Calculating Pressure Drop In Ductwork

Pressure Drop Equation:

\[ PD = \frac{f \times L \times V^2 \times \rho}{2 \times D} \]

Friction Factor (f):

dimensionless

Length (L):

Velocity (V):

m/s

Density (ρ):

kg/m³

Diameter (D):

Pressure Drop (PD):

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1. What is Pressure Drop in Ductwork?

Pressure drop in ductwork refers to the loss of pressure as air flows through a duct system due to friction and other resistances. It is a critical parameter in HVAC system design to ensure proper airflow and system efficiency.

2. How Does the Calculator Work?

The calculator uses the Pressure Drop equation:

\[ PD = \frac{f \times L \times V^2 \times \rho}{2 \times D} \]

Where:

\( f \) — Friction factor (dimensionless)
\( L \) — Length of duct (m)
\( V \) — Velocity of air (m/s)
\( \rho \) — Density of air (kg/m³)
\( D \) — Diameter of duct (m)

Explanation: The equation calculates the pressure loss due to friction in a straight duct section, which is proportional to the friction factor, length, square of velocity, and density, and inversely proportional to the diameter.

3. Importance of Pressure Drop Calculation

Details: Accurate pressure drop calculation is essential for proper HVAC system design, fan selection, energy efficiency optimization, and ensuring adequate airflow throughout the duct system.

4. Using the Calculator

Tips: Enter all values in appropriate units. Friction factor typically ranges from 0.02 to 0.05 for smooth ducts. Standard air density is approximately 1.2 kg/m³ at 20°C.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical friction factor value for ductwork?
A: For smooth metal ducts, friction factor typically ranges from 0.02 to 0.03. For flexible ducts, it can be higher, around 0.04-0.05.

Q2: How does duct shape affect pressure drop?
A: Non-circular ducts have higher pressure drops. The equivalent diameter should be used for rectangular ducts in calculations.

Q3: What are acceptable pressure drop values in HVAC systems?
A: Typical design values range from 0.5-2.0 Pa per meter of duct length, but this varies based on system requirements and energy efficiency goals.

Q4: How does air temperature affect pressure drop?
A: Higher temperatures reduce air density, which decreases pressure drop. The density term in the equation accounts for this effect.

Q5: Are there other factors that contribute to pressure drop?
A: Yes, fittings, elbows, transitions, and other components create additional pressure losses that must be calculated separately and added to the straight duct pressure drop.