The evaporator is the heart of any refrigeration system; it is in this unit that the key physical process occurs - boiling of the refrigerant, accompanied by active absorption of heat from the chambers. Understanding what temperature conditions should be maintained on the surface of the heat exchanger allows the owner of the equipment to independently diagnose many faults without waiting for a technician to arrive. Normal values vary depending on the type of refrigerant, system design and thermostat settings.
If you notice that the food in the refrigerator has stopped freezing or, conversely, has turned into an ice ball, the first thing you need to check is the condition of the evaporator. The temperature of its surface directly affects heat transfer efficiency and the ability of the compressor to maintain a given climate inside the chambers. In modern models with the No Frost system, the parameters will differ significantly from those of classic “crying” refrigerators.
In this article we will analyze in detail the physical basis of the operation of the refrigeration circuit, consider specific digital values for different types of equipment and discuss why deviations from the norm can lead to expensive repairs. It is important to understand that defrost sensor and the thermostat work in conjunction, relying specifically on the temperature of the evaporator.
Physics of the process: why the evaporator is cold
The principle of operation of the refrigerator is based on the property of substances to absorb heat when moving from liquid state into gaseous state. The evaporator, which is a thin tube with fins or channels inside the wall, is supplied liquid refrigerant at low pressure. Once in the rarefaction zone, freon boils at very low temperatures, intensively removing heat from the metal walls of the heat exchanger.
This process leads to the fact that the surface of the evaporator is cooled to values significantly lower than the air temperature in the chamber. The air circulating around the cold tubes or plates gives off its heat and cools itself. The efficiency of this process depends on the temperature difference between the refrigerant and the surrounding environment, as well as on the heat exchange surface area.
If the system is working properly, the refrigerant is completely evaporated within the evaporator circuit before entering the compressor. The entry of the liquid freon fraction into the compressor, the so-called “hydraulic hammer,” can lead to valve failure or jamming of the piston group. Therefore, monitoring the temperature and degree of superheat of the steam at the outlet of the evaporator is critical.
⚠️ Attention: The boiling point of freon depends on the pressure in the system. When diagnosing, you cannot rely only on tactile sensations, since different types of refrigerants (R600a, R134a) have different physical properties.
Normal temperature indicators for different systems
The answer to the question of what the evaporator temperature should be cannot be unambiguous without reference to the type of refrigeration system. In classic models with a drip defrosting system, the evaporator is located in the rear wall of the freezer or refrigerator compartment. Here, the surface temperature can drop to -18...-24°C in the freezer and to 0...-2°C in the refrigerator compartment when the compressor is operating.
In systems No Frost the situation is different. The evaporator is hidden behind a plastic panel, usually at the top of the freezer or behind the back wall. Here, the evaporator temperature during compressor operation reaches values -25...-30°C and lower to ensure rapid freezing of products and efficient operation of the fan. However, the air is supplied to the chamber already heated to the specified parameters.
Particular attention should be paid to dual-circuit systems, where each chamber has its own evaporator. In such units, the temperature in the freezer circuit will be extremely low, while in the refrigerator compartment it will be maintained in the region -5...-10°C, which prevents overdrying of food.
For accurate diagnostics, specialists use a digital thermometer with a probe, applying it to the surface of the tubes after removing the protective panel. It is considered normal if the surface temperature of the evaporator is 5-10 degrees lower than the temperature of the air that should be in the chamber. This provides the necessary heat transfer.
The influence of the type of refrigerant on the boiling point
The characteristics of the working fluid play a decisive role in the thermodynamics of the cycle. In modern household appliances, isobutane (R600a) and tetrafluoroethane (R134a) are most often used. Each of these gases has its own characteristics, which affect the operating temperature of the evaporator and the requirements for system tightness.
Isobutane (R600a) is a natural refrigerant that has excellent refrigeration properties and low pressure requirements in the system. However, it is explosive when mixed with air. The evaporation temperatures of isobutane may be slightly higher than those of synthetic analogues, but its high heat capacity allows it to quickly gain cold. When working with such systems, it is important to consider that gas leak may not be visually noticeable, but will lead to an increase in the temperature of the evaporator.
R134a is a more common gas in older models and industrial equipment. It requires a higher condensing pressure. The evaporator temperature on R134a is usually stable, but if there is air in the system (poor repair), the readings may fluctuate. Air acts as a non-condensable impurity, increasing pressure and disrupting heat exchange.
| Type of refrigerant | Boiling point (at atmospheric pressure) | Operating features evaporator | Risks |
|---|---|---|---|
| R600a (Isobutane) | -11.7°C | Low pressure, high efficiency | Explosion hazard during repairs |
| R134a | -26.3°C | Medium pressure, stability | Moisture sensitivity |
| R12 (Obsolete) | -29.8°C | High discharge temperature | Ozone layer depletion |
Fault diagnosis based on evaporator temperature
Analysis of the surface temperature of the evaporator is one of the most informative methods of primary diagnostics. If you remove the panel and see that the pipes are warm or at room temperature while the compressor is running, this indicates a lack of refrigerant circulation. The cause may be freon leak or compressor failure.
