Methods and systems for cryogenic cooling
Methods and systems are provided for cooling an object with a cryogen having a critical point defined by a critical-point pressure and a critical-point temperature. A pressure of the cryogen is raised above a pressure value determined to provide the cryogen at a reduced molar volume that prevents vapor lock. Thereafter, the cryogen is placed in thermal communication with the object to increase a temperature of the cryogen along a thermodynamic path that maintains the pressure greater than the critical-point pressure for a duration that the cryogen and object are in thermal communication.
1. A method for cooling an object with a cryogen having a critical point defined by a critical-point pressure and a critical-point temperature, the method comprising:
raising a pressure of the cryogen to between 0.8 and 1.2 of its critical-point pressure and providing the cryogen at a temperature within ±10% of its critical-point temperature;
thereafter, reducing a temperature of the cryogen without decreasing the pressure of the cryogen below 0.8 times its critical-point pressure;
thereafter, placing the cryogen in thermal communication with the object to increase the temperature of the cryogen to an ambient temperature along a thermodynamic path that maintains a pressure greater than 0.8 times the critical-point pressure for a duration that the cryogen and object are in thermal communication;
thereafter, removing the cryogen from thermal communication with the object; and
thereafter, reducing the pressure of the cryogen to an ambient pressure.
2. The method recited in claim 1 wherein reducing the temperature of the cryogen comprises placing the cryogen in thermal communication with liquid cryogen having the same chemical structure maintained at substantially the ambient pressure.
3. The method recited in claim 2 wherein the determined pressure value is between 0.8 and 1.2 times the critical-point pressure.
4. The method recited in claim 2 wherein raising the pressure of the cryogen comprises providing the cryogen at a temperature within ±10% of its critical-point temperature.
5. The method recited in claim 1 wherein the cryogen is selected from the group consisting of N 2 , SF 6 , N 2 O, He, and CO 2 .
6. The method recited in claim 1 wherein:
raising the pressure of the cryogen comprises raising the pressure of the cryogen to greater than a pressure value determined to provide the cryogen at a reduced molar volume that prevents vapor lock; and
the thermodynamic path maintains the pressure above the determined pressure value.
7. The method recited in claim 1 wherein raising the pressure of the cryogen comprises:
placing the cryogen within a thermally insulated tank; and
applying heat within the thermally insulated tank at least until a predetermined pressure within the thermally insulated tank is reached.
8. The method recited in claim 1 wherein the cryogen comprises N 2 .
9. The method recited in claim 1 wherein the pressure of the cryogen is substantially constant for the duration that the cryogen and object are in thermal communication.
10. The method recited in claim 1 wherein reducing the temperature of the cryogen comprises placing the cryogen in thermal communication with a second liquid cryogen having a temperature lower than the temperature of the cryogen.
11. The method recited in claim 10 wherein the cryogen and second cryogen are chemically identical.
12. The method recited in claim 10 wherein the second liquid cryogen is substantially at ambient pressure.
13. The method recited in claim 10 wherein the pressure of the cryogen is substantially constant while reducing the temperature of the cryogen.