Air Compressor is a mechanical device which is designed to convert power into portal energy stored in compressed air. The process occurs by roaring the air into a storage tank, which increases its pressure. The compressed air is used for various purposes and applications. The working principle of an air comprises comprises compressing air to a higher pressure. The compressor takes in air through an intake valve and compresses it through a piston and diaphragm. The compressed air is then stored in the pressure tank. There are several types of air compressors, each designed to suit different applications. The most common types include reciprocating compressors, rotary screw compressors, and centrifugal compressors. The applications of air compressors are vast, which makes them indispensable in various sectors. In the manufacturing industry, air compressors are used to power pneumatic tools, operate machinery, and maintain air-driven systems like assembly lines. They are also widely employed in automotive workshops for tasks like tire inflation, power tools, and spray painting. To understand the workings and applications of an air compressor, it is crucial to explore its working principle, types, and the applications of this versatile machine.

In this article, we will discuss working of air compressors, various types, their advantages, and disadvantages. Let us also look at some of their applications. This topic can pose problems in the SSC JE or GATE ME exams if not learnt well. So, let us start learning!

What are Air Compressors?

Compression is a process of increasing the pressure of any state of matter. Air compression is the process of increasing the air pressure above the atmospheric pressure. The value of atmospheric pressure is given as ‘1 atm’ or ‘101,325 Pa (Pascals)’. Or air compressors produce compressed air and they have many applications. The process is used in manufacturing oxygen cylinders, spray cans, etc. Due to these wide applications

The mechanical device used to compress air is called an ‘Air compressor’. Thus, the air compressor is a mechanical pneumatic device powered by IC engines or electric motors to convert the kinetic energy of air to potential energy. The compressed air from the air compressors is usually stored in a high-pressure vessel. An example is shown in the images below.

Fig 1: Typical Air Compressor

Air compressors are usually run by steam engines, IC engines, or electric motors. There is an upper limit provided by engineers to each type of compressor manufactured, crossing which it gets shut down. The pressurized compressed air in the cylinders has kinetic energy when released and can be used in various applications, which are discussed below. An air compressor cannot be considered a pump as the former works with only gas or air while the latter works with liquids only. Let us now learn the basic principle behind the working of these mechanical devices.

Every air compressor follows obeys Boyle’s Law which states, “The product of pressure and volume of the mass of a confined gas is constant and proportional to the temperature.”

Mathematically, \((PV \propto T)\).

A general working principle of these compressors is listed below.

1. The compressor, usually a piston-cylinder arrangement or other types as discussed below, is a work-absorbing device.
2. The air from the atmosphere is sucked into the device.
3. The working element compresses the air to high pressure increasing its volume.
4. According to Boyle’s Law, the temperature is also increased in this process.
5. Then, the compressed air from the compressor is delivered to the high-pressure vessel to store the potential energy in the form of air.

This is represented mathematically and diagrammatically in the image below.

Here,

\((P_1)\) and \((P_2) \)= Inlet and outlet pressure respectively,

\((V_1)\) and \((V_2) \)= Inlet and outlet velocity respectively.

Let us study the types and then the working principle of each type in detail.

Parts of Air Compressor 

An air compressor comprises various key components that work together to convert power into stored compressed air. Each component of the plays a very important role, which ensures its reliability. The core function of an air compressor is to take in the atmospheric air and compress it into a tank. Understanding how the Air compressor works helps for better performance, maintenance, and troubleshooting. Aspirants can check the air compressor parts along with an explanation. 

1. Air Compressor Motor

The motor is the driving force of the air compressor. It helps to convert electrical energy into mechanical energy. The motor is usually electric, but some compressors also use gas and diesel. It is important that the motor is of appropriate size to avoid inefficiencies. Regular maintenance, like lubrication and cooling, helps the air compressor motor to ensure consistent performance. 

2. Air Intake Valve

The air intake valve helps to regulate the amount of atmospheric air into the compression chamber. When compressor is running, the air entail valve opens which allows air to be sucked into the system. It closes when the chamber is filled and prevents air from escaping. It is essential to regularly clean the air intake valve of the air compressor to avoid clogs that can reduce the flow rate. 

