There are two types of connecting air ducts: series connection and parallel connection. In case of parallel connection of air pipelines with a different diameter the flow rate is one and the same in each of these pipelines.

Q=Q1=Q2=Q3 - fig. 1.28

And the fluid velocity of each pipeline depends on its diameter.

In parallel pipelines with branches the flow rate of each pipeline is different and it is the total of all separate branches.

Q=Q1+Q2+Q3 - fig. 1.29

Fig. 1.31 shows how all the separate branches are presented in the HQ diagram where Q is the flow rate and H is the pressure of the system. The chart represent parallel connected individual branches.

Fig. 1.30 represent series connection of individual branches with constant volume of air.

Losses in the separate branches of air ducts with parallel connection are always different. Pressure at the start of the air duct (point “A” fig 1.29) is used to overcome the resistance of each branch. Once all pipeline branches are correctly estimated, the system result characteristic is given - fig. 1.31 (red line) and the diagram can be used to determine what pressure is required in order to eliminate all hydraulic losses and also what the flow rate will be.