風管摩擦壓降

概述

Duct friction pressure loss (major loss) is the energy drop caused by air flowing through ductwork. In galvanized steel circular ducts with turbulent flow, the loss per unit length depends on airflow rate and duct diameter. This page covers the standard empirical formula, reference data for common duct sizes, and practical design limits.

關键公式

Duct Friction Loss (Imperial)

$\Delta h = 0.109136 \times \frac{q^{1.9}}{d^{5.02}}$

where $\Delta h$ is in inches of water gauge per 100 ft of duct, $q$ is airflow in cfm, and $d$ is duct inside diameter in inches.

Air Velocity from Flow Rate

$v = \frac{q}{A} = \frac{q}{\pi (d/2)^2}$

For a circular duct with diameter $d$ in inches and flow $q$ in cfm, velocity in ft/min is:

$v = \frac{576 \, q}{\pi \, d^2}$

Rectangular Duct Equivalent Diameter

For non-circular ducts, replace $d$ with the equivalent diameter:

$d_e = 1.30 \times \frac{(a \cdot b)^{0.625}}{(a + b)^{0.25}}$

where $a$ and $b$ are side dimensions in inches.

變量

Friction Loss Reference Data

Head loss and air velocity for galvanized steel circular ducts with turbulent airflow.

Air Velocity Calculator

Estimate air velocity in a circular duct from flow rate and duct size.

Imperial Units Calculator

The original page included an Imperial units calculator. This migrated version calculates air velocity, straight-duct friction loss per 100 ft, and total friction loss for round galvanized steel duct using the source equation.

SI Units Calculator

This SI calculator is the same equation with metric inputs converted internally to cfm and inches.

單位換算器

Friction Loss Trend — 10 in Duct

The chart below shows how friction loss rises sharply with airflow in a 10-inch duct. Loss scales roughly with $q^{1.9}$, so doubling airflow roughly quadruples the pressure drop.

Air Ducts - Friction Loss Diagram

The original friction loss diagram covers several round duct sizes. This interactive version keeps the same intent by plotting friction loss against air volume for common duct diameters.

还原的原始源表

The following tables are restored from the original source page to preserve the complete reference data.

Air Ducts - Major Friction Head Loss

Air Ducts - Major Friction Head Loss, Normalized Source Rows

The source table alternates friction head-loss rows and air-velocity rows under the same duct-size columns. The normalized table below preserves the same source values in one row per air volume.

原始源圖

The following original source images are preserved to avoid losing visual reference material. When an image contains chart or tabular data, its extracted values are represented in the page tables, calculators, or interactive charts; remaining images are retained as visual source references.

工程要點

• Noise limits. Air velocity should stay below roughly 600–1000 ft/min in residential applications, 1500 ft/min in quiet commercial spaces, and 2000+ ft/min only in industrial settings. Exceeding these limits causes objectionable noise.
• Turbulent flow assumption. The empirical formula above assumes fully developed turbulent flow, which is typical for HVAC duct sizing. It does not apply to laminar or transitional regimes.
• Material. Reference data applies to galvanized steel ducts. Flex duct, lined duct, and fiberglass duct have higher friction factors and require correction.
• Fittings and transitions. The formulas cover major (straight-duct) losses only. Elbows, tees, transitions, dampers, and grilles add minor losses that often dominate in short duct runs. Total system pressure drop must include both.
• Rectangular ducts. Always convert to equivalent circular diameter before using the friction loss formula. Undersized rectangular ducts are a common source of excessive static pressure.
• Economic balance. Larger ducts reduce friction loss (and fan energy) but increase material and installation cost. The $q^{1.9} / d^{5.02}$ relationship means a modest increase in duct diameter yields large reductions in pressure drop.

參考资料

• Engineering ToolBox — Air Ducts Friction Loss