3. Compression Chamber

This helps to compress the air. The compression chamber is responsible for reducing the air volume and increasing its pressure. The efficiency of the compression chamber impacts the overall performance of the compressor. Any damage or leak in the chamber can lead to reduced pressure and inefficiency. It is important to do regular maintenance to avoid such issues.

4. Air Receiver Tank

The air receiver tank in the Air Compressor stores the compressed air once it is created. This tank serves as a buffer, ensuring that the compressor operates continuously at a steady rate. The tank must be periodically inspected for rust, moisture buildup, and leaks to maintain proper functioning.

5. Pressure Switch

The Air Compressor pressure switch is responsible for regulating the system's pressure. It monitors the pressure inside the air receiver tank and activates or deactivates the compressor accordingly. This part ensures that the system maintains consistent pressure for efficient operation.

6. Air Filter

The air filter of Air Compressor is an essential component that prevents dirt, dust, and debris from entering the compressor system. As air is sucked into the compressor, the filter traps contaminants that can cause wear and tear on the internal components. Regular cleaning and replacing the air filter are necessary to ensure that the air entering the system is clean, helping maintain the efficiency and longevity of the compressor.

7. Cooling System

Air Compressor cooling system is responsible for preventing the compressor from overheating during operation. As the compressor works, it generates heat, which can reduce its efficiency and even cause damage if not managed. Air-cooled compressors use fans to blow air over the system, while liquid-cooled compressors circulate coolant to absorb and dissipate heat.

8. Discharge Valve

The discharge valve allows the compressed air to exit the compression chamber and enter the storage tank or system. The Air Compressor discharge valve ensures that the air is released at the proper pressure and flow rate. In most compressors, the discharge valve opens when the desired pressure is reached and closes when the system is at rest. Proper maintenance of the discharge valve is critical, as any damage could lead to air leakage or pressure fluctuations.

9. Safety Valve

The safety valve is a crucial safety feature in an air compressor. It prevents the system from exceeding its maximum pressure capacity, which could lead to dangerous situations or equipment failure. Regular testing and maintenance of the safety valve are vital for the safe operation of the compressor.

10. Oil and Lubrication System 

In oil-lubricated compressors, the oil and lubrication system is responsible for keeping the moving parts, such as pistons and bearings, properly lubricated. It is important to regularly check oil levels and replace the oil to ensure smooth operation and prevent wear or overheating.

Types of Air Compressors

The commonly known types of air compressors are given in the classification tree below.

Fig 3: Classification of Air Compressors

Positive Displacement Air Compressor 

The compression here follows a process of trapping the air, and then compressing it. This reduces the volume while increasing the pressure. The air above the condensing pressure after the rise in pressure level is expelled from the enclosed chamber. A fresh supply of air is then drawn, and the cycle continues.These type of compressors forces air into the chamber. The air is displaced continuously by a rotating mechanical component. Air cavities are formed when the component moves from one place to another.

Piston Air Compressor

A piston air compressor, a true workhorse in the world of compressed air, exemplifies the ingenuity and efficiency of conservative engineering principles. With its robust design and mechanical precision, this exceptional device stands tall as a testament to conservative values.

Reciprocating Compressors

The reciprocating type is a positive displacement air compressor that is similar to IC engines. The only difference is the absence of combustion gases for the piston to work. These have a piston connected to the crankshaft reciprocating inside a cylinder. The suction and delivery valves are at the top of the compressor cylinder.

There are three major types of reciprocating air compressors:

• Single acting
• Double acting 
• Diaphragm type.

Single Acting Reciprocating Air Compressors

Let us refer to the image below as we learn more about the type. These are commonly known as single stage reciprocating air compressors.

