# Equipment Data Sheet

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OPERATING MANUAL - VOLUME A - SECTION A- 03 THEORETICAL ELEMENTS – THE
COMPRESSORS

SCOPE OF APPLICATION                                                                                                                                  2
INTRODUCTION                                                                                                                                          2
COMPRESSION PARAMETERS                                                                                                                                3
    COMPRESSION RATIO ................................................................................................................................. 3
    MULTISTAGE COMPRESSION ..................................................................................................................... 3
    ENERGY OF COMPRESSION ....................................................................................................................... 4
THE VOLUMETRIC COMPRESSORS                                                                                                                            5
DYNAMIC COMPRESSORS                                                                                                                                   8
COMPRESSORS RANGE OF USE                                                                                                                            10

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SCOPE OF APPLICATION

This document provides the general principles about compressor’s operation.

INTRODUCTION

The role of compressor in the ASU process is to provide gas flow under a certain pressure.
There are two great families of compressors:
   Volumetric compressors:
Pressure is obtained by admitting successive volumes of gas in a closed room and driving back them using a
variable component.
   Dynamic compressors:
Kinetic energy is convert in potential energy. The speed of air is transform to provide pressure.

Fig 1

COMPRESSORS

VOLUMETRIC                                                 CONTINUOUS FLOW

RECIPROCATING                            ROTARY                               DYNAMIC                  EJECTOR

WITH ROTATE              WITH LIQUID       PERIFERIC
PISTON                   RING                                  WITH
SCREW

CENTRIFUGAL              AXIAL

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COMPRESSION PARAMETERS

P1 T1                                             P2 > P1
T2 > T1
V1                                              V2 < V1

   COMPRESSION RATIO

Absolute pressure of discharge
Compression ratio                                       3

Note: In case of gas heat is too significant, it would cause ignition of oil and deterioration of machine elements.

   MULTISTAGE COMPRESSION

The multiplication of compression stages, increase the compression ration.

1                            2                           3                      4

R1                           R2                   R3                      R4
P4,T

P0, T                            P1, T                        P2, T                P3, T

Note:

   The compression ratio realized in each cylinder is only a fraction of the total compression ratio. Thus,
temperature rises reached by gas are low, which lead to lubrication and a better mechanical operation. The
intercoolers extract heat produced at each stage, which would not be possible by the only use of a water cylinder.

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   Multistage compression, by lowering temperature of gas between stages, makes possible to approach
isothermal compression: it results a significant power saving.

For compressor with piston:
   Effort on the stem of piston and connecting rod assembly depend on the difference of pressures acting on the
two faces of the piston, which is weak in case of multistage compression.

   The compression ratios are "weak". So, the primary cylinder and consequently the followings received a real flow
more significant than if the compression ratio was total.

   ENERGY OF COMPRESSION

Power necessary for a stage of compression is expressed as follows:
K                   P2
P [kW] =                    x Q x T x Log
efficiency               P1

K = constant                                                 T = Admission temperature of the stage
Q = gas output aspired (Nm3/H)                               P1 = pressure (abs) of admission of the stage
P2 = pressure (abs) of admission of the stage

A simplified formula can be remembered:
P = 0.11xQxlog(P2/P1) : This formula is valid in the following conditions :
    Suction Temperature : 25 °C
    Relative humidity : 75 %
    Ti = 25 °C
    Isothermal efficiency : 66 %

Power consumption increases with gas flow and compression ratio.
However, this power depends on admission temperature for the considered stage: which highlights the importance of
intercoolers efficiency.

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THE VOLUMETRIC COMPRESSORS
Their principle is the reduction of volume to involve increase in pressure.

volumetric compressors

ROTARY             RECIPROCATING

   Rotary compressor with root

Operation:
Two thrusters of lobed form turn in a way synchronized inside a cylinder thanks to a set of
gears.
The gas is not compressed by the machine, but by action of gas present in the collector of
repression.
Used as primary vacuum pump, it provides a vacuum interesting for significant volumes not-
lubricated

Characteristics:
   2 or 3 stages on compression can be assembled
   Flow can go up to 60000 m3/h:
   Air is not contaminated by oil.
   Supports important variations of its compression ratio without affecting its efficiency

   Compressor with screw

Operation:

They are composed of two rotors comprising of the male and female helicoid grooves which gear one on the other, in
a cylindrical casing.

