Converting an old refrigeration unit into a full-fledged air compressor is a classic project for the home workshop, which allows you to get a powerful tool for almost free. Hermetic compressorsinstalled in household refrigerators have the unique ability to create high pressure and operate in long cycles, making them an ideal basis for pneumatic systems. Unlike specialized industrial analogues, such devices operate much quieter and do not require complex maintenance.
The main idea is to use electric motor a piston group already designed to work with refrigerant, but reconfigure them to compress atmospheric air. However, simply pumping air requires changes to the lubrication and filtration system, since the standard design does not allow for work with a moist air environment. It is critical to understand that when working with air, the oil in the crankcase quickly oxidizes and turns into an emulsion, so the standard lubrication system often has to be modified or completely replace.
Creating such a device requires not only technical skills, but also strict adherence to safety measures, since you will be dealing with high pressure and electric current. In this article we will analyze all stages of transformation, from choosing a donor to final setup pressure switch and connecting pneumatic tools. The finished unit will be able to power spray guns, inflate tires or be used for purging systems.
Selection of a suitable compressor unit
The first and most important stage is the search and assessment of the condition of the donor. Not every old refrigerator is suitable for this purpose, so you should pay attention to the exhaust and appearance of the motor. Soviet-made models are considered the best choice, since they were often equipped with more powerful and repairable units with copper windings.
Upon visual inspection, make sure that the case sealed unit has no traces of deep corrosion, dents or, worst of all, traces of soldering and tampering. If the fittings are cut off from the pipes, this is a good sign - it means that the freon has already come out, and the unit is safe for further work in the workshop. However, the presence of rust on the mounting legs may indicate long-term storage in damp conditions, which is dangerous for the internal electrical system.
Checking the functionality is carried out by briefly plugging it into the network. To do this, assemble a simple circuit: connect the starting relay (if it is removable) and a capacitor (if required for a specific model) to the motor contacts. When voltage is applied, a working motor should emit a characteristic hum and begin to suck in air through one of the tubes, and forcefully blow it out from the other.
⚠️ Attention: It is strictly forbidden to check the operation of a unit sealed in a sealed casing without ensuring free air outlet. The pressure inside a closed circuit can instantly rupture welds, which will lead to an explosion and scattering of metal fragments.
It is also worth paying attention to the noise level. Modern imported compressors can operate quieter, but they are often less durable when working with air due to the use of lighter alloys. Units of the SK, FK or KD type are considered the "gold standard" for homemade compressors due to their reliability and availability of spare parts.
Necessary tools and materials for assembly
For successful implementation of the project, you will need not only the compressor itself, but also a number of additional components that will ensure stable operation of the entire system. The basis of the pneumatic system is receiver, the role of which can be played by an old gas cylinder, a fire extinguisher or a specially made container made of a thick-walled pipe. The volume of the receiver directly affects the stability of pressure: the larger it is, the less often the engine will turn on.
In addition to the air tank, it is necessary to prepare a filtration system. A air filtermust be installed at the compressor inlet, which will prevent dust from entering the cylinders. At the outlet, in front of the receiver, an oil-water separator is often installed, especially if you plan to use a compressor for painting, where the presence of oil in the air is unacceptable.
The most important element of safety and automation is pressure switch (pressostat). This device automatically turns off the engine when a preset maximum is reached and turns it on when the pressure drops to a minimum. Without this device, you will have to constantly monitor the pressure gauge and manually control the power, which is inconvenient and dangerous.
- 🔧 Tools: a set of wrenches, screwdrivers, a hacksaw, a drill with drill bits, a soldering iron or a welding machine (for metal connections).
- ⚙️ Accessories: ball valves, check valve (to prevent air from flowing back into the compressor), pressure gauge, adapters and fittings, high-pressure hoses.
- 🛡️ Safety: safety valve (emergency pressure release), which will operate if the pressure switch will fail.
Do not forget about consumables: sealant for threaded connections (for example, FUM tape or anaerobic fixative), oil for compressors (special, not ordinary motor oil) and fasteners for mounting the entire structure on a frame or wheel base.
Preparing the compressor and changing the oil
One of the main technical problems when converting a refrigeration compressor to air is the lubrication system. In the factory version, the oil is located in the lower part of the sealed housing and is sprayed by the crankshaft, lubricating the rubbing pairs. However, when working with air for a long time, the oil oxidizes, loses its properties and turns into an aggressive emulsion, which can coke the valves.
There are two approaches to solving this problem. The first is “lazy”, when the oil is simply changed more often, using special synthetic oils for compressors, and its condition is monitored. The second, more professional option is to convert the system to forced lubrication or use an oil-free compressor (although the latter require regular maintenance of graphite rings). To begin with, it is recommended to simply drain the old mineral oil and fill in fresh compressor oil of the appropriate viscosity.
To change the oil, find a sealed tube at the bottom of the housing (usually there are three of them: two for freon and one thin, sealed, for filling oil). Carefully cut off the tip of this tube and drain the waste into a container. Then, using a syringe or funnel, fill in new oil in the volume recommended by the manufacturer (usually 300-400 ml, but it is better to check the specification for the specific model). After this, the tube must be properly soldered or a threaded fitting with a plug must be installed for ease of future replacement.
It is also recommended at this stage to clean the internal cavities if there is a suspicion of the presence of freon breakdown products or dirt. You can rinse the system several times with a special solvent, but this requires careful subsequent drying, since moisture in the system is unacceptable.
Manufacture and preparation of the receiver
The receiver acts as a buffer, accumulating compressed air and equalizing pressure pulsations created by the piston. The most affordable and reliable option for a homemade compressor is fire extinguisher with a volume of 10-50 liters. Its body is designed for high pressure, it has a convenient neck for installing fittings and, as a rule, is in good condition.
