Should put a dryer on and a/c unit if you replaced the evaporator?
And should you fill the system with nitrogen when brazing a new evaporator coil in?
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should put a dryer on and a/c unit if you replaced the evaporator

It depends upon the reason for the replacement evaporator coil. If the service person have a very good vac pump and lets it run the correct amount of time to boil the moisture out of the system it is not necessary to add a filter dryer BUT if the system has been open for a long time meaning the refrigerant charge was completely lost for a few days then it would be best to add a liquid line drier.

and should you fill the system with nitrogen when brazing a new evaporator coil in?

NO you never fill the system with nitrogen BUT flowing a bit of very low pressure nitrogen during brazing helps to keep scale out of the pipe. When finished the system repair can be bubble checked with nitrogen pressure but I prefer to use the same refrigerant that is used in the system.