Another common scenario is a “crying” evaporator, covered with an uneven layer of frost or ice. If the ice coat occupies less than 30% of the surface, and the rest of the tubes are dry and warm, this is a sign. Freon simply does not reach the end of the circuit, evaporating at the beginning of the path. In this case, the temperature in the chambers will be higher than normal. lack of refrigerant. Freon simply does not reach the end of the circuit, evaporating at the beginning of the path. In this case, the temperature in the chambers will be higher than normal.
The opposite situation is when the entire evaporator is covered with a thick layer of ice, and the temperature in the chamber is not low enough. This indicates problems with the defrost system: burned out, the timer or defrost sensor is faulty. The ice crust works as a heat insulator, blocking the release of cold. Evaporator heating element, the timer or defrost sensor is faulty. The ice crust acts as a heat insulator, blocking the release of cold.
☑️ Diagnostics of the evaporator
⚠️ Attention: If during operation of the compressor you hear gurgling or hissing, and the evaporator temperature is not drops, perhaps an ice plug has formed in the system in the capillary tube or a blockage in the filter-drier.
Problems with the No Frost system and sensors
In refrigerators with the system No Frost evaporator temperature control is fully automated. This process is controlled by a defrost sensor, which is installed directly on the evaporator pipes. Its task is to monitor the heating of the heating element and the temperature of the evaporator during the defrost cycle. If the sensor is “lying,” the system may not turn on the defrost or, conversely, heat the evaporator for too long.
A frequent problem is the failure of the sensor, which shows a temperature higher than the actual one. As a result, even when the evaporator is covered with ice, the “brains” of the refrigerator consider that the defrost was successful and turn on the compressor. Cycle after cycle, ice accumulates, blocking the air passages. As a result, the fan begins to hum, and the temperature in the chambers rises.
To check the sensor, you need to ring it with a multimeter at a certain temperature. The resistance of a working sensor at room temperature is usually several kiloohms, and when cooled below the response threshold (for example, -7°C or -10°C), it should change sharply or break the circuit, depending on the type (NC or NO).
How to check a defrost sensor with a multimeter?
To check, remove the sensor and place it in a glass with cold water and ice. Place the thermometer there. When the water temperature reaches the value indicated on the sensor body (usually around -7°C), the resistance should change. If there are no changes, the sensor is faulty and requires replacement.
The influence of evaporator temperature on electricity consumption
Many users do not think that the evaporator temperature is directly related to electricity bills. The lower the boiling point of the refrigerant (that is, the colder the evaporator), the higher the suction pressure and the more energy the compressor consumes to compress the vapor. However, in refrigerators this parameter is rigidly specified by the design.
Problems begin when, due to leaks in the door seal or frequent openings, warm, humid air enters the chamber. The evaporator becomes covered with ice, its efficiency decreases. The compressor is forced to work longer and harder to compensate for the loss of cold through the ice coat. This leads to overconsumption of electricity up to 30-40%.
It is also worth mentioning overheating. If there is little freon in the system, the steam at the outlet of the evaporator has too high a temperature (high superheat). The compressor is cooled by the suction steam, and if the steam is hot, the motor may overheat, which will shorten its life. The optimal suction overheating usually is 5-10 degrees.
Frequently asked questions about evaporator temperature (FAQ)
Why is the evaporator only half covered with ice?
This is a classic sign lack of refrigerant (leaks). Freon boils and evaporates in the first part of the circuit, without reaching the end. It is necessary to look for the leak, seal it and recharge the system.
Is it possible to replace the evaporator temperature sensor yourself?
Technically, yes, if you have access to the evaporator (the panel has been removed) and the skills to work with a multimeter. However, in some models, access to the sensor is difficult, and incorrect selection of an analogue by resistance will lead to incorrect operation of the refrigerator.
What temperature should be at the outlet of the evaporator before the compressor?
The temperature of the steam at the outlet of the evaporator (at the suction line) should be 5-15°C higher than the boiling point. This ensures that only gas and not liquid enters the compressor. For R600a at a boiling point of -25°C, the compressor pipe may be about -15...-10°C.
Does condenser contamination affect the evaporator temperature?
Yes, indirectly. If the condenser (grid at the back) is clogged with dust, the condensation pressure increases. This disrupts the circulation of freon, and it gives off heat worse, which can lead to a change in the boiling point in the evaporator and a decrease in refrigeration capacity.
Why does the evaporator temperature not drop for a long time after defrosting?
Perhaps there is air or moisture left in the system that interferes with normal circulation, or the thermostat is faulty, which does not give the command to turning on the compressor. Also, the reason may be low compression of the compressor itself.