Supply vs Return Static Loss

[Jamsith]

Dear Friends:

I am still having these doubts everytime when I start to make a static pressure calculation. Some time I dont consider the static pressure for the Return duct. Since very small in the length. But some cases the return duct also need to make same like supply duct. in these I calculated the static loss for the return. but this static losses for the return duct also, should I have to add with the supply duct losses, OR needs to compare individually the return duct losses with the machine ststic same like comparing for the supply duct.

why I am asking the Fan will create the Positive and negative statics, for the supply duct we need the positive static and for the return we need the negative, by this we can't compare the machine static by adding the supply & return together. but the consultant want to do it ? Am I right?? Please let me know,

Thanks

 

[GT-EGR]

Unless you have a return fan, you have to size your supply fan for all the static losses associated with the entire system. Every grille, duct, and accessory, whether before or after the fan, is still an obstacle for the supply fan to overcome.

This is always the case, so even if you ignored return in the past, you only got away with it because it was likely a small number and any safety you had in your supply side calculation just ended up overcoming that resistance on the return side.

The fan static is just a differential, and it will have negative pressure on one side, and positive pressure on the other. But the total difference between those two is your fam static pressure. And depending on how much resistance you have on supply vs return you’ll see more negative or more positive pressure at the fan inlet/outlet, but it will still be the same total static created by the fam.

 

[chicopee]

Why bother with all these calculations. I would plot the fan characteristic curve and the so called system resistance curve on graph paper with the ordinate as the static pressure (inches H2O or other units) and the abscissa as the air flow (CFM or other units); where those two curves intersect, that will be the air delivery at whatever pressure the graph tells you from the fan outlet. The determination of the system resistance will be a "dog" to determine as you will need to make several calculation to get a viable curve which in all likelihood will be nearly quadratic in form.