Balancing scheme
1. On a line diagram of the duct system with its outlets, mark the length in metres from the extreme outlet to each of those before it, in the direction towards the fan, i. e. the extreme outlet is O, and first outlet is the length of main in metres between it and the extreme outlet.
2. Convert these lengths in metres into diameters of pop. If the pop is 150 mm diameter or 0.15 m, and length is 21.3 metres for example, this is 142 diameters.
3. From the balancing chart in Figure 3.76, read against each value in diameters the opening length in percentage of pop diameter. For example, if 140 diameters, the resistance equivalent is 76.5% of pop diameter.
Figure 3.76 Balancing chart 70 SO 90 100 110 120 130 140 Opening length X of branch dtamatar |
Methods of balancing
In blowing systems the connection of an air outlet pop to the main is made at 45°. The development of the hole in the main to attach the pop is of a peculiar shape. Its total length is 1.41 x diameter of pop and is shown in Figure 3.74.
The Table in Figure 3.77 shows three examples worked for a length of ducting with pops of 150 mm, 250 mm and also of graduated diameter from 215 mm to 250 mm diameter. 4. These lengths of opening are then specified on the working drawing for ducting as millimetres, and are usually shown alongside for ducting as millimetres, and are usually shown alongside the pop in a circle thus: (^25) Work to nearest 5 mm. |
Design balancing is based upon alteration of the length of this hole, producing restriction as required and gradually adding resistance to the outlets from those at the extreme end of the main to those near the fan. See Figure 3.75.
M-1.41 0*-i |
Figure 3.74 Hole in main duct for branch |
Compensating langth |
Extram« and Naar fan
Experiments made some time ago by Sturtevant Engineering Company Ltd showed that as the main duct static resistance is increased, equivalent to various lengths of main duct, an exactly similar addition of static resistance had to be inserted into the branch to maintain flow constraint.
They first set up conditions to represent a branch at the extreme end of a main. The air velocity was 7.5 m/s in both the main and the branch. There was no control resistance in the branch.
Conditions were then created to represent the first branch in a system with the main velocity 20 m/s and the branch velocity
7.5 m/s.
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Test: |
A |
B |
C |
0 |
E |
F |
G |
H |
Resistance of main B • F |
Control resistance added D•H |
Difference In pressure velocity A • C |
Regain A-C |
% Regain |
. |
50 |
1175 |
145 |
100 |
70 |
675 |
90 |
50 |
50 |
50 |
106 |
95 |
90 |
2 |
150 |
222 5 |
250 |
205 |
155 |
155 |
106 |
100 |
94 |
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3 |
275 |
345 5 |
375 |
325 |
275 |
275 |
106 |
100 |
94 |
Figure 3.77 Results of balancing tests |
Figure 3.78 Noise from room to room |
The results can be seen in Figure 3.77
As the air velocity attained the branch value at the entrance to the branch, this regain must be passed into the main air stream and is returned in the regain from Ato C. The volume flow in the branch was measured by a Venturi.
Posted in Fans Ventilation A Practical Guide