Abu Dhabi Gas Liquefaction Company Ltd Job Training Mechanical Technician Course Module 9 Compressors

Section 4.1.2
) or knockout drums (see 
Section 6.2
) 
Figure 2.3: Parallel Compressors—Total Capacity 2Q 
Suction 
header 
Discharge 
header 
Compressor unit 1: 
capacity = QNm
3
/h 
Low 
pressure
Low 
pressure
Low 
pressure
Compressor unit 2: 
capacity = QNm
3
/h 
In-flow rate 
= 2QNm
3
/h 
Out-flow rate 
= 2QNm
3
/h 
Medium 
pressure
Medium 
pressure
Medium 
pressure

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 12 of 90
Series connection increases the pressure because all the gas is compressed by the first 
unit, then compressed again by the second. 
Two compressors connected in series: 
• give a greater pressure increase than a single compressor 
• give the same flow rate (capacity) as a single compressor 
Figure 2.4 shows two compressors in series.
Notice that the high pressure (HP) compressor unit is smaller than the low pressure 
(LP) unit. This is because the volume of the gas entering the second unit is smaller 
than that entering the first—it has been compressed. 
The compressor units shown in Figures 2.3 and 2.4 may be separate machines or they 
may be combined in the same casing. 
Compressor units connected in series, as in Figure 2.4, are called stages. Each 
compressor stage increases the gas pressure. 
When compressors are connected in parallel, it is possible to stop one of them and 
maintain flow through the others. This is useful if full capacity is not always wanted, 
if one compressor unit develops a fault or when routine maintenance is needed. 
When compressors are connected in series, all flow stops as soon as any one 
compressor stops. 
Low pressure 
compressor unit 
capacity = QNm
3
/h
High pressure 
compressor unit 
capacity = QNm
3
/h 
Low 
pressure
Medium 
pressure
High 
pressure
In-flow rate 
= QNm
3
/h 
Out-flow rate 
= QNm
3
/h