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

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Extram« and Naar fan

3.7.3 Balancing tests

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.

0 B 020 m/s *.. 7~ ‘

7.5 m/s

~7~ 7.5 m/s 0 No control

7.5 m/s

Loss from main to point D or H estimated at 17.5 Pa and checked approximately

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
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.

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