Many DIYers and engineering enthusiasts often wonder about recycling old units, trying to figure out what kind of vacuum a refrigerator compressor can create. This interest is quite justified, because piston mechanism, designed for pumping freon, is theoretically capable of operating in air pumping mode. However, expectations often diverge from harsh reality when it comes to vacuum numbers.
Before looking at specific numbers, it is necessary to understand the basic principle of operation. Unlike specialized vacuum pumps, the refrigeration unit is designed to work with a closed circuit and refrigerant, and not for deep evacuation of the atmosphere. Sealed housing (kupak) is not designed to operate under conditions of significant pressure differences outside and inside for a long time.
However, for simple household needs, such as vacuum packaging of products or pumping air out of small containers, this may be quite sufficient. It is only important to clearly understand the limits of the equipment’s capabilities so as not to expect the performance of an industrial installation from a household device.
Theoretical limits and design of the mechanism
To understand what residual pressure the unit is capable of producing, you need to consider its design. Most household refrigerators use crank-type piston compressors. They create pressure through the forward movement of a piston, which compresses the gas and pushes it through the discharge valve. The design of the valve group plays a decisive role here.
Inlet and outlet valves in such devices are often thin metal plates (petals). These plate valves are designed for a certain viscosity and density of freon. When working with air, which has a different density and may contain moisture, the effectiveness of the valves is reduced. Gaps that are invisible when working with refrigerant become critical when creating a vacuum.
⚠️ Attention: The design of the valve group does not allow achieving deep vacuum. Reed valves begin to let gas back in when a certain vacuum threshold is reached, which physically limits the minimum pressure in the system.
In addition, the presence of internal leaks should be taken into account. B sealed casing the motor is in a gaseous environment, and some of the compressed air can leak through the shaft seals or micro-slits back into the suction cavity. This phenomenon is called “overflow” and is the main enemy of deep vacuuming.
Real pressure indicators: numbers and facts
When we talk about vacuum, it is important to operate with specific units of measurement. Atmospheric pressure at sea level is approximately 1013 mbar (or 760 mmHg). Specialized two-stage vacuum pumps can lower this figure to 0.005 mbar. But what does refrigeration compressor reality show?
Practical measurements show that one standard household unit is capable of creating a vacuum corresponding to a residual pressure in the range from 50 to 100 mbar (absolute pressure). As a percentage of atmospheric air, this is approximately 5-10% of the initial air volume. This is enough to boil water at room temperature or tightly pack meat, but not enough for serious technical tasks.
Below is a table comparing the capabilities of different types of pumps for clarity:
| Equipment type | Residual pressure (mbar) | Degree vacuum | Applicability |
|---|---|---|---|
| Refrigeration compressor (1st stage) | 50 - 100 mbar | Low | Packaging, simple experiments |
| Car compressor (modified) | 200 - 400 mbar | Very low | Tire inflation, aerography |
| Vacuum pump (1 stage) | 0.5 - 1.0 mbar | Average | Air conditioning, HVAC |
| Vacuum pump (2 stages) | 0.005 - 0.01 mbar | High | Refrigeration equipment, laboratories |
As can be seen from the data, vacuum performance the refrigerator is an order of magnitude lower than that of specialized equipment. An attempt to use it for tasks requiring deep vacuum is doomed to failure due to design limitations.
Factors limiting performance
Why doesn't the motor pump more? The first limiting factor is the amount of dead space. This is the volume between the piston at top dead center and the valve group. The air remaining in this space is not completely removed, but expands during the next stroke, mixing with a new portion of gas. powerful in appearance Doesn't the engine pump harder? The first limiting factor is the amount of dead space. This is the volume between the piston at top dead center and the valve group. The air remaining in this space is not completely removed, but expands during the next stroke, mixing with a new portion of gas.
The second factor is heating. When working to create a vacuum, the compressor experiences increased load. The lack of pumped refrigerant, which normally cools the windings and mechanical parts, leads to rapid overheating. Thermal protection may operate more often, interrupting the pumping process.
The third factor is lubrication. Compressors use oil, which at low pressure can begin to actively evaporate (oil waste). This leads not only to loss of lubricating properties, but also to contamination of the pumped-out medium with oil mist. If you need pure vacuum, you will have to install oil separator at the outlet.
Connection diagrams for operation in vacuum mode
In order to use the unit as a vacuum pump, it is necessary to correctly determine the terminals. The standard connection diagram requires voltage to be supplied to the working and starting windings. It is important not to confuse the phasing, otherwise the shaft will not rotate, and humming and heating will be ensured.
