The question of how many atmospheres a refrigerator compressor should show arises for most craftsmen and equipment owners at the first signs of lack of cold. Understanding the physical processes inside the refrigerant system is the key to correct diagnosis, because the ability of freon to remove heat from the chambers depends on it. Many people mistakenly believe that there is a single figure for all models, but the reality is dictated by the physical properties of the refrigerant used and the ambient temperature. pressure The ability of freon to remove heat from the chambers depends. Many people mistakenly believe that there is a single figure for all models, but the reality is dictated by the physical properties of the refrigerant used and the ambient temperature.
Normal operating pressure in the circuit is a dynamic parameter that changes depending on the stage of the cooling cycle. When the engine starts, the readings on the pressure gauge may jump until the system reaches the design mode. It is important to distinguish between static pressure (when the compressor is turned off and the temperatures have balanced) and operating pressure (during the gas circulation). It is this data that allows you to understand whether there is leakage or a blockage in the system.
To accurately determine the condition of the equipment, it is necessary to take into account the type of freon. Modern environmentally friendly gases, such as R600a (isobutane), operate at significantly lower rates than their predecessors R134a or old R12. An attempt to pump into the system isobutane up to 6-8 atmospheres, guided by the table for R12, will lead to critical overload of the compressor and possible rupture of the pipelines. Therefore, before connecting the equipment, always check the markings on the unit.
Physics of the process and types of refrigerants
The principle of operation of the refrigeration circuit is based on the compression and expansion of gas. The compressor compresses the refrigerant, turning it into a hot, high-pressure liquid, which then passes through a condenser and capillary tube. In the evaporator, where the most important action for the user occurs - cooling, the pressure drops sharply, and the freon boils, turning into gas. It is this boiling process that removes heat from the products.
Different types of refrigerants have different boiling points at atmospheric pressure and different pressure-temperature relationships. For example, R134a (tetrafluoroethane), which is widely used in household refrigerators in recent decades, is characterized by average performance indicators. It is safe, non-flammable, but requires high-quality oil in the system and careful vacuuming, as it is sensitive to moisture.
The situation changes dramatically with the arrival of R600a. This hydrocarbon gas has excellent cooling performance but is flammable. Its operating pressure is significantly lower. If in a system with R134a there can be 9-10 atmospheres at the outlet from the compressor (discharge), then in a system with isobutane these values will be 30-40% lower at the same condensation temperature.
Below is a table showing the approximate dependence of saturated vapor pressure on temperature for the most common types of freons. These data are relevant for a static state (when the refrigerator is turned off and warmed up to room temperature).
| Air temperature (°C) | Pressure R134a (Bar) | Pressure R600a (Bar) | Pressure R12 (Bar) |
|---|---|---|---|
| 20 | 5.7 | 2.5 | 5.6 |
| 25 | 6.6 | 3.0 | 6.5 |
| 30 | 7.7 | 3.8 | 7.5 |
| 40 | 10.1 | 5.2 | 9.6 |
| 50 | 13.0 | 7.0 | 12.2 |
Normal pressure indicators in operating mode
When the refrigerator is started, the picture changes. The pressure on the suction side (low) and discharge side (high) begins to differ. For a working household refrigerator operating at R134a, the suction pressure is usually from 0.06 to 0.1 MPa (approximately 0.6 - 1.0 atm or 0.8 - 1.5 Bar, depending on the temperature in the chamber). However, the pressure gauge often shows negative readings (vacuum) or readings just above atmospheric, which is normal for low temperature evaporators.
The discharge pressure (compressor outlet) is significantly higher. In the condenser, the gas must have time to give off heat. A range of 8 to 11 Bar (atmospheres) for R134a at room temperature is considered normal. If the pressure gauge needle shows significantly less, for example, 4-5 atmospheres, this may indicate underfilling or low compressor performance.
For systems on R600a the numbers will be different. The suction pressure is often in a deep vacuum or close to zero. Discharge pressure rarely exceeds 6-7 bar. Exceeding these values may indicate a clogging capillary tube or the presence of air in the system (air lock), which is extremely dangerous due to the risk of ignition.
⚠️ Attention: Pressure indicators directly depend on the temperature in the room. In winter, when the room is +18°C, the pressure in the system will be lower than in summer at +28°C. Do not try to compensate for this by adding freon, guided only by the pressure gauge numbers without taking into account the temperature.
Tools for measurement and preparation
To carry out high-quality diagnostics, desire and a screwdriver are not enough. You will need a specialized refrigerator repair kit. The central element is pressure station. It consists of a manifold with two pressure gauges (red for high pressure and blue for low) and a set of hoses.
In addition, you need:
— Pipe cutter for accurate cutting into a copper tube without the formation of chips.
— Burner with solder for sealing joints.
— Vacuum pump to remove moisture and air before filling.
— Electronic scales for precise dosage refrigerant (charging “by eye” by pressure is unacceptable for modern systems).
