Parallel running of fans

In some applications, for example, for garage ventilation, fans are designed in parallel connection. The above-indicated solution enables one to operate only one fan during low demands for air replacement, turn on the second fan, if the sensors of air quality are turned on, and increase the flow rate. The indicated solution has several important conditions; if they are not adhered to, the required parameters of the equipment are not achieved; in a worst case scenario this could lead to the destruction of fans.

The same output characteristics are a condition for obtaining the maximum flow rate and efficiency in the region of maximal flow rates.

The working point must lie in a region close to the maximum flow rate of fans (where the equipment runs with good efficiency). The working point must be known perfectly and its position must take into account all the unfavourable operational states, such as, for example, the increase of resistances of clogged filtration chambers, operational changes of network lengths, etc.

The working point close to the point of air flow separation (forbidden region) leads to the fluctuation of the working point, strong vibrations and destruction of the rotor wheel. The minimum recommended pressure reserve is 15% below the separation point of the air flow (for details see the main EDV catalogue, axial-flow fans, furthermore, Theory chapter…).

Individual parallel branches should be designed of the maximum possible and at the same time identical length (resistances must be identical). The distribution of resistances must the largest possible in parallel branches (at least 40% of the overall resistance) and the smallest possible in common branches.

The pipes must be separated and connected so that flows influence each other to a minimum extent. Connection of parallel fans through the flap valves directly to the common line of a large size is extremely demanding as for the tightness of valves, their regulation and starting the running of fans (see the main EDV catalogue, Theory chapter… for further details).

Fans must be separated with tight flap valves
, otherwise the rotor wheel of the disconnected fan starts running in the opposite direction due to a “false draught” and is destroyed when connected with the sensor to the net.

Fans must be equipped with suitable control
with soft start up and the properly designed control of opening the flap valves (during the fan’s start up into a closed flap valve, the fan runs in the unstable forbidden region).

We recommend using the sensor of fan vibrations in the event of important installations as a preventive diagnostic measure. The sensor of vibrations may help to find the defect of bearings, operation of fans in the forbidden region or in an unsuitable working point, etc.

Details to the design of the parallel running of fans can be found in learning texts (ČVUT FS), specialized monograph “Fans” by Ing. J. Čermák CSc. 1974, or, as the case may be, in other specialized publications.
We do not provide any warranty for fans destroyed under the above-indicated circumstances.

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