Refrigerators and Heat Pumps – Principle and Working

Refrigerators and Heat Pumps: Heat pumps, air conditioners, and refrigerators transfer heat from the colder region to the hotter region. They are heat engines that run backward. We say backward and not reverse because except for Carnot engines, all heat engines can be run backward but cannot be truly reversed.

In the kitchen of every home in India, we find a refrigerator. Every ten minutes, we hear the motor of the refrigerator turn on and keeps things cold magically. We will be throwing out our food instead of saving them for another day if there is no refrigerator. The refrigerator is a miracle of modern living that has changed our life.

The main reason for having a refrigerator is to keep food cold. The fundamental idea behind refrigeration is to slow down the activity of decaying bacteria so that it takes a longer time for the bacteria to spoil the food. For example, bacteria spoil milk in two or three hours if the milk is left out on the kitchen slab at room temperature. By reducing the milk temperature in the refrigerator, it will stay fresh for a week or few days.zing the milk can stop the bacteria activity altogether, and the milk can last for many days.zing and refrigeration are two of the most common forms of food preservation techniques used today.

The process of refrigeration and how the process of cooling takes place in a normal refrigerator is explained in detail in this article. The article also discusses the coefficient of performance of a refrigerator.

Study Newton’s Law Of Cooling Here

What is Thermodynamics?

Thermodynamics is a branch of Classical Physics that deals with converting heat energy into mechanical energy and exchanging heat energy between bodies.

Note: Nicolas Leonard Sadi Carnot is often described as the “Father of Thermodynamics.”

Thermodynamic Processes and their Types

The process in which the state of a system changes and involves a change of thermodynamic variables such as pressure \(P\), volume \(V\), and temperature \(T\) is known as the thermodynamic process. Some important processes are:

1. Isothermal process: Constant temperature
2. Adiabatic process: Heat supplied is zero
3. Isobaric process: Constant pressure
4. Isochoric: Constant volume
5. Cyclic and non-cyclic process  
6. Reversible and irreversible process

Reversible, Irreversible, Cyclic, and Non-cyclic Process

1. Reversible process: The reversible process is one in which all changes occurring in the direct process are exactly repeated in the opposite order. For example, whatever heat is absorbed in the direct process of a reaction, the same amount of heat should be given out in the reverse process of the reaction. Whatever work is done on the working substance in the direct process, the same amount of work is done in the reverse process.
   Note: A reversible process can never be realised practically. It is an ideal concept. There is always a loss of heat in a reversible process due to friction, conduction, radiation, etc.
2. Irreversible process: A process that is not reversible exactly is called an irreversible process. All-natural processes such as radiation, conduction, and radioactive decay, etc., are irreversible. All experimental processes such as Joule-Thomson expansion, expansion, and electrical heating of a wire are also irreversible.
3. Cyclic and Non-cyclic process: A cyclic process occurs when a series of changes occur, and the system comes to its initial state. In a non-cyclic process, the series of changes (reactions) involved do not return the system to its initial state. In the cyclic process, the \(P – V\) graph is a closed curve, as shown in the figure below, and the area enclosed by the closed path represents the work done during the process. If the curve is clockwise, work done is positive, and if the curve is anticlockwise, work done is negative.

In the non-cyclic process, work done depends upon the series of changes involved or the path chosen and can be calculated by the area enclosed between the curve and \(x\)-axis on the \(P – V\) diagram.

Second Law of Thermodynamics

The first law of thermodynamics only explains the equivalence of heat and work. It doesn’t explain why heat flows from hot body to cold body and why the converse is not possible. It can’t explain why the efficiency of a heat engine is always less than unity. It also doesn’t explain why cold water on stirring gets hotter, but there is no effect on stirring hot water in a beaker. The second law of thermodynamics provides answers to all these questions.

The Second law of thermodynamics proposes that “any spontaneously occurring process will always lead to an increase in the entropy \(\left( {\rm{S}} \right)\) of the universe.” The law explains that the entropy of an isolated system will never decrease over time.

Note:

1. The second law of thermodynamics is also known as the Law of Increased Entropy.
2. Entropy is a measure of randomness or disorder of the system.

Clausius statement: States that “A self-acting machine can’t transfer heat from a colder body to a hotter body without the help of an external agency.”
From Clausius’s statement, it is understandable that heat cannot flow from a cold body to a hot body unless an external agent works. This statement is in good agreement with our experiences in physics. For example, electric current cannot flow from a conductor at the lower electric potential to that at higher potential unless external work is done. Also, a body at a lower gravitational potential level cannot move up to a higher level without an external agent’s work.

