Thermal performance in the built environment -- Determination of air flow rate in building applications by field measuring methods

In the cooling and heating loads of a building, the air taken in from outside account for a large portion of the entire load; in order to estimate this load, it is necessary to correctly grasp the air flow rate of ventilation and air-conditioning systems. This International Standard stipulates the methods for measuring the rate of air flow through the ducts in a steadily operating ventilation and air-conditioning system and in the air control ports including air diffuser, suction opening, and exhaust opening.

Performance thermique des bâtiments -- Détermination du taux de renouvellement d'air dans les bâtiments par des méthodes de mesure sur site

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Publication Date
19-Jan-2015
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9093 - International Standard confirmed
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04-Jun-2020
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ISO 16956:2015 - Thermal performance in the built environment -- Determination of air flow rate in building applications by field measuring methods
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INTERNATIONAL ISO
STANDARD 16956
First edition
2015-02-01
Thermal performance in the built
environment — Determination of air
flow rate in building applications by
field measuring methods
Performance thermique des bâtiments — Détermination du taux de
renouvellement d’air dans les bâtiments par des méthodes de mesure
sur site
Reference number
ISO 16956:2015(E)
ISO 2015
---------------------- Page: 1 ----------------------
ISO 16956:2015(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2015

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form

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Published in Switzerland
ii © ISO 2015 – All rights reserved
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ISO 16956:2015(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 2

5 Types and selection of measurement method ....................................................................................................................... 2

5.1 Types of measurement methods and their application ....................................................................................... 2

5.2 Selection of measurement method ........................................................................................................................................ 2

6 Basic specifications measuring instruments and utilization methods .......................................................2

6.1 General ........................................................................................................................................................................................................... 2

6.2 Thermal anemometer ....................................................................................................................................................................... 2

6.3 Pitot tube and manometer ............................................................................................................................................................ 3

6.4 Vane-type anemometer ................................................................................................................................................................... 3

7 Field measuring methods of air flow rate of ventilation and air conditioning systems ............3

7.1 Multipoint air velocity measurement method ............................................................................................................. 3

7.1.1 Measurement in a duct ............................................................................................................................................... 4

7.1.2 Measurement method at duct connection of air-conditioning system ............................. 6

7.1.3 Selection of measuring instruments ............................................................................................................... 7

7.2 Tracer gas measurement method ........................................................................................................................................... 7

7.2.1 Formula .................................................................................................................................................................................... 7

7.2.2 Tracer gas ............................................................................................................................................................................... 8

7.2.3 Procedures for measuring air flow rate ....................................................................................................... 8

7.2.4 Tracer gas injection procedures ......................................................................................................................... 9

7.2.5 Tracer gas sampling procedures .....................................................................................................................10

7.3 Flow hood method ............................................................................................................................................................................10

7.3.1 General...................................................................................................................................................................................10

7.3.2 Equipment composition .........................................................................................................................................10

7.3.3 Measurement procedures .....................................................................................................................................11

7.4 Pressure compensation measurement method .......................................................................................................11

7.4.1 Equipment composition .........................................................................................................................................12

7.4.2 Measurement procedures .....................................................................................................................................13

7.4.3 Effective application range ...................................................................................................................................13

7.5 Pressure difference measurement method .................................................................................................................13

7.5.1 Measuring equipment ..............................................................................................................................................13

7.5.2 Selection of measuring instruments ............................................................................................................14

7.5.3 Measuring procedures .............................................................................................................................................14

8 Uncertainty ..............................................................................................................................................................................................................15

8.1 Uncertainty of each measurement ......................................................................................................................................15

8.1.1 Multipoint air velocity measurement method ....................................................................................15

8.1.2 Tracer gas measurement method...................................................................................................................15

8.1.3 Flow hood method ......................................................................................................................................................15

8.1.4 Pressure-loss compensation measurement method .....................................................................15

8.1.5 Pressure difference measurement method ............................................................................................15

