Obviously it will be impossible to predict the air entering the system when it is open to the atmosphere. However if the temperature of the exposed parts of the system is above the dew point temperature or if the exposure time is short, the amount actually entering the system will be small. If a new dryer filter is installed in the liquid line each time the system is opened for maintenance, the dryer, which normally has sufficient absorption capacity, will bring the system moisture to a sufficiently low level.
When the exposure period is too long during maintenance, the amount of moisture present may be greater than the effective capacity of the dryer. In this case the vacuum is the only effective means of extracting large quantities of accumulated moisture from the system. In order to perform satisfactory dehydration, evacuate under pressures at such low levels that the moisture can be easily vaporized.
