During the operation of any household appliance, be it a classic model or a modern one Side-by-Sidea complex process of energy transfer occurs, which directly affects the microclimate in the room. Understanding how to determine the amount of heat given off by a refrigerator is necessary not only for solving school problems in physics, but also for competently designing ventilation systems in warehouses or small kitchens. Many people mistakenly believe that the unit only cools the internal chamber, forgetting that, according to the law of conservation of energy, heat does not disappear anywhere, but is only redistributed.
In fact, the refrigerator works like a heat pump, pumping energy from the internal volume to the external environment. Amount of heatemitted into the room always exceeds the amount of cold generated inside due to the operation of the electric motor compressor. If you are planning to install powerful refrigeration equipment in a confined space without ventilation, ignoring this factor will lead to overheating and accelerated wear of the equipment.
In this article we will analyze the physical basis of the process, learn how to use formulas for calculations and consider the practical aspects of heat transfer of various systems. You will learn why the rear grille heats up and how it relates to power consumption. It is important to understand: the refrigerator does not produce cold, it only takes thermal energy from the food and air inside the chamber, adding to this volume the heat released during the operation of the compressor.
Physical basis of the operation of the refrigeration circuit
To correctly To determine the amount of heat, it is necessary to refer to the first law of thermodynamics. The refrigeration cycle is based on the circulation of refrigerant (freon) in a closed circuit, which alternately changes its state of aggregation. In the evaporator located inside the chamber, the liquid refrigerant boils at low pressure, actively absorbing heat from the surrounding space. This process is called phase transition, and it is what ensures the cooling of products.
After the evaporator, the gaseous refrigerant enters the compressor, where it is compressed. Here, electrical energy is converted into mechanical work, which leads to a sharp increase in gas temperature and pressure. Next, the hot gas enters the condenser (black radiator at the back or on the sides of the case), where it releases the accumulated energy into the environment, turning back into a liquid state.
- 🌡️ The evaporation of the refrigerant inside the chamber is accompanied by the absorption of heat from the internal volume.
- ⚙️ Compression of the gas in the compressor adds additional thermal energy to the system energy equivalent to the operation of the electric motor.
- 🔥 Condensation in the heat exchanger ensures the release of the total heat (taken from the chamber + received from the motor) into the room.
Thus, the thermal balance of the system is based on the sum of two quantities: the heat taken from the cooled bodies and the work done on refrigerant. Heat transfer in the condenser occurs due to convection and radiation, so the efficiency of this process directly depends on the ambient temperature.
Calculation formula and energy balance
To accurately determine the amount of heat released by the refrigeration machine into the environment, the fundamental heat balance equation is used. According to the first law of thermodynamics for cyclic processes, the amount of heat given off to the heater (in our case, the air in the room) is equal to the sum of the heat taken from the refrigerator (inner chamber) and the work expended on compressing the refrigerant.
Mathematically, this is expressed by the formula: Q_det = Q_internal + A_slave, where Q_det is the required amount of heat given off by the condenser, Q_internal is the amount of heat received from the cooled bodies, and A_work is the work done by the compressor per cycle. Technical specifications often include the concept cooling capacity, which exactly corresponds to the parameter Q_internal.
⚠️ Attention: In real operating conditions, the compressor does not operate continuously. It is turned on and off by a thermostat or inverter electronics. Therefore, to calculate the average heat gain per day, it is necessary to take into account the working time coefficient (WFC), which usually ranges from 0.3 to 0.5 for household models.
If we consider the process from the point of view of consumed electricity, then work A_work can be replaced by energy consumed from the network for the same period of time (provided that the engine efficiency is close to unity, which is acceptable simplification for household calculations). Consequently, the refrigerator always heats the room more than it cools its internal volume.
Why does the sum not always converge under ideal conditions?
In real calculations, part of the energy can be lost to vibration, sound and heating of the housing outside the condenser, however, for engineering calculations of heat inflows, these losses are usually neglected, considering the system closed.
The influence of the type of defrosting system on heat transfer
The type of defrosting system installed in the refrigerator has a significant impact on the amount of heat released. In models with manual defrosting or a drip system (Drop System), heat exchange occurs more evenly during the compressor operating cycle. However, in systems No Frost (or Full No Frost) the picture is complemented by the operation of powerful heating elements (thermoelectric heaters).
Periodically, based on a signal from a timer or cycle counter, the defrost mode is turned on. At this moment, the compressor stops, but the heating elements begin to actively heat the evaporator to melt the frozen ice. All the energy expended in this process also ends up in the room, increasing the overall heat gain. This means that No Frost gives more heat into the room than a drip model of similar cooling capacity.
Let's consider the main sources of heat in different systems:
- ❄️ Drip system: the main source is the condenser and the operation of the compressor, heating elements are absent or minimal (only in tray).
- 💨 No Frost: the energy of the heating elements of the evaporator, blower fan and often the heating element of the drip tray is added.
- ⚡ Inverter models: operate in a wider power range, which can smooth out peak heat transfer values, but does not change the overall energy balance.
