To prevent the pre-coolers from icing up, the first pre-cooler cooled the air to around 10 degrees above freezing point, to liquefy the water vapour in the air. Then LOX would have been injected into the airflow to drop the temperature to flash freezing the water into microscopic ice crystals, sufficiently cold that they wouldn't melt due to kinetic heating if they struck the second pre-cooler elements. A water trap could have been added after the first pre-cooler if operating conditions resulted in an excess of moisture.