Replacing a compressor in a refrigerator is a complex technical task that requires accurate calculations, and not just selecting a unit that is suitable in size. Many craftsmen make the mistake of installing the first motor they come across, which physically fits into a niche, which often leads to rapid failure of a new part or spoilage of products. Refrigeration compressor is the heart of the system, and its performance must strictly correspond to the volume of the evaporator, the type of refrigerant and the operating temperature.
Incorrectly selected power can lead to the refrigerator working non-stop, not reaching the set temperature, or, conversely, turning on too rarely, causing defrosting. In this article we will analyze the physical principles, formulas and tables that will help you perform competently compressor calculation. Understanding these processes is necessary in order for the equipment to serve for many years without complaints.
Before starting mathematical calculations, it is necessary to collect initial data. You will need to know the make of the old unit (if it is still there), the type of refrigerant used (for example R134a or R600a) and the desired temperature in the chamber. Only with this information can you proceed to the analysis of technical characteristics.
Determination of the required cooling capacity
The main parameter that needs to be calculated is the cooling capacity, measured in Watts (W) or kilocalories per hour. This indicator determines how much heat the compressor is able to remove from the internal volume of the chamber per unit of time. Compressor power directly depends on the temperature difference between the evaporator and the condenser, as well as on the type of freon.
For household refrigerators, there is a rule of thumb: for every 100 liters of chamber volume it is required approximately 100-120 W cooling capacity under standard conditions. However, this is a very rough approximation. A more accurate calculation requires taking into account the coefficient of performance (COP) of the system and heat losses through the walls of the housing. If you change the compressor to a more powerful one than the standard one, this may upset the balance of the system.
⚠️ Attention: Installing a compressor with excess power will increase the pressure in the system, which can cause rupture of the condenser tubes or damage to the door seals due to too low a temperature.
When calculating, it is also important to take into account the climate class of the refrigerator. Devices designed for tropical climates must operate at higher ambient temperatures, which requires greater output from the motor. At the same time, using tropical compressor in a temperate climate without adjusting the thermostat may be ineffective.
Analysis of the type of refrigerant and its influence on the choice
The type of refrigerant is a critical factor when selecting a compressor. Different gases have different thermodynamic properties, such as boiling and condensation pressures. For example, R134a (tetrafluoroethane) requires a higher operating pressure compared to R600a (isobutane), which is explosive, but environmentally friendly.
The use of a compressor designed for one type of freon with another gas is strictly prohibited without completely reconfiguring the system and changing the oil. Oils for different refrigerants also differ: for R134a synthetic oil Polyolester (POE)is usually used, and for mineral oils and R12 traditional compositions are used. Mixing oils can lead to the formation of acid and blockage of the capillary system.
Modern refrigerators are increasingly using R600a. These compressors often have lower capacity per volume but greater efficiency. When replacing, it is important to pay attention to the labeling, which indicates the acceptable type of refrigerant. Ignoring this requirement leads to the fact that refrigeration circuit ceases to perform its function.
Why can’t you mix refrigerants?
Mixing different refrigerants changes their physical properties, which leads to unstable pressure in the system, overheating compressor and possible destruction of internal components. In addition, some mixtures can become explosive.
If you are not sure about the type of gas, look at the sticker inside the refrigerator compartment or at the nameplate of the old compressor. The type of refrigerant and its amount in grams are always indicated there. This information is necessary not only for selecting a motor, but also for subsequent refueling of the system.
Calculation of the working time coefficient
One of the key indicators of the efficiency of the refrigerator is the working time coefficient (LOC). It shows how much of the time the compressor is on. A normal value is considered to be between 0.3 and 0.5 (or 30-50%). If switching coefficient above 0.7, this indicates an overload of the unit or insufficient tightness of the chambers.
To calculate the required performance of a new compressor, you can use the following logic: if the old motor worked with a coefficient of 0.4, then the new one should provide a similar mode. The formula for an approximate calculation looks like this: P_req = Q_load / K_rvwhere P_req is the required power, Q_load is the heat inflow, K_rv is the desired working time coefficient.
The working time coefficient is too low (less than 0.2) also harmful, as it leads to frequent engine starts, which wears out the starting relay and windings. A coefficient that is too high indicates that the compressor cannot cope with the heat flow, and the food begins to deteriorate.
When selecting an analogue, try to find a model with similar performance indicators. If you install a motor that is too weak, it will run constantly, quickly overheat and burn out. A unit that is too powerful will cycle frequently, which will also shorten its service life.
Table of correspondence between power and chamber volume
To simplify the selection task, you can use averaged data that relates the volume of the refrigeration chamber to the required cooling capacity of the compressor. These data are relevant for a working heat exchanger and normal operating conditions.
| Chamber volume (liters) | Required power (W) | Typical refrigerant | Approximate compressor model |
|---|---|---|---|
| 100 - 150 | 90 - 110 | R134a / R600a | Aspera NB5212 |
| 200 - 250 | 130 - 160 | R134a | Secop (Danfoss) BD35F |
| 300 - 350 | 180 - 220 | R134a / R404a | Embraco EGLE 80CL |
| 400+ (two-compressor) | 250+ (per circuit) | R600a | Secop NL11FT |
This table is for reference only. Real cooling capacity depends on many factors, including the design of the evaporator (crying or No Frost), the quality of the seals and the room temperature. No Frost systems require more power, since part of the energy is spent on operating the fans and compensating for losses when opening the door.
