Specifying correct orifice sizes for gas based cryosurgical probes
Gas based cryosurgical probes use the Joule-Thomson (JT) effect to achieve rapid freezing at the probe tip. The JT effect is a thermodynamic process that occurs when high pressure gas is flowed through a small orifice into a large chamber, causing a pressure drop.
Cryogenic gases such as Carbon Dioxide or Nitrous Oxide cool rapidly when this process is applied.
The JT effect has been used for many years in Cryosurgical medical probes, and is an effective way of producing temperatures in the region of -70 Deg C. The cooling process is almost instant and the defrost is just as quick, which makes it good for procedures where several short freeze cycles are required.
The shape of the JT orifice is important in achieving consistent and reliable performance, generally the orifice shape will be circular or annular. The annular orifice is susceptible to changes in flow rate as the orifice size can vary as the internal parts contract due to cooling.
An additional problem is that they are very susceptible to blocking either by debris or moisture. The width of the annular orifice is only a few microns so any particle sizes larger than this will block it. Whereas, the hole diameter is much larger than the width of the annular gap so much larger particles can pass through it. The hole is also less susceptible to variation in size due to cooling.
Setting the flow rate level is critical to getting the best performance out of the probes, in terms of freeze temperature, ice growth, reliability and avoiding unwanted freezing. The flow rate required will be dependent on the type of gas, the system design and size of the probe.
We have carried out extensive testing in order to characterise the anticipated flow rates for different pressures and systems. From this we are able to predict the hole size needed to supply the desired flow rate for each particular application. We have found that a difference of 0.01mm in diameter can make as much as 1ltr/min difference in flow rate.
Therefore, we require precision nozzles to set the flow rate which have long lead-times and large minimum order quantities. Having a high degree of confidence that the nozzle size chosen will deliver the desired flow rate reduces costs and lead-times of projects.