Kelvin’s statement: States that “A body or system can’t perform continuous work by cooling it to a temperature lower than the temperature of the coldest one of its surroundings.” Work done by the engine will result in heating of the surroundings and cooling of the source more and more. Hence, a Carnot heat engine cannot work if the source and sink are at the same temperature.

Kelvin-Planck’s statement: States that “It is impossible to design an engine that extracts heat and fully utilises it into work without giving any other effect.”
A particular amount of heat can never be converted completely into work because the engine has to return some amount of heat to the sink. Any engine essentially requires a source as well as a sink. The efficiency of any engine is always less than unity because heat cannot be fully converted into work.

Heat Engine

A heat engine is a very important machine that converts applied heat into necessary work continuously by a cyclic process.

The essential parts of a heat engine are:
Source: It is a reservoir of heat that has high temperatures and infinite thermal capacity. It should be noted that any quantity of heat can be taken from the source.
Working substance: Steam, petrol, etc.
Sink: It is a reservoir of heat that has low temperature and infinite thermal capacity. It should be noted that any amount of heat can be given to the sink.

Refrigerator and Heat Pumps

Refrigerator

A machine used to maintain the food substances at low temperatures and prevent their spoilage is called a refrigerator. The food materials kept at low temperatures have a longer shelf life. The refrigerator generally maintains the perishable items at a lower temperature range.

Heat Pump

A device that transports heat from one place to another is called a heat pump. It is a machine that helps heat transfer from a cold region to a hot region in a refrigerator. A heat pump can be used to cool and heat a region. It is essentially a heating unit and a refrigerator all in one.

Main Components of a Refrigerator

Following are the five main components of a refrigerator:

1. Refrigerant:  CFCs, Isobutane, ammonia.
2. Compressor: Compresses the vapour and maintains the flow of refrigerant (coolant) in the refrigerator cycle.
3. Condenser Coil: Cools the high-pressure vapour and converts it into high-pressure liquid (fluid).
4. Expansion device or throttling device: This component expands the high-pressure liquid and reduces its pressure and temperature.
5. Evaporator coil: This component takes the heat from the air inside the fridge and makes it cool.

Let’s discuss the function of each component in detail:

1. Refrigerant
   It is also called the working fluid for the refrigerator. It takes the heat from the inner region of the refrigerator and transfers it to the outer region. Most commonly used refrigerants are CFCs, isobutane (used in modern fridges), and ammonia (toxic gas and not used in the modern fridge).
2. Compressor
   It circulates the refrigerant (coolant), compresses the refrigerant, and increases its temperature and pressure during the working cycle of the fridge. The compressor is called the heart (the important part) of the fridge.
3. Condenser Coil
   It is present outside on the backside of the refrigerator. It has grill tubes and looks like a vehicle radiator. Its main purpose is to cool (condense) the hot and high-pressure gases coming from the compressor. When the hot refrigerant gases pass through the condenser coil, it gets cool down by the room’s cool air and converts into a high-pressure liquid.
4. Expansion Device or Throttling Device
   As its name indicates, it expands the high-pressure liquid refrigerant and reduces its pressure and temperature—the temperature drops to \({20^{\rm{o}}}{\rm{C}}\) and pressure to \(0.\)6 bar.
5. Evaporator Coil
   An evaporator coil is found inside the fridge. It takes the heat from the air present inside the fridge and makes it cooler. This cold air inside the fridge takes the heat from the food materials and lowers their temperature.

Working Principle of Refrigerator

It works on the principle of thermal equilibrium. When a cold body comes in contact with a hot body, heat flows from a hot body to a cold body until the bodies attain the same temperature. In the same way, a refrigerant at a low temperature is allowed to pass through the fridge compartment. When the refrigerant comes in contact with the air, it takes the heat from it and lowers its temperature. This process keeps continuing, and the temperature inside the fridge lowers down and down and keeps the food or perishable items at lower temperature and slows down their spoilage time and can be stored for weeks or months.

A refrigerator is a heat engine and heat pump which runs in the reverse direction.

It essentially consists of three parts:
Source: At higher temperature \({T_1}\).
Working substance: It is called refrigerant or coolant. Liquid ammonia, CFC, and Freon work as working substances.
Sink: At lower temperature \({T_2}\).

Heat \({Q_2}\) is taken from a sink (contents of the refrigerator) at a lower temperature \({T_2}\) by the working substance. A net amount of work \(W\) is done on it by an external agent (compressor), and a larger amount of heat \({Q_1}\) is given to a hot body at temperature \({T_1}\) (usually atmosphere). Thus, it can transfer heat from a cold to a hot body at the expense of mechanical energy supplied by an external agent. The cold body is thus cooled more and more.

