Chapter 6: Duct Design & Air Distribution in HVAC

 

Chapter 6: Duct Design & Air Distribution in HVAC

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🔹 6.1 What is a Duct System in HVAC?

A duct is a passageway made of metal, fiberglass, or flexible plastic that delivers and removes air in HVAC systems.

➡️ Purpose: To distribute conditioned air (cooled/heated) to rooms and return stale air back to the system.

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🔹 6.2 Types of Ducts

Type	Description & Use
Rectangular Ducts	Common in commercial buildings; easy to install in tight spaces
Round Ducts	More efficient airflow, less pressure loss
Flexible Ducts	Used in short runs, residential false ceilings
Spiral Ducts	Aesthetic & low leakage, used in exposed ceiling designs

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🔹 6.3 Components of Air Distribution System

Component	Function
Supply Duct	Delivers conditioned air to rooms
Return Duct	Brings used air back to the air handler
Grilles/Registers	Control airflow into/out of rooms
Diffusers	Evenly distribute air in space
Dampers	Regulate or block airflow in ductwork
Plenums	Air distribution chambers connected to ducts

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🔹 6.4 Duct Design Principles

Designing ductwork properly ensures:

Efficient air delivery

Quiet operation
Minimum energy loss

➡️ Key Factors in Duct Design:

1. Airflow Rate (CFM) – Cubic Feet per Minute

2. Velocity of Air (fpm) – Ideal: 600–1200 fpm (feet per minute)

3. Friction Loss (inches of water) – Caused by bends and length

4. Duct Material – Galvanized steel, aluminum, flexible plastic

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🔹 6.5 Duct Sizing Methods

✅ 1. Equal Friction Method (Most Common)

All duct sections have same friction loss per unit length.

✅ 2. Velocity Reduction Method

High velocity in main duct, reduced in branches.

✅ 3. Static Regain Method (Used in high-rise buildings)

Adjusts duct size to regain static pressure and balance airflow.

📌 Formula to Calculate Air Flow (CFM):

$$CFM=\text{Velocity (fpm)}\times \text{Area of duct (sq ft)}$$

🔹 6.6 Air Distribution Techniques

Type	Application	Placement
Ceiling Diffusers	Offices, homes, hospitals	Ceiling center or corner
Side Wall Grilles	Homes, classrooms	On walls, just below ceiling
Floor Registers	Residential, under windows	In the floor

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🔹 6.7 Duct Design Considerations

1. Noise Control – Use acoustic lining to reduce duct noise.

2. Leakage Prevention – Seal joints, use proper insulation.

3. Fire Safety – Install fire dampers at wall penetrations.

4. Air Balancing – Adjust dampers to ensure even airflow to all rooms.

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🛠️ Practical Application

Check your home or office for supply and return grilles.

Feel air velocity and note which area gets more or less airflow.
Observe duct shapes in malls (round, spiral, rectangular).

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📄 Assignment Questions

1. List and explain different types of ducts.

2. What are dampers, and why are they used in HVAC systems?

3. Explain the equal friction method of duct sizing.

4. How is air balancing achieved in a duct system?

5. What materials are used for duct fabrication?

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💼 Interview Questions

1. What is the purpose of a return duct?

2. How do you determine the size of a duct for a room?

3. What are the common problems in duct systems?

4. Explain the role of diffusers and grilles.

5. What steps are taken to reduce duct noise?