What is A Refrigerator?
The working of the refrigerator is simple to understand, in this type of system, we have a source through which heat is taken from the surroundings and another system as a sink through which heat gets disposed of.
A refrigerator we have at our homes works on cooling through the evaporation principle and there are three steps by which a refrigerator or a fridge works:
Cool refrigerant is passed around food items placed inside the fridge.
A refrigerant absorbs heat from these food items.
A refrigerant transfers the heat to the relatively cool surroundings.
Refrigerator Thermodynamics
The working principle of a refrigerator is based on the second law of thermodynamics. So, there are the following five steps because of which working conditions are observed:
First: the Evaporator coil removes heat from the object.
Second: there is an expansion valve that evaporates the gas.
Third: A cold liquid gas is passed through the compressor.
Fourth: Hot gas is disposed of in the surroundings.
Fifth: A metallic coil dissolves heat into the gas.
These five are repeated endlessly and we get chilled food items.
Components of A Refrigerator
There are five components in a refrigerator; these are as follows:
Expansion valve
Compressor
Evaporator
Condenser
A specialized gas called refrigerant
Refrigerator Working Principle
Do you what is the refrigerator working principle? Well! A refrigerator works on the principle of vapor compression refrigeration cycle in which there are four components viz: expansion valve, compressor, evaporator, condenser, and they all are connected with copper/steel tubes.
The refrigerator has a low boiling point that is about - 20℃. The freezer is surrounded by the evaporator tube so, when we keep any food item in the refrigerator, the evaporator tube absorbs the heat from this item after this, a liquid refrigerant is converted into vapor.
Now, the vapors are compressed by the compressor to high pressure and temperature. The heat generated is then rejected to the atmosphere from the compressor from vapor to liquid state. Thereafter, this heat again passes through the expansion valve and moves to the evaporator, and so on. This cycle keeps on repeating and cools down the air inside the fridge.
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Working Principle of Refrigerator With Diagram
Expansive Valve
The expansive valve is also known as the flow control device. An expansion valve controls the flow of the liquid refrigerant or simply the ‘coolant’ into the evaporator. This device is very small in size and also sensitive to temperature changes of the refrigerant.
Compressor
The compressor comprises a motor that ‘captures in’ the refrigerant from the evaporator and compresses it in a cylinder to generate a hot, high-pressure gas.
Below is the image for the compressor inside the standard refrigerator:
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Evaporator
The evaporator is an exact part that cools the foodstuff kept inside a refrigerator. It consists of finned tubes that are made of metals with high thermal conductivity to maximize heat transfer that absorbs heat expelled via a coil by a fan. The evaporator absorbs/seizes heat from the foodstuff kept inside the fridge, and as a result of this heat, the liquid refrigerant vaporizes.
Condenser
A condenser contains a coiled set of tubes with external fins and is located at the backside of the refrigerator.
A condenser helps in the liquefication of the gaseous refrigerant by absorbing its heat and instantaneously expelling it to the surroundings.
Now, as the heat of the refrigerant is disposed of, its temperature drops to condensation temperature, and it again changes its state from vapor to liquid.
The below image shows the condenser coils that we can see at the rear of our refrigerators:
(Image to be added soon)
Now, we are left with one more component and that is refrigerant or simply a coolant, so let’s understand its working as well:
Refrigerant
Refrigerants are basically referred to as the coolant, it is the liquid that keeps the refrigeration cycle continued.
The liquid which is discussed above is actually a specially designed chemical that is capable of alternating or changing its state between being a hot gas and a cool liquid.
In the 20th century, fluorocarbons, especially CFCs, were a common choice as a refrigerant, since they have caused havoc in our environment, so, now they’re being replaced by eco-friendly refrigerants, such as ammonia, HFC-134a, R-290, R-600A, etc.
Difference Between Heat Pump and Refrigerator
The difference between a heat pump and refrigerator is simple; let’s understand it:
A heat pump working principle is that a device transfers heat energy from a source of heat to the thermal reservoir. Heat pumps move thermal energy in the opposite direction of spontaneous heat transfer, by absorbing heat from a cold space and releasing it to a warmer one; however, a refrigerator is a household appliance that is passed a cooler temperature than the external environment. It makes the objects cooler than the normal temperature.