8.2 Analysis of uncertainty .................................................................................................................................................................15

9 Measurement report .....................................................................................................................................................................................16

9.1 Information related to measured object ........................................................................................................................16

9.2 Items related to measuring method ..................................................................................................................................16

9.3 Measurement results ......................................................................................................................................................................16

Annex A (normative) Position for cross-section measurement in a duct using multipoint

measurement method ..................................................................................................................................................................................17

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ISO 16956:2015(E)

Annex B (normative) Accuracy of air velocity measurement instrument ..................................................................21

Annex C (informative) Types of tracer gas ...................................................................................................................................................22

Bibliography .............................................................................................................................................................................................................................23

iv © ISO 2015 – All rights reserved
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ISO 16956:2015(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 on the meaning of ISO specific terms and expressions related to conformity

assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers

to Trade (TBT), see the following URL: Foreword — Supplementary information.

The committee responsible for this document is ISO/TC 163, Thermal performance and energy use in the

built environment, Subcommittee SC 1, Test and measurement methods.
© ISO 2015 – All rights reserved v
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INTERNATIONAL STANDARD ISO 16956:2015(E)
Thermal performance in the built environment —
Determination of air flow rate in building applications by
field measuring methods
1 Scope

In the cooling and heating loads of a building, the air taken in from outside account for a large portion

of the entire load; in order to estimate this load, it is necessary to correctly grasp the air flow rate

of ventilation and air-conditioning systems. This International Standard stipulates the methods for

measuring the rate of air flow through the ducts in a steadily operating ventilation and air-conditioning

system and in the air control ports including air diffuser, suction opening, and exhaust opening.

2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 5168, Measurement of fluid flow — Procedures for the evaluation of uncertainties

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
orifice plate
thin plate having a hole, or holes, bored through it

Note 1 to entry: These are used for measuring the difference in static pressure of the flow before and after the disc

and obtaining the air flow rate in the duct by multiplying by a predetermined coefficient.

3.2
tracer gas
gas used to measure its concentration varying in the air

Note 1 to entry: This gas is mixed with a sufficiently small amount of air so as not to affect the flow, and the

amount of air is determined by measuring the gas concentration diluted in the air.

3.3
volumetric concentration
ratio of the volume of the specific gas to the unit volume of the mixture of air
Note 1 to entry: It is expressed in cubic meters per cubic meters or 10-6 ∙ vol.
3.4
mass flow
mass of air or tracer gas flowing in unit time
Note 1 to entry: It is expressed in mg per second or kg per hour.
3.5
volumetric flow
volume of air flowing in unit time
Note 1 to entry: It is expressed in cubic meters per hour.
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ISO 16956:2015(E)
4 Symbols and abbreviated terms

Among the symbols used in this International Standard, only those common to all items are shown

below. Individual symbols are explained at the relevant sub clauses.
Symbol Quantity Unit

Area of evaluation section of air flow rate. Sectional area of duct and sectional area of

A m
air control port

Number of divisions for measurement of evaluation section. The number of lattice-like

N -

divisions of rectangular duct section and radial number of divisions in round duct.

Average air velocity of section of measured portion.

The section is divided into n and if the time average air velocity at the centre of each is

v , the following formula is applied:
V m/s
v= v
∑ i
i=1
5 Types and selection of measurement method
5.1 Types of measurement methods and their application

The measurement methods covered in this International Standard are the multipoint air velocity

measurement method, tracer gas measurement method for air flow rate in air duct, flow hood method,

pressure compensation measurement method, and pressure difference measurement method between

outlet and inlet. The method selected should be suitable for the purpose, as well as for the field conditions.

5.2 Selection of measurement method

The measurement method is selected considering the following items specified in Clause 7 and Clause 8:

a) ventilation/air-conditioning equipment subject to measurement;
b) position for measurement;
c) measurement period;
d) accuracy;

e) practicability (equipment size and composition simplicity, preparation, ease of data processing, and

cost).
6 Basic specifications measuring instruments and utilization methods
6.1 General

The following describes air velocity measuring instruments common to various measurement methods.