The refrigerator’s performance is expressed employing “coefficient of performance” \(\beta \) defined as the ratio of the heat extracted from the cold body to the work needed to transfer it to the hot body.

i.e. \(\beta = \frac{{{\rm{Heat}}\,{\rm{extracted}}}}{{{\rm{work}}\,{\rm{done}}}} = \frac{{{Q_2}}}{W} = \frac{{{Q_2}}}{{{Q_1} – {Q_2}}}\)
\(\therefore \,\beta = \frac{{{Q_2}}}{{{Q_1} – {Q_2}}}\)

A perfect refrigerator transfers heat from cold to a hot body without doing work.

i.e., \(W = 0\) so that \({Q_1} = {Q_2}\) and hence \(\beta = \infty \)

Carnot Refrigerator

For Carnot refrigerator \(\frac{{{Q_1}}}{{{Q_2}}} = \frac{{{T_1}}}{{{T_2}}}\)

\(\therefore \,\frac{{{Q_1} – {Q_2}}}{{{Q_2}}} = \frac{{{T_1} – {T_2}}}{{{T_2}}}\) or \(\frac{{{Q_2}}}{{{Q_1} – {Q_2}}} = \frac{{{T_2}}}{{{T_1} – {T_2}}}\)

So the coefficient of performance \(\beta = \frac{{{T_2}}}{{{T_1} – {T_2}}}\)

Where \({T_1} = \) temperature of surrounding, \({T_2} = \) temperature of cold body,
It is clear that \(\beta = 0\) when \({T_2} = 0\)
i.e. the coefficient of performance will be zero if the cold body is at a temperature equal to absolute zero.

Relation between the Coefficient of Performance and Efficiency of a Refrigerator

We know \(\beta = \frac{{{Q_2}}}{{{Q_1} – {Q_2}}}\) or \(\beta = \frac{{{Q_2}/{Q_1}}}{{1 – {Q_2}/{Q_1}}}\,…..(i)\)

But the efficiency \(\eta = 1 – \frac{{{Q_2}}}{{{Q_1}}}\) or \(\frac{{{Q_2}}}{{{Q_1}}} = 1 – \eta \,…..(ii)\)

From \((i)\) and \((ii)\), we get  \(\beta = \frac{{1 – \eta }}{\eta }\)

Summary

1. Heat pumps, air conditioners, and refrigerators transfer heat from the colder region to the hotter region.
2. Clausius statement states that “It is impossible for a self-acting machine to transfer heat from a colder body to a hotter one without the aid of an external agency.”
3. A refrigerator or heat pump is a heat engine run in the reverse direction.
4. For the Carnot refrigerator \(\frac{{{Q_1}}}{{{Q_2}}} = \frac{{{T_1}}}{{{T_2}}}\), coefficient of performance \(\beta = \frac{{{T_2}}}{{{T_1} – {T_2}}}\), where \({T_1} = \) temperature of surrounding, \({T_2} = \) temperature of the cold body.

FAQs on Refrigerators and Heat pumps

Q.1. What is the relation between heat pump and refrigerator?
Ans: A refrigeration system cools the external fluid flowing through the evaporator, whereas a heat pump heats the external fluid flowing through the condenser. The main difference between a refrigerator and a heat pump is in operation regarding cooling or heating.

Q.2. How does a refrigerator work?
Ans: A refrigerator or heat pump is a heat engine run in the reverse direction.

Q.3. What is a refrigerator’s ideal temperature?
Ans: A refrigerator should not potentiallyze the contents, and hence the ideal temperature for most refrigerators used for domestic purposes is between \(35\) to \(38\) degrees Fahrenheit.

Q.4. What does the fan in a fridge accomplish?
Ans: There are two fans in a refrigerator, one located under the refrigerator and another inside the refrigerator. The former forces air through the exterior refrigerator coils, and the latter forces air to move around the coils, which improves the cooling efficiency.

Q.5. Why is the backside of a refrigerator painted black?
Ans: Black colour works well as a heat radiator. It is the perfect colour for all external refrigeration coils.

Q.6. What is the coefficient of Performance (COP) of a refrigerator?
Ans: “Coefficient of performance” \(\beta \) is defined as the ratio of the heat extracted from the cold body to the work needed to transfer it to the hot body.
i.e. \(\beta = \frac{{{\rm{Heat}}\,{\rm{extracted}}}}{{{\rm{work}}\,{\rm{done}}}} = \frac{{{Q_2}}}{W} = \frac{{{Q_2}}}{{{Q_1} – {Q_2}}}\)

Study About Combustion And Flame Here

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