| 摘要 |
A liquid cryogen is supplied, preferably by means of a reservoir (12), to a dispensing tube (36,38,40) so that the liquid cryogen tends to flow from the dispensing tube. A heating coil (44,46,48) wrapped around the dispensing tube heats the dispensing tube so that the liquid cryogen undergoes nucleate boiling within the dispensing tube to form a vapor block, and thereby at least impede the liquid cryogen from flowing from the dispensing tube. Flow is re-established within the dispensing tube by terminating the heating of the dispensing tube, e.g. by turning off electrical power supplied to the heating coil. The heating of the dispensing tube can be sufficient to stop the flow of liquid cryogen. Additionally, the dispensing tube can be cyclically heated by a timing circuit (56) connected to the heating coil. Such cyclical heating is used to throttle the flow of the liquid cryogen from the dispensing tube. <IMAGE> |