6.2 Thermal anemometer

In the category of thermal anemometers, there are the hot-wire anemometer and the semiconductor

anemometer. The hot-wire anemometer has less resistance against flow and is suitable for multipoint

air velocity measurement in air duct when measuring very low air velocity. Attention should be paid

to the directivity of the sensor. The sensor should be calibrated as necessary to avoid errors due to

deterioration and adhesion of dust.
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ISO 16956:2015(E)

Since the semiconductor anemometer has improved accuracy beyond 1 m/s and the distance between

the sensor and transducer can be extended, it is suitable for multipoint air velocity measurement for

fixed setting in air duct for permanent installation.
6.3 Pitot tube and manometer

The Pitot tube has less resistance to flow and is suitable for multipoint measurement, but to be accurate,

the flow velocity in the straight-pipe section should be 4 m/s or higher. However, when the cross-section

of the duct is small, the Pitot tube is not used.

When the air velocity is calculated from the dynamic pressure, it is obtained by using the density of

measured air depending on air temperature and atmospheric pressure as given in Formula (1):

vP= (/2 ρ) (1)
where
v is the air velocity, in meter per second;
ρ is the air density, in kg per cubic meters;
P is the dynamic pressure, in Pa.

The result of Formula (1) is multiplied by the compensation coefficient k of the Pitot tube, if it is shown. Air

density, ρ, is obtained from measured air temperature θ and atmospheric pressure P, using Formula (2):

T P 273,15 P
ρρ=⋅ ⋅=1,293 ⋅ (2)
T P 273,15+θ 101325
atm
where

ρ is the density when dry air temperature Θ is 0,0 °C (= 1,293), in kg per cubic meters;

T is the thermodynamic temperature of dry air, in K;
Θ is the air temperature, in °C;
T is the thermodynamic temperature when Θ is 0,0 °C (273,15), in K;
P is the atmospheric pressure, in Pa;
P 1 atmospheric pressure (101 325); in Pa.
atm
6.4 Vane-type anemometer

The vane-type anemometer generally cannot be used for multipoint measurement because the vane is

too large. If it is used, it is advisable to select a mini-vane anemometer with the measured portion made

smaller.
7 Field measuring methods of air flow rate of ventilation and air conditioning
systems
7.1 Multipoint air velocity measurement method

The multipoint air velocity measurement method obtains the air flow rate by measuring the average

air velocity in the duct and multiplying it by the sectional area of the duct. For both a round duct and

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ISO 16956:2015(E)

rectangular duct, the cross-section is divided into multiple equal areas, the representative air velocity

is measured and the average air velocity is calculated. The air flow rate is expressed by Formula (3):

QS=×3600 ⋅v (3)
where
Q is the air flow rate in the duct, in cubic meters per hour;
S is the duct sectional area of measured portion, in square meters;

v is the average air velocity of cross-section of measured portion, in meter per second.

7.1.1 Measurement in a duct
7.1.1.1 Position for cross-section measurement

The position for cross-section measurement is selected in accordance with Annex A. Normally, it is a

place having a straight section of not less than six times the equivalent diameter, D , of the duct on the

upstream side in the air flow direction. When precise measurement is required, a straightening or wire

grid is provided upstream.

In the case of a rectangular duct, the equivalent diameter, D , is calculated by Formula (4):

Da=+2/ba()b (4)
where

D is the equivalent diameter when a rectangular duct is converted to a round duct, in meter;

a is the width of rectangular duct, in meter;
b is the height of rectangular duct, in meter.
7.1.1.2 Simplified measurement method

