The process of preparing the refrigeration circuit for operation is a fundamental step in the repair of climate control equipment. Many amateur craftsmen underestimate the importance of deep vacuumrelying only on short-term air removal. However, inside the system, in addition to atmospheric air, there is always water vapor, which, when the temperature drops, turns into ice or, what is even more dangerous, enters into a chemical reaction with oil and refrigerant.
The formation of acids and slags inside the compressor is a direct consequence of poor-quality preparation of the circuit. Moisture remaining in the system can freeze in the capillary tube, causing an ice block that will completely stop the circulation of freon. That is why the pumping procedure must be carried out strictly according to technology, using specialized equipment and observing time intervals.
In this article we will analyze in detail the physics of the process, the necessary equipment and a step-by-step algorithm of actions that will allow you to avoid repeated equipment failures. High-quality evacuation is a guarantee of long and stable operation compressor and the entire refrigeration unit as a whole.
Physics of the process: why air and moisture are the enemies of the system
Atmospheric air consists of a mixture of gases, the main components of which are nitrogen and oxygen. When these gases enter the closed circuit of the refrigerator, they create excess pressure, which interferes with the normal condensation of the refrigerant. Non-condensable gases accumulate in the condenser, increasing the temperature and discharge pressure, which leads to overload of the compressor.
However, the most dangerous component is moisture. Water has a unique property: at normal atmospheric pressure it boils at 100°C, but in deep vacuum the boiling point drops sharply. At a pressure of 600 microns (microns of mercury), water already boils at a temperature of about 13°C. This means that by pumping out the air, we simultaneously force the moisture to evaporate and be removed from the system in a gaseous state.
⚠️ Attention: Residual moisture in the system leads to hydrolysis of freon (especially R134a), resulting in the formation of aggressive acids. These acids destroy the compressor motor windings and cause corrosion of metal parts from the inside.
The process of moisture evaporation takes time. If you simply run the vacuum pump quickly, you will remove most of the air, but the moisture adsorbed on the walls of the tubes and in the oil will remain. Therefore, the concept of “vacuum” in the context of refrigeration technology is not just the absence of air, it is a state of the system in which all free and bound moisture has been removed.
Necessary equipment for high-quality vacuuming
A conventional household pump is not enough to perform the work. A specialized tool capable of creating and maintaining a deep vacuum is required. The main device is a two-stage vacuum pump. Single-stage models often cannot achieve the required residual pressure values (less than 500 microns) required to remove moisture from modern systems with narrow capillary tubes.
A critical element is the pressure station (manifold). It should be equipped with a vacuum gauge, preferably an electronic one, that shows the pressure in microns, and not just in bars or PSI. Conventional dial pressure gauges have low accuracy in the high vacuum zone and do not allow control of the drying process of the system.
- 🛠️ Vacuum pump: Capacity from 45 to 100 l/min for household refrigerators. It is mandatory to have a check valve to prevent oil from being thrown into the circuit when stopped.
- 📏 Gauge manifold: Preferably with three hoses (blue, red, yellow) and the ability to connect a vacuum gauge.
- 🔗 Hoses: Special hoses for refrigerants with low pressure losses and high-quality fittings.
You will also need tools for working with metal: a torch (gas or oxygen-propane), solder (phosphorus-copper or silver-containing) and a pipe cutter. To protect the system from moisture from the air during soldering, nitrogen is used, although simple repairs often involve purging with the refrigerant itself, which is less effective, but acceptable.
Preparation of the refrigeration circuit for pumping
Before starting vacuuming, you must make sure that the system is tight. Trying to pump out a leaky circuit is a waste of time and pump resources. If there was a leak in the system, it must be found and eliminated using nitrogen. The pressure during pressure testing is usually 15-20 bar, and it must be maintained for at least 24 hours (or less when using a helium leak detector). pressure tests nitrogen. Pressure testing is usually 15-20 bar and should be maintained for a minimum of 24 hours (or less if using a helium leak detector).
If a compressor has burned out in the system, a mandatory procedure is to flush the circuit with nitrogen under pressure to remove combustion products and acidic residues. In such cases, it is often necessary to replace the filter-drier and, possibly, install an additional press shopter (trap) on the suction line.
The preparation process includes the following steps:
- Removing the old filter-drier. This element is disposable and is saturated with moisture upon contact with air.
- Installation of a new filter-drier. Soldering must be done quickly so that the zeolite inside the filter does not have time to absorb moisture from the atmosphere.
- Connecting the pressure gauge station to the service fitting (usually on the suction line of the compressor).
⚠️ Attention: Never leave the new filter drier open to the air for more than 10-15 minutes. If you did not have time to solder it right away, plug the holes with special caps or temporarily close them with a piece of rubber from the hose.
Step-by-step instructions: the vacuum process
Now let's move directly to the pumping technology. This process requires care and consistency of actions. First make sure all valves on the gauge station are closed. Connect the yellow manifold hose to the vacuum pump and the blue (low pressure) hose to the refrigerator service port.
Open the valve on the blue hose and turn on the vacuum pump. The pressure gauge needle should sharply go negative. If an electronic vacuum gauge is used, you will see the reading drop quickly. At the initial stage, the main volume of air is removed. The initial pumping time depends on the volume of the system and the degree of moisture contamination, but usually is at least 15-20 minutes for household refrigerators.
Sequence of actions:1. Open the low pressure valve on the manifold.
