When to use: Visualize airflow velocity and pressure drop through a branched duct network. Color-coded velocity bands (green/amber/yellow/red) flag over-sized or undersized ducts. Animated particles show airflow direction and relative speed in each segment.
This simulator visualizes airflow velocity and pressure drop through a branched duct network, using animated particles and color-coded velocity bands to identify undersized or oversized duct segments. Engineers use it to verify duct designs comply with SMACNA velocity limits and to balance branch flows for uniform diffuser delivery.
Duct airflow velocity is calculated as V (fpm) = CFM / A (ft²), where A is the duct cross-sectional area. Pressure drop for round ducts is calculated using the Darcy-Weisbach equation: ΔP = f × (L/D) × (V/4005)², where f is the friction factor (approximately 0.018 for sheet metal), L is duct length in feet, and D is diameter in feet. The term (V/4005)² is the velocity pressure in inches of water gauge.
The equal friction method targets a uniform pressure drop per 100 feet of duct, typically 0.08–0.1 in. w.g./100 ft for main trunks. Branch ducts serving individual diffusers are sized to deliver their required CFM within the same pressure budget. The static regain method alternatively sizes downstream ducts to recover velocity pressure as static pressure, resulting in self-balancing systems.
In a branched network, each branch and end run must have approximately equal total pressure drop from the fan to the diffuser for natural balance. Dampers compensate for paths that are shorter or lower in resistance than the index circuit.
SMACNA (Sheet Metal and Air Conditioning Contractors National Association) HVAC Duct Construction Standards govern duct gauge, seam type, reinforcement, and maximum pressure ratings. SMACNA also provides friction loss charts and fitting loss coefficients (C-values) for duct design. ACCA Manual D is the residential equivalent for duct design.
ASHRAE Standard 90.1 sets duct leakage requirements — commercial ductwork must be sealed to Leakage Class 6 or better for systems above 2 in. w.g. static pressure. ASHRAE Handbook of Fundamentals Chapter 21 provides the friction charts and fitting data used in manual calculations.
Velocity limits protect against noise, excessive pressure drop, and erosion. Supply air trunks in commercial systems should target 600–900 fpm; branch ducts serving diffusers should stay below 600 fpm to avoid noise at the diffuser. Return air ducts have lower velocity limits (400–600 fpm) because any leakage pulls unconditioned air inward rather than losing conditioned air.
Duct leakage is a significant source of system inefficiency. A poorly sealed duct system can lose 15–25% of supply air to unconditioned spaces. ASHRAE 90.1 and many local codes require pressure testing of ductwork above certain system sizes. Using sheet metal with longitudinal seam locks and mastic sealant or UL-listed tape significantly reduces leakage compared to slip joints.
Set the total supply CFM for the system and adjust the main trunk diameter. Watch velocity colors update — green indicates acceptable velocity below 600 fpm, amber is moderate (600–900 fpm), yellow is high (900–1200 fpm), and red flags excessive velocity above 1200 fpm that will generate noise and excessive pressure drop.
Adjust individual branch duct diameters in the branch table to resize each segment. The system summary shows trunk pressure drop and maximum branch pressure drop — these should be approximately equal for a balanced system. Animated airflow particles reflect relative velocity in each segment.
Main supply trunks in commercial buildings are typically limited to 800–1,200 fpm. Branch ducts to diffusers should be 400–700 fpm to control noise at the terminal device. Return air main ducts operate at 600–800 fpm. High-velocity systems (used in older VAV systems) may exceed 2,500 fpm in mains but require sound attenuation.
The equal friction method sizes all duct segments to produce the same pressure drop per 100 feet, typically 0.08–0.1 in. w.g./100 ft. Starting from the fan, each successive duct is sized so that the cumulative pressure drop from fan to any diffuser is approximately equal. This simplifies design but requires dampers to balance shorter paths.
Balancing involves adjusting volume dampers at each branch or diffuser to equalize pressure drop across all paths from the fan. The index circuit — the longest or highest-resistance path — is left fully open, and all shorter paths are throttled to match its resistance. TAB (Testing, Adjusting, and Balancing) contractors use digital manometers and calibrated flow hoods to perform final balancing.
Duct noise results from high air velocity, turbulence at fittings, and vibration from fans transmitted through ductwork. Velocity above 1,000 fpm in branch ducts creates hiss at diffusers; sharp elbows and abrupt transitions create turbulence noise. Solutions include reducing velocity, using long-radius elbows with turning vanes, adding flexible connections at AHU outlets, and lining ducts with acoustical insulation.
Supply duct leakage in unconditioned spaces (attics, crawl spaces) loses conditioned air without delivering it to zones — this acts like a direct energy loss and also causes pressure imbalances that pull outdoor air into the building through envelope gaps. Studies show that sealing ductwork from leakage class 48 to leakage class 6 can reduce HVAC energy use by 10–20% in typical buildings.
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