Basic Concepts of Refrigeration and Air Conditioning [PDF]


Refrigeration and air conditioning are two of the most essential technologies in modern-day living. They are responsible for maintaining a comfortable indoor environment, preserving food etc. Despite their importance, many people are unaware of the basic concepts underlying these technologies. In this article, we aim to shed light on these fundamental concepts, starting with the principles of thermodynamics and heat transfer. We’ll then delve into the working principles of refrigeration and air conditioning systems, explaining the critical components and their functions. By the end of this article, readers will have a comprehensive understanding of how refrigeration and air conditioning work and the importance of these technologies in our daily lives.

Intensive Properties VS Extensive Properties

  • Intensive properties do not depend on the size of the system e.g. pressure and temperature etc.
  • Extensive properties depend on the size of the system e,g, volume, internal energy, enthalpy, etc.
  • Entropy increases in an irreversible process due to two reason which is due to heat addition and due to friction i.e. conversion of mechanical forms of energy into heat.
  • Throttling process is an irreversible adiabatic process. It is used to reduce the pressure of the fluid by introducing a restriction to the flow.
  • Since enthalpy is a function of temperature only, the temperature also remains constant during the throttling process.

Basic Concepts of Refrigeration and Air Conditioning

  • Expansion of liquid with flashing : This process is accompanied by an increase in Entropy along with a drop in pressure due to which volume increases and a part of the liquid is vaporized, thus cooling the liquid.
  • Reversible adiabatic expansion of gas : This method is used with permanent gases such as air.
  • Irreversible adiabatic expansion of a real gas : Real gases produce a substantial decrease in temperature under certain condition, which corresponds to initial low temperature and high pressure of the gas.
  • Thermoelectric cooling : Cooling is produced at one junction of two dissimilar metals if a current is passed through them. The phenomenon is called the Peltier effect.
  • Adiabatic Demagnetization : Cooling could be produced by the adiabatic demagnetization of a paramagnetic salt.
Introduction and Basic Concepts of Refrigeration and Air Conditioning

Properties of air

Cp = 1.005

R = 0.287 kJ/kg.K

M = 28.966

Cv = 0.718 kJ/kg.K

γ = 1.4

What is Refrigeration and Air Conditioning?

Refrigeration and air conditioning are two closely related processes that deal with the transfer of heat from one location to another. Refrigeration is the process of removing heat from a space or substance to lower its temperature. Air conditioning involves both cooling and heating of air to maintain a comfortable indoor environment.

What is Refrigeration and Air Conditioning?
Credits :

In general, refrigeration and air conditioning systems work by compressing and expanding gases to create temperature changes. The refrigerant, which is the substance that absorbs and releases heat in these systems, is circulated through a closed loop that includes a compressor, condenser, evaporator, and expansion valve. The compressor compresses the refrigerant, which increases its temperature and pressure. The condenser then releases heat from the refrigerant to the surrounding air or water, which causes it to condense into a liquid. The liquid refrigerant then passes through an expansion valve, which reduces its pressure and causes it to evaporate back into a gas. This process absorbs heat from the surrounding environment, which cools the air or substance being refrigerated.

Applications of Refrigeration and Air Conditioning

Refrigeration and air conditioning systems have a wide range of applications in various industries and everyday life. Following are some of the most common applications:

  • Food preservation: Refrigerators and freezers are used to keep food fresh and prevent it from spoiling. The cold temperature slows down the growth of bacteria and other microorganisms that cause food to spoil, extending its shelf life.
  • Transportation of perishable good: It is used to transfer materials such as fruits, vegetables, dairy products, and meat, from one place to another. Refrigerated trucks and containers are designed to maintain a constant temperature, ensuring that the food remains fresh during transit.
  • Medical industry: Hospitals and healthcare facilities depend on these systems to store and transport vaccines, blood, and other medical supplies that need to be kept at specific temperatures. Air conditioning systems are also used to control the temperature and humidity levels in operating rooms, patient rooms, and other areas of hospitals to ensure a comfortable and safe environment for patients and staff.
  • Manufacturing industry: Refrigeration and air conditioning systems are used to control the temperature and humidity levels in production facilities, warehouses, and laboratories. These systems help maintain optimal conditions for the production of sensitive products such as pharmaceuticals, electronics, and chemicals.
  • Maintain Comfort Conditions and Air quality: Air conditioning systems help regulate the temperature and humidity levels, providing a comfortable environment during hot and humid summers. They also filter the air, removing pollutants, allergens, and other harmful particles, improving the indoor air quality.

