I always just leave the evaporator filled and let it freeze. It has sat for 3 weeks like that before and the sap was still good. My evaporator is a raised flue and there hasn't been any damage to the flues even with temps well below zero. If you have a drop flue you may want to drain it and let the sap freeze in buckets. Trying to keep it thawed will let the bacteria grow and the sap will spoil.