When refrigerant is being evacuated from the chiller cooler or condenser vessels, any liquid refrigerant left in a vessel will flash off, lowering the temperature in that vessel enough to freeze the fluid (usually water) flowing through the cooler or condenser tubes. This event, called tube freeze-up, can cause extensive damage to the chiller; therefore, all liquid refrigerant must be removed from a vessel before evacuation of refrigerant vapor is started. If all the liquid cannot be removed, then the cooler water and condenser water pumps must be operated throughout the process of evacuating refrigerant vapor to keep fluid moving through the cooler and condenser tubes.