Fluid Mechanics and Machinery

Md. Mohiuddin

Lecturer

Department of Mechanical Engineering

ME 3219

Compressor

What is an Air compressor?

• A machine to compress the air and to raise its pressure.
• The air compression sucks air from the atmosphere, compresses it, and then delivers the same under high pressure to a storage vessel.
• From there, it may be conveyed by the pipeline to a place where the supply of compressed air is required.
• Compressor is driven by a prime mover.
• Compressed air is used for
1. Pneumatic drills
2. Paint spraying
3. Supercharging of internal combustion engine
4. Gas turbine plants
5. Jet engines
6. Air motos, etc.

Classification

• Consider that the piston is at the top of its stroke (Figure a). This position is called the top dead center position of the piston.
• In this position, the suction valve is held closed because of the pressure in the clearance space between the top of the piston and the cylinder head.
• The discharge valve is also held closed because of the cylinder head pressure acting on the top of it.
• When the piston moves downward as shown in Figure (b), the air left in the clearance space expands.
• Thus the volume of the cylinder increases and the pressure inside the cylinder decreases.

Single Stage Reciprocating Air Compressor

• When the pressure becomes slightly less than the suction or atmospheric pressure, the suction valve gets opened and the vapor refrigerant flows into the cylinder.
• This flow continues until the piston reaches the bottom of its stroke (bottom dead center)
• At the bottom of the stroke, as shown in Figure (c) the suction valve closes because of spring action.
• Now when the piston moves upward as shown in Figure (d), the volume of the cylinder decreases and pressure inside the cylinder increases.
• When the pressure inside the cylinder becomes greater than that on the top of the discharge valve, the discharge valve gets opened the vapor refrigerant is discharged and the cycle is repeated.

Single Stage Reciprocating Air Compressor

1. To provide a sufficient amount of compressed air the required cylinder size becomes too large.
2. Due to compression, there is a rise in the temperature of the sir. It is difficult to reject heat from the air in the short time available during compression.
3. Sometimes, the temperature of air, at the end of compression is too high. It may heat the cylinder head or burn the lubrication oil.

Drawbacks of Single Stage Reciprocating Compressor

Two-stage Reciprocating Air Compressor

• At first, the fresh air is sucked from the atmosphere in the low-pressure cylinder during its suction stroke at atmospheric pressure and temperature.
• The air, after compression in the low-pressure cylinder is delivered to the intercooler at the intermediate pressure.
• Due to compression of air in the low pressure compressor, there is an increase in air temperature.
• Therefore, the air is sent to the intercooler, where the air is cooled at constant pressure (the outlet pressure of low pressure compressor)

• After that, the air is sucked into the high pressure cylinder during its suction stroke.
• Finally, the air, after further compression in the high-pressure cylinder is discharged from the compressor to a storage or where it will be needed.

Roots Blower

• The mechanical energy is provided to one of the rotors from some external source.
• The other rotor (lobe) is driven by the first rotor.
• As the rotor rotates, the air at atmospheric pressure is trapped in the pockets formed between the lobes and casing.
• The rotary motion of the lobes delivers the entrapped air into the receiver.
• Thus more and more flow of air into the receiver increases its pressure.
• Finally, the air at a higher pressure is delivered from the receiver.

Single Stationary Blade type Rotary Compressor

Single Stationary Blade type Rotary Compressor

• The system consists of a fixed cylinder, a roller, and a shaft.
• The shaft is located with an eccentricity where the roller is mounted.
• A blade maintains constant contact with the roller through the spring, which serves as a sealing blade by consistently separating the suction and discharge ports.
• Figure (a) shows the ending of the intake stroke and the beginning of the compression stroke.
• During the roller's rotation, the air ahead of the roller undergoes compression, while fresh intake air is drawn into the cylinder, illustrated in Figure (b).
• As the roller progresses toward the midpoint, shown in Figure (c), additional air is drawn in, while compressed air is discharged.
• By the end of the compression stroke, shown in Figure (d), a significant portion of the compressed air exits through the discharge port.
• This cycle repeats, gradually elevating the pressure and temperature of the low-pressure and temperature air.

Vane Blower Compressor

• The compressor consists of a rotor, placed eccentrically, with radial slots, which is splined to the drive shaft and rotates inside a cam ring.
• Each slot contains a vane that can slide freely along the slot.
• Centrifugal force keeps the vanes in contact with the inner surface of the cam ring as the rotor turns.
• During the first half of the rotor's revolution, the volume between the rotor and cam ring increases. This expansion in volume causes a reduction in pressure, creating a suction effect. The suction process allows air to flow through the inlet port and fill the void created by the expanding volume.
• As the rotor continues to rotate through the second half of its revolution, the surface of the cam ring pushes the vanes back into their slots. The reduction in volume, caused by the vanes being pushed back, pushes the trapped air through the discharge port.
• The continuous rotation of the rotor creates a cyclical suction and discharge process, allowing the compressor to deliver a constant flow of fluid.

Screw Compressor

• A screw compressor works with a male and female rotor that rotate in the opposite direction.
• This movement draws in air that is compressed as the space between the rotors and their housing decreases.
• The compressed air is then displaced to the outlet.

Centrifugal Compressor

• A basic centrifugal blower compressor comprises a rotor (or impeller) with symmetrically fitted curved vanes.
• The rotor rotates within an airtight volute casing, containing inlet and outlet ports.
• The compressor's casing is designed to convert the kinetic energy of air into pressure energy before exiting the casing.
• Mechanical energy is supplied to the rotor from an external source.
• During rotation, the rotor draws air through its eye, elevates its pressure through centrifugal force, and directs the air over the diffuser.
• The air's pressure further increases while passing through the diffuser and casing, ultimately delivering high-pressure air to the receiver.
• the air enters the impeller radially and exits the vanes axially.
• Both the curved vanes and the diffuser are designed to allow the air to enter and exit their tips tangentially, without shock.
• To minimize frictional losses, the surfaces are made exceptionally smooth

Advantages of Centrifugal Compressors over Reciprocating Compressors

1. Since the centrifugal compressor has no valves, pistons, cylinders, connecting rods, etc., therefore the working life of these compressors is longer as compared to reciprocating compressors.
2. These compressors operate with little or no vibration as there are no unbalanced masses.
3. The operation of centrifugal compressors is quiet and calm.
4. The centrifugal compressors run at high speeds, therefore these can be directly connected to electric motors or steam turbines.
5. Because of the high speed, these compressors can handle large volumes of air compared to reciprocating compressors.
6. The efficiency of these compressors is considerably high.
7. Require less floor space.

Advantages of Centrifugal Compressors over Reciprocating Compressors

1. Since the centrifugal compressor has no valves, pistons, cylinders, connecting rods, etc., therefore the working life of these compressors is longer as compared to reciprocating compressors.
2. These compressors operate with little or no vibration as there are no unbalanced masses.
3. The operation of centrifugal compressors is quiet and calm.
4. The centrifugal compressors run at high speeds, therefore these can be directly connected to electric motors or steam turbines.
5. Because of the high speed, these compressors can handle large volumes of air compared to reciprocating compressors.
6. The efficiency of these compressors is considerably high.
7. Require less floor space.

Difference Between Compressor and Blower

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