When securing a flash type evaporator for an extended period, what is the recommended action?

When securing a flash type evaporator for an extended period, completely draining the unit is indeed the recommended action. This practice is crucial for several reasons.

Draining the evaporator helps prevent corrosion and sediment buildup that can occur when water is left in the system. Water, particularly if it contains impurities, can lead to the formation of scale inside the evaporator, which can compromise its efficiency and longevity. Additionally, any water left inside can freeze in cold conditions, leading to damage in various components or seals.

By ensuring that the unit is completely emptied of water, you mitigate the risk of these issues occurring during the period of inactivity. This preventive measure extends the life of the evaporator and maintains its operational integrity when it is brought back into service.

The other options, while they can have their uses in different contexts, do not address the critical need for draining the evaporator to prevent internal damage and ensure cleanliness when the unit is not in operation.