FAQs on Refrigerator Heat Pump
1. What is a refrigerator and what is its primary function in thermodynamics?
A refrigerator is a device that operates on a thermodynamic cycle to transfer heat from a colder space (the refrigerated compartment) to a warmer space (the surrounding environment). Its primary function is to maintain a temperature lower than that of its surroundings by continuously extracting heat from the cold reservoir (inside the fridge) and rejecting it to the hot reservoir (the room).
2. What are the essential components of a refrigerator and what role does each play?
A refrigerator operates using a closed-loop system with five main components, each performing a specific role in the cooling cycle:
- Compressor: This is the heart of the system. It pressurises the refrigerant gas, significantly increasing its temperature.
- Condenser: Located on the back of the fridge, these coils allow the hot, high-pressure refrigerant gas to release heat into the surrounding air, causing it to condense into a liquid.
- Expansion Valve (or Capillary Tube): The high-pressure liquid refrigerant passes through this narrow valve, causing a sudden drop in pressure and temperature.
- Evaporator: These coils are inside the freezer compartment. The cold, low-pressure liquid refrigerant absorbs heat from the inside of the refrigerator, causing it to boil and turn back into a gas. This absorption of heat is what cools the food.
- Refrigerant: A specialised fluid with a low boiling point that circulates through the system, changing state between liquid and gas to absorb and release heat.
3. How does a refrigerator work based on the principles of thermodynamics?
A refrigerator works on the principle of the vapour compression refrigeration cycle, which is an application of the Second Law of Thermodynamics. The cycle involves these key steps:
- Evaporation: A cold liquid refrigerant flows through the evaporator coils inside the fridge. It absorbs heat from the contents, causing it to boil and transform into a low-pressure gas.
- Compression: The compressor draws in this gas and compresses it, increasing its pressure and temperature.
- Condensation: The hot, high-pressure gas moves to the condenser coils on the outside. Here, it releases its heat to the surrounding room and condenses back into a high-pressure liquid.
- Expansion: This liquid then flows through the expansion valve, where its pressure and temperature drop drastically, making it very cold. This cold refrigerant then re-enters the evaporator to repeat the cycle.
4. What is the fundamental difference between a refrigerator and a heat pump?
While both a refrigerator and a heat pump operate on the same thermodynamic cycle, their intended purpose is opposite. The key difference lies in the desired effect:
- A refrigerator's main goal is to cool a specific, enclosed space. Its desired effect is the amount of heat extracted from the cold reservoir (the inside of the fridge).
- A heat pump's main goal is to heat a space. Its desired effect is the amount of heat delivered to the hot reservoir (the room to be heated). Essentially, a heat pump is a refrigerator working in reverse to provide heating.
5. How is the efficiency of a refrigerator and a heat pump measured and compared?
The efficiency of these devices is not measured by traditional efficiency percentages but by the Coefficient of Performance (COP). The COP is the ratio of the desired thermal output to the required work input.
- For a refrigerator, the COP is the ratio of heat extracted from the cold reservoir (Qc) to the work done (W). Formula: COP_ref = Qc / W.
- For a heat pump, the COP is the ratio of heat delivered to the hot reservoir (Qh) to the work done (W). Formula: COP_hp = Qh / W.
Since Qh = Qc + W, the COP of a heat pump is always greater than the COP of a refrigerator operating between the same two temperatures, with the relation being COP_hp = COP_ref + 1.
6. What are some examples of modern refrigerants, and why were older types like CFCs phased out?
Older refrigerants, most notably Chlorofluorocarbons (CFCs) like R-12, were phased out under the Montreal Protocol. This was because they were found to cause significant damage to the Earth's ozone layer when released into the atmosphere. Modern refrigerators use more environmentally friendly alternatives, such as:
- Hydrofluorocarbons (HFCs): For example, R-134a. These do not deplete the ozone layer but are potent greenhouse gases.
- Hydrocarbons (HCs): For example, R-600a (isobutane) and R-290 (propane). These have very low global warming potential and are now commonly used in domestic refrigerators.
7. Why can't you cool a room by leaving a refrigerator door open?
You cannot cool a room by leaving a refrigerator door open because a refrigerator is not a closed system in relation to the room. While it removes heat from its interior (the cold reservoir), it releases that same heat—plus additional heat generated by the work done by its motor and compressor—into the room from its condenser coils (the hot reservoir). According to the Second Law of Thermodynamics, heat is simply moved from one place to another with an input of work. The net effect is that the refrigerator releases more heat into the room than it absorbs, causing the overall room temperature to increase.
