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Beginners Guide to Sheet Metal Duct Fabrication Process

Sheet Metal Duct Fabrication

A sheet metal duct plays an integral role in the HVAC systems.

Therefore, fabricating sheet metal duct is critical in HVAC systems for efficient airflow and circulation.

Today’s guide takes you through the fundamental aspects about the sheet metal duct fabrication process – from sheet metal duct design, and choosing material to various fabrication procedures.

So, if you want to be an expert in fabricating sheet metal duct systems, read this guide.

Step 1: Designing Sheet Metal Duct for Fabrication

A sheet metal duct system or ductwork is an assembly of many parts and components. Therefore, the design process may vary slightly depending on application requirements.

So, when you assemble all HVAC accessories, you get a complete system such as this:

Duct Network
Duct Network

But, before we explore the duct design process, let’s have a quick recap:

What is a Duct in HVAC?

It is a tube, pipe, or channel in the HVAC system for free airflow.

Therefore, when you connect these channels or tubes using various accessories, you have a ductwork system.

Duct System
Duct System

Generally, the HVAC duct systems are available as:

  1. Flexible ductwork – these are bendable HVAC ducts. They are suitable for places that require bending. Moreover, they are made from flexible wire or coil covered with flexible material
  2. Rigid duct work – as the name suggests, these ducts do not bend. They are available in rectangular or cylindrical shapes.

For the scope of this guide, we will only focus on sheet metal ducts.

Round Sheet Metal Duct
Round Sheet Metal Duct
Rectangular Sheet Metal Duct
Rectangular Sheet Metal Duct
Spiral Sheet Metal Duct
Spiral Sheet Metal Duct

Apart from these, we also have oval-shaped duct systems. Ideally, these are the most common HVAC duct designs you can consider.

Sheet Metal Duct Design Considerations

Design is a critical stage in sheet metal duct fabrication. It determines the cost, efficiency, and practicality of the final HVAC ductwork system.

There are certain aspects you must consider during the design process such as:

1. Determine the Air Flow Characteristics

As you size the ductwork system, it is critical to evaluate:

  • Amount of air that will flow in the HVAC duct, which is normally in cubic feet/minute
  • The duct size whose measurements are in square feet
  • Rate at which air should flow in the HVAC duct system can be determined in feet per minute

Well, there are other airflow characteristics you should analyze such as;

  • Pressure – these include both gauge and absolute pressure
  • Air density
  • Analyze airflow principles

Additionally, you should also know the pressure in the duct system. Here, you can evaluate:

  • Static pressure
  • Total pressure
  • Velocity pressure

After determining all these parameters, it will be easier to design a specific sheet metal duct system for your applications.

2. Duct Components

There are many components that make the ductwork system.

Even as you design the duct, you should figure out how to integrate it with other HVAC accessories such as:

  • Duct boots
  • Duct adapters
  • Duct angles
  • Duct caps
  • Duct cleats
  • Duct collars
  • Duct corners
  • Duct couplings
  • Duct elbows
  • Duct reducers or duct increases
  • Duct plenums
  • Duct tees
  • Duct wyes
  • Duct air return box
  • Duct air return frame
  • Duct transitions

Depending on the HVAC duct design, it may have more components.

Ideally, these components will also determine the duct design. Remember they should join seamlessly for optimal airflow.

Additionally, they may affect the airflow characteristics. Of course, all these determine the duct sizing in the long run.

3. Duct Classification

Classification criteria form a fundamental aspect in duct design. Broadly, you may classify ducts depending on:

  • Velocity where you can have high, medium, or high velocities
  • Pressure classifications – again you may have low, medium, and high pressure

4. HVAC Duct Shapes

Duct Shapes
Duct Shapes

When it comes to fabricating sheet metal ductwork, you will have:

Round Duct Design

It is an efficient ductwork due to low resistance to airflow.

When you compare ducts with the same capacity, round ducts require less sheet metal for fabrication. Of course, this is not the same case in rectangular ducts and square ducts.

Therefore, when it comes to the fabrication cost, the round ductwork will be affordable.

Generally, round ducts are known for:

  • Less pressure drops
  • Characterized by better airflow
  • Low dirt accumulation thereby reducing bacterial growth
  • Superior acoustic performance
  • May require less insulation
  • They come in the longer length

Remember, you can fabricate round ducts using plain sheet metal or with spiral lock seams.

