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Primary/secondary air circuit

Primary/secondary air circuit

Primary/secondary air circuit

(OP)
Hello there,

I would like your support with the following problem:

1.    I have to deal with a close loop (from now on " primary") made of a 600 x 600 mm square duct in which flows air at 120 °C temperature. Thermal losses are minimized with proper insulation: this way I have assumed temperature is constant in the primary. A centrifugal fan blows the air in the primary. The fan is capable to ensure a flow rate of 10000 m3/hr with enough head to overcome pressure drop of the primary. The primary is generously sized to keep pressure drop as low as possible.
2.    No. 5 centrifugal fans suck air from the primary, blow it into their own secondary circuit  (from now on "secondary") and return to primary. Each of the above mentioned fans is capable to ensure a flow rate of 2000 m3/hr with enough head to overcome just the pressure drop of their secondary. Automatic (on/off) valves are fitted both upstream each fan (in the secondary inlet) and in the secondary outlet. When a secondary fan is switched on the automatic valves of the corresponding secondary are switched on too. When the secondary fan stops the automatic valves of the corresponding secondary close.

Since any secondary fan is activated randomly, I was wondering if there could be a mutual interference which could affect the fan performance (i.e. vary the air flow rate of the secondary circuit)? Primary/secondary circuits arranged this way for water or diathermic oil do not produce interference, but is incompressible while air is not.
Would it be better to use a frequency inverter for the primary fan, in order to vary the primary flow rate according to the number of secondary fans activated? (An alternative option could be the use of modulating valve in the primary).
Any comment appreciated.

Thanks
 

RE: Primary/secondary air circuit

Interference, assumed to be a transient flow and pressure effect as the secondary fans are started, might be set up, until the primary flows can be diverted into the secondary loop and loads become balanced from loop 1 to loop 2.  Obviously, if a second loop fan starts extremely fast in relation to what the first loop can supply, that IMO would create some "interference".

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Primary/secondary air circuit

(OP)
BigInch,

Thanks for response.

The frequency inverter on the primary fan should adjust the primary flow to ensure enough air to meet each secondary loop requirement. I thought (but maybe this analogy is not correct) the primary duct could be considered as a "water reservoir", from which each secondary loop draws up its own water flow. If the water reservoir is fed with a flow rate at least equal to that required by the sum of the secondary fans activated at the mean time, the system should be balanced.
 

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