Problem: HVAC 13, 1100 CFM of air flows in a 14 inch round duct.
Given: 1100 CFM of air flows in a 14 inch round duct; 200 CFM; 1100 CFM and it's a 14 inch round duct
Approach: The duct reduces the 12 ventures after a branch takeoff of 200 CFM.
Calc: We have a duct with 1100 CFM and it's a 14 inch round duct.
Calc: So here's our 1100 CFM and we'll call this state one.
Result: So this is closest to minus 0.02 so 0.02 inches of water decrease.
Office Hours
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Student questions asked in live office hours about this problem
OH 97: HVAC: HVAC-13
Q: In HVAC-13 (duct branch takeoff), why can you assume that the total pressures are equal at the branch junction?
A: This is a direct application of conservation of energy — at a duct branch junction, the total pressure is the same regardless of which path you take, because energy must balance at the split. Think of it like Bernoulli: the total pressure can't just appear or disappear at the junction.
OH 98: HVAC: HVAC-13
Q: In HVAC-13, are velocity pressure and static pressure inversely related — the velocity pressure increases at the branch, so does static pressure decrease?
A: Yes, inversely related: total pressure = static pressure + velocity pressure, so if velocity pressure increases, static pressure must decrease to keep total pressure constant. That's exactly what's happening at the branch takeoff — flow accelerates into the branch, trading static pressure for velocity pressure.