The spiral lock seams make the round duct more rigid. Furthermore, by adding spiral seams, you can use sheet metal with lighter gauges.

Finally, the leakages in spiral ducts are less. When it comes to sealing leakages, the process is easier in round duct systems than in square or rectangular ducts.

Rectangular Duct Design

When it comes to fitting in most buildings, these duct designs are easy to install.

You can easily customize rectangular ducts by adjusting the width-to-height ratio in case there are space constraints.

Normally, we refer to the width-to-height ratio as the aspect ratio in HVAC. Therefore, even as you design the rectangular duct, the aspect ratio should not go above 4.

Ducts with aspect ratios above 4 are:

  • Not efficient since you will use more material
  • Prone to high-pressure loses

In most cases, a 2 or 3 aspect ratio will serve you well.

Even as you choose these ducts, it is important to pay attention to the following critical elements:

  • Joints that are difficult to seal
  • High-pressure drop
  • High expense in terms of material when you compare rectangular to round duct
  • If not properly designed, you may have a high aspect ratio
Oval Duct Design

In case there is limited space, you can manipulate the height of the round duct. With this design, you will still enjoy the benefits of round HVAC duct systems.

However, you may experience the following challenges:

  • HVAC duct accessories are difficult to find and fabricate
  • Handling large sizes can be a challenge
  • Interestingly, these ducts may become more circular when you subject them to extremely high pressure
  • Where you have a large aspect ratio, joining the oval ducts can be difficult.

At times, you may wish to replace rectangular ducts with round ducts and vice-versa.

In such situations, your design must factor in the equivalent diameter.

With the HVAC duct fabrication, the equivalent diameter is the diameter of a circular duct system with the capacity to deliver a pressure drop in the rectangular ducts. Of course, this should be under the same airflow characteristics.

1. Sizing the Duct for HVAC

Broadly, you can use these methods to size ducts for the HVAC system:

  • Equal friction method
  • Velocity reduction method
  • Static regain method

As you can see, designing sheet metal ducts involves many considerations and arithmetic. For that reason, we may have not covered all that in detail here.

However, here are some useful resources for you:

Step 2: Choosing Sheet Metal Duct Material

Although aluminum and galvanized steel remain key materials in sheet metal duct fabrication, manufacturers are adopting other metals for specialized applications.

Remember, there are certain environments such as highly acidic or corrosive environments. Such require high corrosion-resistant materials.

Let’s look at some materials you can consider for your sheet metal HVAC duct fabrication:

Galvanized Steel

Galvanized steel ducts are common in most industrial and domestic applications.

You can consider either the:

  • ASTM A653 or
  • Coating C90

Galvanized steel ducts are corrosion-resistant. They can also withstand higher temperatures.

You can learn more about Galvanized Sheet Metal Fabrication Processes.


Aluminum ductwork is suitable for clean rooms. The most common alloys for aluminum ductwork fabrications are:

  • ASTM B209
  • 1100 Series
  • 3003 Series
  • 5052 Series

Additionally, you can use aluminum ductwork for moisture-laden air. They are also perfect for exhaust systems or ornamental HVAC duct systems.

Aluminum ductworks are good for high strength and lightweight. You can strengthen aluminum and bend it multiple times.

Additionally, you can install aluminum ducts without requiring much structural support.

You can learn more about Aluminum Sheet Metal Fabrication Process.

Stainless Steel

Stainless steel ducting systems are also popular in modern HVAC systems. They are perfect for air with high moisture content, exhaust fumes from the kitchen, etc.

For stainless steel ducts, you can consider these grades:

  • ASTM A167
  • Class 302/304

At times, you may apply certain surface finishes such as No. 2B in case you are fabricating concealed duct. Alternatively, the No. 4 surface finish is popular for stainless steel ducts that are exposed.

The superior corrosion-resistant property makes stainless steel ductwork suitable for most harsh environments.

You can learn more about Stainless Steel Sheet Metal Fabrication.


Copper duct systems are popular in specialized applications. Of course, they may also be decorative.

Copper ducts are common for exhaust systems in some chemical industries.

