The question of how many liters of air a compressor removed from an old household refrigerator can pump is one of the most popular among home craftsmen and garage craftsmen. Interest in converting these devices into compressors for airbrushing, tire inflation or pneumatic tools is due to their availability, low cost and relatively quiet operation compared to piston analogues. However, answering the main question of the article, it is necessary to immediately clarify: a household refrigerator compressor is not designed to create a large volume of compressed air per unit of time; its element is maintaining pressure in a closed circuit.
The typical performance of such units ranges from 30 to 50 liters per minute, provided they operate with free exhaust (without resistance). If you connect a receiver and create a pressure of 2-6 atmospheres, the actual performance drops to 10-20 liters per minute. This is absolutely not enough for working with a screwdriver or spray gun, which requires 200-300 liters, but is quite enough for local airbrushing or inflating bicycle tires. Understanding these limitations is key before starting to assemble a homemade compressor station.
In this article we will examine in detail the technical characteristics of various types of motor-compressors, methods for calculating actual performance and the nuances of their operation as air blowers. You will learn why the refrigerator's passport data does not coincide with the actual output indicators, and how to choose the right equipment for your tasks so as not to be disappointed with the result after assembling the system.
Fundamental differences between refrigeration compressors and air compressors
The main misconception lies in the design of the unit itself. Hermetic compressorwhich you see on the back wall of the refrigerator, designed to circulate refrigerant (freon) in a closed system where the volume of gas is strictly limited and constant. It works in the mode of “pushing” steam through the narrow tubes of the condenser and evaporator, creating a pressure difference, but without accumulating volume. Unlike piston air compressors with an open cylinder, there is no air cooling system and effective heat removal when compressing large masses of gas.
The second important difference is the lubrication system. In a refrigeration unit, oil is in the crankcase and is sprayed or supplied under pressure to lubricate the rubbing pairs, being in direct contact with the refrigerant. When pumping air, the oil begins to actively burn and is thrown into the outlet pipe along with the air flow. This requires the mandatory installation oil separator or the use of special filters if you plan to use air for painting or airbrushing, where oil ingress is unacceptable.
⚠️ Attention: When converting a refrigeration compressor to air, it is necessary to take into account that the oil in the crankcase may foam during intensive work, which will lead to water hammer and failure of the mechanism. The oil level must be monitored regularly.
In addition, refrigeration compressors do not have forced air cooling of the cylinder head, since in normal operation they are cooled by freon itself and natural convection. When working to compress air to high pressures (more than 4-6 atmospheres), the temperature at the outlet and on the housing can reach critical values, causing thermal deformation of the valves or even melting of parts.
Why can’t you just remove the compressor and plug it into an outlet?
A simple connection to an outlet without a starting capacitor and a protection relay will lead to the engine it will hum, but will not start, or the winding will burn out. Also, without a pressure switch, the compressor will pump air until the hoses rupture or the engine stops.
Actual performance: numbers and calculations
To understand how many liters a particular unit pumps, you need to refer to the markings. The nameplate usually indicates the type of refrigerant and power, but rarely the volume of gas pumped. Engineers use the concept of “refrigerant capacity,” which for household models is approximately 0.08–0.12 cm³ (cubic centimeters) per piston stroke. Converting these values into liters of inlet air, we obtain average values.
The actual performance is highly dependent on the outlet pressure. With free exhaust (0 bar), the compressor can produce the stated 40-50 l/min. However, as soon as the pressure in the receiver reaches 2 atmospheres, productivity will drop by about 40-50%. At 4 atmospheres it will be no more than 20-25% of the original. This is a fundamental characteristic of low-power piston systems.
To accurately calculate the required pumping time for the reservoir, you can use the following formula, taking into account the volume of the reservoir and the required pressure:
T = (V (P2 - P1)) / (Q 60)Where T is the time in minutes, V is the volume of the reservoir in liters, P2 and P1 — final and initial pressure in atmospheres (or bars), and Q — compressor performance in liters per minute. For example, to pump a 20-liter receiver from 0 to 4 atmospheres with a compressor with a capacity of 40 l/min, it will take approximately 2 minutes (taking into account the drop in productivity - about 3-4 minutes).
Table of performance depending on type and power
Different models of refrigerators are equipped with compressors of different capacities. The larger the volume of the refrigerator compartment and the lower the energy efficiency class (older models), the more powerful the motor usually costs. Below is a summary table that allows you to evaluate the potential of different types of units.
| Compressor type/power | Refrigerator volume (l) | Performance (l/min, 0 atm) | Max. operating pressure (atm) | Recommended application |
|---|---|---|---|---|
| Low power (up to 100 W) | 100 - 200 | 25 - 35 | 4 - 6 | Airbrush, tire inflation |
| Medium power (100-140 W) | 200 - 350 | 35 - 50 | 6 - 8 | Washing, blowing, pneumatic tools (short-term) |
| High power (>140 W) | > 350 | 50 - 70 | 8 - 10 | Assembling homemade stations |
| Inverter (linear) | Any | Unstable | Depends on the controller | Not recommended for modification |
It is worth noting that inverter compressors, which are becoming increasingly popular in modern refrigerators, are extremely difficult adapt to work with air. They require complex electronics to control the speed and often do not have a standard starting winding. For garage homemade products, the good old units with a start relay and a capacitor are best suited.
