Many users of household appliances, noticing condensation on external tubes or hearing a characteristic gurgling sound, wonder what exactly is circulating inside the closed circuit of the refrigeration unit. The answer to this question seems obvious, but the cooling system is more complex than just a pipe with gas. Understanding the physical processes occurring in refrigeration circuithelps not only in operation, but also in proper maintenance of equipment.
Inside a copper or aluminum pipeline there is not one substance, but a complex mixture of components under pressure. The main mass is refrigerant, which constantly changes its state of aggregation. In addition, there is a special synthetic or mineral oil necessary to lubricate the compressor. It is important to understand that the system is sealed, and the entry of air or moisture inside is strictly unacceptable.
If you have ever wondered why you can’t just cut the tube or try to refuel the device yourself, then this article will give comprehensive answers. We will analyze the chemical composition of the working fluid, its properties and the role of each component in the cooling cycle. This knowledge will protect you from mistakes that can lead to breakdown of expensive equipment.
⚠️ Attention: Refrigerants used in modern refrigerators can be flammable (propane-butane mixtures) or toxic when burned. Any work to depressurize the circuit requires a license and special tools.
Main content: the role of the refrigerant
The main character in the system is refrigerant (often called freon, although this is only a trademark of one of types of substances). This substance has the unique ability to boil at very low temperatures, even below minus 30 degrees Celsius, at normal atmospheric pressure. Inside the closed loop, the pressure is higher, which allows the refrigerant to condense into liquid at room temperature in the condenser.
As it circulates through the system, the substance does the work of transferring heat from the inner chamber to the outside. In the evaporator, which is located inside the freezer or refrigerator, the pressure drops sharply and the liquid boils, turning into steam. This process phase transition is accompanied by active absorption of thermal energy from products lying on the shelves.
Modern industry uses various types of gases, which are environmentally friendly and efficient. Older models could run on R12 freon, which destroyed the ozone layer. Now manufacturers have switched to safer analogues, such as R600a (isobutane) or R134a. The choice of a specific gas depends on the design of the compressor and the volume of the internal chamber.
Cost Please note that the amount of refrigerant is strictly regulated by the manufacturer. A lack of gas will cause the refrigerator to stop freezing, and an excess can cause overload and failure. compressor. That is why independent “refueling” without the use of scales and a pressure gauge station is impossible.
Lubricants and their circulation
The second critical component located in the pipes is compressor oil. It is necessary to lubricate the rubbing parts of the compressor motor, which is the heart of the refrigerator. However, the oil does not remain only in the compressor crankcase; a small part of it is always in circulation along with the refrigerant.
The oil dissolves in the refrigerant and moves throughout the circuit, providing lubrication to the valves and piston group. Modern systems use synthetic oils based on polyesters (POE), which are highly hygroscopic, that is, they actively absorb moisture from the air. This makes the system extremely sensitive to depressurization.
If there is too much oil in the pipes, this may disrupt heat transfer. Oil has less thermal conductivity than metal, and its settling on the walls of the evaporator or condenser creates an insulating film. This reduces the efficiency of the entire unit. Therefore, the design also includes special trap loops that return the lubricant back to the compressor. oil separators and special trap loops that return the lubricant back to the compressor.
It is important to understand the difference between the types of oils for different freons. Mineral oils (used with R12) and synthetic oils (used with R134a, R600a) require different chemical compositions. Mixing them is unacceptable, as this can lead to the formation of acids and blockage of the capillary tube.
The physical state of the substance in different nodes
To understand what exactly is in a particular section of the pipe at the moment, you need to consider the cooling cycle. The system constantly undergoes processes of compression, condensation, throttling and evaporation. Depending on the section of the circuit, the substance can be gas, liquid or a mixture of both.
At the outlet of the compressor, the refrigerant is superheated steam under high pressure. Its temperature can reach 80-90 degrees Celsius. Moving along the condenser (the grill on the back or sides of the case), the gas gives off heat to the environment and gradually turns into liquid. At the bottom of the condenser, liquid freon already flows.
Next, the liquid passes through the filter drier and enters the capillary tube. Here there is a sharp decrease in pressure. At the outlet of the capillary, a mixture of liquid and gas enters the evaporator at a very low temperature. It is in this state that the substance actively “takes away” the cold. After the evaporator, only gaseous refrigerant again enters the compressor suction pipe.
Why are the tubes sometimes covered with frost?
Frost on the tubes outside the freezer (usually at the inlet to the compressor) indicates that liquid freon is entering the compressor. This phenomenon is called “liquid hydraulic shock” and is extremely dangerous for the engine. Reasons: lack of freon, malfunction of the thermostat or blockage.