2. Turn on the vacuum pump.
3. Wait 20-30 minutes for deep pumping.
4. Close the valve on the manifold.
5. Turn off the pump.
6. Monitor the readings for 15-30 minutes.
After the initial pumping, it is necessary to check for leaks. Close the valve on the manifold and turn off the pump. Monitor the pressure gauge readings. If the arrow begins to creep up (the pressure increases), it means that there is a leak in the system or the valve is not completely closed. If the pressure is stable in the deep vacuum zone, the system is sealed.
☑️ Checklist before refueling
An important point is triple vacuum method. It is used to ensure guaranteed removal of moisture. The essence of the method: after the first pumping, the system is briefly filled with a small amount of refrigerant (or nitrogen) to atmospheric pressure, then evacuated again. This helps to “pull out” moisture from hard-to-reach places where it could be preserved.
Control of residual pressure and exposure time
How long does it take to vacuum the refrigerator? This is the most frequently asked question. The time depends on the pump power, ambient temperature and system volume. However, the main criterion is not time, but residual pressure. For household refrigerators with R134a or R600a refrigerants, the norm is to achieve a pressure of 200-500 microns (0.2-0.5 mm Hg).
Ordinary pressure gauges do not see such values, showing simply “-1 bar” or “76 cm Hg.” Therefore, experienced craftsmen focus on the operating time of the pump after reaching the maximum vacuum for a given pump. It is recommended to keep the pump on for another 15-20 minutes after the pressure gauge has stopped showing changes.
| System type | Minimum pumping time | Recommended pressure (micron) | Features |
|---|---|---|---|
| Domestic refrigerator (R134a) | 30-40 minutes | < 500 | Careful drying required |
| Domestic refrigerator (R600a) | 30-40 minutes | < 500 | Explosive gas, ventilation required |
| Industrial display case | 60+ minutes | < 300 | Large volume, long pipeline |
| System after engine combustion | 60+ minutes + purge | < 300 | Triple vacuum required |
If after turning off the pump the pressure begins to slowly increase, but stops at a certain level (for example, 1000-1500 microns) and then remains stable, this may indicate evaporation of residual moisture. In this case, the pumping process should be continued. If the pressure rises to atmospheric pressure, look for a leak.
Why can pressure rise after the pump is turned off?
An increase in pressure after the pump is turned off can be caused by three reasons: 1. Leaks in the circuit (air leaks). 2. Evaporation of moisture from the oil or from the walls of the tubes (boiling at low pressure). 3. Oil return from the vacuum pump itself through the check valve (if it is faulty). To diagnose, close the valve on the manifold: if the pressure rises between the pump and the valve, the pump is at fault. If there is a problem in the circuit or moisture between the valve and the refrigerator.
Typical errors and troubleshooting
One of the most common mistakes is using dirty oil in the vacuum pump. The oil in the pump is hygroscopic and absorbs moisture from the pumped air. If the oil has become milky in color, its effectiveness drops sharply, and it itself begins to release moisture back into the system. The oil needs to be changed regularly, especially after working with wet systems.
Another mistake is the hoses for connecting the pump are too thin or long. The capacity of the hose limits the pumping speed. Use hoses with a diameter of at least 1/4 inch and the shortest possible length.
- ❌ Lack of filter-drier: An attempt to refill the refrigerator without replacing the filter after opening the circuit is doomed to failure. The zeolite of the old filter is already saturated.
- ❌ Ignoring temperature: In a cold room, moisture evaporates worse. It is recommended to slightly warm up the condenser or compressor with a hairdryer (carefully!) during evacuation to speed up the evaporation of moisture.
- ❌ Incorrect shutdown: If you first turn off the pump and then close the valve, oil from the pump can be drawn into the refrigerator circuit.
⚠️ Attention: When working with refrigerators using isobutane (R600a), remember that this is a flammable gas. Evacuation in this case also removes remaining air, preventing the formation of an explosive mixture during subsequent refueling. Work in a ventilated area.
Properly performed vacuuming is the key to ensuring that your repair will be durable. Do not save time at this stage, since calling again and redoing the work will cost much more than half an hour of extra pump operation.
Frequently asked questions (FAQ)
Is it possible to vacuum a refrigerator with a regular car pump?
No, a regular car pump is designed to create pressure (compression), not a vacuum. Even if it is capable of creating some vacuum, its performance and vacuum depth are not sufficient to remove moisture from the refrigeration circuit. You need a specialized vacuum pump.
Do you need to heat the compressor during evacuation?
Heating the compressor (for example, with a hairdryer or turning it on for a short time, if it is safe for its windings in a vacuum) promotes the evaporation of moisture dissolved in the oil. However, this must be done carefully. A safer method is to simply increase the exposure time under vacuum.
What to do if the vacuum does not hold and the arrow creeps up?
This is a sign of leakage. You should recheck all soldered seams with soapy water (bubbles will indicate a leak) or use an electronic leak detector. Also check that the valves on the pressure gauge station are tightly closed.
How often should you change the oil in the vacuum pump?
The oil is changed when it changes color (becomes cloudy, whitish or dark) or after each complex repair associated with a large amount of moisture or engine combustion. Visual control is the best indicator.
Is it possible to fill the refrigerator if the vacuum gauge shows not 0, but a little higher?
If the pressure is stable and does not increase after turning off the pump, and the system was dry, then a slight deviation is acceptable. However, you need to strive for the highest possible vacuum that your pump can provide (usually this is a zone below 500 microns).