Difference between Refrigeration and Air Conditioning

Refrigeration is primarily used to cool and preserve perishable goods, such as food and medicines, by removing heat from their surroundings.Air conditioning, on the other hand, is used to regulate the temperature and humidity of indoor spaces for human comfort and health.
It works by compressing and expanding a refrigerant gas, which absorbs heat from the environment and releases it outside.It works by circulating air over a refrigerated coil, which removes moisture and heat from the air and releases it outside.
Refrigeration systems typically consist of a compressor, a condenser, an evaporator, and a metering device, all of which work together to remove heat from the refrigerated space and maintain a constant temperature.Air conditioning systems can be either central or individual, depending on the size and layout of the building, and can include additional features such as air filtration, dehumidification, and ventilation.
Refrigeration is primarily used for commercial and industrial applications, such as food storage and transportationAir conditioning is more commonly used in residential and commercial buildings, such as homes, offices, and hospitals.
Refrigeration systems typically operate at lower temperatures and require more insulation and specialized components, such as refrigerated trucks and freezers.Air conditioning systems, on the other hand, operate at higher temperatures and are designed to control humidity as well as temperature.

Refrigerating Machine / Heat Pump

  • It contains evaporator, compressor, condenser, expander
  • The process followed in the cycle is as follows

Heat Q0 is absorbed in the evaporator by the evaporation of a liquid refrigerant at a low-pressure P0 and corresponding low saturation temperature T0. The evaporated refrigerant vapor is compressed to a high-pressure Pk in the compressor consuming work W. Heat Qk is rejected from the condenser to the surrounding

The heat rejected to the surroundings equals the heat absorbed from the cold body or refrigeration produced plus the work done or mechanical energy consumed. A reversible heat engine may be converted into a refrigerating machine if it runs in the reversed direction. There is no difference in the cycle of operations between a refrigerating machine and a heat pump. The same machine can be utilized either.

  • To absorb heat from the cold body at temperature T0 and reject it to the surroundings at temperature Tk ≥ Ta ( i.e. refrigerator)
  • To absorb heat from the surroundings at temperature T0 ≤ Ta and rejects it to the hot body at temperature Tk (i.e. heat pump)

The main difference between two is in their operating temperature. A refrigerating machine is operated between the ambient temperature Ta and a low-temperature T0. A heat pump is operated between the ambient temperature T0 and a high-temperature Tk.

Difference in their functions

In a refrigerating machine, the heat exchanger that absorbs heat is connected to the conditional space. A refrigerating machine that is used for cooling in summer is to be used as a heat pump for heating in winter, it will be necessary either,

  • To rotate the machine by 1800 to interchange the positions of the two heat exchangers between the space and surroundings.
  • To exchange the functions of the two heat exchangers by the operation of valves e.g. a four-way valve in a window type air conditioner.

Coefficient of performance (COP)

COP = Energy Ratio = Useful heat / work

(COP)net = Q0 / W = Q0 / (Qk – Q0)

(COP)pump = Qk / W = Qk / (Qk – Q0)

(COP)pump = 1 + (COP)net

If the refrigerator/ heat pump cycle is reversed, it becomes a heat engine.

The thermal efficiency of the heat engine:

ηth = W/ Qk = (Qk – Q0)/Qk

(COP)pump = Qk /(Qk – Q0) = 1/ ηth

(COP)ref = (1/ηth)-1

For vapor compression system ξ is of the order of 3 for air conditioning application. For air cycle refrigeration system ξ ≈ 1 and for vapor absorption systems, it is well below unity. Steam ejector machines have still lower values. For the purpose of heating, it is far more economical to use a heat pump rather than an electric resistance heater. The value of ξ for air conditioning applications can range between 3 and 5.

Reverse Carnot cycle

A reversible Carnot cycle has the maximum COP

Heat absorbed from cold body Q0 = T0 ΔS

Heat rejected to hot body Qk = Tk ΔS

Work done W = Qk – Q0 = (Tk – T0)ΔS

COP for cooling = Q0 / W = T0 / (Tk-T0)

COP for heating = Qk / W = Tk / (Tk-T0)

Carnot COP depends on the operating temperature Tk and T0 only. It does not depend on the working substance used. The lowest possible refrigeration temperature is T0 = 0 of which ξ = 0. The highest possible refrigeration temperature is T0 = Tk i.e. when the refrigeration temperature is equal to the temperature of surroundings of which ξ= ∞

  • COP for cooling varies between 0 and ∞
  • COP for heating varies between 1 and ∞

To obtain maximum possible COP in any application

  • The cold body temperature T0 should be as high as possible
  • The hot body temperature Tk should be as low as possible.

The selection of temperature Tk depends on the surrounding medium used for heat rejection viz. air, water, and ground.

Water as a cooling medium is preferable to air as it affords a lower value of Tk because of following reasons:

  • It is available at a temperature lower than that of air. Its temperature approaches the wet bulb temperature of the surrounding air. This is the limiting temperature to which heated water can be cooled in a cooling tower or a spray pond.
  • The specific heat of water is about four times that of air.
  • Water has a higher heat transfer coefficient than air mainly because of its high thermal conductivity.

Thus the use of water as a cooling medium results in a lower Tk, higher COP and lower power consumption in a refrigerating plant. Temperature approaching zero have been obtained using adiabatic demagnetization on a limited scale in laboratories.

  • Refrigerating efficiency :

ηc = (COP)c / (COP)c, carnot

  • Heating efficiency:

ηc = (COP)h / (COP)h, carnot

Video on Air Conditioner


What refrigerator temperature is the best?

