ISO 21773:2021

INTERNATIONAL ISO
STANDARD 21773
First edition
Methods of test and characterization
of performance for energy recovery
components
Méthode d'essai et caractérisation des performances des composants
récupérateurs d’énergie
PROOF/ÉPREUVE
Reference number
ISO 21773:2021(E)
©
ISO 2021

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ISO 21773:2021(E)

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© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 21773:2021(E)

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 4
5 Metrics . 5
5.1 General . 5
5.2 Effectiveness . 6
5.3 Pressure drop . 6
5.3.1 Measured pressure drop . 6
5.3.2 Standardized pressure drop . 6
5.4 Recovery efficiency ratio . 7
5.5 Outside air correction factor . 8
5.6 Exhaust air transfer ratio . 8
5.7 Sensible energy transfer rate for the supply airstream . 8
5.8 Humidity transfer rate for the supply airstream . 8
5.9 Total energy transfer rate for the supply airstream . 8
6 General test requirements . 9
6.1 Test apparatus . 9
6.2 Installation . 9
6.3 Static pressures . 9
6.4 Instrument calibration . 9
7 Effectiveness tests .10
7.1 Test requirements .10
7.2 Stability limits when testing effectiveness .10
7.3 Data collection period .11
7.4 Data sampling rates .11
7.5 Temperature and humidity conditions: inlets to exchanger .12
7.6 Test temperature limits .12
8 Pressure drop tests .12
9 Leakage tests .13
9.1 General test requirements .13
9.2 Outside air correction factor .13
9.3 Exhaust air transfer ratio .13
10 Uncertainty limits.13
10.1 General .13
10.2 Uncertainty limits for effectiveness tests .14
10.3 Uncertainty limits for RER .14
10.4 Uncertainty limits for measured pressure drop tests .14
10.5 Uncertainty limits for leakage tests .14
11 Inequality limits .15
11.1 General .15
11.2 Inequality limits for thermal tests.15
11.3 Inequality limits for leakage tests .15
12 Reporting of test results .15
12.1 Pressure drop test results .16
12.2 Leakage test results .16
12.3 Thermal test results .16
12.4 Uncertainties .16
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ISO 21773:2021(E)

Annex A (informative) Example of test data collection and calculation of metrics .17
Annex B (informative) Best practices .21
Annex C (informative) Expression of performance metrics for use in calculation of system
performance . .32
Annex D (informative) Inequality limits for use when condensate flow rate can be measured .37
Annex E (informative) Expressions, used in other standards, related to effectiveness as
defined in this document .38
Bibliography .44
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ISO 21773:2021(E)

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 documents 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).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
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 86, Refrigeration and air-conditioning,
Subcommittee SC 6, Testing and rating of air-conditioners and heat pumps.
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.
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INTERNATIONAL STANDARD ISO 21773:2021(E)
Methods of test and characterization of performance for
energy recovery components
1 Scope
This document specifies methods for testing and characterizing the performance of air-to-air heat/
energy exchangers when used as devices to transfer heat or heat and water vapor between two
airstreams used in ventilation systems. It also specifies methods to characterize the performance of
exchangers for use in calculation of the energy performance of buildings. This document is applicable to:
— fixed-plate exchangers (also known as recuperators),
— rotary exchangers, including heat wheels and total energy wheels (also known as regenerators),
— heat pipe exchangers using a heat transfer medium, excluding those using mechanical pumping.
This document does not provide a method for measuring the response of exchangers to the formation
of frost.
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 3966, Measurement of fluid flow in closed conduits — Velocity area method using Pitot static tubes
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 5801, Fans — Performance testing using standardized airways
ISO 13253, Ducted air-conditioners and air-to-air heat pumps — Testing and rating for performance
ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
effectiveness
actual energy transfer rate (sensible, latent, or total) divided by the maximum possible energy
transfer rate
Note 1 to entry: The formula for effectiveness is given in 5.2.
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ISO 21773:2021(E)