Copper is popular for its ductility, heat conductivity, electrical conductivity, and anti-microbial properties.

Here is more information on Copper Sheet Metal Fabrication.

Carbon Steel

Carbon steel ductwork is a perfect choice for high-temperature applications. However, they may require special coating for industrial applications.

You can learn more about Steel Sheet Metal Fabrication.

Depending on your metal ductwork requirement, you can use any other sheet metal. However, you must evaluate the fabrication and maintenance costs.

Step 3: Sheet Metal Duct Fabrication Techniques

When it comes to fabricating metal ductwork, the process may vary slightly depending on the type and configurations.

Ideally, all processes remain the same except when it comes to shaping the metal ductwork.

Let’s look at how metal ducts are made:

Round Sheet Metal Duct Fabrication

Fabricating sheet metal ducts may vary slightly depending on the configuration.

For instance:

Fabricating Spiral Metal Ductwork

The process involves:

  1. Choose sheet metal duct material – it can be aluminum, stainless steel, or galvanized steel
  2. Cut the sheet metal to make strips – the metal strip dimensions will depend on the duct design
  3. You will then feed the metal strip into the spiral duct-making machine.
  4. Usually, spiral duct making machines have inner and outer welding capabilities. However, the machine design may vary.
  5. As you feed the strip metal into the machine, it coils and welds at the same time. Usually, this is a continuous process producing spiral ductwork.
Spiral Duct making Process
Spiral Duct making Process

Fabricating Round Metal Duct without Spiral Lock Seam

Again here, making round ductwork is straightforward:

  1. Choose ductwork material, which can be stainless steel, aluminum, or galvanized steel
  2. Cut the sheet metal using a laser cutter, plasma cutter, or any suitable technique to recommended dimensions
  3. Form sheet metal into a circular shape – the sheet should form a complete circle, leaving a small section for welding
  4. Weld the two sections – you will form a complete and continuous conduit, which will be the round or circular duct. Alternatively, you can use a seam to join the sheet metal duct sections.

Closely related to these two is the oval duct fabrication process. The process is similar to the two above.

Ideally, an oval duct is a round or circular duct that has been “stretched”. Therefore, you will start by making a round duct.  Then, use a special machine to make the oval ductwork.

Fabricating Rectangle Sheet Metal Duct & Square Sheet Metal Duct

The difference between rectangle sheet metal ducts and square sheet metal ducts is in the dimensions. However, the ductwork fabrication process is the same.

Normally, the fabrication process involves:

  1. Choose suitable metal duct material – you can choose galvanized steel, aluminum, copper, or stainless steel
  2. Cut sheet metal according to the dimensions in the design (step 3). In most cases, to form a rectangular duct or square duct, you will form two sheet metals. Then, you will join them to form a complete system. You can use a plasma cutter, laser cutter, or any other appropriate sheet metal cutting tool.
  3. It involves forming the two sheet metals into “L” shapes. There should be overlapping edges to allow for joining or assembly.
  4. Join the two pieces together by riveting or welding. You can use an appropriate mechanism such as sliding the hemmed section and securing it with screws or fasteners. Whichever method you choose, the joint must be secure.

Here are some common joining mechanisms:

Sheet Metal Duct Assembly
Sheet Metal Duct Assembly

Note: The sheet metal duct making process may vary depending on the machine and design requirements. For any inquiries, on custom sheet metal duct fabrication, you can contact KDM technical team.

After successful metal duct fabrication, you can apply a suitable surface finish. Normally, it will depend on where you intend to use the metal duct.

Some available options include:

  • Painting
  • Polishing
  • Powder coating
  • Sandblasting or any other custom surface finish.

Here are detailed guides on:

Finally, you must subject the sheet metal duct to various quality testing. Some quality checks include:

  • Uniform surface finish
  • Consistent dimensions
  • Welded joints quality
  • Check surface roughness
  • Check for the possibility of leakages
  • Where possible, they must comply with local quality and safety standards


In short, sheet metal duct fabrication requires you to choose a design, have the right dimensions, select material, and adopt the right manufacturing process.

With the right machine and expertise, it can be a simple process.

At KDM, we fabricate HVAC ductwork systems and accessories – talk to our team now.

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