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Series AFG Averaging Flow Grid Specifications - Installation and Operating Instructions - page 5 / 6

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5 / 6

Reading No.

12

3

4

5

Pv

250 230

260

240

212

Pv

15.81 15 17

.

16.12

15.49

14.56

15.43

Pv

96 92

99

96

90

Pv

9.80 9.59

9.95

9.80

9.49

9.73

Pv

48 46

47

51

45

Pv

6.93 6.78

6.86

7.14

6.71

6.88

DUCT AREA A = 250

X 500 mm = 0.125 m2

Pitot Tube Traverse

Average Average

Flow-

Mag-

Flow

Readings Pv Pa

of

of

grid

Factor

Constant

& Pv

Pv

Pv

Diff

Δp = M

K1 =

page 5

238.08

514.2

2.160

0.1095

94.67

199.8

2.110

0.1108

47.33

98.1

2.072

0.1118

2. Prepare holes in the duct wall upstream of the AFG Flow Grid for the Pitot static tube traverse to give an adequate survey of the duct velocity pattern and mean duct velocity.

AVERAGE k1 = 0.1107

Calibration on Site: To achieve the best possible accuracy, the AFG Flow Grids must be calibrated on site and the following method of site calibration and subsequent calculation is advised.

Table 6: Example: K Factor Calculation (Metric)

Any valid method of determining the volume flow rate may be used in establishing the flow rate to differential pressure characteristic.

PITOT STATIC TUBE TRAVERSE DATA

The following method and theory applies to the use of Pitot static tubes, as the primary means of determining volume flow rate by the velocity traverse technique.

To calibrate an AFG Flow Grid: 1. Install the AFG Flow Grid as described (See installation pages 2 & 3) and connect to a suitable manometer.

Δp

Pv

Ax1.291

CF M

3. Operate the system to give a typical flow rate through the Flow Grid and take records of Pitot static tube traverse readings and the Flow Grid differential pressure readings.

4. If possible, arrange the system flow rate to be changed to give additional sets of readings covering the range over which the sys- tem is intended to be used.

5. The theory of the AFG Flow Grid for normal atmosphere condi- tions is shown on page 4.

Diff Δp Pa

25

50

100

200

300

400

500

600

Δp

5

7.07

10

14.14

17.32

20.0

22.36

24.43

ΔpM3/s

Table 7: Graph Plotting Information Derived from Traverse Data (metric)

MX Flow Grid

2.704

2.475

2.214

1.917

1.565

1.107

0.783

Vol Flow Rate Q = K1

0.553

(Δp)/(Pv)= M (See page 4).

Imperial Units (example- using AFG Flow Grid in duct size 10” x 20”)

Measure the ambient temperature ‘t’, the ambient barometric pressure ‘B’ and the duct static pressure Ps. Calculate the cor- rection factor CF in the appropriate units.

Hence calculate the flow constant ‘K’ for each set of readings taken.

Table 5: Calculating Flow Constant K

Table 8: Example: K Factor Calculation (Imperial)

Pitot Tube Traverse

Average Average

Flow-

Mag-

Readings Pv Pa

of

of

grid

Factor

& Pv

Pv

Pv

Diff

Δp = M

PITOT STATIC TUBE TRAVERSE DATA

Δp

34 5

Pv

Flow Constant K1 = Ax4005

CF M

Reading

No.

12

SI Units

Imperial Units

0.95 0.99

1.03 1.06 0.99

0.975 0.995

1.015 1.030 0.996

Pv

Pv

1.002

1.004

2.169

2.160

3781

K1 = A x 1.291

CF M

K1 = A x 4005

CF M

Where = Δp = X Flowgrid differential pressure M = X Flow grid magnification factor Pv = duct mean velocity pressure Pa A = cross section area of duct m2 CF = correction factor Take the average K1 and plot the curve of Q v Δp

Δp = X Flowgrid differential pressure in. wg. M = X Flowgrid magnification factor Pv = duct mean velocity pressure wg A = cross section area of duct ft2 CF = correction factor

Where Q = K1

Δp

Q = K1

Δp

Q = volume flowrate ft3/min

Q = volume flowrate m3/s

6. Suggested format for results using worked examples:

Pv

0.48 0.50

0.52 0.53 0.49

Pv

0.693 0.707

0.721 0.728 0.70

Pv

0.23

0.25

0.25 0.24 0.22

Pv 0.480 0.50

0.50 0.49 0.469 0.488

DUCT AREA A = 10˝

X 20˝ = 1.388 ft2

0.710

0.504

1.063

2.110

3828

0.238

0.493

2.072

3863

AVERAGE k1 = 3824

Diff Δp Pa

0.5

1.0

1.5

2.0

2.5

Δp

0.707

1.0

1.225

1.414

1.581

Δpft3/min

Table 9: Graph Plotting Information Derived from Traverse Data (Imperial)

MX Flow Grid

6046

5408

4684

3824

Vol Flow Rate Q = K1

2703

Metric Units (example- using AFG Flow Grid in duct size 250 mm x 500 mm)

(see Fig. 17 for Graph)

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