Patent ID: 8534079

Claim:
A freezer for using liquid cryogen as a refrigerant comprising: a) an inner vessel defining a storage chamber; b) an outer jacket generally surrounding the inner vessel so that an insulation space is defined there between; c) a heat exchanger positioned in the storage chamber, said heat exchanger having an outlet and an inlet adapted to communicate with a supply of the liquid cryogen refrigerant so that the liquid cryogen refrigerant may flow through the heat exchanger to cool the storage chamber while being vaporized; d) a purge line in communication with the outlet of the heat exchanger, said purge line including a purge outlet positioned adjacent to an exterior of the heat exchanger; e) a purge valve positioned within the purge line so that the vaporized liquid cryogen from the heat exchanger may be selectively directed to the exterior of the heat exchanger to reduce ice formation on the heat exchanger; f) an exhaust line in communication with the outlet of the heat exchanger and the purge line; g) the exhaust line having an exhaust vent; h) an exhaust valve positioned within the exhaust line; i) a feed line in communication with the inlet of the heat exchanger and adapted to communicate with the supply of liquid cryogen; j) as bypass line in communication with the feed line; k) a bypass valve positioned in the bypass line; l) a feed temperature sensor in communication with the feed line; m) a purge gas temperature sensor in communication with the purge line; n) an exhaust gas temperature sensor in communication with the exhaust line; o) a chamber temperature sensor in communication with the storage chamber; p) a controller in communication with the feed, purge gas, exhaust gas and chamber temperature sensors and the bypass, purge and exhaust valves, said controller programmed to: i. open the bypass valve when a temperature of gas flowing through the feed line is higher than a temperature of the storage chamber; ii. close the bypass valve when the temperature of gas flowing through the feed line is lower than the temperature of the storage chamber; iii. open the purge valve and close the exhaust valve when a temperature of gas flowing through the purge line is greater than a minimum desired temperature of the storage chamber; iv. close the purge valve and open the exhaust valve when the temperature of gas flowing through the purge line is lower than the minimum desired temperature of the storage chamber; v. close the exhaust valve when a temperature of gas flowing through the exhaust line is lower than the minimum desired temperature of the storage chamber by a predetermined amount; and vi. close all valves when a temperature of the storage chamber is less than the minimum desired temperature.