Practical guide Air flow measurements in ducts according to DIN EN 12599. Air flow measurements in ducts according to DIN EN 12599

Introduction.

Nowadays, we spend most of the day Ensuring adequate air exchange and in closed rooms. This is why heat- thus determining the volume flow is an ing, ventilation and air conditioning important quality factor when it comes (HVAC) systems are installed which are to commissioning and operating HVAC intended to ensure pleasant ambient systems. The reliable determination of conditions indoors. Ventilation is of air velocity in ducts is one of the most particular importance here. Firstly, it challenging measurements which a is not only used to provide fresh air, ventilation and air conditioning techni- but also for extracting pollutants, for cian has to carry out. instance removing excess humidity from rooms.

2 Contents:

1. Importance of air velocity...... 04 2. Measurement of the correct air velocity...... 05 3. The right measuring location...... 06 3.1 Flow profiles in the duct...... 07 3.2 Distance from disruptions...... 08 4. The measurement method...... 10 4.1 Trivial method...... 11 4.2 Centroidal axis method...... 12 4.3 Calculation of the volume flow...... 13 5. Evaluation of the readings ...... 14 6. The measurement report ...... 20

3 Air flow measurements in ducts according to DIN EN 12599

1. Importance of air velocity

In line with the motto: "The more, the On the other hand, too low a volume merrier", HVAC systems are often flow can also be problematic. The operated with air volumes that are too people in the room have too little high. This excessive requirement leads fresh air to breathe in. The indoor air to increased operating costs. Energy is "stale", because the CO2 content expenditure for the fan rises, because in the room is too high. Low volume a larger volume of air has to be moved flows can also have negative impacts through the system. However, costs on the system's hygiene: there is the are also incurred for conditioning risk of germ formation in the system the air (cooling, heating, humidifying when movement of the humidified air or dehumidifying) and these can be in the ducts is too slow. A correctly set reduced when the system is set cor- HVAC system therefore not only helps rectly. In addition, a high air exchange make the indoor climate comfortable, often leads to draughts occurring in but also helps save costs. the room, making people feel uncom- fortable.

4 2. Measurement of the correct air velocity

The key parameter for evaluating the In order to meet quality requirements functional capability of the HVAC when it comes to determining volume system is the air volume flow. This is flow, there are different standards all the product of flow velocity and duct over the world dealing with the correct area. Since, in practice, flow velocity in measurement of flow velocities. In the duct cross-section is not the same, addition to EN 12599, which is the an individual point measurement leading standard in Germany and does not suffice when it comes to large parts of Europe, there are also determining the average air velocity. EN 16211 and ASHRAE 111. What all Disruptors, such as dampers, elbows methods have in common is that the and the like, have an influence on measuring points are distributed over the velocity profile in the duct, which the duct cross-section according to means a so-called grid measurement the size of the duct in line with defined has to be carried out at several specifications, that a distinction locations in the duct. is made between rectangular and round ducts and that the readings are averaged.

We will now go into the correct measurement of volume flow according to EN 12599.

5 Air flow measurements in ducts according to DIN EN 12599

3. The right measuring location

The decisive factor when it comes to • Minimum distances from disruptions meaningful measurements is selecting must be adhered to: at least 6-times a suitable measuring point. This is the hydraulic diameter downstream, already established by the system and 2-times the hydraulic diameter planner in the execution plan (project upstream plan). The following criteria must be • The measuring points must be taken into account here: easily accessible and there must be • Air flow measuring points must be enough space available for handling allowed for on all main ducts and the measuring instrument. on supply lines to rooms with high • The flow must be free of any return requirements. flow or swirling

Round Rectangular Square

D = diameter h 2×duct length×duct width D = Dh = edge length h duct length+duct width

Calculation of the hydraulic diameter Dh for different duct shapes.

6 3.1. Flow profiles in the duct

Air which flows through a duct does Turbulent flow. not have a uniform velocity. As a In this case, the flow velocities are to rule, the air in the middle flows a large extent identical right across faster than at the duct wall. There the duct diameter, but the velocity are greater resistances at the duct does drastically fall at the duct wall. wall due to friction and these have to However, the flow lines are non- be overcome. A distinction is made directional, in other words the air between two basic flow profiles: moves chaotically and with a high degree of friction. Laminar flow. Laminar flow involves a uniform air All mixed forms between these two flow with the flow lines running parallel ideal forms are possible, with every to one another. There is no turbulence disruptor (such as dampers, elbows, and a distinct maximum velocity in the valves, volume flow regulators, etc.) middle of the duct. The average flow changing the flow profile. velocity is approximately at a third of In practice, a so-called grid the duct diameter. As soon as the air measurement over the whole duct velocity rises, laminar flow increasingly cross-section is indispensable for converts into turbulent flow. reproducible results.

