In apparatus to convert cryogenic fluid to gas, a vaporizer having passages to pass the cool or cold cryogenic fluid in heat transfer relation with warming gas flowing downwardly through the vaporizer, the vaporizer having surfaces on which ice collects and from which ice falls to the base of the vaporizer and collects in a pile, and removing fluid flow control means operating to direct flow of removing fluid at the ice pile with sufficient force to cause removal of such ice in the pile relative to the vaporizer base.