How can we avoid air entry and subsequent liquefying/freezing in the LH2 vent line?

It is normal for some air ingress to occur from the vent stack outlet. This is not a hazard if the stack has been properly designed to withstand an internal explosion or fire. Once hydrogen flow from a device is initiated, it will sweep out any air that might be in the stack. Generally, if the vent rate is insufficient to sweep the air out, then it’s also insufficient to freeze or liquefy air in the stack. However, it’s important to prevent air from being pulled into the stack from a venturi effect, so leaks or holes where air can enter the vent piping should be eliminated.