3.2
exhaust air transfer ratio
EATR
tracer gas concentration difference between the leaving supply air (3.12) and the entering supply air
(3.11), divided by the tracer gas concentration difference between the entering exhaust air (3.13) and the
entering supply air (3.11), which quantifies the air quantity transferred from the exhaust to the supply
Note 1 to entry: The formula for EATR is given in 5.6.
Note 2 to entry: It can be expressed as a percentage for rating purposes, but is used as a ratio in the calculation
of RER (3.6).
3.3
fixed-plate exchanger
exchanger with multiple alternate airflow channels, separated by a heat or heat and water vapor
transfer plate(s) and connected to supply and exhaust airstreams
3.4
heat pipe exchanger
exchanger with an array of finned and sealed tubes that are placed in side-by-side supply and
exhaust airstreams, which may include an internal wick structure in each tube, and filled with a
heat transfer medium
Note 1 to entry: Thermosiphon exchangers are a subset (or type) of heat pipe exchanger in which the heat transfer
medium moves by gravitational forces only.
3.5
outside air correction factor
OACF
factor defined as the entering supply air (3.11) divided by the leaving supply air (3.12)
Note 1 to entry: The formula for OACF is given in 5.5.
3.6
recovery efficiency ratio
RER
ratio of the recovered energy rate divided by the sum of the calculated combined fan power and the
auxiliary power
Note 1 to entry: The formula for RER is given in 5.4.
Note 2 to entry: RER can be characterized as gross, or as net in which case EATR (3.2) is accounted for.
3.7
rotary exchanger
exchanger with porous discs, fabricated from materials with heat or heat and water vapor retention
capacity, that are regenerated by collocated supply and exhaust airstreams
3.8
standard air
3 -5
dry air with a density of 1,204 3 kg/m and a dynamic viscosity of 1,824 7 x 10 kg/(m∙s)
Note 1 to entry: These conditions approximate dry air at 20 °C and 101,325 kPa absolute.
3.9
station
location in the test apparatus at which conditions such a temperature, humidity, pressure or airflows
are measured
Note 1 to entry: indicated in Figure 1 as 1, 2, 3 and 4.
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ISO 21773:2021(E)

3.10
static pressure differential
static pressure at supply outlet minus the static pressure at exhaust inlet
Note 1 to entry: A positive pressure differential occurs when the static pressure at station (3.9) 2 is higher than
the static pressure at station 3. A negative pressure differential occurs when the static pressure at station 2 is
lower than the static pressure at station 3.
3.11
entering supply air
supply air inlet
outdoor airflow
OA
outside air entering the exchanger
Note 1 to entry: Indicated in Figure 1 as 1.
3.12
leaving supply air
supply air outlet
supply airflow
SA
outside air after passing through the exchanger
Note 1 to entry: Indicated in Figure 1 as 2.
3.13
entering exhaust air
exhaust air inlet
return airflow
RA
indoor air entering the exchanger
Note 1 to entry: Indicated in Figure 1 as 3.
3.14
leaving exhaust air
exhaust air outlet
exhaust airflow
EA
indoor air after passing through the exchanger
Note 1 to entry: Indicated in Figure 1 as 4.
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ISO 21773:2021(E)

Key
1 entering supply air 2 leaving supply air
3 entering exhaust air 4 leaving exhaust air
5 exchanger
Figure 1 — Schematic diagram of airflows for heat and energy recovery exchangers
4 Symbols and abbreviated terms
Symbol Term Units
-6
C Tracer gas concentration at station i (i = 1, 2, 3, 4) 10
i
C Specific heat of condensate at its measured temperature kj/kg
p
C Specific heat of dry air at station i (i = 1, 2, 3, 4) J/(kg⋅K)
p,i
δT Maximum deviation of any temperature reading of T from T K
i i AVE,i
δW Maximum deviation of any humidity ratio reading of W in from W kg water / kg dry
i i AVE,i
air
ΔP Pressure drop through the exchanger, exhaust air stream, measured Pa
e
ΔP Pressure drop through the exchanger, exhaust air stream, at reference Pa
e,ref
conditions
ΔP Pressure drop through the exchanger, supply air stream, measured Pa
s
ΔP Pressure drop through the exchanger, supply air stream, at reference Pa
s,ref
conditions
∆ps Static pressure differential Pa
2,3
ΔT Temperature change in the supply airstream °C or K
1-2
ΔW Humidity change in the supply airstream kg water / kg dry
1-2
air
ε Effectiveness %
ε Sensible effectiveness %
sensible
ε Latent effectiveness %
latent
ε Total effectiveness %
total
a
F Outside air correction factor (OACF) 1
oac
h Enthalpy of air at station i (i = 1, 2, 3, 4) kJ/kg dry air
i
h Heat of vaporization of water J/kg
fg
a
Some quantities of dimension 1 are defined as ratios of two quantities of the same kind. The coherent derived unit is
the number 1 (ISO 80000-1:2009, 3.8).
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ISO 21773:2021(E)