Before use, the fire extinguisher must be prepared. First make sure it is completely empty and there is no residual pressure. Then unscrew the bell and the shut-off device. The inner surface of the container must be thoroughly rinsed with water and detergent to remove any remaining powder that can have an abrasive effect on the compressor mechanism or clog the pneumatic tool. After washing, the container must be completely dried to prevent corrosion.
In the upper part of the receiver (into the former hole for the shut-off device), a metal fitting (squeeze) with a thread is welded or screwed into which all the fittings will be screwed. If you are using a gas cylinder, the technology is similar, but requires greater precautions when cutting and welding due to the explosiveness of residual gas. Be sure to remove the valve and steam the cylinder before any welding work.
| Receiver type | Volume (liters) | Working pressure (Bar) | Difficulty of preparation |
|---|---|---|---|
| Fire extinguisher | 10 - 50 | 15 - 20 | Low |
| Gas cylinder | 20 - 50 | 16 - 25 | High |
| Truck receiver | 20 - 40 | 10 - 15 | Medium |
| Pipe (welded) | Any | Depends on the wall | High (welding required) |
After preparing the housing, a tap is installed in it to drain condensate at the lowest point. This is a mandatory requirement: when air is compressed, moisture is released from it, which accumulates at the bottom of the receiver and causes corrosion. Regular drainage of water will prolong the life of your system.
Assembling a pneumatic circuit and connecting
Assembling the entire system into a single block requires sequential connection of all elements according to the diagram. Air from the atmosphere passes through the inlet filter, then enters the compressor, is compressed and enters the receiver through a check valve. From the receiver, air goes to the pressure switch (to control the motor) and to the outlet valve (for the consumer).
The key element here is check valve. It is installed between the compressor and the receiver and prevents compressed air from returning back to the compressor after it stops. Without this valve, the next time the engine is started, it will try to push through the already compressed air, which will lead to overload and combustion of the windings or the starting relay.
The pressure switch is connected in two ways: electrical and pneumatic. The pneumatic port is connected to the receiver (often through a separate elbow or tee). The electrical circuit implies an interruption of the motor power supply phase through the relay contacts. When the pressure in the receiver reaches the upper limit (for example, 8 Bar), the relay opens the contacts and the engine stops. When the pressure drops to the lower limit (for example, 6 Bar), the contacts close and the cycle repeats.
☑️ Checking the system assembly
Don't forget to install a pressure gauge to visually monitor the pressure in the system. It is best to place it at the outlet of the receiver, before the adjusting gearbox (if there is one), in order to see the real pressure in the tank.
Configuration, testing and safety
The first start-up of the assembled compressor is the most crucial moment. Carry out it in a well-ventilated area, away from flammable materials. Before plugging in, check all electrical connections again for short circuits and insulation. Make sure that the outlet valve is closed and the safety valve is working properly.
Turn on the compressor and monitor the pressure gauge readings. The engine should run smoothly, without strong vibration or extraneous knocking. When the cut-off pressure (which you set on the relay) is reached, the engine should stop. If this does not happen, immediately turn off the power manually - this means that the pressure switch is faulty or incorrectly configured, and there is a risk of rupture of the receiver.
⚠️ Attention: Never leave a working homemade compressor unattended. The risk of system depressurization or electronics fire during homemade assembly is higher than with factory products.
Check the system for air leaks. Apply soapy water to all connections: the appearance of bubbles will indicate a leak. Also check the compressor heating temperature. During prolonged operation, the housing can heat up to 60-80 degrees, but it should not smoke or emit a smell of burnt insulation.
What to do if the compressor hums but does not start?
Most often the problem is in the starting relay or capacitor. Try to carefully (using a screwdriver with an insulated handle) briefly close the contacts of the starting winding at the moment of switching on. If the engine starts, change the starting device. Also, the cause may be jamming of the piston group due to thickened oil.
Frequent problems and their solutions
During operation, you may encounter a number of typical problems. One of them is overheating compressor. This may be caused by insufficient receiver volume (the engine is turned on too often), low unit performance for the selected task, or poor ventilation. Solution: increase the volume of the receiver or organize forced blowing of the motor with a fan.
Another common problem is oil getting into the pneumatic tool. If you use a compressor for painting, it is fatal. In this case, it is necessary to install an additional oil-moisture separator (cyclone type) at the outlet of the receiver. Also check the oil level in the crankcase: it may be too much.
Noise and vibration may be a sign of bearing wear or imbalance. Make sure that the compressor is securely attached to the frame via rubber dampers. Sometimes simply replacing the oil with a less viscous one or, conversely, a higher quality synthetic one helps.
Conclusion
Creating a compressor from a refrigeration unit is a great way to get an inexpensive and reliable source of compressed air for your garage. Subject to assembly technology and safety measures, such a device can serve for years, performing most of the tasks that more expensive store-bought analogs can handle.
Is it possible to use a compressor from a No Frost refrigerator?
Technically it is possible, but not recommended. Compressors of No Frost systems often have a different design and lower performance compared to classic models. In addition, they may be more sensitive to overheating when working with air.
What is the maximum pressure such a compressor can develop?
Standard household compressors are designed for a freon condensation pressure of about 15-25 Bar. However, for safe long-term operation, it is recommended to limit the operating pressure in the receiver to 6-10 Bar.
Is it necessary to change the oil when switching from freon to air?
Yes, definitely. Mineral oil compatible with freon may not have the necessary lubricating properties for air and oxidizes more quickly. It is better to use specialized compressor oils.
Why does the compressor get very hot?
Heating up to 70-90 degrees for piston compressors is the norm. If the temperature is higher, check the ventilation, oil level and correct operation of the valves. Perhaps the gaps are broken or the rings are worn out.