PURPOSE: To avoid the risks of deformation and breaking down of a heat exchanger when it is restarted by an arrangement wherein, during rest periods, heat is introduced at the hot end and cold is introduced at the cold end of the heat exchanger so as to maintain these two ends at temperatures which are relatively close to those corresponding to the active period.

CONSTITUTION: On one vertical surface of a heat exchanger, gaseous nitrogen is introduced into a box 3 through a duct 4 with a stop valve 5. On the lower surface of the heat exchanger, cold waste gas is introduced into a box 6 through a duct 7 with a stop valve 8. On the other vertical surface of the heat exchanger, a box 9 is normally used as the outlet of air to be rectified after being cooled and the air constitutes the heating fluid for the heat exchanger and exits the box 9 through a duct 10. When production of nitrogen is interrupted, the stop valves 5, 8 are closed and a stop valve 15 is opened to feed gaseous nitrogen to all ducts and to feed a hot flow of room temperature to all ducts of the heat exchanger at the hot end through a supporting section 11. Consequently, a temperature gradient corresponding to normal operation of facility can be sustained between the hot and cold ends of the heat exchanger during rest periods with very small nitrogen consumption.