Symbol Term Units
ṁ Measured condensate flow rate kg/s
condensate
ṁ Mass flow rate of dry air at station i (i = 1, 2, 3, 4) kg/s
i
a
m /m Ratio of supply air outlet mass flow rate to exhaust air inlet mass flow rate 1
s e
a
η Combined efficiencies of the supply and exhaust air fan and drive 1
fs,fe
ps Static pressure at station i (i = 1, 2, 3, 4) Pa
i
q Auxiliary power input to the exchanger (e.g. to rotate a wheel) kW
aux
Q Humidity transfer rate kg water/(kg dry
latent
air ∙ s)
Q Sensible energy transfer rate W
sensible
Q Total energy transfer rate W
total
3
Q Leaving supply volume flow rates m /s
2
3
Q Entering exhaust volume flow rates m /s
3
3
ρ Dry air density at station i (i = 1, 2, 3, 4) kg/m
i
a
R Exhaust air transfer ratio (EATR) 1
eat
R Gross recovery efficiency ratio (gross RER) W/W
rer,gross
R Net recovery efficiency ratio (net RER) W/W
rer,net
Θ Purge angle °
T Average value of temperature readings taken at station i (i = 1, 2, 3, 4) dur- °C
AVE,i
ing a measurement period
T Measured temperature of the condensate °C
condensate
T Temperature efficiency %
e
T Dry-bulb temperature at station i (i = 1, 2, 3, 4) °C
i
T Wet-bulb temperature at station i (i = 1, 2, 3, 4) °C
WB,i
a
U Expanded relative uncertainty 1
W Average value of humidity readings taken at station i (i = 1, 2, 3, 4) during a kg water/kg dry
AVE,i
measurement period air
W Humidity efficiency %
e
W Humidity at station i (i = 1, 2, 3, 4) kg water/kg dry
i
air
μ Dynamic viscosity at station i (i = 1, 2 3 or 4) kg/(m∙s)
i
-5
μ Dynamic viscosity of standard air = 1,824 7 x 10 kg/(m∙s)
s
a
Some quantities of dimension 1 are defined as ratios of two quantities of the same kind. The coherent derived unit is
the number 1 (ISO 80000-1:2009, 3.8).
5 Metrics
5.1 General
The performance of an air-to-air heat/energy exchanger is primarily characterized by its sensible,
latent, and total effectiveness [see Formulae (1), (2) and (3)] its pressure drops [see Formulae (4), (5),
(6) and (7)], its recovery efficiency ratio [see Formulae (8) and (9)], the outside air correction factor [see
Formula (10)], and its exhaust air transfer ratio [see Formula (11)]. Formulae (1) to (3) reproduced with
permission from ANSI/ASHRAE 84:2020. Formulae (4) through (11) are based on formulae in ANSI/
ASHRAE 84-2020 with permission from ANSI/ASHRAE. Annex E provides guidance on equivalence
between the metrics provided in this document and related metrics in use in certain other standards.
Derived metrics that are needed for use in calculating the performance of complete systems include
sensible energy transfer rate (see Formula (12)), humidity transfer rate (see Formula (13) and enthalpy
transfer rate (see Formula (14)).
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ISO 21773:2021(E)

See Clause 4 for the units of different quantities.
5.2 Effectiveness
The sensible, latent, and total effectiveness (ε , ε and ε ) are defined by Formulae (1), (2)
sensible latent total
and (3):

mC TC− T
()
21pp,,12 2
ε = (1)
sensible

mC TC− T
()
min pp,,11 33

mh Wh− W
()
21fg,,12fg 2
ε = (2)
latent

mh Wh− W
()
minfgf,,11 g 33

mh −h
()
21 2
ε = (3)
total

mh −h
()
min 13
where
ṁ is the mass flow rate at station i (i = 1, 2 or 3)
i
ṁ is the lesser of ṁ and ṁ
min 2 3
C is the specific heat of dry air at station i (i = 1, 2 or 3)
p,i
h is the heat of vaporization of water at station i (i = 1, 2 or 3)
fg,i
T is the dry-bulb temperature at station i (i = 1, 2 or 3)
i
W is the humidity at station i (i = 1, 2 or 3)
i
h is the enthalpy at station i (i = 1, 2 or 3)
i
5.3 Pressure drop
5.3.1 Measured pressure drop
The air friction pressure drops (ΔP and ΔP ) at specific conditions and air mass flow rate through the
s e
exchanger are defined by Formulae (4) and (5):
ΔPp=−sps (4)
s 12
ΔPp=−sps (5)
e 34
where ps is the static pressure at station i (i = 1, 2, 3 or 4).
i
5.3.2 Standardized pressure drop
Air friction pressure drops at reference conditions (ΔP and ΔP )can be determined by Formulae (6
s,ref e,ref
and (7):
 ρ  μ   ρ  μ 
1 s 2 s
ΔPp= sp  − s    (6)
sr, ef 1 2
     
