Refrigeration – Storage of solidified or liquified gas – Liquified gas transferred as liquid
Patent
1998-04-13
1999-09-07
Kilner, Christopher B.
Refrigeration
Storage of solidified or liquified gas
Liquified gas transferred as liquid
62 512, F17C 702, F25B 1902
Patent
active
059469200
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and apparatus for controlling the rate of supply of a liquid cryogen. The invention is particularly suitable for use in the supply of liquid cryogen to cryosurgical apparatus, but it is not limited exclusively to this.
FIG. 1 illustrates schematically a conventional type of controllable cryogen supply. The liquid cryogen, typically Nitrogen, is contained in a vacuum insulated pressure vessel 10 which is generally sealed except for an outlet tube 12 extending below the surface of the liquid. The extreme volatility of the liquid results in the liquid boiling in the vessel 10 (or in a separate heat exchange circuit communicating with the vessel), which in turn causes a build up of pressure above the liquid surface. This gaseous pressure acts as a propellant to drive the liquid out through the tube 12.
A control valve 14 is fitted in the outlet tube 12 downstream of the vessel 10 for regulating the output flow rate as desired. In order to ensure that the pressure of the Nitrogen gas in the vessel does not reach too high a level, a pressure safety valve, or relief valve, 16 coupled to the vessel 10 opens automatically at a predetermined pressure to allow excess pressure to escape.
The control valve 14 is needed in this design so that the output rate can be controlled. In the field of cryosurgery, the flow rate determines the magnitude of the freezing effect. Not only does this vary depending on particular surgical operation to be carried out, but it is often necessary to increase or decrease the magnitude of the freezing effect at different times as part of the cryosurgical treatment.
However, there are significant problems associated with placing a control valve 14 in the flow line of the liquid cryogen. Firstly, the valve has to be designed to operate at the extremely low temperature of the liquid cryogen without risk of the valve freezing open or closed. Typically, the operating temperature would be less than -196.degree. C. for liquid Nitrogen. The cost of many such valves is prohibitively expensive, and only two types of valve are in general use today. These are: (i) a manually operable tap which is not suitable for automated operation by electronic control; and (ii) an electromagnetic on/off valve, which can only be controlled to regulate the flow rate by being switched repeatedly between the on (open) and off (closed) states.
Not only are cryogen valves expensive, they also reduce the efficiency of, and introduce unpredictability in the performance of cryosurgical instruments. The valve mechanism represents a considerable thermal mass in contact with the liquid cryogen flowing through the valve, causing undesirable heat gain (i.e. the thermal mass acts as a sink to the cooling capacity of the liquid cryogen). Heat gain is a problem associated with both types of valve discussed above, but it is particularly associated with the electromagnetic valve because heat is also generated by the flow of electric current in the valve mechanism
A further problem associated with the electromagnetic valve is that gas bubbles are produced in the liquid cryogen. The gas bubbles are caused by contact of the cryogen with the electrically driven shutter of the valve mechanism when the valve is closed. Upon such contact, localised boiling of the liquid occurs, creating a gas bubble. The valve has to be driven repeatedly to control the liquid flow rate, and bubbles are thus created continuously in the stream of liquid. Furthermore, when low flow rates are required, the valve has to held shut for a considerably longer period than it is open, which results in much larger gas bubbles.
Particularly in the field of cryosurgery, uniform laminar flow of cryogen in the supply tube is desired, as this enables better phase separation in an instrument such as a cryogenic probe, and enables efficient nuclear boiling to be established and maintained. However, the gas bubbles and the turbulence created by the intermittent operation of the electromagnetic valve disrupt the desired laminar flow, and reduce
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Kilner Christopher B.
Spembly Cryosurgery Limited
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