Its the first time I see filters on cooler evaporator. What on earth for? They block the air movement, collecting dirt.

It is much easier and cheaper to wash the evaporator on a regular intervals.
At this time of the year when ambient is dropping very low, the condenser fans pressure control have to be checked for keeping good head pressure.

Another and maybe the best way to avoid freezing on large evaporators is using EEV to keepmevaporating temp above freezing. [it can save a few compressors]