ESP Calculation 3

[TabRazesMechEngr]

Hi there,

Just wanted to ask a simple question but couldnt find a satisfactory answer yet. Actually, while calculating the esp in a duct, we usually take the index route or the critical route. But my question is that shouldnt the esp be the sum of all the routes including the index route cuz the air is being supplied to all the diffuser outlets at once, so why is the index route only considered leaving out the rest?? I understand that index route has the highest pressure drop but arent the other routes also having some pressure drop, the total esp should be = esp (index route) + esp for the other routes. And similarly for the pump head calculations as well. But we only consider the index route, why is it, can anyone please explain it. Thanks

 

[BronYrAur]

You are calculating the pressure drop that the fan or pump must overcome. If it can overcome the pressure drop of the critical route, then it has enough pressure to overcome the other routes. Your routes are in parallel to one another. During the balancing process, you will artificially add pressure drops to the non-critical routes via damper or circuit setters. The individual flows are additive, but not the individual pressure drops.

 

[MTES93]

Think of pressure developed by the fan in a similar manner as the voltage provided by a battery..

 

[GT-EGR]

If the pressure were additive, it would be saying that the fan has to have enough power to move the air all the way through one path and then again all the way through a second path, and so on. But really, if you think about just one "molecule" of air, taking a trip through the whole duct system, it just needs the ability to make it from the beginning to the end one single time. Simultaneous (in parallel) to that, all the other "molecules" are also traveling from the beginning to the same or different end, and they all utilize that same single pressure/force created by the fan.

 

[urgross]

If you are driving on a tollway, and you drive the full length, do you pay the toll for going the whole distance down the tollway, or do you pay the toll for every exit on the tollway plus the final toll?

 

[lukaiENG]

good analogy urgross!

 

[lilliput1]

The pressure drop is from the inlet to the outlet. In the ductwork system the pressure at the end of all air outlet is the same and same with all the inlet. When air is pushed by the fan and if the ductwork runs are not balanced more air will go through the least resistance. The duct paths of least resistance need damper to regulate the air going through. The fan if sized to deliver the design cfm at the index run therefore would be adequate so long as the paths of least resistance are properly dampered to allow their corresponding design air cfm to pass through and kill of the corresponding excess pressure.