Compressors V1

7/29/2019 Compressors V1

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A Guide To Compressors

Compressor Basics


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A Guide To Compressed AirCompressor Basics

Page 220 August 2013

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Types of Compressor

DisplacementThese type of compressor seal a volume of a gas

and then reduce the enclosed volume.

Diaphragm Compressors

This is the most common type of compressor and

come in single acting, double acting, oil lubricated or

oil free variants.

Piston Compressors

Diaphragm compressors have mechanically or

hydraulically acting diaphragms. Mechanical

diaphragms are better at lower pressures and small

flows while hydraulic diaphragms are commonly used

at higher pressures.

Twin Screw Compressors

Twin screw compressor can be used in varying

pressure conditions. They have male and female

rotors that rotate in opposite directions while the

volume between them decreases between them and

the housing.

Oil-free Screw Compressors

These are twin screw compressors that do not have

any cooling liquid inside the compression area. The

rotors do not come into contact with each other or the

compressor housing, removing the need for cooling.

Liquid-injected Screw Compressors

These are basically oil-free screw compressors with

a cooling liquid being used in the compression area

and in the bearings. The purpose of liquid is to cool,

lubricate and prevent leakage. Oil is commonly used

for it good lubricating and sealing properties.

Tooth Compressors

Tooth compressor operate with 2 rotors rotating inopposite direction. They draw air in the inlet and then

compress the air by reducing the volume of the

chamber it is trapped in. During this time the inlet is

sealed by the rotors, and after compression the outlet

is revealed by the rotors. Temperature difference

between the inlet and outlet is the limiting factor with

this compressor.


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Types of Compressor

DisplacementScroll Compressors

These are usually oil free and produce little sound

and vibration. The compression is done by eccentric

rotating spirals that form air pockets with reducing

volumes.

Vane Compressors

Vane compressor have to eccentric rotors that trap

air in a volume and as they rotate will reduce the

volume of the trapped air before letting it out of the

outlet. They are usually oil lubricated.

Root BlowersThis is a valve-less compressor without any internal

compression. Air comes in to the intake and then into

the compression chamber, to be released to outletand back into the housing. These are very noisy and

not energy efficient and are only really used as

vacuum pumps and pneumatic conveyance.

Root Blower Compressor

Rotor set of double tooth compressor


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Types of Compressor

DynamicThese are sometimes called turbo compressors andcome in axial and radial designs. Dynamic

compressors work for a constant pressure at a

constant f low.

Centrifugal Compressors

These can be identified has the resulting flow of

compressed air has a radial flow to it. They operateby taking air into the top end of an impeller and then

forcing it using centrifugal force, into the lower

volume bottom end of the impeller.

They usually operate in multiple phases with a

maximum compression ratio of 3. RPM of 15,000-

100,000 are not uncommon so oil film bearings are

preferred to roller bearings. In oil free variants active

magnetic bearings can be utilised.

Every centrifugal compressor has to be sealed toreduce leakage around the shaft, at the point it

enters the compressor housing. Labyrinth seals, ring

seals and controlled gap seals are the most

commonly used.

Axial Compressors

Axial compressors have axial flow, in which the

gases go along the shaft through rotating and

stationary blades. The rotating blades increase the

velocity of the gas and the stationary blades are used

to convert the kinetic energy into pressure.

Axial compressors are usually smaller and lighter

than centrifugal compressor, and can therefore

operate at higher speeds. They work best at constant

high volume flow rates and a moderate pressure.

Modern Centrifugal Compressor


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Types of Compressor

OtherVacuum Pumps

As the name suggests, these are used to compress

vacuums back to atmospheric pressure and higher.

Typically vacuum pumps will have a very high

compression ratio and this means that multi-stage

compression is common around the pressure range

of 0.1 bar(a) to 1 bar(a).

Booster Compressors

These compressors also work at a high compression

ratio and are normally utilised when a large pressure

loss has occurred due to a long pipeline or there is a

sub-process that requires a high pressure.

The compressors can be single stage or multi-stage,

displacement or dynamic. Their power requirements

increase with pressure ratio, and as the pressureratio goes up the mass flow rate drops. The

compression curves for booster compressors tend to

be the same as the curves for vacuum pumps.

Pressure Intensifiers

This is a type of booster compressor and is driven by

the compressed air medium itself, which is commonly

called the propellant. They are used in very

specialised applications and are capable of

converting 7 bar to 200 bar in a single stage or 1700

bar in multi stage. The pressure intensifiers are usedwhere there is a very small volume flow rate. The

pressure intensifier is more likely to be used with

inert gases as with gases such as air, self-ignition is

a hazard.


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Drying of Compressed Air

Drying Compressed AirAll air has some water content. Hotter air will havemore water content than colder air. It is important

considered the water in the air during compression

as compression will increase the amount of water in

the air, which can cause problems later on in pipe

work and equipment. Therefore methods of drying

are commonly used after compression. The Pressure

Dew Point (PDP) is the temperature which the water

in compressed air condensates back to liquid.

The dew point at atmospheric pressure cannot be

compared directly to the dew point of compressed

air. Using a filter to remove moisture does not work

as further cooling of the compressed air will lead to

further condensation of water. The rule normally is

the lower the dew point of the air the greater the cost

of the drying equipment.

