the inner wall of the transfer pipe above a critical heat flux temperature, and valving means: the transfer pipe having smooth contours at changes in cross section or direction of flow and at the valving means, when open, whereby a liquid cryogen is conveyed through the transfer pipe by Leidenfrost flow.

Conveniently the heating means comprises a heating wire helically wound around the transfer pipe and through which an electric current is passed. The heat so generated is distributed over the external surface of the transfer pipe wall by a flow of gas passing over the heating wire, thereby ensuring that the temperature of the inside wall of the transfer pipe is everywhere above the critical heat flux temperature.

Desirably the gas is exhaust gas from the exit of the transfer pipe and the gas is contained in an envelope enclosing the transfer pipe. The heating wire may conveniently be fixed to the surface of the envelope.

The system may advantageously be used to deliver liquid cryogen to, and remove exhaust gas from, a surgical cryoprobe. Most advantageously the envelope within the surgical cryoprobe is a polytetrafluoroethylene (PTFE) envelope on which the heating wire is wound, the PTFE envelope being contained together with an endoscope within a containing tube.

The invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 is a three dimensional cutaway representation of a cryogen transfer system; Figure 2 is a section along a transfer pipe illustrating Leidenfrost flow; Figure 3 is a diagrammatical partial crosssectional representation of a cryogen transfer system used to supply a cryoprobe and including an endoscope; and Figure 4 is a cross-section through a cryogen transfer system within a cryoprobe including an endoscope.

In Figure 1 a cryogen transfer pipe 1 has cryogen in the form of droplets dispersed in gas 2 flowing through it in the direction indicated, a helical heating wire 3 is heated by an electric current and consequently heats a countercurrent flow of exhaust gas 4 which subsequently supplies heat to the outside of the transfer pipe 1. The exhaust gas is contained by an envelope 5. The effect of heating the wall of the transfer pipe is shown in Figure 2, the heated transfer pipe-wall 1 causes gas 20 to be formed between the pipe-wall and the liquid cryogen. The gas 20 has the effect of substantially insulating the liquid cryogen 2 from the hot transfer pipe-wall, thereby substantially reducing the rate of boiling of liquid cryogen.

Figure 3 shows the transfer pipe system 30 feeding liquid cryogen to, and removing exhaust gas from a cryoprobe 31. The liquid cryogen is boiled in a miniature heat exhanger 32 at the end of the cryoprobe 31 thereby providing a freezing tip. The electrical power for the transfer pipe heating wire is supplied through leads 33 from an electrical supply 34 which is controlled by means of a three term proportional temperature controlled (not shown) with a temperature sensor 35 situated at the exit of the exhaust gas from the envelope 5.

The heating power supplied to the heating wire is of the order of 25W per metre of transfer pipe, the temperature sensor has a reference temperature of 00C which in the particular embodiment means that the temperature of the inside surface of the transfer pipe is kept above about -700C.

A second electrical supply 36 supplies electricity along leads 37 to a heating wire within the cryoprobe 31 in response to a second temperature sensor 38 situated in the exhaust gas flow near to its exit from the cryoprobe. The reference temperature and the level of heat input are, in practice, known roughly from development tests and are adjusted to give the optimum freezing performance once the cryoprobe system is assembled and operational. An endoscope 39 is provided in order to view the freezing tip 32 and the area to which it is being applied within a body cavity 40. The cryogen flow is controlled from a console at the entrance to the transfer pipe indicated generally by 41.

Figure 4 shows a configuration whereby the endoscope 39 may be included together with the liquid cryogen transfer system within a common containing tube 42 in a manner which reduces the likelihood of damage to the endoscope due to the proximity of cryogenic temperatures and furthermore removes the need for extra thermal insulation around the outside of the cryoprobe.

The envelope is made of polytetrafluoroethylene (PTFE) 43 which contains the exhaust gas 4 outside the transfer pipe 1 which contains the liquid cryogen 2 and the insulating gas 20. The heating wire 3 is wound onto the outside of the PTFE envelope, then the whole of this is included with the endoscope into the containing tube 42.

Claims

1. A liquid cryogen transfer system including: a transfer pipe having a heating means adapted to maintain the inner wall of the transfer pipe above a critical heat flux temperature, and a valving means: the transfer pipe having smooth contours at changes in cross-section or direction of flow and at the valving means, when open, whereby a liquid cryogen is conveyed through the transfer pipe by Leidenfrost flow.

2. A liquid cryogen transfer system as claimed in claim 1 in which the heating means comprises a heating wire wound around the transfer pipe and through which an electric current is passed.

3. A liquid cryogen transfer system as claimed in claim 2 in which the heat generated by the heating wire is distributed over the external surface of the transfer pipe wall by a flow of gas passing over the heating wire.

4. A liquid cryogen transfer system as claimed in claim 3 in which the flow of gas is provided by exhaust gas from the exit of the transfer pipe.

5. A liquid cryogen transfer system as claimed in claim 4 in which the exhaust gas is contained in an envelope enclosing the transfer pipe.

6. A liquid cryogen transfer system as claimed in claim 5 in which the heating wire is fixed helically to the surface of the envelope.

7. A liquid cryogen transfer system as claimed in claim 6 for use in a surgical cryoprobe.

8. A liquid cryogen transfer system as claimed in claim 7 in which the envelope is contained in a containing tube.

9. A liquid cryogen transfer system as claimed in claim 8 in which the containing tube also contains an endoscope.

10. A liquid cryogen transfer system as claimed in any one of claims 5 to 9 in which the envelope is a polytetrafluoroethylene envelope.

1 A liquid cryogen transfer system as claimed in any preceding claim in which the transfer pipe is circular in cross-section.

12. A liquid cryogen transfer system substantially as hereinbefore described with reference to the accompanying drawings.