When When calculating heat inflows for warehouses with a large number of equipment, this nuance becomes critical. Heat release from a block of No Frost refrigerators can be 15-20% higher than from equipment with a drip system, due to the cyclic operation of defrost heaters.
Practical calculation of heat inflows in indoors
For engineers and designers, it is important not only to know the formula, but also to be able to apply it to assess the load on the air conditioning system. If you install several refrigerators in one room, the combined heat release can create a “greenhouse” effect. The calculation is based on the passport data, which indicates the power consumption and cooling capacity.
Let's imagine that we have a refrigerator with a power consumption of 150 W and a cooling capacity of 100 W (conventional units for example). For one hour of compressor operation, it will consume 150 Wh of electricity. According to the law of conservation, the following will go into the room: 100 W (heat from the chamber) + 150 W (motor operation) = 250 W of heat. However, if the compressor operated for only 20 minutes (1/3 hour), then the actual heat gain will be less.
Below is a table with approximate heat transfer data for various types of equipment during continuous operation:
| Equipment type | Power consumption (average) | Cooling capacity | Total heat gain (Q_department) |
|---|---|---|---|
| Domestic refrigerator (150 l) | 100 W | 70 W | ~170 W |
| Two-chamber No Frost | 180 W | 130 W | ~310 W |
| Industrial wardrobe | 400 W | 300 W | ~700 W |
| Chest freezer | 120 W | 80 W | ~200 W |
It is important to note that the table shows data for the moment the compressor operates. To obtain the average daily value, it is necessary to multiply the obtained figures by the switching factor, which depends on the load of the chambers, the temperature in the room and the frequency of opening the doors.
☑️ Checking operating conditions
Factors influencing the efficiency of heat transfer
The efficiency of heat transfer to the environment is not a constant value. It directly depends on the operating conditions and technical condition of the unit. If heat transfer is disrupted, the compressor is forced to run longer, consuming more electricity and generating even more heat, creating a vicious cycle of overheating.
One of the key factors is the ambient temperature. The hotter the room, the worse the condenser works, since the temperature difference between the hot gas and air decreases. This leads to an increase in condensation pressure, an increase in the load on compressor and, as a consequence, an increase in heat transfer. The cleanliness of the heat exchanger is also critically important.
Dust, animal hair and grease settling on the black grille at the back or in the base of the refrigerator act as a heat insulator. Thermal conductivity such a layer is extremely low, which forces the system to operate in emergency mode. In the instructions you can often find a requirement to leave gaps between the case and the wall, and this is not just a recommendation, but a physical necessity for organizing convection flows.
⚠️ Attention: Installing the refrigerator in a niche without ventilation ducts provided by the manufacturer or building it into furniture without gaps leads to a local increase in temperature. This can increase energy consumption by up to 30% and reduce the life of the compressor by half.
Comparison of theoretical and real indicators
In practice, determining the exact amount of heat given off by a refrigerator is more difficult than in a physics textbook. Real conditions make their own adjustments: frequent opening of doors, loading with warm food, changes in network voltage. All this leads to the fact that the actual heat gain may differ significantly from the calculated one according to the passport data.
In addition, part of the heat is removed not only through the condenser, but also through the walls of the case, especially if the refrigerator operates in intensive freezing mode. The walls can heat up to 40-50 degrees Celsius, actively releasing energy into the room. In modern models with the system Multi Air Flow the distribution of temperatures and, accordingly, heat flows becomes even more difficult to analyze without specialized equipment.
For accurate measurements in laboratory conditions, calorimeters or thermal imagers are used, which allow visualizing heat flows. However, for everyday needs it is enough to understand the principle: Q_otd > Q_internal. Any attempt to use a refrigerator to cool the kitchen (by opening the door) is doomed to failure, since you will heat the room by running the motor more than cooling it with an open chamber.
Frequently asked questions (FAQ)
Can a refrigerator cool the kitchen if the door is opened?
No, this is physically impossible. When the door is open, the refrigerator will work continuously, trying to cool the kitchen volume. However, the heat it throws out from the rear (the sum of the heat from inside and the work of the motor) will be greater than the cold coming out of the chamber. As a result, the temperature in the kitchen will rise.
Why is the back wall of the refrigerator hot?
This is a normal operating process. In the condenser, located on the back wall or side panels, the hot refrigerant releases heat to the environment, changing from a gaseous state to a liquid state. The temperature can reach 50-60 degrees.
Does the color of the refrigerator affect the amount of heat given off?
Theoretically, black color has a higher emissivity than white, and should give off heat better by radiation. However, in domestic conditions, the main mechanism of heat transfer is convection (air movement), so the influence of color on the overall efficiency of heat transfer is negligible.
How to calculate the heat flow for a warehouse with 10 refrigerators?
It is necessary to sum up the power consumption of all units of equipment (in Watts) and multiply by the simultaneous operation coefficient (usually 0.6-0.8 for a warehouse) and the safety factor. The resulting figure in Watts should be taken into account when selecting the power of the air conditioner.