1. It is compised by two independ systems! If one of the system broke down, it will still not affect the normal operation of the other system. At the same tiime, the broken one can be repaired on the condition of not affect the normal one. 

2. The designed "X" type whiche can provide a much efficient heat exchane ratio. This kind of design can have a balance frost process and it is more easy to defrost as well.