Before connecting pressure gauges, you must make sure that the system is sealed. If there was a leak in the refrigerator, simply connecting a pressure gauge will show 0 atmospheres. In this case, the leak is first searched for using nitrogen or a leak detector, then the hole is sealed, and only after that the system is evacuated.
☑️ Preparing for pressure measurement
Step-by-step instructions: how to check pressure
The measurement process requires accuracy. First you need to find the process pipe. In older models it could be sealed, in newer ones a Schrader valve (similar to a car nipple) is often installed. If there is no valve, carefully cut the tube with a pipe cutter.
Next, connect the low pressure hose (blue) to the pipe. If a station with a Schrader valve is used, the connection is instantaneous. If the tube is cut, a fitting is put on it and clamped. After this, you can briefly run the compressor to displace any remaining gas or air from the hose, and connect the pressure gauge.
Start the refrigerator. Let it run for 10-15 minutes to stabilize the processes. Take readings from the low pressure gauge. Compare them to the boiling point table for your type of freon. If the suction pressure is too low (deep vacuum) and the refrigerator does not freeze, a leak is likely. If the pressure is high, the thermostat may be faulty or there is an excess of freon in the system. clogging or leak. If the pressure is high, the thermostat may be faulty or there is an excess of freon in the system.
⚠️ Attention: Never open the circuit if there is pressure left in the system. Gas escaping under pressure can cause frostbite to the skin or damage the eyes. Always bleed off the refrigerant in a well-ventilated area or through a special container.
Typical faults and their effect on pressure
Analysis of pressure gauge readings allows you to diagnose a breakdown with high accuracy. Let's look at the main scenarios. If the suction pressure gauge needle shows zero or a value close to it, and the compressor hums but does not pump, most likely the valves inside the motor have broken down or a dead hole has formed in the filter drier. clogging in the filter drier.
If the suction pressure is normal, but the discharge pressure (red pressure gauge) is critically high (above 15-18 Bar for R134a), this indicates that the gas cannot circulate. Often the culprit is a condenser clogged with dirt or an ice plug in the capillary tube. In such cases, the compressor operates with overload and may burn out.
The opposite situation also happens: the pressure on both sides is the same and low. This is a sure sign leaks. The freon has come out and the compressor is circulating air. It is also worth paying attention to the color of the oil in the filter-drier when disassembling the system: if it turns black and smells like burning, it means that the compressor was overheated, which often leads to coking of the capillary.
What is an “air lock” and how does it affect pressure?
Air that gets into the system during poor-quality repairs does not condense as easily as freon. It accumulates in the condenser, creating additional pressure. This causes the pressure gauge to show too high values and the refrigerator to operate at low efficiency. It can only be eliminated by complete refilling after deep evacuation.
The influence of ambient temperature
The factor of external temperature cannot be ignored. Pressure in a closed volume is directly proportional to temperature. If you carry out diagnostics in the summer in a hot workshop, where it is +30°C, the static pressure in a working R134a can reach 7-8 Bar. In winter in an unheated garage at +10°C it will drop to 4 bar. This does not mean that there is a leak in the refrigerator in winter, and an excess in summer.
The operating pressure is also adjusted. In hot weather, the condenser releases heat worse, and the discharge pressure increases. The compressor is forced to work harder. In a cold room, the refrigerator may not turn on at all (if there is no No Frost system with heating or a bypass valve), and the pressure in the system will tend to atmospheric.
When refueling the refrigerator, it is important to take this factor into account. Refueling is carried out strictly according to the weight indicated on the nameplate, and not according to the pressure gauge. The pressure gauge serves only as an indicator of the process: if, when gaining weight, the pressure does not increase or increases too quickly, it means there is a problem in the system (air, oil, wrong type of gas).
What pressure should be in the refrigerator that is turned off?
In the refrigerator that is turned off and warmed up to room temperature, the pressure in the entire system (both at the inlet and outlet) is the same. It corresponds to the saturated vapor pressure of a particular freon at a given temperature. For R134a at +25°C this is about 6.5 Bar, for R600a - about 3 Bar. If the pressure is significantly lower, it means there is a leak.
Is it possible to fill an R134a refrigerator with R12 freon?
Absolutely not. These refrigerants require different types of compressor oil (synthetic for R134a and mineral for R12). Mixing oils will lead to the formation of an acidic environment, destruction of the winding insulation and failure of the compressor. They also have different operating pressures and characteristics.
Why does the pressure gauge show negative pressure?
A negative value on the low pressure gauge (blue) means that a vacuum has been created in the system. This is a normal situation for a working evaporator, especially in a freezer. The compressor actively “pulls” freon vapor, creating a vacuum. If the vacuum is too deep, this may indicate undercharging or problems with the capillary tube.
Is high pressure in the compressor dangerous?
Yes, excessive pressure on the discharge side (above the calculated 12-14 Bar for household models) is dangerous. It creates a colossal load on the compressor valves and can lead to rupture of the weakest link - often these are the welds of the condenser or the motor housing itself. High pressure is often caused by blockages or the presence of air in the system.