Fig 4: Single Stage Reciprocating Air Compressor Schematic

In this type, one end of the piston is used for suction and compression. The first stroke draws the air inside the compressor chamber followed by the compression in the second stroke. The other end of the piston is free and is not open for work. As seen from the image above, the induction (suction) valve intakes the air at atmospheric pressure. The work is done by only one end of the piston, usually the upper part, in the compressor. Water jacket maintains the temperature well below the operating temperature of the air, which may increase according to Boyle’s law.

The other side of the piston indirectly helps the stroke by aiding the opening and closing of the crankcase. The piston works by the air pushing it down in one direction, and the spring action by the crankshaft brings the piston back to its normal position. This increases the pressure above the atmospheric pressure of air. This is supplied through the delivery valve. Thus, the work is done by only one piston in these types of air compressors for the supply of compressed air. 

• Advantages of Single Acting Reciprocating Compressors:
  • The compressor has a simple working
  • It is Easier to repair
• Disadvantages of Single Acting Reciprocating Compressors:
  • The system should run at full potential even for a small amount of work output.
• Application of Single Acting Reciprocating Compressors:
  • Commonly used in woodworking, food production in the bakery, and metalwork.
  • They are commonly known as gas air compressors.

Double Acting Reciprocating Air Compressors

A schematic diagram of a typical double acting reciprocating air compressor is shown below. These are usually categorized under multi-stage reciprocating air compressors, but let us restrict the study to double acting/stage compressors concerned with the AE/JE mechanical engineering exams.

Fig 5: Double Acting Reciprocating Air Compressor Schematic

This is a well-known type of positive displacement compressor. Here, unlike single acting, both sides of the piston are used to intake and compress the air. The piston moves downward and draws the air. It then moves upward by compressing the air and pushing it towards the delivery (exhaust) valve. Here, both sides of the piston are used in the compressor chamber for the total work of air compression. The volume of the air is reduced by the end of the process. Water jackets are provided to maintain the temperature of the air while the compressor is working.

The operation of the induction and delivery valve depends on the piston movement of the air compressor.

• Advantages of Double Acting Reciprocating Compressors:
  • Greater control over the compression process
  • Only the first stage could be used to produce energy
• Disadvantages of Double Acting Reciprocating Compressors:
  • Higher capital investment
  • Complicated repair process
• Applications of Double Acting Reciprocating Compressors:
  • It is well-known to be used in various airforce applications in the military

Diaphragm Reciprocating Air Compressors

The diaphragm type of air compressor is a classic of the positive displacement category. It has piston rings and a seal with backup as shown in the image below. In this type, a moving membrane (just like a human diaphragm) is driven by the rod and crankshaft to intake the air. 

Fig 6: Diaphragm Type Reciprocating Air Compressors Schematic

The membrane and the plate are in contact with the air, unlike the other types where the piston is in contact. Usually, the diaphragm or the membrane is manufactured to withstand the stress of the compressing action. As these are commonly used with toxic gases as operating gases, the diaphragm should be non-reactive. The oil space is used to maintain the system temperature while the perforated plates let the oil lubricate the piston-cylinder arrangement. 

This concludes the discussion on the reciprocating type.

• Advantages of Diaphragm Reciprocating Compressors:
  • No air leakage and contamination during the process
  • Low vibration
• Disadvantages of Double Acting Reciprocating Compressors:
  • Requires a large area to install.
• Applications of Double Acting Reciprocating Compressors:
  • Widely used for compressing toxic gases and rare gases such as hydrogen, nitrogen, etc.

Rotary Air Compressors

Unlike other types of compressors that use piston-cylinder movement, Rotary air compressors use the action of gears to compress air. These compressors involve the rotating element developing a liquid seal. This creates a suction at the inlet.

The air is displaced positively by the mechanical component in this type of compressor. There are three major types of this compressor: Lobe type or Roots Blower, Screw type, and Vane type.

Lobe Type or Roots Blower Rotary Air Compressors

A typical roots blower with two lobes is shown in the image below.

