22 Duct

Updated on 01/04/26 by Mananjay MahatoShare on WhatsApp

1. Fundamentals of Ducting

A duct is a passage used in air conditioning to deliver and remove air. These are basically “pipes” for air.

  • Purpose: To transport conditioned air to the target space and return room air back to the AC plant.

  • Classification by Pressure:

    • Low Pressure: Static pressure up to 50 mm (2 inches) of water gauge. Velocity < 600 m/min.

    • Medium Pressure: Static pressure up to 150 mm (6 inches) of water gauge. Velocity 600–1200 m/min.

    • High Pressure: Static pressure up to 250 mm (10 inches) of water gauge. Velocity > 1200 m/min.

2. Duct Materials

Exam questions often focus on the properties and applications of these materials:

  • Galvanized Iron (G.I.): Most common. Coated with zinc to prevent rusting. Standard for most commercial HVAC.

  • Aluminum: Lightweight and corrosion-resistant. Used in high-humidity areas (like coastal regions) or where weight is a factor.

  • Stainless Steel: Used in labs, kitchens, or chemical plants where high hygiene or chemical resistance is required.

  • Fiberglass (Duct Board): Provides built-in thermal insulation and sound absorption.

  • Flexible Ducts: Made of rubber/plastic with wire reinforcement. Used for final connections to diffusers to handle misalignments.

3. Shape and Design

The shape of a duct affects friction loss and space management.

  • Circular (Round) Duct: * Fact: Most efficient. Has the minimum friction loss and requires the least material for a given cross-sectional area.

    • Drawback: Difficult to fit in false ceilings with low heights.

  • Rectangular Duct: * Easier to install in tight spaces (above false ceilings).

    • Aspect Ratio: The ratio of the longer side to the shorter side. Ideally kept below 4:1. Higher ratios increase friction and noise.

  • Square Duct: A compromise between round and rectangular.

4. Duct Design Methods

For competitive exams like ISRO/DRDO, remember these three methods:

  1. Velocity Reduction Method: Arbitrarily reducing air velocity in each sub-duct. Simple but not very accurate.

  2. Equal Friction Method: Most widely used. The pressure drop per meter of duct length is kept constant throughout the system.

  3. Static Regain Method: Converts velocity pressure into static pressure to offset friction losses. Best for long, complex systems.

5. Important Formulas & Technical Terms

  • Continuity Equation: $Q = A \times V$

    (Where $Q$ = Airflow (CFM/m³/h), $A$ = Area, $V$ = Velocity)

  • Equivalent Diameter ($D_e$): Used to find a round duct size that has the same friction loss as a rectangular duct.

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

  • Static Pressure: The outward pressure exerted by air on duct walls (measured by a U-tube manometer).

  • Velocity Pressure: The pressure due to the speed of air.

  • Total Pressure: Sum of Static + Velocity pressure.

6. Insulation and Accessories

  • Thermal Insulation: Applied to prevent heat gain (in cooling) or heat loss (in heating). Common materials: Glass wool, Nitrile rubber, Thermocol.

  • Vapor Barrier: Essential on the outside of cooling duct insulation to prevent condensation (sweating).

  • Dampers: * Volume Control Damper (VCD): Regulates air flow quantity.

    • Fire Damper: Automatically closes using a fusible link to stop fire/smoke spread.

  • Diffusers & Grilles: * Grille: Used for return air; usually has no volume control.

    • Diffuser: Used for supply air; spreads air in multiple directions.

7. Quick Revision Table for Exams

Feature Round Duct Rectangular Duct
Material Usage Least (Economical) More
Friction Loss Lowest Higher
Space Required High vertical space Low (can be flat)
Rigidity Naturally strong Needs “cross-breaking” for strength
Noise Lower Higher

Key Exam Tips:

  • Manometer: Used to measure duct static pressure.

  • Anemometer: Used to measure air velocity at the duct outlet.

  • Pitot Tube: Used to measure velocity pressure inside the duct.

22 Duct