A refrigerator is a kind of home heat pump, which, with the help of coolants, transfers heat from the inside of its body to the outside. A mixture of gases called “freon” is used as a coolant. Each coolant has its own properties and symbol (see section 2 Refrigerants).
The composition of the refrigerator
Compressor is the heart of any refrigerator, causing the refrigerant to circulate through the system and transfer heat. At its core, it is an electric pump located in a sealed housing and operating in a refrigerant environment.
Evaporator - an aluminum sheet with tubes or just tubes, which is glued to the back of the refrigerator compartment and acts as a heat absorber.
Condenser - iron tubes located on the back of the refrigerator or behind the side walls, which are separated by a heat-insulating casing from its interior. A condenser is needed to remove heat from the refrigerant, which is removed due to contact with the surrounding air or its natural circulation.
Cycle control system is a system that prevents constant operation of the refrigerator, as this can lead to freezing of food (milk will freeze, sausage will turn to stone), and will lead to significant waste of electricity. Therefore, all household refrigerators have their own cycle control system, which regulates the operating cycle relative to a given temperature, upon reaching which the compressor is turned off, and when it changes, the compressor is turned on.
Also, this system can control not only the temperature, but also the operating time of the compressor. Periodically defrosting the refrigerator to increase the heat absorption coefficient of the evaporator, thereby preventing large accumulations of snow and ice, which will then need to be thawed and removed manually (see section 4, cycle control systems).
Capillary tube - artificially narrowing the flow area of the circuit to 0.63-0.8 mm to create a pressure difference.
The principle of operation of the refrigerator
Before considering the basic operating principle of a refrigerator, it is necessary to define the two main processes that occur in any refrigerator or heat pump.
Evaporation is the process of transition of a liquid substance into a gaseous one, which occurs on the surface of the substance due to the fact that the molecules (in our case, the refrigerant) begin to fill with energy sufficient to overcome the force of attraction from other molecules, separating from the main part of the substance. At the same time, the energy of the remaining molecules becomes less and the liquid cools. This process is the reverse of condensation.
Condensation is the process of transition from a gaseous state to a liquid state, where it is worth noting such a concept as the “dew point”. The dew point is the gas temperature at which dew begins to form.
The Carnot Cycle
Section A-B The compressor pumps refrigerant into the condenser, but due to the small flow area of the capillary tube, most of it begins to accumulate in the condenser rather than going straight to the evaporator. At the same time, the pressure begins to increase, and the gaseous refrigerant first begins to condense on the walls, and then completely turns into liquid.
Section B-C When the refrigerant transitions from one state of aggregation to another, heat is released and the condenser begins to heat up.
Section C-D When leaving the capillary tube, the vapor-liquid mixture “refrigerant” receives a larger volume than it had before entering the condenser, due to which the concentration and pressure decrease.
Section D-A With a decrease in concentration and pressure, the “refrigerant” boils, and it turns from a liquid back into a gas, while beginning to re-absorb heat from the refrigeration chamber.
This cycle lasts until the specified temperature parameters in the chamber are reached and is called the “Carnot cycle”.
Temperature distribution systems
Some refrigerator models use the principle of natural convection - air mixing and natural temperature distribution (1), due to which cold air narrows and becomes heavier compared to heated air. And if the source of cold is placed on top, then colder air masses will gradually flow from it, which will displace warmer ones, rising upward, where the evaporator is located, which begins to cool them. And this happens while the cooling cycle is ongoing.
In addition to natural convection, the principle of artificial air mixing (3) is very often used in modern refrigerators. With such a system, a cold source can be located below (usually a freezer) and a fan can drive cold air upward into the refrigerator compartment. Very often this channel has a partition, which regulates the heat distribution between the compartments. This system is called NO FROST and has many features, which we will talk about in the lesson “Cycle Control Systems”.
There are refrigerator models that combine these two systems (2). At the same time, the evaporator of the refrigeration compartment is of a weeping type, i.e., it does not have a forced source of convection, and the freezer is No Frost (fan, defrost heating element, sensors and timer).