With the simplified measurement method, as shown in A.4, the measured cross-section is divided into

four equal parts and the four centres and the centre of the whole, five points in total, are selected as

the measuring points. The arithmetic average of the five points is used as the average air velocity. If the

required conditions are met, including securing the length of the straight-pipe section, the uncertainty

can be minimized (approximately ±10 %) even with this method. If, however, a sufficient straight-pipe

length cannot be secured upstream from the measurement position, the measuring points are increased

to a number closer to the precise measurement method.
7.1.1.3 Precise measurement method
a) In the case of a rectangular cross-section

With the precise measurement method, the rectangular cross-section is divided into multiple equal

rectangles in such a way that the length of one side will be about 15 cm or less and the air velocity is

measured at their centres. However, the number of divisions need not exceed 64.
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ISO 16956:2015(E)
Key
1 measuring point
Figure 1 — Air velocity measuring points of rectangular duct
b) In the case of a round cross-section

Figure 2 shows the measuring points in the precise measurement method for a round cross-section. The

measured cross-section is divided into n doughnut-shaped equal areas, which are then divided into four

or more equal sectors with a common straight line passing through the centre of the cross-section, and

the measuring point is provided at the centre of each figure. The distance r from the centre of the cross-

section of the measuring point in area i is given by Formula (5):
i−12/
rR= (5)
where
R is the radius of cross-section, in meter;
r is the distance from the centre of cross-section to measuring point, in meter;
n is the number of divisions in diameter direction of cross-section;

i is the measuring point position number in radial direction from centre of cross-section

The relationship between R and n is as follows.
n = 2 when R < 0,13 m
n = 3 when R ≤ 0,15 m
n = 4 when R ≤ 0,30 m
n = 5 when R ≤ 0,50 m
n = 6 when R ≥ 0,75 m
When R > 0,75, 1 is added every 0,25 m.
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ISO 16956:2015(E)
Figure 2 — Air velocity measuring points of round duct
Formula (5) and the above conditions are arranged in Table 1, which may be used.
Table 1 — Measuring positions of round cross-section
Distance r from centre of
Diameter of Number of
Number of
cross-section (mm)
round cross-section measuring
divisions
(m) points
r R R R R
1 2 3 4 5
0,075 2 8 20 30 - - -
0,100 2 8 25 45 - - -
0,125 2 8 30 55 - - -
0,150 2 8 35 65 - - -
0,175 2 8 45 75 - - -
0,200 2 8 50 85 - - -
0,225 2 8 55 95 - - -
0,250 3 12 50 90 115 - -
0,300 3 12 60 105 135 - -
0,350 4 16 60 105 140 165 -
0,400 4 16 70 120 160 185 -
0,450 4 16 80 135 175 210 -
0,500 4 16 90 155 195 235 -
0,550 4 16 95 170 215 255 -
0,600 4 16 105 185 235 280 -
0,650 5 20 100 180 230 270 310
0,700 5 20 110 190 245 290 330
0,750 5 20 120 205 265 315 355
7.1.1.4 Semi-precise measurement method for round cross-section

With the semi-precise measurement method, for a round cross-section of 0,3 m or less in diameter

(R < 0,15 m), the number of divisions of cross-section n = 2 is adopted in Figure 2. That is, the measurement

[6]
can be made at eight measuring points.
7.1.2 Measurement method at duct connection of air-conditioning system

When the amount of intake air from outside and the amount of air returned are measured at the duct

connection of the air-conditioning system, the position selected should have less velocity distribution

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ISO 16956:2015(E)

at the connection on the suction side. With the measuring points of air velocity as per 7.1.1.3 a), the

measurement is made at the centre points of 16 or more equal-area zones and the average air velocity

is determined. Refer to Annex A.
7.1.3 Selection of measuring instruments

For cross-section air velocity measurement, the thermal anemometer (hot-wire anemometer and semi-

conductor anemometer), Pitot tube, precise manometer, vane-type anemometer, etc. are used.

