Parul Institute of Technology Subject Code 151903 Name

Parul Institute of Technology Subject Code : 151903 Name Of Subject : Fluid Power Engineering Name of Unit : Compressor Name of Topic : Types of compressor

Compressors

Presentation Outline Introduction Types Compressor System References Sub: Flow through pipes Topic: Types of compressor

Introduction • Compressors used to increase the pressure of a gas (compressible fluid) • Examples – Increase the pressure for instrument air systems (to get control valves to operate), transport gases such as hydrogen, nitrogen, fuel gas, etc. in a chemical plant

Types of Compressors • Positive Displacement (PD) : Operate by trapping a specific volume of gas and forcing it into a smaller volume – 2 Basic Designs for PD Compressors • Rotary • Reciprocating • Centrifugal : Operate by accelerating the gas and converting the energy to pressure – 2 Basic Designs for Centrifugal Compressors • Centrifugal • Axial

Positive Displacement Compressors: Rotary Design • Rotary compressors (get their name from the rotating motion of the transfer element) compress gases with lobes, screws, and vanes into smaller volumes. • 4 Primary Types of Rotary Compressors: – Rotary Screw – Sliding Vane – Lobe – Liquid Ring

Rotary Screw Compressors • Commonly used in industry. • It operates with 2 helical rotors that rotate toward each other, causing the teeth to mesh. • As the left rotor turns clockwise, the right rotor rotates counterclockwise. This forces the gases to become trapped in the central cavity. • The 2 rotors are attached to a drive shaft and drive that provide energy to operate the compressor. • Have an inlet suction line and outlet discharge port.

Rotary Screw Compressor

Sliding Vane Compressors • Uses a slightly off-center rotor with sliding vanes to compress gas. • Inlet gas flows into the vanes when they are fully extended and form the largest pocket. As the vanes turn toward the discharge port, the gases are compressed. • As the volume decreases, the pressure increases until maximum compression is achieved. Then the gas is discharged out the compressor.

Sliding Vane Compressor

Lobe Compressors • Characterized by 2 kidney-bean shaped impellers used to trap and transfer gases. • The 2 impellers move in opposite directions on parallel mounted shafts as the lobes sweep across the suction port. • Compressed gases are released into the discharge line. • The lobes do not touch each other. A few thousands of an inch clearing exists between the casing and lobes.

Lobe Compressors • Designed to have constant volume discharge pressures and constant speed drivers. • Lobe Compressors can be used as compressors or vacuum pumps.

Lobe Compressor

Liquid Ring Compressors • It has one moving transfer element and a casing that is filled with water or seal liquid. • As the rotor turns, the fluid is centrifugally forced to the outer wall of the elliptical casing. An air pocket is formed in the center of the casing. • As the liquid ring compressor rotates, a small % of the liquid escapes out the discharge port. Make up water or seal liquid is added to the compressor during operation. The liquid helps cool the compressed gases.

Liquid Ring Compressors • Used to compress hazardous and toxic gases as well as hot gases.

Positive Displacement Reciprocating Compressors • Most common type of compressors. • Work by trapping and compressing specific volumes of gas between a piston and a cylinder wall. • The back and forth motion incorporated by a reciprocating compressor pulls gas in on the suction (or intake) stroke and discharges it on the other. • Spring-loaded suction and discharge valves open/close automatically as the piston moves up and down in the cylinder chamber.

Positive Displacement Reciprocating Compressors • Basic Parts of are: – Piston – Connecting Rod – Crankshaft – Diver – Piston Rings – Suction Line – Discharge Line – Spring -Loaded Suction and Discharge Valves

Positive Displacement: Reciprocating Compressors • Can have 1 to 4 cylinders. One shown only has one cylinder.

Multistage Compressors • Discharge from Stage 1 is suction for Stage 2

Centrifugal Compressors • Centrifugal compressors accelerates the velocity of the gases (increases kinetic energy) which is then converted into pressure as the gas flow leaves the volute and enters the discharge pipe. • Usually operate at speeds > 3, 000 rpm. • Deliver much higher flow rates than positive displacement compressors.

Centrifugal Compressors • 2 Types of Centrifugal Compressors – Single- Stage : Compress the gas once • Use for high gas flow rates, low discharge pressures – Multi- Stage : Take the discharge of one stage and pass it to the suction of another stage • Use for high gas flow rates, high discharge pressures

Centrifugal Compressors • Basic Components – Impellers, Vanes, Volutes, Suction Eyes, Discharge lines, Diffuser Plates, Seals, Shaft, Casing – Suction Vane Tips = Part of the impeller vane that comes into contact with gas first. – Discharge Vane Tips = Part of the impeller vane that comes into contact with gas last

Centrifugal Compressor

Centrifugal Compressor: Axial Design • Composed of a rotor that has rows of fanlike blades. • In industry, axial compressors are used alot high flows and pressures are needed. • Gas flow is moves along the shaft. • Rotating blades attached to a shaft push gases over stationary blades called stators. • Stator blades are attached to the casing.

Centrifugal Compressor: Axial Design • As the gas velocity is increased by the rotating blades, the stator blades slow it down. As the gas slows, kinetic energy is converted into pressure. • Gas velocity increases as it moves from stage to stage until it reaches the discharge. • Multi-Stage axial compressors can generate very high flow rates and discharge pressures. • Axial compressors are usually limited to 16 stages (due to temperature/material limitations) • Pound for pound, axial compressors are lighter, more efficient, and smaller than centrifugal compressors.

Axial Compressor

Typical Compressor System Other equipment needed in a process system.

Typical Compressor System • Safety valves and pressure relief valves used to remove excess pressure that could damage equipment and people. • Silencers are mounted on the inlet and outlet of a compressor to ‘reduce’ the noise. Compressors are very noisy. Exxon had one for a refinery light ends stream nicknamed “Old Snort” by the technicians.

Typical Compressor System • Demister removes moisture (liquid) from the gas stream. The liquid falls to the bottom of the demister and is removed. The clean gases goes out the top of the demister. • Dryer sometimes used on the compressor discharge line to remove any liquids (moisture). Silica gel and molecular sieves (3 A mole sieve) often used.

Typical Compressor Start Up Procedures • Check valve line up on the compressor and associated equipment. • Check compressor oil levels and bearing cooling water systems. • Be sure all the compressor controls are set correctly. • Turn on the compressor. • Monitor equipment and process until conditions ‘steady’ out.

Reference • “ The Process Technology Handbook”, by Charles E. Thomas, UHAI Publishing, Berne, NY, 1997.