Question of System Curve

[funnyguy]

Dear all,

I've designed a ventilation system, which comprise 6 branch from the main ductwork, and each branch is end cap with motorized on/off damper & diffuser.
Since the required flowrate and total pressure for critical path of the captioned ductwork is identified. System curve can be plotted with relation P=kQ^2 by the such value.
However, if three branch outlet is kept no airflow status by the motorized damper (off position) and all these three branch outlet is not included in the critical path,
I would like to know, would the new operation point still on the original "System Curve" or a new system curve would generate in such occurance.

Thanks for help!

 

[remp]

In my opinion there would definatly be a different system curve. How different would depend on the difference in loss coefficients at the branches,i.e. System 1: damper open and air is going down the branch and System 2: damper closed and where the air pass by the and does not go down the branch....look up loss coefficient chart for fittings (ASHRAE) and check branches for straight thro' or tee off.

 

[lilliput1]

It will be a different system curve. Break up the worst case run in portions with corresponding CFM at each portions. Use the equation at each portion using the correspondinf CFM^2 ratio & sum up your total pressure drop. You can easily do this on spreadsheet. Each section would be a column. Each row can be a case of run identifying the corresponding CFM at each corresponding sections. Set up one row as the base, worst case CFM scenario. You can the set up the calculations on another another row using the section Pb=Pa x CFMb^2/CFMa^2. Using CFMa from the base CFM row & CFMb from the CFM row of the scenario you want to calculate.
You can do this system by both air & water flows.

 

[funnyguy]

dear lilliput1,

Thanks for your kind opinion, since I've identified the critical path and calculate with the corresponding total pressure drop ventilation system.

Then, is it accurate to generate the "System Curve" by using the value "total pressure drop" and the design flowrate of the captioned system.

Thanks a lot

 

[lilliput1]

Yes. What happens is when balanced the pressure loss from each inlet branch to the fan suction are equal because the differential pressure = athmospheric pressure - fan suction pressure. Similarly the pressure loss from the fan discharge to each termination of each supply branch is equal = fan discharge presure - athmospheric pressure.
The balancer must adjust dampers in branches to kill off excess pressure available in the low loss runs so eventually the loss through them is each equal to the loss through the worst run. Then when you do part load analysis of particular branches you must take note of this adjusted damper PD1. Note that PD2 through it with new flow will equal PD1 x CFM2^2/CFM1^2.

 

[funnyguy]

dear lilliput1,

And I would like to know, does the "total pressure drop" of branch ductwork (Tee-off from Main ductwork) has influence when predicting the system curve?
Since when calculating the pressure drop of "Critical Path", pressure drop of such branch ductwork is ignored.

Thanks

 

[lilliput1]

Air will then follow the path of less resistance until flows become such that PD to each branch will be equal when all branch terminals at at one pressure, athmospheric. Understanding is the key to engineering.