Creating a homemade compressor based on a unit from an old refrigerator is a popular solution for a home workshop, allowing you to obtain a source of compressed air for aerography, inflating tires or operating pneumatic tools. The main element of such a system is receiver, which acts as a buffer that accumulates air and stabilizes the pressure in the circuit. Without this reservoir, flow pulsations would make working with the tool impossible, and the motor itself would quickly fail due to frequent on and off cycles.
The assembly process requires not only mechanical connection of pipes, but also competent electrical switching so that the automation controls the motor within specified limits. You have to connect three main units: itself compressor unit, the air container and the control unit with pressure switch. Errors at the design or installation stage can lead to a decrease in the performance of the entire installation or even an emergency, so it is important to strictly follow proven technical solutions.
In this article we will look in detail at how to properly integrate the receiver into the system, which fittings to choose for tightness and how to configure pressure thresholds for efficient operation. You will learn about the nuances of preparing a container, methods for removing condensate, and the specifics of connecting an unloading solenoid valve. It is critical to use a container that has passed hydraulic tests and has a safety margin of at least 3 times the operating pressure of the system.
Preparation of components and selection of container
The first step is to carefully prepare all elements future installation. The basis is a compressor removed from the refrigerator, which must be cleaned of dirt, old oil removed (if you plan to use it in oil-free mode, although this is rare for refrigeration units) and check its performance. Receiver most often old fire extinguishers, gas cylinders or special containers that can withstand high pressure are used. It is important to make sure that the inner surface of the cylinder does not have through corrosion.
To connect the components you will need a set of fittings, hoses and shut-off valves. The standard set includes threaded adapters, a check valve that will prevent air from flowing back into the compressor when it stops, and a pressure gauge for visual control. Brass fittings preferable over steel ones, as they are less susceptible to corrosion and are easier to process. Also, do not forget about hoses designed for pressure higher than the planned operating pressure.
Particular attention should be paid to preparing the installation site. The compressor unit vibrates during operation, so it is better to secure it on a wooden or rubber base. The receiver should be positioned vertically, which will facilitate draining the condensate through the drain valve at the lowest point. Before assembly, it is recommended to treat all threaded connections with sealant or use FUM tape to ensure complete tightness of the circuit.
⚠️ Attention: Never use containers containing flammable liquids or gases for the receiver without first thoroughly cleaning and degassing. Residual vapors combined with a spark from an electric motor or static electricity can lead to an explosion.
Connection diagram and mechanical assembly
Mechanical assembly of the system begins with inserting the fittings into the receiver. At the top of the cylinder it is necessary to organize at least three outlets: for supplying air from the compressor, for taking air to the consumer, and for installing a pressure gauge with a pressure switch. At the lowest point, a tap must be welded or screwed in to drain the condensate, since when air is compressed, water is formed from atmospheric moisture, which must be removed regularly.
The connection between the compressor and the receiver is made through a check valve. This element is critical: it allows air to flow in only one direction - into the cylinder. Without it, when the engine stops, compressed air from the receiver would rush back into the piston group, creating resistance the next time it starts and potentially damaging the compressor valves. A tee is often installed between the check valve and the receiver to connect the unloading valve.
Next, an air preparation unit (filter-moisture separator, reducer, lubricator) is mounted if you plan to use a tool sensitive to air quality, for example, a spray gun. After mechanical assembly of all components, an initial leak test is carried out. To do this, the system is pumped with air to operating pressure and all joints are washed with soapy water. The appearance of bubbles will indicate leaks that need to be eliminated.
☑️ Mechanical assembly checklist
Electrical circuit and automation control
The most difficult part of the assembly is connecting the electrics, which will automatically turn the engine on and off depending on the pressure in the receiver. The heart of this system is pressure switch (pressostat). Inside it there are contacts that open the circuit when the upper pressure threshold is reached and close when it drops to the lower limit. The standard connection diagram requires care, as it works with a mains voltage of 220V.
The phase wire from the network is supplied through a switch (toggle switch) to the input terminals of the pressure switch. From the output terminals of the relay, the phase goes to one of the contacts of the starting capacitor or directly to the motor winding, depending on the design. The neutral wire usually goes directly to the motor, bypassing the switching contacts of the relay, but passes through the switch. It is important to use wires with a cross-section corresponding to the current consumption of the motor, usually at least 1.5 mm² for copper.
The unloading solenoid valve deserves special attention (if it is provided for in the design). It is connected in parallel to the engine or through a separate relay in order to open only when the compressor stops, bleeding air from the outlet pipe to the check valve. This makes it easier to restart the engine by relieving the load on the piston group. Incorrect commutation can lead to burnout of the relay contacts or valve coil.