7 Air flow measurements in ducts according to DIN EN 12599

Laminar Turbulent

Max A(m2)

1/3D mean Min

Laminar and turbulent air flow velocity. Different flow profiles are generated, depending on the flow velocity.

3.2. Distance from disruptions

Ideal flow profiles are almost can be considerably reduced. exclusively found in very long ducts In practice, dampers, valves, which run in a straight line and where elbows and other bends prevent the there are no disruptions. For this development of a consistent flow. reason, minimum distances from In unfavourable circumstances, this disruptions have to be adhered to. results in the maximum of the flow profile not being in the middle of Where the distance from disruptions the duct, but being shifted towards is sufficient, the number of measuring the edge, in more problematic points which have to be measured circumstances there may also be spread across the duct cross-section return flows or areas with no flow.

8 As a rule, return flows are diminished distorted that a large number of after a distance of 2-times the measuring points are required to keep hydraulic diameter from the disruption, the measurement uncertainty low. however the flow profile is so strongly 5 m/s 5 m/s 5 m/s 10 m/s 10 m/s 10 m/s

Distance Distance Distance 7 x D h 2 x Dh 1 x Dh

Balanced flow Distorted flow profile profile D=250 Flow profile with return flow AIR

Irregularities in the flow profile are diminished as the distance from the disruption increases. The greater the dis- tance from the disruption, the more uniform the flow profile and the more precise the measurement or the smaller the number of measuring points required.

9 Air flow measurements in ducts according to DIN EN 12599

4. The measurement method

The representative average flow round ducts. DIN EN 12599 envisages value in the duct cross-section the following two measurement has to be established to determine methods: the air volume flow. To do this, the • the trivial method for measurements measurement area is split into partial in air ducts with a rectangular or areas and the velocity is determined square cross-section at the central point of the partial • the centroidal axis method for areas. This method is called grid measurements in ducts with a measurement. The method for dividing circular cross-section the duct cross-section into partial areas is different for rectangular and

Grid A i y

xi

Duct Measurement plane B

Division of the duct cross-section according to the trivial method. The measuring points are at the centre points of the areas.

10 4.1. Trivial method

The trivial method does not assume the number of measuring points has any special velocity distribution in the to be increased accordingly. It is large duct. The duct cross-section is simply enough when the reading fluctuations divided into several measurement within a partial area are so small that areas with an identical size. The the values measured at the centre measuring point is in the middle of the points can be considered to be mean partial area. values in the context of the specified measuring accuracy. Where there is a uniform velocity profile, this enables a meaningful The reading for air volume flow for the measurement result to be achieved whole duct is then calculated as an even with a small number of arithmetic mean from the partial area measuring points. Where there are readings. larger differences in the flow velocities,

11 Air flow measurements in ducts according to DIN EN 12599

4.2. Centroidal axis method

The procedure for the centroidal axis axis is the radius (y) which bisects the method which should be used in round partial area. ducts is similar. It involves the circular duct cross-section being divided into Since it cannot always be assumed rings of an equal area and a circle in that the flow will move forwards in a the middle. The measuring location in rotationally symmetrical way in the the ring area and in the inner circle is duct, two measurement planes should on the centroidal axis of each partial be selected for round ducts which are area. In this respect, the centroidal at 90° angles to one another.

Measuring Rings points

Centroidal axes

Determination of the measuring locations of a circular cross-section with the centroidal axis method.

12 4.3. Calculation of the volume flow

The velocity readings determined via the trivial method or centroidal axis method should be used to calculate the average flow velocity, from which the air volume flow should then be calculated. The calculation is carried out using the following formula:

V = A v 3600

V = volume flow in m³/h v = mean flow velocity in m/s A = flow cross-section in m²

Example: A cross-section A of 0.5 m² and a measured mean velocity of 4 m/s gives a volume flow of 7200 m³/h

Determination of the volume flow based on the mean flow velocity and the duct cross-section