When it comes to keeping your food fresh and safe to eat, one of the most important factors is the temperature at which it is stored. Refrigerators are designed to maintain a consistent temperature that will keep your food at a safe level, but the question is, what temperature is the best? The answer to that question depends on a few factors, including the type of food you are storing and how quickly you plan to consume it.

In general, the ideal temperature range for a refrigerator is between 35 and 38 degrees Fahrenheit. This temperature range will help to slow the growth of bacteria and other microorganisms that can cause food spoilage and foodborne illness. However, it’s important to note that some foods may require slightly different storage temperatures.

It’s also important to keep in mind that the temperature of your refrigerator can fluctuate depending on a variety of factors, including how often you open and close the door, the ambient temperature in your kitchen, and how full your refrigerator is.

How does refrigeration work?

Refrigeration is a process that plays a crucial role in our daily lives. It is used to preserve food, keep drinks cold, and even to maintain the temperature of certain medicines. The basic principle behind refrigeration is the transfer of heat from one area to another. This is achieved through the use of a refrigeration system, which consists of various components such as a compressor, evaporator, condenser, and expansion valve.

In simple terms, the refrigeration system works by removing heat from a designated area and transferring it to another. This is done by compressing a refrigerant gas, such as Freon or R-134a, which increases its temperature and pressure. The hot gas is then passed through a condenser, where it is cooled and converted back to a liquid state. The liquid refrigerant is then passed through an expansion valve, which lowers its pressure and temperature.

As the refrigerant passes through the evaporator, it absorbs heat from the surrounding area and evaporates, turning back into a gas. The heat is then released through the condenser, and the cycle repeats. This continuous process of heat transfer results in the desired cooling effect inside the refrigerator.

In conclusion, refrigeration is a complex process that involves the transfer of heat from one area to another through the use of a refrigeration system. Understanding how refrigeration works can help us better appreciate the technology that we rely on every day.

Who invented the refrigerator?

The invention of the refrigerator can be traced back to the early 1800s, when various inventors were experimenting with ways to artificially create cold temperatures. However, it was not until 1834 that the first practical refrigerator was invented by Jacob Perkins, an American inventor and engineer. Perkins’ refrigerator used a closed-cycle system that involved compressing and expanding a refrigerant gas to produce cooling. Although his invention was innovative, it was also expensive and not widely adopted.

In the years that followed, other inventors made further improvements to the refrigerator, including the development of electric refrigerators in the early 1900s. One of the most significant advances in refrigeration technology came in 1927, when General Electric introduced the first refrigerator with a hermetically sealed compressor. This innovation made refrigeration more efficient and reliable, and paved the way for the widespread use of refrigerators in households and businesses.

Today, refrigerators are an essential part of modern life, used for storing food and beverages at safe temperatures. They come in a variety of sizes and styles, from small dorm room fridges to large commercial refrigerators. The invention of the refrigerator has had a significant impact on the way we live and has revolutionized the food industry by enabling the safe transport and storage of perishable goods.

When was air conditioning invented?

Air conditioning is a technological innovation that has revolutionized the way we live and work. It has become an integral part of our daily lives, providing us with cool and comfortable temperatures during the hot summer months. But have you ever wondered when air conditioning was invented? & Who invented air conditioning? The first modern air conditioning system was invented in 1902 by Willis Haviland Carrier, an American engineer. Carrier’s invention was initially intended to control the humidity levels in a printing plant, but it soon became apparent that his invention had a much broader application. Over the years, air conditioning technology has evolved significantly, and today, we have highly advanced and sophisticated air conditioning systems that are more energy-efficient and eco-friendly.

What is ductless air conditioning?

Ductless air conditioning is also known as mini-split air conditioning. It is a type of cooling system that does not require ductwork to distribute cool air throughout a building. It consists of an outdoor unit that is connected to one or more indoor units that are mounted on the wall or ceiling. The indoor units blow cool air directly into the room, providing precise temperature control and energy efficiency. Ductless air conditioning systems are becoming increasingly popular due to their versatility and ease of installation. They can be used in a variety of settings, including homes, offices, and commercial buildings. They are also ideal for use in older homes or buildings that do not have existing ductwork, as they eliminate the need for expensive and disruptive ductwork installation.

What is central air conditioning?

Central air conditioning is a popular cooling system that is designed to regulate the temperature of an entire house or building. Unlike traditional window units, central air conditioning operates through a network of ducts. These ducts are connected to a central unit that is responsible for cooling and distributing the air throughout the space. Central air conditioning is a highly efficient and effective way to keep a large area cool and comfortable during hot weather. It is also much quieter than window units, and it does not obstruct the view or take up valuable floor space.


In conclusion, refrigeration and air conditioning are essential in our daily lives, especially during the hot summer months. The development of these technologies has significantly improved our quality of life by providing us with a comfortable indoor environment. With the increasing demand for these technologies, it is crucial to ensure that they are energy-efficient and environmentally friendly. The use of natural refrigerants and the implementation of innovative designs can significantly reduce the impact of refrigeration and air conditioning on the environment. As we move towards a more sustainable future, it is imperative that we adopt new technologies and practices that minimize our carbon footprint.

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