Please note that for two-chamber refrigerators with one compressor, the calculation is based on the total volume, but taking into account a more complex cold distribution system. In such cases, high-power compressors with two outputs or a flow switching system are often used.
⚠️ Attention: Compressor manufacturers may change the markings of models. Always check the technical specifications (cylinder volume, power) in official catalogs, and do not rely only on the model name.
Electrical parameters and starting currents
When replacing a motor, it is important to take into account not only the mechanical power, but also the electrical characteristics. The supply voltage, current frequency and, most importantly, the starting current must match the wiring and starting relay of your refrigerator. Starting relay is selected strictly for the specific compressor model.
There are compressors with low starting torque (LST) and high starting torque (HST). For home refrigerators, LST models are most often used, as they work with a capillary tube that equalizes pressure when idle. If you install the HST compressor in a system with a capillary tube without additional valves, this may lead to difficult starting.
☑️ Checking electrical compatibility
Also It is worth paying attention to the power consumption in Watts. If the new compressor consumes significantly more electricity, this can lead to overheating of the wires and melting of the insulation at the joints. Electric motor must be coordinated with the rest of the elements of the electrical circuit of the refrigerator.
Inverter technologies and their features
Inverter compressors are increasingly found in modern refrigerators. They differ from traditional linear models in that they do not turn off completely, but smoothly regulate the shaft rotation speed. Inverter motor allows you to maintain the temperature with high accuracy and operates much quieter.
Replacing a conventional compressor with an inverter one (and vice versa) requires a complete replacement of the control unit (board). Simply installing an inverter motor in a refrigerator with a regular board will not work, since the inverter requires a special speed control signal. This makes their interchangeability extremely difficult and often economically impractical when repairing older models.
The advantage of inverter systems is the absence of large starting currents, which reduces the load on the network. However, diagnosing and repairing such systems require specialized equipment and in-depth knowledge of electronics. When calculating the efficiency of the inverter, its ability to operate in a wide range of speeds is taken into account.
Practical installation recommendations
After the calculations have been made and the compressor selected, it is critical to perform the installation correctly. Any mistake at this stage can undo all previous efforts. Before soldering the tubes, it is necessary to purge the system with nitrogen to remove moisture and residual old oil.
When soldering, use solder containing silver and be sure to supply nitrogen inside the tubes to prevent the formation of oxides (scale). Scale, getting into the capillary tube or filter drier, will cause blockage, and the refrigerator will stop freezing, even if compressor power is calculated perfectly.
Be sure to replace the filter drier every time the system depressurizes. It absorbs moisture from the air and reusing an old filter is unacceptable. After assembling the system, it is necessary to carry out evacuation for at least 15-20 minutes with a powerful pump.
Why is evacuation necessary?
Evacuation removes air and, most importantly, water vapor from the system. At low temperatures, water freezes in the capillary, forming an ice plug, and also reacts with oil and refrigerant, forming acid.
Refill the refrigerant strictly by weight, using electronic scales. A lack of freon will lead to overheating of the compressor, and an excess will lead to an increase in condensation pressure and a drop in efficiency. The exact dose of refilling is always indicated on the factory sticker inside the refrigerator or on the compressor nameplate.
Common mistakes when making your own selection
The most A common mistake is trying to select a compressor only by the appearance and size of the connecting pipes. The dimensions may be the same, but the performance may differ significantly. The second mistake is ignoring the type of oil. If there was mineral oil in the system, and you fill in synthetic oil (or vice versa), coagulation will occur and the system will clog.
Thermal conditions are also often forgotten. The compressor must have a sufficient safety margin. Operating at maximum capacity reduces service life. If you are replacing the compressor in an old refrigerator, make sure that the condenser (the black grille on the back) is clean and not clogged with dust, otherwise even the new motor will overload.
You should not try to save on the brand. Compressors from well-known manufacturers (Secop, Embraco, Aspera, Danfoss) undergo strict quality control. Cheap analogues often have actual characteristics lower than declared, which leads to rapid failure.
Is it possible to install a compressor of greater power than originally installed?
Technically this is possible, but it is not recommended without replacing the evaporator and condenser. A compressor of greater power will pump out freon faster than the evaporator can cool it, which will lead to freezing of the suction tube and liquid freon entering the cylinder (water hammer), which will destroy the compressor.
How to find out the power of an old burnt-out compressor?
Find the model of the old compressor (for example, on tag or on the Internet for labeling). Technical datasheets (passports) for this model always indicate the refrigeration capacity in Watts under standard conditions (usually at a boiling point of -23.3°C or -25°C).
Does the length of the capillary tube affect the calculation?
Yes, the length and diameter of the capillary tube directly affect the pressure in the system. When replacing a compressor with a model with different characteristics, it may be necessary to select a new capillary length to ensure the correct balance of pressure and performance.
Why does the new compressor get hotter than the old one?
This may be due to several factors: incorrect freon charging (underfilling or overfilling), presence of air in the system, condenser malfunction (poor heat transfer) or operation in conditions different from the calculated ones (for example, the room temperature is too high).
Is it necessary to flush the system when replacing a compressor?
Yes, if the old compressor burned out, oil combustion products and acidic compounds could remain in the system. Washing with a special solvent or nitrogen under pressure is mandatory, otherwise the new compressor will burn out in a short time.