and activates a stretcher 315 containing motor and reduction gear 116 and rotary to linear motion converter 118 to stretch the heat exchanger 302 to re open airflow passages as heretofore described when stretch reaches a limit a defrosting cycle of the heat exchanger is activated 210 when defrosting is complete the stretch of the heat exchanger is relaxed 212 to allow the helical heat exchanger to return to an unstretched state an alternative embodiment as illustrated in fig4 has multiple wedgelike heat exchange surfaces 402 404 some of these heat exchange surfaces 402 form a first multiple wedge surface and are fixed to a frame not shown of the heat exchanger a second group of these heat exchange surfaces 404 form a second multiple wedge surface interdigitated with heat exchange surfaces 402 of the first multiple wedge surface heat exchange surfaces 404 of the second multiple wedge surface are fixed to a movable element 406 of the heat exchanger heat exchange surfaces 402 404 are either fabricated from microchannel tubing or are fabricated from thermally conductive fins in thermal contact with coolant tubes 405 movable element 406 is attached to an actuator 410 that typically incorporates a motor reduction gear and one or more rotary to linear motion converters similar to those previously discussed between wedges 402 and 404 are gas passages 406 a controller 412 has sensors 414 for monitoring for airflow obstruction and is coupled to drive actuator 410 in operation the controller 412 initially drives the movable element 406 to a position such that gas passages 406 are narrow as moisture condenses out of the gas such that ice accumulates on heat exchange surfaces 402 404 sensors 414 detect airflow obstruction in response to the airflow obstruction controller 412 causes the actuator.