Working pressure in a refrigerator compressor: standards and diagnostics

The question of how many atmospheres should be in the compressor from the refrigerator is one of the most common when diagnosing cooling system malfunctions. Many amateur craftsmen and owners of household appliances mistakenly believe that there is a single universal figure that applies to all models of units. However, the real picture is much more complex and depends on many physical parameters, including the type of refrigerant used, the design of the compressor and the ambient temperature.

Understanding the processes occurring inside the refrigeration circuit is critical for correct diagnosis. If you are planning to check or refill the system yourself, ignoring technical nuances can lead to the failure of expensive equipment. In this article, we will look in detail at how pressure is measured, what indicators are normal for different freons, and why the pressure gauge needle can behave unpredictably.

It is worth noting right away that the pressure in the system is a dynamic, not a static quantity. It changes depending on whether the motor is currently running or at rest. Static pressure is balanced throughout the system, while the working one is divided into high (at the outlet of the compressor) and low (at the inlet). It is these parameters that we will analyze to determine the health of your refrigerator.

Physics of the process: why the pressure changes

First, you need to understand the basic principles of operation of the refrigeration machine. The compressor acts as the heart of the system, pumping refrigerant and creating a pressure differential. At the outlet of the compressor, freon is under high pressure and has a high temperature. Passing through the condenser, it cools down and turns into a liquid state, after which it is throttled in the capillary tube, where the pressure drops sharply.

In the evaporator located inside the chamber, the refrigerant boils at low pressure, actively absorbing heat from the products. That is why the readings on a pressure gauge connected to different points in the circuit will be radically different. Suction pressure (low side) is usually significantly lower than atmospheric pressure, while the discharge (high side) can exceed it several times.

It is important to understand that saturated vapor pressure refrigerant directly depends on temperature. This is the fundamental law of thermodynamics. If the room is hot, the pressure in a standing (not working) refrigerator will be higher than in a cold garage in winter. Therefore, when diagnosing, always take into account the ambient temperature.

Dependence of indicators on the type of refrigerant

One ​​of the key factors that determines the operating parameters of the system is the brand of gas used. In modern and Soviet household appliances, three main types of refrigerants are most often found: R12, R134a and R600a. Each of them has unique physical and chemical properties, which dictates different pressure requirements in the circuit.

Old Soviet refrigerators often worked on R12 (freon-12). This gas is characterized by relatively high operating pressures. Modern units have switched to more environmentally friendly, but demanding installation quality substances. For example, R134a requires cleanliness of the system and the use of synthetic oils, and R600a (isobutane) is explosive, but effective.

⚠️ Attention: Never mix different types freons and do not fill the system with gas not intended for this compressor model. This will lead to destruction of the oil, coking of the capillary tube and guaranteed failure of the compressor.

Below is a table showing approximate pressure values in the system at an ambient temperature of +25°C for various types of refrigerants at rest and in operation.

As can be seen from the data, isobutane (R600a) operates at significantly lower pressures than its predecessors. This means that refilling it requires a more accurate scale, and not just a pressure gauge, since the volume of gas in the system is minimal (often less than 100 grams).

Diagnostics by low and high pressure

When connecting a pressure gauge station, the master gets the opportunity to assess the state of the system in real time. Low pressure (blue pressure gauge) is connected to the compressor suction tube. Normally, when the engine is running, it should be from 0.4 to 1.2 atmospheres, depending on the type of freon and the temperature in the chamber.

If the low pressure gauge needle shows values ​​below normal (closer to vacuum), this may indicate lack of freon or a clogged capillary tube. The cause may also be inefficient operation of the compressor, which is not able to create sufficient vacuum. In such cases, there is often a lack of cold in the evaporator.

High pressure (red pressure gauge) is measured on the discharge line. An excessive increase in this indicator (>15-18 atmospheres for household models) indicates problems with heat transfer. This may be caused by contamination of the condenser (the grille at the back of the refrigerator), a malfunction of the fan (in No Frost models) or the presence of air in the system.

Particular attention should be paid to the pressure difference. If the compressor hums, but there is no difference between suction and discharge (the pressures are equal), this is a sure sign compression. The piston group is worn out, the valves do not hold, and the unit requires replacement.

The influence of temperature on pressure gauge readings

The temperature factor plays a decisive role in diagnosis. As already mentioned, saturated vapor pressure depends on temperature. If you are carrying out repairs in an unheated room in winter, the static pressure readings will be significantly lower than the table values ​​for +25°C.

When the refrigerator is turned on, the suction pressure will begin to drop as the evaporator cools. This is a normal process. However, if the system is frozen or has an ice jam, the pressure can drop to a deep vacuum even with sufficient freon. In such cases, defrosting the system is required.

For accurate diagnosis, professionals use PT (Pressure-Temperature) tables. Knowing the boiling point of freon at a given pressure, you can determine how efficiently heat transfer occurs. For example, if at a pressure corresponding to the boiling point of -20°C, the evaporator tube is warm, then the refrigerant does not boil and does not take away heat.

Instructions for safely checking pressure

Checking the pressure in the compressor requires special equipment: a pressure gauge station, hoses and, possibly, scales for refilling. The procedure should be carried out strictly in a ventilated area, away from open fire, especially if used R600a.

First, you need to gain access to the process fitting (soldered copper tube with or without a needle). If there is no fitting, it must be carefully cut into the copper process pipe using a special valve. Then the low pressure hose (blue) of the pressure gauge station is connected.

After connecting the pressure gauge, you can turn on the refrigerator. Follow the arrow: it should start moving. If the suction pressure drops sharply into a vacuum and no cold appears, the system is empty or clogged. If the pressure rises to normal values, but the refrigerator does not freeze, there may be a problem with the compression of the motor itself.

⚠️ Attention: When working with R600a (isobutane), it is strictly forbidden to use tools that produce a spark. A gas concentration in the air of only 1.3% is already explosive. Make sure that there is forced ventilation in the room.

It is also important to monitor the current consumption of the compressor with an ammeter. Pressure and current must correlate. High pressure at low current may indicate inefficiency, and low pressure at high current may indicate mechanical problems or peddling due to lack of load.

Common errors when measuring and refilling

One ​​of the most common mistakes is trying to determine the amount of freon solely by pressure. In capillary tube systems, the amount of refrigerant is critical. Recharging even by 10-20 grams can lead to liquid freon entering the compressor, causing water hammer and destruction of the valves.

The second mistake is ignoring the operating time. The pressure does not stabilize immediately. The refrigerator must operate for at least 15-20 minutes for the heat exchangers to return to operating mode. Measurements taken in the first minute of work do not have diagnostic value.

The third mistake is the use of low-quality air or nitrogen for purging. There should be no moisture remaining in the system. Water in the circuit with R134a freon and synthetic oil forms an acid that corrodes the compressor windings from the inside. Vacuuming is a mandatory step before any refueling.