Truck mounted direct fired vaporizers are fire tube burner-generated hot gas heated water bath vaporizers. Hot gas is generated by a burner system and circulated through heat exchangers heating water retained in a ballast tank to the required temperature; cryogen is vaporized through tube bundles by direct contact with hot water. These are generally used for the higher capacity requirements. It is commonly used in pipeline back up service or other high volume systems such as steel mills, or LNG peak-shaving and LNG send-out station duty.