Compressors

CM4120Julie King

Presentation Outline

IntroductionTypesCompressor SystemReferences

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

2 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.

Mulitstage 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 (3A 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.