ISO 4376:2024

International
Standard
ISO 4376
First edition
Cycle energy requirement —
2024-11
Test method
Exigence d'énergie de cycle — Essais de réception
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols, abbreviations and subscripts . . 3
4.1 Symbols and abbreviations .3
4.2 Subscripts . .3
5 Measuring equipment, methods and accuracy . 3
5.1 General .3
5.2 Measurement of pressure and temperature .4
5.3 Measurement of compressor package actual flow rate.4
5.4 Measurement of compressor package power input .4
5.5 Measurement logging frequency .4
5.6 Throttle valve .4
5.7 Non-return valve .4
6 Test procedure . 4
6.1 CER test conditions and limitations .5
6.2 Cycle energy requirement test method .6
6.2.1 (Optional) Full load actual volume flow and package power input .6
6.2.2 Minimum actual volume flow and package power input .6
6.2.3 Idle power .6
6.2.4 Logging data .6
6.2.5 Cycling modes and determination of cycle.6
6.3 Calculation of cycle energy requirement .8
6.4 Test report .9
Annex A (informative) Cycle energy requirement — Simplified estimation.10
Annex B (informative) Cycle energy requirement — Compressor control strategies .11
Bibliography .23

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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This document was prepared by Technical Committee ISO/TC 118, Compressors and pneumatic tools,
machines and equipment, Subcommittee SC 6, Air compressors and compressed air systems.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
This document was developed to provide guidance to determine the cycle energy requirement for all types
of compressors. The types of compressors included in the consideration include dynamic and positive
displacement designs. Applicable pressures include low pressure (e.g. blowers), typical air network
pressures in industry and higher pressures needed for special application. Applications covered include but
are not limited to standard industrial air production and gas compression for industrial and other purposes.
Compressors are used in almost all types of industries and in processes such as energy production and water
treatment. In most industrial facilities, small and large, compressed air is an expected utility. In industry
and elsewhere many processes demand gases to be compressed to certain pressures.
Compressing a gas is energy intensive and growing attention to the environmental impact has encouraged
manufacturers of compressors to continuously raise the energy efficiency of its products.
The need for compressed gas usually varies with time. While some types of compressor can adapt to changes
of demand by delivering variable amounts of gas all compressors will at some point change from gas delivery
to no delivery and back. Such a no delivery mode, called idle mode, usually means the compressor is left
running being ready to resume delivery on short notice while still consuming energy.
Until now performance data is typically given and evaluated for a steady state design point. It is also
customary to provide data for the idle mode when the delivered amount of gas is zero. What is not provided at
present time is the energy consumed in switching from idle mode to delivery and vice versa. Taken together
the energy required for these two events combined can be referred to as the cycle energy requirement (CER).

v
International Standard ISO 4376:2024(en)
Cycle energy requirement — Test method
1 Scope
This document applies to electrically driven positive displacement and dynamic compressors.
This document defines and describes the test method to evaluate the cycle energy requirement.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 5167-1, Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section
conduits running full — Part 1: General principles and requirements
ISO 9300, Measurement of gas flow by means of critical flow nozzles
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
standard inlet point
location at which gas enters the compressor package
3.2
standard discharge point
discharge point for a packaged compressor is the terminal outlet
3.3
inlet pressure
absolute pressure of the gas at the standard inlet point (3.1)
3.4
inlet temperature
total temperature at the standard inlet point (3.1) of the compressor
3.5
compressor package
compressor unit with prime mover, transmission, fully piped and wired and generally includes all ancillary
items necessary for effective operation
3.6
compressor package power input
sum of the electrical power inputs to the prime mover and all other ancillary and auxiliary items included in
the compressor package (3.5)
3.7
compressor package actual volume flow rate
actual volume flow rate of gas, compressed and delivered at the standard discharge point (3.2), referred to
conditions of total temperature, total pressure and composition prevailing at the standard inlet point (3.1)
Note 1 to entry: Composition can refer to humidity, for instance.
3.8
thermal steady state
state in which the variation in the difference between inlet and outlet temperatures is within 1 K for a period
of three minutes or more
3.9
idle power consumption
stable steady-state power consumption of the compressor at zero volume flow rate or at pressure ratio of
one, the compressor shall always be able to reach this state independent of the number of load-idle cycles
Note 1 to entry: The idle power consumption can be zero.
3.10
rated discharge pressure
total pressure at the standard discharge point (3.2) where performance is measured.
3.11
offload discharge pressure
total pressure at the standard discharge point (3.2) when the compressor package (3.5) transitions to idle state
Note 1 to entry: The offload discharge pressure shall be between 100 % and 110 % of the rated discharge pressure.
3.12
minimum actual volume flow rate
lowest actual volume flow rate at which the compressor can run stable at the specified offload discharge
pressure (3.10)
EXAMPLE For a centrifugal this can correspond to the surge anticipation limit at the offload discharge pressure.
For a variable speed screw compressor this can correspond to the lowest speed point at the offload discharge pressure
3.13
measured power
power measured by a power meter
...