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1- Aspiration of a quantity of air, defined by volume between the screws
2- Reduction of air in this volume
3- Repression of this air.

Characteristics:

3
   For oil lubrication: flow can go up to 10000 m /h
   For water or dry air lubrication : flow can go up to
3
30000 m /h.
   Several stages can be assembled

   Rotary compressor with pallets

Operation:
Longitudinal fins slide radially in a rotor whose axis is eccentric from the cylinder.

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(Fig1) A radial paddle wheel is motionless in a partially filled up cylindrical body of liquid.
(Fig2) At a sufficient speed, due to the centrifugal force liquid forms a concentric ring with the pump housing.
(Fig3) The volume of the cells delimited by the blades and the liquid ring do not vary.
However, if the wheel is offset compared to the pump housing, this volume varies:
-   It grows regularly and it is in depression in the straight lines part of the figure.
-   It decrease regularly and it is in pressure in the left part.
(Fig4) Side faces of the pump housing being provided with openings in the shape of crescent the volume of gas is
aspired thanks to the increase in the cells, and is driven back reciprocally thanks to their reduction.

Characteristics:
   Flow can go up to 6000 m3/h

   Alternate compressor with piston

Operation:

A piston carrying out a back and forth pass inside a cylinder.

Admission             In phase of admission, the induction valve opens and the piston moves, enlarging
the volume of compression room.

Compression

In phase of compression, the valves are closed and the piston moves
reducing the volume of compression room.

In phase of exhaust, the exhaust valve opens, the piston completes to move,
reducing the volume of compression room.

Exhaust
Relaxation

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In phase of relaxation of died volume, the valves are closed and the piston moves increasing the volume of room
compression

DYNAMIC COMPRESSORS

By opposite to volumetric compressors, dynamic compressors transform an increase speed of a gas (kinetic energy)
in increasing pressure.

Axial Compressors

Dynamic compressors                         Centrifugal Compressors

Thereafter we will develop only axial compressors and centrifugal ones, the axial-radial compressors consisting of
assembly in series of several axial stages centrifugal wheels.

Note :
when high pressure is required for a very high flow rate ( > 250 000 Nm3/h ), axial and centrifugal technologies are
combined:
   First stages are axial to accept the total flow rate
   Air is admitted in centrifugal stages, when the flow has been reduced enough to be compressed.

   Axial compressor

aubage                 aubage
s                      s
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Operation
In axial compressor, gas circulates in a way parallel with the rotor axis. Can treat significant flows.
The axial compressors consist of a cylindrical or slightly conical rotor on which is assembled a certain number of
lines bladings decreasing height as one advances in compression. The rotor lines of bladings turn between the stator
lines of bladings which rectify gas flow tangentially.

   Dynamic centrifugal compressor

Operation
The centrifugal compressors are made up with a shaft on
which centrifugal wheels are assembled.
-   After its passage in a wheel, gas pass in a diffuser.
-   An element of the stator transforms its speed in
pressure.
-   Gas can enter the following wheels

Fluid
Characteristics:                                                                                           Fluid
   They are machines offering the highest compression
ratios. It is possible to obtain discharge pressures about
50bar in the case of oxygen or inert gases and 100bar for air.
   The volume throughputs treated are significant.
   The isothermal efficiency obtained by these compressors is very satisfactory.

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COMPRESSORS RANGE OF USE

1000
Pressure (bar g)

Reciprocating compressors
100

Centrifugal &
10
Screw compressor                               axial compressor
1
0
1
Liquid ring                    Roots compressor
1            compressor

0

0
-0,8
-
3
0.                            100      500       1000           10000   100000        Flow m /h
8

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