ISO 18501:2025

International
Standard
ISO 18501
First edition
Performance rating of positive
2025-03
displacement refrigerant
compressor
Détermination des performances des compresseurs frigorifiques à
déplacement positif
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Rating requirements . 4
4.1 General requirements .4
4.2 Rating conditions .4
5 Published rating . 5
5.1 General requirements .5
5.2 Individual published rating .6
5.3 Number of individual published ratings requirement .6
5.4 Polynomial equations .6
5.5 Published data.7
5.6 Application envelope .8
6 Tolerance . . 8
7 Marking provisions .10
Annex A (informative) Method to handle zeotropic mixtures .11
Annex B (informative) Superheat correction .13
Bibliography . 14

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 86, Refrigeration and air-conditioning,
Subcommittee SC 4, Testing and rating of refrigerant compressors.
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iv
International Standard ISO 18501:2025(en)
Performance rating of positive displacement refrigerant
compressor
1 Scope
This document specifies normative references, terms and definitions, rating requirements, published
rating, tolerance and marking provisions of positive displacement refrigerant compressors. This document
applies to positive displacement refrigerant compressors and their performance in air-conditioning,
heating, refrigeration and dehumidification applications. Single stage, two-stage and refrigerant-injected
compressors without intermediate temperature load (only one evaporator) are included in this document.
Subcritical operation and transcritical operation are also included in this document.
Compressors for other applications, compression types and operations can also refer to this document.
The manufacturer is solely responsible for the determination of values to be used in the published product
information.
This document is intended as guidance for the industry, including manufacturers, engineers, installers
and service contractors. It defines the minimum amount of information in a standard form to enable the
evaluation and comparison of different compressors for use in an application.
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 18976, Testing of refrigerant compressors
ISO 817, Refrigerants — Designation and safety classification
IEC 60038, IEC standard voltages
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18976 and the following 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
positive displacement refrigerant compressor
machine that increases the pressure of a refrigerant vapour by reducing the compression chamber volume
3.2
fixed displacement compressor
compressor with only one displacement capacity
3.3
modulating compressor
compressor with more than one displacement capacity, either continuous or discrete modulating capacities

3.3.1
continuous modulating compressor
compressor with more than four displacement capacities or infinitely variable displacement capacities
Note 1 to entry: This includes but is not limited to variable speed, continuously variable mechanical unloading or
cyclic modulation of discrete.
3.3.2
discrete modulating compressor
compressor with two to four discrete displacement capacities
3.4
subcooling
difference between the bubble point temperature of the refrigerant corresponding to its pressure and the
temperature of the liquid refrigerant
3.5
superheat
difference between the temperature of the vapour refrigerant and its dew-point temperature corresponding
to its pressure
3.6
refrigerating capacity
product of the refrigerant mass flow at the compressor inlet port and the difference between the specific
refrigerant enthalpy at the compressor inlet port and the specific enthalpy of fluid entering the evaporator
expansion device
3.7
heating capacity
product of the refrigerant mass flow leaving the compressor discharge port and the difference of the specific
refrigerant enthalpy at the discharge port and the outlet of the condenser or gas cooler
3.8
power input
time rate of energy usage of the compressor plus any accessories required to sustain operation of the
compressor at the rating condition
3.9
refrigerating coefficient of performance
ratio of the refrigerating capacity to the power input
3.10
heating coefficient of performance
ratio of heating capacity (3.7) to the power input
3.11
intermediate port
inlet port for vapour or liquid within a compression chamber at a pressure between suction and discharge
pressure
3.12
interstage port
inlet port for vapour or liquid between two stages of compression
3.13
economizer
heat exchanger or flash tank that is used to lower the liquid specific enthalpy of the refrigerant entering the
evaporator while producing vapour that is reintroduced to the compression process

3.14
economized operation
system operation mode which reduces the specific enthalpy of the refrigerant entering the evaporator and
produces vapour which is injected into the compression process
Note 1 to entry: For economized operation with heat exchanger economizer, partial high-pressure liquid refrigerant is
throttled to intermediate pressure to subcool the remaining refrigerant. The vapourized refrigerant can be piped to
the intermediate port or interstage port of the compressor.
Note 2 to entry: For economized operation with flash tank economizer, all refrigerant at outlet condenser is throttled
to intermediate pressure in the flash tank. The saturated liquid goes to evaporator loop and saturated vapour can be
piped to the intermediate port or interstage port of the compressor.
3.15
variable frequency drive
VFD
power electronic device supplying an electric motor with a variable frequency and voltage
3.16
subcritical operation
operating condition with discharge pressure below the critical pressure of the refrigerant
3.17
transcritical operation
operating condition with discharge pressure above and the suction pressure below the critical pressure
3.18
liquid injection
injection of high-pressure side liquid refrigerant into the compressor
3.19
step
level of modulation achieved by changing the displacement capacity
EXAMPLE Compressor operating frequency, compressor rotation speed, frequency of variable frequency drive
and mechanical unloading setting.
3.19.1
maximum step
step producing the highest displacement capacity of the modulating compressor defined by the manufacturer
3.19.2
minimum step
step producing the lowest displacement capacity of the modulating compressor defined by the manufacturer
3.19.3
reduced step
step producing less displacement capacity than the maximum step of the modulating compressor
3.20
fluid
liquid, gas or vapour including the state of appearance close to and above the critical pressure
3.21
publish
transfer information to any entity decided by the manufacturer

4 Rating requirements
4.1 General requirements
All published ratings shall be based on tests conducted in accordance with ISO 18976.
Published ratings shall be established with an ambient temperature around the compressor and its
accessories of 35 °C. If rating at ambient temperatures other than 35 °C, the actual ambient temperature
used to establish the rating shall be stated
...