There are usually three contacts on the block: common (C), working (R) and starting (S). To start, you need a capacitor connected between the working and starting terminals. After starting, the capacitor can be turned off if a circuit with a starting relay is used, or left in the circuit if the capacitance is selected for operation. Start relay The refrigerator can also be used to automatically turn off the starting winding.
Here are the basic preparation steps:
- 🔌 Determine the winding contacts using a multimeter (between the operating and starting resistance should be maximum).
- ⚙️ Connect a capacitor of appropriate capacity (usually 20-30 uF for household models).
- 💨 Find the pipes: suction (usually larger diameter) will be the inlet for the vacuum, and the discharge one will be the outlet.
- 🛡️ Install a filter drier at the inlet so that moisture from the air does not get inside the mechanism.
⚠️ Attention: Never leave the compressor running without pressure control. If you completely block the outlet or inlet, the pressure inside may rise to critical values, which will lead to rupture of hoses or damage to the seals.
☑️ Check before starting
Myths about cascade connection of compressors
There is a popular theory among DIYers: if you connect two or three compressors in series, you can achieve a deep vacuum. Allegedly, the first will pump out up to 100 mbar, the second up to 10 mbar, and the third up to 1 mbar. In practice, this cascade circuit works extremely inefficiently with conventional refrigeration units.
The problem lies in performance. When the second compressor tries to pump out rarefied gas from the first, its actual performance (volume flow) drops catastrophically. It simply does not have time to remove the gas coming from the first stage due to the low density of the medium. As a result, you get a system with huge energy consumption and a minimal increase in efficiency.
Why does the cascade not work?
The fact is that the compressor is a volumetric machine. It pumps a certain volume of gas per cycle. If the gas is rarefied (few molecules in volume), then the mass of the pumped substance is small. The second stage, working with an already rarefied gas, is physically unable to create a sufficient pressure drop for further deep evacuation without a special design of the cylinders.
Moreover, with a series connection, the requirements for the tightness of the entire system sharply increase. Each additional joint is a potential leak, which, in a deep vacuum, negates all efforts. To obtain a deep vacuum, it is better to use one specialized two-stage pumpthan to have a garden of three refrigerators.
Domestic use and safety measures
Despite the limitations, where can such a vacuum degasser be effectively used? An excellent option is vacuum packaging products. A pressure of 50-80 mbar is enough to remove air from a bag of meat or vegetables, significantly extending their shelf life in the refrigerator.
The device is also suitable for degassing epoxy resins or silicone in small volumes. Air bubbles will rise and burst under the influence of the created vacuum. However, for large volumes of resin, the process will take a long time due to the low productivity of the unit.
When operating, follow the following rules:
- 🔥 Monitor the temperature of the housing: it should not heat above 80-90 degrees.
- 💧 Use a moisture separator: water contained in the air, condenses inside and mixes with oil, forming an emulsion, which leads to corrosion.
- 🔊 Noise insulation: a running compressor creates strong vibration noise, so it is better to place it in a separate room or box.
⚠️ Attention: Oil characteristics and load ratings may vary depending on the compressor model (R12, R134a, R600a). Before long-term operation in non-standard mode, check the technical documentation of the manufacturer of the specific model.
Frequently asked questions (FAQ)
Is it possible to use a compressor from an old refrigerator to vacuum the air conditioner?
No, it is not possible. To vacuum the lines of air conditioners and refrigeration units, a residual pressure of no more than 0.5-1 mbar (500 microns) is required. The refrigerator compressor produces a maximum of 50 mbar, which is 50-100 times worse than required. The remaining moisture and air will lead to the formation of acid in the system and failure of the compressor.
Does the compressor get very hot when operating under vacuum?
Yes, very much. In normal mode, the refrigerant cools the windings. There is no cooling when pumping air. Without additional fan blowing, the unit can burn out within 15-20 minutes of continuous operation. It is recommended to take breaks or install a temperature sensor.
Why does oil flow from the compressor when operating under vacuum?
When a vacuum is created inside the crankcase, the oil begins to actively foam and be carried away by the air flow. This is normal for piston groups without special separators. To avoid oil loss and contamination of the object being evacuated, be sure to install an oil separator or trap filter at the outlet.
What is the maximum vacuum in percentage it creates?
In terms of percentages of atmospheric pressure, a good working compressor creates a vacuum of about 90-95%. That is, it removes 90-95% of the air volume. The remaining 5-10% cannot be removed due to structural gaps and backflows through the valves.
Is it necessary to change the oil in the compressor during refurbishment?
It is advisable. Old oil could pick up moisture and freon breakdown products. To work with air, it is better to fill in fresh vacuum oil or high-quality compressor oil of the same viscosity as it was originally. This will extend the life of the mechanism and improve the vacuum performance.