The nuances of the refrigeration compressor starting relay
Refrigeration compressors often have a starting relay that briefly connects the starting winding. When converting into a compressor unit, it is important not to disrupt the operation of this unit. If the relay is thermal, it may not have time to cool down when turned on frequently, so sometimes it is replaced with solid-state analogues or forced cooling is provided.
⚠️ Attention: Carry out all electrical connection work only with the power completely turned off! The presence of capacitors in the circuit requires caution, as they can retain a charge after being turned off.
Setting the pressure and testing the system
After assembly and checking for leaks, you need to set up the pressure switch. On the body of most industrial relays there are two adjusting nuts for a slotted screwdriver or wrench. The large nut (often labeled P or Range) sets the cut-off pressure (upper limit). The smaller nut (designated as ΔP or Differential) sets the pressure difference, that is, it determines how much the pressure must drop for the compressor to turn on again.
The setting process is as follows: you set the desired maximum pressure, for example, 6 atmospheres, by rotating the main nut. Then you set the delta to, for example, 2 atmospheres. This means that the compressor will turn off at 6 atm and turn on at 4 atm. A delta that is too small will result in the engine turning on frequently, which is harmful to its service life, and a delta that is too large will lead to a drop in pressure in the tool below a comfortable level.
Testing is carried out in several stages. First, the system is inflated without connecting a tool, observing the pressure gauge readings and the moment the relay operates. Then they check the operation under load by connecting a pneumatic tool. It is important to monitor the temperature of the compressor housing and cylinder head. During prolonged operation, they should not heat up to temperatures that are dangerous to the lubricant or surrounding materials.
Maintenance and typical problems
Operation of a homemade compressor requires regular maintenance. The main problem of such systems is the accumulation of condensate in the receiver. Water, mixing with oil (if the compressor is oil-based), forms an emulsion that can clog channels or get into the tool. Condensate must be drained after each operating session, opening the bottom tap until air and moisture are completely released.
Overheating of the compressor is a common problem. Refrigeration units are designed to operate on a cycle of 20% run time and 80% rest. When working intensively, they overheat. The solution is to install an additional finned radiator on the output tube or organize forced airflow with a fan. It is also worth monitoring the level of noise and vibration by tightening the fastening bolts.
The table below shows common malfunctions and methods for eliminating them:
| Problem | Possible cause | Remedy |
|---|---|---|
| The compressor does not turn on | The thermal relay has tripped or there is no power | Check the circuit, let it cool, check the contacts |
| Frequent on/off | Small pressure delta or leakage in system | Adjust the relay, check the tightness of the fittings |
| Pressure drop under load | Air filter clogged or low capacity | Replace the filter, check the compressor valves |
| Knock or vibration | Loose fasteners or wear of bearings | Tighten the bolts, troubleshoot the unit |
⚠️ Attention: Technical characteristics of compressors and equipment safety requirements may vary depending on the manufacturer and model of the unit. Always check the technical documentation for a specific compressor unit before making changes to the electrical circuit.
FAQ: Frequently Asked Questions
What is the minimum receiver volume needed for airbrushing?
For airbrushing, where a stable flow without pulsations is important, the minimum recommended receiver volume is 25-50 liters. Smaller volumes (for example, 10 liters) will require very frequent activation of the compressor, which will create noise and vibrations that interfere with the artist’s work.
Is it possible to use a compressor without a receiver?
Technically, you can start the system, but it will not work effectively. The air flow will be pulsating, which will ruin the quality of the paint or the work of the tool. In addition, the motor will turn on every time the tap is opened, which will quickly damage it. A receiver is required.
What kind of oil should I fill into the compressor from the refrigerator?
In most cases, adding oil is not required, since these compressors are sealed and designed for their entire service life without replacement. If replacement is necessary (for example, after combustion of the windings), use special synthetic oil for refrigeration compressors (for example, POE or Mineral depending on the type of refrigerant with which the unit worked), and not regular motor oil.
Why does the compressor hum, but not pumps?
This is a sign of a valve group malfunction. Either the plate valves in the cylinder head have broken, or the gasket between the head and the block has burned out. The compressor needs to be disassembled, valves and gaskets replaced.
Is it safe to keep the unit under pressure?
It is not recommended to store the unit under pressure. After completing the work, bleed the air from the receiver. Prolonged exposure of metal under tension accelerates the processes of fatigue and corrosion, especially in the presence of condensation.