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Low-pressure chillers and vacuum behavior
Type III changes the mental model: low-pressure appliances normally operate in a vacuum, so leaks pull air and moisture inward. This 25-question mode focuses on purge behavior, leak testing, water-freezing prevention, recovery, oil handling, and the correct recharge sequence.
A low-pressure chiller can operate below atmospheric pressure. Instead of refrigerant always pushing outward through a leak, air and moisture can be drawn into the machine. That is why excessive purge-unit operation can indicate leakage and why water temperature matters during evacuation: reducing pressure also reduces water's boiling point and can freeze water in tubes.
How pressure, boiling temperature, air, moisture, and noncondensables behave below atmospheric pressure.
Excessive purging, controlled heat, system pressurization devices, nitrogen limitations, and leak-test pressure.
Liquid-first recovery, vapor removal, pressure rebound, water circulation, and heating oil to reduce refrigerant release.
Introduce vapor before liquid to prevent freezing and charge a centrifugal through the evaporator charging valve.
| Condition | What can happen | Exam decision |
|---|---|---|
| System pressure below atmosphere | Air and moisture can leak inward. | Excessive purge operation can be a sign of leakage. |
| Pressure drops during evacuation | The boiling point of water falls. | Circulate or remove water to prevent tube freezing. |
| Liquid is introduced too early during charging | Rapid boiling can freeze water in the tubes. | Introduce vapor before liquid. |
| Pressure rises after reaching the target | Liquid refrigerant may remain in the system or oil. | Wait, observe, and continue recovery as appropriate. |
EPA's evacuation table lists low-pressure appliances at 25 mm Hg absolute. Compare all categories in the recovery and evacuation levels reference.
What does unusually frequent purge-unit operation commonly suggest on a low-pressure chiller?
A is correct. A low-pressure system can draw air and moisture inward. The purge unit removes noncondensables, so excessive operation is a useful leak clue.
Why should water be circulated or removed from a chiller while refrigerant is being evacuated?
B is correct. Lower pressure lowers water's boiling temperature. The resulting cooling can freeze water and damage tubes if it is not circulated or removed.
What should be introduced first when recharging an evacuated low-pressure chiller?
C is correct. Vapor is introduced first to raise system pressure and reduce the chance that liquid refrigerant will boil violently and freeze water in the tubes.
What level does EPA's evacuation table list for a low-pressure appliance?
D is correct. Low-pressure appliances use an absolute-pressure target: 25 mm Hg absolute. Do not confuse it with inches of mercury vacuum used for other rows.
Below atmospheric pressure, a low-pressure system can pull air and moisture inward.
The low-pressure target is stated as 25 mm Hg absolute, not 25 inches Hg vacuum.
Start with vapor so liquid does not boil rapidly and freeze water in the tubes.
A rising pressure after recovery can reveal refrigerant still trapped in liquid or oil.
Take 25 Type III questions, then review the pressure, purge, freezing, recovery, and charging decisions that need another pass.
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