Evacuating a refrigerating system step by step: dehydration and standing vacuum test
How to evacuate a refrigerating system: the right equipment, a target of 500 microns (0.67 mbar), a standing test that tells moisture from leaks, triple evacuation with nitrogen and common mistakes.
Key points
- Evacuation removes air and moisture from the system after pressure testing and before the refrigerant is charged [1][3].
- Measure the vacuum with an electronic vacuum gauge on the system, not with the manifold gauges.
- A common target is below 500 microns (0.67 mbar), or the value given by the manufacturer [1].
- A standing test with the pump isolated tells moisture from a leak: the pressure levels off with moisture but keeps rising with a leak [1].
- If the system is wet: break the vacuum with dry nitrogen and evacuate again, several times if needed [2].
Why evacuate?
Air and moisture in a refrigerating system lead to higher condensing pressure, lower capacity and chemical problems. Moisture reacts with refrigerant and oil and forms acids, especially with polyolester oils (POE), which absorb moisture easily. Free water can freeze in the expansion valve. That is why the system must be emptied of air and moisture before the refrigerant is charged. EN 378-2 requires the system to be dried and evacuated before charging [3].
Equipment
- A vacuum pump with enough capacity for the system volume, with clean, dry oil. Change the oil often: wet oil gives a poorer ultimate vacuum.
- Short hoses with a large diameter. A short, wide hose evacuates much faster than a long, narrow one.
- Remove the valve cores (Schrader) during evacuation with a core removal tool so that the flow is not restricted.
- An electronic vacuum gauge mounted on the system, as far from the pump as possible. Manifold gauges cannot show a deep vacuum.
- Shut-off valves so that the pump can be isolated from the system during the standing test.
Step by step
- Make sure the system has been pressure tested and tightness tested. Evacuation is not a good way to find leaks.
- Connect the pump to both the high and low pressure sides with short, wide hoses. Remove the valve cores and connect the vacuum gauge directly on the system.
- Open all valves in the system, including solenoid valves. Use a magnet or energise the coil so that no part is shut in.
- Start the pump and evacuate until the gauge shows less than the target value, often 500 microns (0.67 mbar), or the value given by the manufacturer [1]. Large systems and cold surroundings take longer.
- Close the valve between the pump and the system and switch off the pump. Carry out a standing test for at least 10–15 minutes, longer for large systems, and watch the pressure.
- Assess the result using the table below.
- When the test is passed: break the vacuum with refrigerant, refit the valve cores and charge the system according to the manufacturer's instructions.
How to read the standing test
| What the gauge shows | Likely cause | Action |
|---|---|---|
| The pressure rises a little and levels off below about 1,000 microns | The system is dry and tight | Passed (follow the manufacturer's limit) |
| The pressure rises and levels off at a higher level, about 1,000–1,500 microns | Moisture boiling off | Keep evacuating, or break with nitrogen and evacuate again |
| The pressure keeps rising towards atmospheric pressure | Leak | Break with nitrogen, pressurise and search for the leak |
The limits in the table are a guide. Which level counts as passed depends on the system and the manufacturer's requirements [1].
Triple evacuation with nitrogen
If the system has taken in a lot of moisture, for example after a failure or after being left open, it can take a long time to remove the moisture with vacuum alone. The vacuum can then be broken with dry nitrogen to a slight positive pressure, the gas left to pick up moisture for a while, released and the system evacuated again. The procedure is often repeated three times, hence the name. Nitrogen does not dry in itself, but it helps the vacuum pump remove the moisture faster [2].
Common mistakes
- Relying on the pump having run for a certain time instead of measuring the vacuum.
- Measuring at the pump instead of on the system. The gauge then shows a better value than the system actually has.
- Forgetting a closed solenoid or shut-off valve so that part of the system is not evacuated.
- Using a pump with old, wet oil.
- Skipping the standing test and charging a system that leaks.