7.2 Tracer gas measurement method

The tracer gas measurement method estimates the flow rate in the duct by steadily injecting a fixed

amount of tracer gas with known concentration and measuring of dilution of concentration at the

downstream in the duct. The measurement items are gas concentration in the air on the upstream

and downstream sides of the tracer gas injection point and the injection rate of the tracer gas. As

preconditions for this measurement, the tracer gas shall be sufficiently mixed at the point where the air

is collected and the concentration distribution on the inner cross-section of the duct shall be lower than

the accuracy of the measuring instrument. It is also necessary to use a gas that is not adsorbed onto the

inner surfaces of the duct or sampling tube.
7.2.1 Formula

A fixed amount of tracer gas of known concentration is continuously injected into air duct, as shown in

Figure 3, and the mass conservation law for the tracer gas is applied at a point sufficiently downstream

from the injection position.
The flow rate in the duct can be obtained by Formula (6):
()CC−
f = ⋅ f (6)
U I
()CC−
The mass flow rate F can be determined by Formula (7):
()CC−
F = ⋅F (7)
U I
()CC−
where
F is the mass flow rate in duct, in kg per hour;
f is the air flow rate in duct, in cubic meters per hour;
F is the tracer gas injection rate (weight unit), in kilogram per hour;
f is the tracer gas injection rate (volume unit), in cubic meter per hour;

C is the volume concentration on upstream side of tracer gas, in cubic meter per cubic meter;

C is the volume concentration on downstream side of tracer gas, in cubic meter per cubic meter;

C is the volume concentration of injected tracer gas, in cubic meter per cubic meter;

ρ is the density of injected tracer gas, in kilogram per cubic meter;
ρ is the density of tracer gas on upstream side, in kilogram per cubic meter.
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ISO 16956:2015(E)
Key
1 sampling
2 tracer gas injection
Figure 3 — Concept of tracer gas measurement method for air flow rate in duct
7.2.2 Tracer gas

The tracer gas normally used for this measurement method shall not be a substance whose discharge

into the atmosphere is prohibited and its concentration level during measurement shall not be toxic to

humans. In comparison with the amount of tracer gas, the absorption and adsorption of tracer gas on

the wall surface and in the sampling tube shall be negligibly small. For selection, see Annex C.

7.2.3 Procedures for measuring air flow rate
For the tracer gas method, the following procedures are used:

a) a fixed amount of tracer gas of known concentration is continuously injected into the duct;

b) at a portion where the gas is uniformly mixed downstream from the tracer gas injection point (more

than 10 times the diameter of the duct), the tracer gas concentration is measured at a minimum N+1

points (Table 2) including one point at the centre of the duct cross-section and the centre of each

area when the duct cross-section is divided into N areas (see Figure 4);

c) if return air is included in the duct system, fluctuation in concentration is expected upstream

from the tracer gas injection point and thus, the concentration of N areas should also be measured

upstream from the injection point to confirm that the distribution is small;
d) the tracer gas injection rate is recorded;
e) either the air flow rate or mass flow rate is calculated, as required.
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ISO 16956:2015(E)
Key
1 air in duct/analyser
2 tracer gas
3 sampling

l for a straight pipe, the yardstick is more than 10 times the diameter; it may be shorter if there is a bend, etc.

that enforce mixing

Figure 4 — Tracer gas injection and sampling position, example of a duct sectional area smaller

than 0,2 m

Table 2 shows the “number of areas dividing a cross-section and number of sampling points” by the size

of duct sectional area, obtained through error analysis to decrease measurement errors in the air flow

rate in the duct.
[ ]
Table 2 — Minimum number of sampling points downstream 7
Duct sectional area Number of areas Number of sampling points
m N N+1
0,2 or less 4 5
0,2 to 2,3 12 13
2,3 or more 20 21
7.2.4 Tracer gas injection procedures
The tracer gas is injected into the duct as follows.

a) The tracer gas is injected upstream away from the sampling point so that it is uniformly mixed in

the duct or a portion is selected with one or more bends on the downstream side of the injection

point and multiple injection points are used. When the injection is made into the supply air duct,

care should be taken so that the tracer gas will not flow out through the peripherical clearance

around
...

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