Abstract: Coolant gas recovery and reuse from a heat exchanger is automatically interrupted when the oxygen content of the used gas exceeds a predetermined maximum concentration and is recommenced when the oxygen content falls below a predetermined minimum concentration. The mass flow of coolant gas introduced into the passageway is controlled independently of its composition and, during normal operation, the mass flow ratio of coolant gas withdrawn for recovery to coolant gas introduced into the heat exchanger is maintained constant. This ratio can be ramped up to the constant level in a predetermined manner on start up of fiber cooling and following recovery interruption. The invention provides both methods and apparatus and has particular application to Optical Fiber manufacture.