Process of Drying


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A Guide To Compressed AirCompressor Basics Page 820 August 2013


After-cooler

This is basically a heat exchanger that will cool down

the compressed air and condensate the water

content in the air. They can be found to be utilising

water or air as a coolant and usually come with a

separator supplied and have automatic drainage.

Usually between 80-90% of the water content is

removed. After-coolers are now normally sold

inclusive with modern compressors.

Over-Compression

One of the easiest drying methods available. Air is

compressed to above working pressure, increasing

the amount of water vapour in the air. The air is

cooled and water is then separated. The air will then

be allowed to get to working pressure with a lower

PDP. Due to high energy consumption, this method is

only suitable for low flow rates.

Absorption Drying

This is a chemical process that is rarely used. An

absorbent such as sodium chloride or sulphuric acid

is used, meaning corrosion must be taken into

account.

Relationship between dew point and pressure dew point


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Membrane Dryers

In a membrane dryer a cylinder contains thousands

of hollow polymer fibres with an inner coating.

Filtered, wet, compressed air enters the cylinder and

the membrane coating allows water vapour to

permeate through the membrane wall and collect

between the fibres.

The air continues along the cylinder and has minimal

pressure loss. Another benefit of membrane drying is

that minimal maintenance is required and they havea low power requirement.

Refrigerant Dryer

Refrigerant drying is a process that cools the air to

remove large amounts of water. As the name

suggests refrigerants are used in this process, and

after cooling and condensing the compressed air is

reheated to room temperature to preventcondensation on the outside of the pipe.

They operate between dew points of 2 and 10oC.

Legislation on the permitted refrigerants to be used in

this process states that only refrigerants with a low

Global Warming Potential (GWP) can be used.

Typical parameter changes


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Filters

The particles in the air stream that are smaller than

1mm can be removed using filters that will utilise 3

different mechanisms. These are impaction,

interception and diffusion.

Impaction occurs when large particles at high gas

velocities do not follow the streamline around the

fibres and instead impact with a fibre. This

mechanism occurs with relatively larger particles and

is more important the larger the particle becomes.

Interception is when a particle does follow the

streamline, but the radius of the particle is large the

distance between the fibres.

Particle diffusion occurs when a very small particle

does not follow streamlines, because of Brownian

motion and moves randomly across the flow.This mechanism is more and more important when

the particle sizes are smaller.

The particle separating efficiency of a filter is always

a compromise. Particle filters will have different

efficiencies at different particle sizes.

All types of filtering result in a pressure drop and the

finer the mesh of the filter the greater the pressure

drop across the filter. Furthermore the finer the filter

the often it will become clogged and need to bereplaced.


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Control and Regulation

Pressure Relief

Applications will normally require a constant pressure

to be supplied into the system. This means that the

air going into the system needs to be regulated. Thiscan be done by various different means. The method

used will depend on the type of compressor,

acceptable pressure variation and acceptable energy

consumption.

In a typical system energy consumption can

represent 80% of the life cycle cost of a compressor,

so the regulation system choice if done correctly can

make a big difference. In an ideal world the

compressors full capacity would match the requiredin the system.

This is the most well known method for pressure

regulation, and exist in spring loaded and serve valve

variants. The flow has to work against the spring or

servo resulting in a lot of work having to be done by

the compressor, meaning higher energy costs.

Bypass

This is the same as pressure relief but the excess

gas is not released into the atmosphere but taken

back to the compressor inlet.

Throttling the InletThis works by reducing the flow rate through the

inlet. The reduced flow rate is achieved by increasing

the pressure ratio across the compressor. The

method is limited to a small range and creates a high

energy requirement.

Pressure Relief with Throttled InletThis method is the most commonly used as it has a

maximum regulation range with low energy

consumption. The inlet valve is kept closed, with asmall opening used as a blow off valve that releases

discharge air from the compressor. The compressor

therefore has a vacuum in the inlet and low counter

pressure. This system requires an air receiver.

Start/stop

Compressors below 10kW are sometimes controlled

by turning the compressor off when the upper level of

required pressure is on and turning the compressorback on when the lower level is reached.

This method requires that there is a large difference

between the upper and lower levels. The method is

energy efficient if the number of starts is kept low.


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Control and Regulation

Speed Regulation

Speed regulation involves controlling the speed ofthe motor that is connected to the compressor. This

can control the flow rate.

It is a good efficient method for controlling outgoing

pressure and lowering energy consumption.

Variable Discharge Port

This method is related to screw compressors and

controls the flow rate by moving the outlet along thescrew. This method is not common and can have

high power consumptions.

Suction Valve Unloading

Piston compressors are sometimes regulated by

mechanically forcing the inlet valves open. This

means air is pump in and out of the inlet valve.

Energy loss is minimal, often less than 10%.

Load-Unload-Stop

This method is used for compressors with a capacity

of greater than 5kW. It is a combination of the

stop/start method and other off loading system.


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Dimensioning Installations

Compressor Unit


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Air Treatment


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Cooling and Energy Recovery


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Compressor Room and Air Distribution


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Electrical


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Sound


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A Guide To Compressed Air

Compressor Basics

Page 19

20 August 2013

Maintenance and Economy

General Cost


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A Guide To Compressed Air

Compressor Basics

Page 20

20 August 2013

Maintenance and Economy

Possible Savings

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