The table below shows changes in the state of the substance depending on the system unit:
| System unit | Aggregation state | Pressure | Temperature |
|---|---|---|---|
| Compressor discharge | Gas (steam) | High | High (+70...+90°C) |
| Condenser (middle) | Liquid + Gas | High | Medium (+40...+50°C) |
| Capillary tube | Liquid | Falling | Cooling |
| Evaporator | Liquid -> Gas | Low | Low (-15...-24°C) |
Types of refrigerants: from R12 to R600a
The evolution of refrigeration equipment is directly related to changes in the chemical composition of what is in the pipes. For a long time, the standard was Freon R12 (dichlorodifluoromethane). It was a stable, non-toxic and non-flammable gas. However, later it turned out that when it enters the atmosphere, it causes colossal harm to the Earth’s ozone layer, which led to the Montreal Protocol and the ban on its production.
Replaced by R134a (tetrafluoroethane). It is ozone friendly, but has a higher boiling point and requires more efficient capacitors. In addition, it is incompatible with mineral oils, which required a complete overhaul of compressor production technologies. In the pipes of refrigerators with this gas, the pressure is higher than that of their predecessors.
The modern trend is the use of hydrocarbons, in particular isobutane (R600a). This substance is highly soluble in oil, energy efficient and environmentally friendly. However, it has a serious drawback - flammability. At concentrations in air from 1.3% to 8.4% the mixture becomes explosive. Therefore, when repairing such refrigerators, the use of open flame and sparking tools is prohibited.
Vacuum and air: enemies of the system
You can often hear the opinion that there is a vacuum in the refrigerator pipes. This is a misconception. A vacuum can only be created there temporarily, during the repair process, before filling with a new portion of freon. In operating condition, there should always be a pressure greater than atmospheric pressure inside the circuit, even when the compressor is turned off and the system has warmed up.
Air getting inside the system is an emergency. Air contains nitrogen, oxygen and, worst of all, water vapor. Nitrogen is a non-condensable gas. It does not liquefy at refrigerator operating temperatures and pressures. Its presence leads to a sharp increase in pressure in the condenser, overloading the compressor and increasing the discharge temperature.
Water vapor contained in the air, when in contact with refrigerant and oil, can form acids that corrode the metal from the inside. In addition, at low temperatures, moisture freezes, forming an ice plug in the capillary tube or filter drier. This completely stops the circulation of the refrigerant.
⚠️ Attention: If, when opening the circuit, you hear a hissing sound of air being sucked in, it means that the pressure in the system was below atmospheric (which is rare, but happens when cooling) or the system was empty. In any case, the presence of air requires complete evacuation of the system for at least 30-40 minutes with a powerful pump.
Hazards and precautions during maintenance
Understanding what is in the pipes dictates strict safety rules. Firstly, there is pressure. Even when turned off, the system can maintain a pressure of several atmospheres. Abrupt opening of the tube can lead to splashing of oil and rapid release of gas, which is dangerous for the eyes and skin.
Secondly, this is a chemical hazard. Although modern freons are low-toxic, in a closed room with a large leak they displace oxygen. When in contact with an open flame (for example, a gas stove), some types of freons can decompose to form phosgene a chemical warfare agent with a suffocating effect.
Thirdly, there is a fire hazard. As mentioned, isobutane (R600a) burns. When carrying out soldering work on pipes, it is necessary to thoroughly ventilate the room and ensure that there are no leaks before using the torch. A spark from a soldering iron or electric motor can cause ignition of the accumulated gas.
☑️ Safety when repairing a refrigerator
To diagnose the contents and condition of the system, technicians use leak detectors, pressure gauges and vacuum gauges. These devices allow you to determine whether there is gas in the pipes, what pressure there is and whether the system is sucking air through microcracks.
Frequently asked questions about the contents of the refrigeration circuit
Can freon run out in the refrigerator?
Freon itself does not disappear and is not consumed, since it circulates in a closed, sealed circuit. If the refrigerator stops freezing due to a lack of gas, it means that depressurization (leakage) has occurred through a crack in the pipe, evaporator, or a poor-quality factory seam. Simply “refueling” it without finding and eliminating the leak is pointless, the gas will escape again.
Why are the tubes at the back hot if it is cold inside?
This is a normal physical process. In the condenser tubes (at the back or on the sides) the process of heat transfer takes place, which was taken from the refrigerator chambers. The hot gas is compressed by the compressor, cools in the grate, turning into a liquid, and this is why the outer walls or grate heat up. The better they give off heat, the more efficiently the refrigerator operates.
Is it dangerous to breathe freon vapors?
Short-term inhalation of small concentrations of modern freons (R134a, R600a) usually does not lead to severe poisoning, but can cause dizziness, nausea or irritation of the mucous membranes. However, in high concentrations they displace oxygen, causing suffocation. When burned, freons release toxic substances. Therefore, in case of a strong leak, it is necessary to open the windows and leave the room.
What will happen if you cut the tube of a working refrigerator?
There will be a sudden release of gas and oil under pressure. The compressor will start running idle, sucking in air, which will lead to instant overheating and burnout of the windings. In addition, moisture and dirt from the air that gets inside will damage the system. Repairs after such an intervention will be complex and expensive, requiring replacement of the filter, compressor (often) and complete flushing of the circuit.