13 Air flow measurements in ducts according to DIN EN 12599

5. Evaluation of the readings

There is a requirement in DIN EN therefore always be seen in connection 12599 for the accuracy of the air with the distance from disruptions, volume flow to be determined with a because these are decisive when it measurement uncertainty of ±10%. comes to irregularity in the profile. Here, the question that now has to be asked is how accurate was the Step 1: Determining the measurement which has just been irregularity of the flow profile carried out. DIN EN 12599 also The required number of measuring provides answers to this. points in a specified duct cross- section depends on the irregularity In addition to the uncertainty of the (distortion) of the flow profile. The measuring instrument and the probe diagram below shows an empirical used, the irregularity of the flow profile relationship between the relative is a crucial factor for determining distance a/Dh (distance from the the total error. Where there is a disruption expressed as quantity large profile irregularity, the required of hydraulic diameters) and the measurement uncertainty of ±10% irregularity U of the flow profile (as a can only be achieved with a number percentage). It can be seen that the of measuring points that is just as profile irregularity diminishes as the large, but this is very time-consuming. distance increases. The number of measuring points must

14 a/Dh

10 Determination of the irregu-

8 larity U of the flow profile ac- 7 cording to the distance from 6 the disruption.

5 Example: 4 For a measurement at a distance of two-times the 3 hydraulic diameter, U is 40% (reading direction see yellow 2 arrows). On the other hand, for a = 4 Dh, U is under 20% (green arrows).

1 4 5 6 7 8 9 10 20 30 40 50 %

15 Air flow measurements in ducts according to DIN EN 12599

Step 2: Determining the number of read off the number of necessary required measuring points measuring points required to adhere With the value for U which has been to a particular specified measuring determined using the diagram, from accuracy. the following table you can now

Uncertainty of the measurement τ as a % Number of u measuring Irregularity of the profile U as a % points 2 10 20 30 40 50

4 6 12 20 28 36 42

5 5 11 17 24 31 36

6 5 10 15 21 27 32

8 4 8 13 18 23 27

10 3 7 12 16 20 24

20 2 5 8 11 14 16

30 2 4 7 9 11 14

50 1 3 5 7 8 10

100 1 2 3 5 6 7

200 1 1 2 3 4 5

Uncertainty of the measurement depending on the number of measuring points.

Example: For U = 40% and a specified measurement uncertainty of τU = ±15%, 20 measuring points are needed (yellow marks, reading direction from top to bottom and then to the left). For U = 20%, 8 measuring points suffice (green marks).

16 Tip: As you increase the measuring distance from the disruption, you can reduce the number of required measuring points and thus the effort involved in the measurement, without impairing the measuring accuracy.

Step 3: Calculating the irregularity duct cross-section into four quadrants of the flow profile with an equal area and determine the Using your readings, you can now arithmetic mean of the readings for check the irregularity of the flow profile each of the quadrants. arithmetically. To do this, split the

17 Air flow measurements in ducts according to DIN EN 12599

x x x x V1 V2 Division of the duct x x x x cross-section into four quadrants. A mean value is generated in each of x x x x the quadrants from the measured flow velocities. V3 V4 x x x x

The highest and lowest mean value will according to the following formula: give the irregularity of the flow profile

v – v U = max min 2 v

U (*100) = irregularity of the flow profile as a %

Vmax (m/s) = maximum of the arithmetic mean of all four quadrants

Vmin (m/s) = minimum of the arithmetic mean of all four quadrants

V (m/s) = arithmetic mean of the velocity in the whole cross-section

18 Step 4: Total error calculation Uncertainty due to influences at according to DIN EN 12599 the measuring point and measuring In addition to the measurement instrument errors (the accuracy of uncertainty due to (flow) influences at the measuring instrument and/or the the measuring point, there are other probes) have the greatest influence in possible error sources which may this respect. With modern measuring need to be taken into account: instruments, like the testo 400, these are automatically taken into • Measurement uncertainty when account when the total uncertainty is reading off calculated, thus supporting standard- • Measurement uncertainty of the compliant performance of the mean value (with fluctuating measurement and documentation of measurement parameter) the results. • Error of the measuring instrument display (measuring instrument error) • Measurement uncertainties of the material values, e.g. air density • Uncertainties with conversion

19 Air flow measurements in ducts according to DIN EN 12599

6. Evaluation of the readings

A full measurement report must be issued to the client, at the latest at the time of handover. This measurement report covers details of the building and project name, the specific measuring location, the agreed target value, the measuring instrument used, the recorded readings and the uncertainty of the measurement results, along with the date and place of the measurement. The testo 400 enables you to complete this measurement report directly in the measuring instrument and to send it by e-mail. It can’t get any more effective than that. Subject to change, including technical modifications. modifications. change, including technical Subject to

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