Fig 7: Roots Blower Rotary Air Compressor

There are two rotors in the compressor, and generally one of them is connected to the external drive that drives the other rotor. The shape, technically known as the lobe, of these rotors, follows a cycloid or involute profile. The mating between these two imparts accurate sealing to the delivery side. Low-pressure air is compressed by the action converting it to high-pressure air by the end of the process.

• Advantages of Double Acting Reciprocating Compressors:
  • Simple in design compared with other rotary air compressors.
  • Efficiency in operating at lower rpm.
• Disadvantages of Double Acting Reciprocating Compressors:
  • Extremely susceptible to the wear of internal components
• Applications of Double Acting Reciprocating Compressors:
  • They are commonly used in the pulp and paper industry.
  • It is also used in pharmaceutical industries.

Screw Type Rotary Air Compressors

The screw type rotary air compressor uses a rotary type positive displacement mechanism. The explanation is understood with the reference to the image below.

Fig 8: Screw Type Rotary Air Compressor

The compressor comprises closely meshing rotors of spiral shape to compress the air. In practical scenarios, while running the system dry, a pair of timing gears are used to ensure precise alignment. This also ensures resistance to rapid wear. The lubricating oil forms a connection in the space between the rotors in an oil-flooded rotary screw air compressor. This oil acts as a transfer medium of mechanical energy between the rotors to aid in the rotation of one powered by the other.

• Advantages of Double Acting Reciprocating Compressors:
  • Less noisy while under operation
  • Outlet air temperature would be comparatively low
• Disadvantages of Double Acting Reciprocating Compressors:
  • This compressor has to be used frequently to avoid rust
  • Needs excessive maintenance
• Applications of Double Acting Reciprocating Compressors:
  • Small air compressors or mini air compressors of this type are used as mobile units to fill the air.
  • They are also used in the food packaging industry.

Vane Type Rotary Air Compressors

The vane type compressor consists of a set of vanes mounted on a rotor as shown in the image below.

Fig 9: Vane Type Rotary Air Compressors

The vanes on the rotor inside the chamber are of variable length. They are in contact with the walls of the casing as the rotor rotates. The air from the inlet is drawn and compressed with this action. This completes our discussion on the first category of air compressors. Let us move ahead with the next major category.

• Advantages of Double Acting Reciprocating Compressors:
  • Easy to maintain
  •  Simple design
• Disadvantages of Double Acting Reciprocating Compressors:
  • Lubricant may contaminate the air
• Applications of Double Acting Reciprocating Compressors:
  • Used in the agricultural industry to supply compressed air to operate farm equipment.

Dynamic Displacement Air Compressors

A steadily flowing air is impacted by kinetic energy by the mechanical elements in a compressor of this type, which increases the pressure of the flowing air. There are two types: Centrifugal and Axial Compressors.

Centrifugal Air Compressors

The rotational energy is imparted to the air by centrifugal air compressors in the possess of compression.

Fig 10: Centrifugal Compressor

The rotational energy by the vanes is converted into a centrifugal force that pushes the air towards the discharge side. The airflow is normal to the axis of the rotating shaft in this type. The vanes are of two types: Diffuser and Impeller. Impeller vanes are curved which aids the flow of air. Diffuser vanes are fixed and are used to aid in the conversion of the rotational kinetic energy of air coming from impeller vanes to pressure energy. The components in figure 10 are the main components of a centrifugal air compressor. Centrifugal compressors are sometimes referred to as rotary compressors due to their construction.

• Advantages of Double Acting Reciprocating Compressors:
  • Reliable and less expensive
  •  Ability to handle high-pressure operations.
• Disadvantages of Double Acting Reciprocating Compressors:
  • Less flexible and stable at higher compressions
  •  Generates more vibrations.
• Applications of Double Acting Reciprocating Compressors:
  • Mainly used by gas turbines and refrigeration compressors

Axial Air Compressors

The axial air compressor, a widely known dynamic compressor, is a mechanical device employed to increase the pressure of the air continuously in the system. In comparison with the centrifugal compressor, the axial compressors are small in size and lightweight. 

Let us have an idea of these types of compressors using the image below.

