The entry of moisture into a cooling system in the form of ice or water is extremely rare. It is the invisible water vapor in the ambient air that produces the real danger.
The ability of air to contain water vapor is greater the higher its temperature. On a hot, humid summer day, the air can be really full of moisture. The relative humidity determines the dew point, ie the temperature at which the moisture contained in the air begins to condense.
Although the fact that steam exists as part of the air around us, it acts completely independent of the air. The movement of the water vapor can not be contained by the movement of the air.
Normally, it is considered an excellent practice to introduce refrigerant under positive pressure which does not allow air to enter the exposed lines. This provides reasonable protection against air ingress but is not sufficient protection against moisture ingress and, if the evaporator is at a lower temperature than the dew point of the air, the water vapor will condense inside the same . This process will continue until the evaporator temperature rises above the corresponding dew temperature or until the system is closed again.