ρ μ ρ μ
     
s 1 s 2
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ISO 21773:2021(E)

 ρ  μ   ρ  μ 
3 s 4 s
ΔPp= sp  − s    (7)
er, ef 3 4
     
ρ μ ρ μ
     
s 3 s 4
where
3
ρ is the density at station i (i = 1, 2, 3 or 4) kg/m
i
3
ρ is the standard density of air = 1,2043 kg/m
s
μ is the dynamic viscosity at station i (i = 1, 2 3 or 4) kg/(m∙s)
i
-5
μ is the dynamic viscosity of standard air = 1,8247 x 10 kg/(m∙s)
s
5.4 Recovery efficiency ratio
a) The gross recovery efficiency ratio (R ) of a heat/energy exchanger is defined by Formula (8):
rer,gross

mh −h
21 2
R = (8)
rerg, ross
ΔΔPQ PQ
s 23e
+ +q
aux
1000⋅ηη1000⋅
fans,,fane
b) The net recovery efficiency ratio (R ) of a heat/energy exchanger is defined by Formula (9):
rer,net
hR− h
()
23eat

mh −
21
1−R
()
eat
R = (9)
rern, et
ΔΔPQ PQ
s 23e
+ +q
aux
1000⋅η 11000⋅η
fans, fane,
where
ΔP and ΔP are the measured pressure drops across the supply and exhaust sides of the exchanger,
s e
respectively
Q and Q are the leaving supply and entering exhaust volume flow rates
2 3
η and η is the supply and exhaust air fan and drive combined efficiencies
fs fe
q is the total auxiliary power input to the exchanger (e.g. to rotate a regenerative wheel,
aux.
a pump, and to operate controls)
R is the exhaust air transfer ratio (EATR) expressed as a ratio
eat
In laboratory testing of heat/energy exchangers it is not usually possible to measure the power required
to move air through the exchanger directly, as the blowers in the test system also are required to
overcome friction pressure of the conditioning equipment, flow measurement equipment, etc. Therefore,
the power required to move air through the exchanger shall be calculated, based on a reference fan and
drive total efficiency which is selected for the purposes of comparison of one exchanger to another. For
example, a performance rating agency could elect to use a reference fan and drive total efficiency of
0,50 in the calculation of RER for all the exchangers for which it provides ratings.
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ISO 21773:2021(E)

5.5 Outside air correction factor
The outside air correction factor (F ) of a heat/energy exchanger at a specific operating condition is
oac
defined by Formula (10):

m
1
F = (10)
oac

m
2
where ṁ are the mass flow rates at stations 1 and 2
1,2
5.6 Exhaust air transfer ratio
The exhaust air transfer ratio (R ) of a heat/energy exchanger at a specific operating condition is
eat
defined by Formula (11):
CC−
21
R = (11)
eat
CC−
31
where C Are the concentration of tracer gas at stations i (i = 1, 2, 3 or 4) during the test described in 9.3.
i
NOTE To express exhaust air transfer ratio as a percentage, multiply by 100.
5.7 Sensible energy transfer rate for the supply airstream
Sensible energy transfer rate (Q ) into or out of the supply airstream for an exchanger at a specific
sensible
operating condition is defined by Formula (12):

Qm=⋅ Tc −Tc (12)
()
sensible 21 pp,,12 2
where
T are the temperatures at stations 1 and 2
1-2
c are the specific heats of dry air at stations 1 and 2
p1,2
5.8 Humidity transfer rate for the supply airstream
Humidity transfer rate (Q ) into or out of the supply airstream for an exchanger at a specific
latent
operating condition is defined by Formula (13):

Qm=⋅ΔW (13)
latent 21−2
where ΔW is the humidity change for the supply airstream.
1-2
5.9 Total energy transfer rate for the supply airstream
Total energy transfer rate (Q ) into or out of the supply airstream for an exchanger at a specific
total
operating condition is defi
...