The influence of pressure on the volume of pumped air
Many beginners mistakenly believe that if the compressor pumps 50 liters per minute, then it can fill a 50-liter cylinder in one minute. This is wrong. With every second of operation, the pressure in the receiver increases, and it becomes increasingly difficult for the compressor to push a new portion of air inside. The performance curve is rapidly going down.
When a pressure reaches 6-8 atmospheres (maximum for most household models), the actual air supply can be only 5-10 liters per minute. At this moment, the compressor runs almost idle, only maintaining pressure, but not creating a new flow. That is why, to work with pneumatic tools that consume 150-200 liters per minute, one refrigeration compressor is physically not enough - it simply will not have time to restore the pressure.
⚠️ Attention: Long-term operation of a refrigeration compressor at pressures above 8-10 atmospheres is prohibited. Oil seals, cuffs and gaskets are not designed for such loads and may not withstand them, which will lead to depressurization or explosion.
To increase the efficiency of the system, it is necessary to use receiver —a container for accumulating compressed air. The large volume of the receiver allows the compressor to operate in a gentle mode: quickly pump up the tank to maximum pressure, turn off and slowly consume air until the pressure drops to a minimum. This prevents overheating and prolongs the life of the unit.
Limitations and risks of operation
Using a refrigeration compressor for other purposes carries certain risks. The first and most important thing is overheating. As mentioned earlier, the lack of active cooling and working with a different working medium (air instead of freon) lead to higher temperature conditions. The case can heat up to 90-110°C, which is dangerous for plastic elements and wire insulation.
The second risk is related to moisture resistance. Air, unlike freon, contains moisture. When compressed, moisture condenses inside the system, entering the oil. The water-oil emulsion loses its lubricating properties, which leads to accelerated wear of rubbing parts, cylinder corrosion and piston jamming. Regular oil changes and installation of a moisture separator are mandatory.
☑️ Checking the readiness of the compressor for rework
It is also worth considering noise and vibration. Although refrigeration compressors are considered quiet, when running on air, the noise level can increase due to the absence of a sound-absorbing casing (if removed) and the operation of the valves under load. Vibration can lead to the destruction of the soldering of copper tubes if they are not securely fastened.
Practical tips for assembling a compressor station
If you decide to assemble a compressor, start by selecting a receiver. Old fire extinguishers with a capacity of 10-20 liters or gas cylinders are ideal. Be sure to install pressure gauge to control pressure and a pressure switch, which will automatically turn off the engine when the upper limit is reached (for example, 6 atm) and turn on when it drops to the lower limit (4 atm).
To connect tools, use standard fittings and hoses designed for a pressure of at least 10-15 atmospheres. Don't skimp on shut-off valves. At the outlet of the compressor, in front of the receiver, it is advisable to install a check valve so that the air from the receiver does not flow back into the compressor after it stops, which could cause reverse rotation and damage to the mechanism.
Be sure to provide a filtration system. At the entrance to the compressor (on the suction tube) it is worth installing an air filter (you can use a car filter) so that dust and abrasive particles do not get inside the cylinder. The output is a moisture-oil separator, especially if painting is planned.
Conclusion: is the game worth the candle?
To summarize, we can say that a refrigeration compressor is an excellent, cheap and accessible source of compressed air for small volumes of work. It's ideal for the garage, where you need to pump up tires, blow dust off electronics, or do artistic painting. Its productivity of 30-50 liters per minute is enough for these purposes, especially when paired with a receiver.
However, you should not expect miracles from it. It is absolutely not suitable for serious work with pneumatic tools (grinders, impact wrenches, large-volume spray guns). In such cases, it is better to consider purchasing a specialized air compressor. But as a first step into the world of pneumatics and an excellent weekend project, remodeling a refrigeration unit remains unrivaled in terms of price and experience.
Is it possible to use a compressor from a refrigerator to paint a car?
Only for local repairs of small parts. To paint the entire body, its productivity (30-50 l/min) is not enough even with a large receiver - the torch will “spit” and will lie unevenly. A minimum of 200-300 l/min is required.
What kind of oil should be poured into the converted compressor?
It is best to use the original refrigeration oil of the same viscosity as was in the crankcase, or special compressor oil (for example, KS-19). It is not recommended to use motor vehicle oil due to the risk of carbon deposits and foaming.
Why does the compressor get very hot during operation?
This is normal for the air compression mode. Temperature of 80-90°C on the case is acceptable. If it gets hotter, check for leaks, cleanliness of the cooling fins (if there is a casing) and whether you are trying to pump up too much pressure.
Do you need to change the oil after the modification?
Not necessary if the compressor is working properly. However, if you are planning intensive use, it is better to replace the old oil with fresh one, since decay products and moisture may have accumulated in the old oil over the years of operation of the refrigerator.