The cooler is the use of cold flow nitrogen and dirty nitrogen, so that after condensing the saturated liquid liquid air and liquid nitrogen and other liquid temperature is lower than the condensing pressure corresponding to the saturated liquid temperature, its role is mainly dependent on the deployment of cold and recovery of cold, to increase the upper tower reflux, so as to improve the distillation working conditions,in addition, the subcooler can reduce the rate of gasification after throttling.