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
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
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
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
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 at a specific time under specific conditions
3.14
ideal load-idle cycle
principle of operation where the transition from load to idle and idle to load is instantaneous at the offload
discharge pressure (3.10) and minimum flow rate without consuming any additional energy
3.15
cycle energy requirement
CER
additional energy compared to what would be consumed during the ideal load-idle cycle (3.14) during
switching of states at offload discharge pressure (3.10) and minimum flow rate

4 Symbols, abbreviations and subscripts
4.1 Symbols and abbreviations
Symbol/ abbrevi- Term SI unit Other practical units
ation
CER cycle energy requirement J MJ, kJ
p pressure Pa MPa, bar, mbar
P power W MW, kW
T temperature K -°C
V volume m l
3 3 3
q volume flow rate m /s l/s, m /min, m /h
V
f frequency Hz
t time s min
4.2 Subscripts
Subscript Term Remark
i Inlet Inlet condition / ambient
d Discharge
1 compressor switches to the idle state
2 zero-flow and steady state achieved
3 compressor switches to the load state
4 compressor is delivering flow downstream
of non-return valve
NRV Non-return valve
UNRV Upstream of Non-Return Valve
DNRV Downstream of Non-Return Valve
V Volume
L Load Machine running in load
min Minimum
L1 Rated
L2 Offload Maximum allowable working pressure
max Maximum
MEAS Measured
IDLE Idle Machine off-load, no compressed air delivered
to customer
EST Estimation
5 Measuring equipment, methods and accuracy
5.1 General
The equipment and methods given in this document are not intended to restrict the use of other equipment
and methods with the same or better accuracy.
All inspection, measuring, test equipment and devices that can affect the test shall be calibrated and adjusted
at prescribed intervals, or prior to use, against certified equipment having a known valid relationship to
nationally recognized standards.

5.2 Measurement of pressure and temperature
Pressure measurement shall have an accuracy of ±1 % at the measured value.
Temperature measurement shall have an accuracy of ±1 K.
The following characteristics shall be measured:
— package inlet pressure (at standard inlet point);
— package inlet temperature (at standard inlet point);
— package discharge pressure (at standard discharge point);
— pressure upstream of the non-return valve;
— pressure downstream of the non-return valve;
If the compressor package does not have an internal non-return valve, one can be installed downstream of
the compressor package with the pressure measurement on both sides to allow for non-invasive measuring
of cycle energy requirement.
5.3 Measurement of compressor package actual flow rate
The actual delivered flow rate of the compressor shall be measured by performing a test as indicated in both
ISO 5167-1 and ISO 9300.
5.4 Measurement of compressor package power input
The compressor package power input measurement shall have an accuracy of ±1 % at the measured value.
5.5 Measurement logging frequency
The logging frequency of the pressure and power measurements shall be at least 10 Hz.
5.6 Throttle valve
An adjustable throttle valve is required downstream of the compressor package.
5.7 Non-return valve
If a non-return valve (check valve) is not part of the compressor package, one shall be installed downstream
for testing purposes.
6 Test procedure
Cycle energy requirement (CER) is the additional energy compared to what would be consumed during the
ideal load-idle cycle during switching of states at offload discharge pressure and minimum flow rate, e.g.,
pressurizing the internals first when switching to load until the non-return valve is opened, e.g., venting the
internals when switching to idle until an idle state is reached.
Figure 1 shows an example of the Cycle Energy Requirement compared to an ideal load-idle cycle where at
time t the compressor switches to idle and at time t the product finally delivers air back to the customer at
1 4
the requested pressure.
A full compressor package cycle includes a venting phase (t to t ), a steady state zero flow phase (t to t )
1 2 2 3
and a loading phase (t to t )
3 4
Key
t time (s)
t compressor switches to the idle state
t zero-flow and steady state achieved
t compressor switches to the load state
t compressor is delivering flow downstream of non-return valve
P power (kW)
CER cycle energy requirement (kJ)
typical load-idle cycle
idealized load-idle cycle
cycle energy requirement
Figure 1 — Example cycle energy requirement
6.1 CER test conditions and limitations
The CER test conditions shall be as close as is reasonably possible to the conditions specified in Table 1 and
not exceed the deviations given in Table 2.
Table 1 — Specified CER test conditions
Quantity Unit Value
Package inlet pressure bara 1
Package inlet temperature °C 20

Table 2 — CER test limitations
Quantity Maximum deviations from speci- Maximum permissible fluctuation
fied values from average during any set of read-
ings
Package inlet pressure ±10 % ±1 %
Package inlet temperature ±10 K ±2 K
6.2 Cycle energy requirement test method
The compressor package
...