The compressor has a drum on which the rotating blades are mounted. The fixed blades are to direct the flow and achieve high speeds. The air radially enters the impeller and is discharged axially from the compressor. In this system, the air is allowed to pass through in a parallel direction due to the rotating blades. When the axial flow compressors are combined with gas turbines, they can generate electricity.

• Advantages of Double Acting Reciprocating Compressors:
  • Higher flow rate and greater pressure ratio
  • Higher thrust and fuel efficiency
• Disadvantages of Double Acting Reciprocating Compressors:
  • Bulky construction
  • High starting power
• Applications of Double Acting Reciprocating Compressors:
  • These compressors are used in the construction of jet engines, high-speed ship engines, and small-scale power stations

Important Terminologies in Air Compressors

• Free Air Delivery (FAD): The volume of air in the compressor after the reduction of volume, considering the intake temperature and pressure, is called ‘Free air delivery or FAD’. It is expressed in \((m^3)/min\).
• Volumetric efficiency: The ratio of the actual volume of air drawn by the piston or membrane of the air compressor to the swept volume per cycle of the compression process is called the Volumetric efficiency. It can be reduced by the resistance offered by the suction valve and the increase in the temperature of the air. Mathematically, \(({\eta_{vol}}={\frac{Swept \ Volume}{Actual \ Volume}}={\frac{V_2}{V_1}})\)

Here, (V1) and (V2) are volume of air at induction valve and delivery valve respectively.

• Isothermal efficiency: In the case of isothermal compression in an air compressor, the ratio of work required by the compressor to the actual indicated work done is called isothermal efficiency. The indicated work is represented by the P-v diagram of the respective compressor. Mathematically, 

\(({\eta_{iso}}={\frac{Isothermal \ Work}{Actual \ Work}})\)

•  Isentropic efficiency: The ratio of isentropic work done by the air compressor to the actual work required by it is the isentropic efficiency. It is commonly used in the discussion of refrigerator compressors. Mathematically, \(({\eta_{ise}}={\frac{Isentropic \ Work}{Actual \ Work}})\)
•  Mechanical efficiency: The ratio of indicated compressor power to the actual power to be given to the shaft is called mechanical efficiency. The shaft power may see a loss due to friction. Mathematically, \(({\eta_{me}}={\frac{Indicated \ Power}{Actual \ Power}})\)
• Clearance ratio: The ratio of the clearance volume of the air compressor to the total swept volume, which is denoted by ‘K’ or ‘C’, is called the clearance ratio.

Mathematically, \((K \ or \ C = \frac{Clearance \ Volume}{Total \ Swept \ Volume})\).

•  Pressure ratio: The ratio of the discharge pressure of the compressor to its suction pressure is known as the pressure ratio. 

Difference between Pumps and Air Compressors

Pumps are devices that move liquids or fluids, while air compressors are designed to increase the pressure and density of air, supplying it for various applications like power tools or pneumatic systems.

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Solved Examples of Air Compressors

Now that we have studied most of the concepts on Air compressors, let us solve one problem to have an understanding of the whole concept.

Q1. Consider a reciprocating compressor of stroke 200 mm and a bore diameter of 150 mm. What will be the actual swept volume if the volumetric efficiency of the compressor is 85%?

Solution: Given, Stroke length, L = 200 mm = 0.2 m,  Diameter of Bore, d = 150 mm = 0.15 m.

\((\eta_{vol}) = 85%\)

Compression chamber volume, \((V_1 = \pi \times {d^2} \times {L}\) = 0.003534 \(m^3)\)

We have, \(({\eta_{vol}}={\frac{Swept \ Volume}{Actual \ Volume}}={\frac{V_2}{V_1}})\)

\((implies V_2 = V_1 \times \eta_{vol}\) = 0.02356 x 0.85 = 0.003004 \(m^3\) = 3004 \(cm^3)\).
There are many problems with various types of compressors. You can practice it through our AE/JE mechanical engineering mock tests following the teaching of our Super Teachers helping you crack AE/JE ME exams. 

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