Rectangular vs. Round Ductwork Expansion Joints: Engineering Analysis for Maximum Airflow Efficiency

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When engineers design commercial HVAC systems, industrial ventilation networks, or even power plant duct systems, they often face a quiet but expensive decision: choosing between rectangular and round ductwork expansion joints. On paper, the choice seems obvious—match the joint to the duct shape. In practice, however, aerodynamics tells a different story. Expansion joints are not neutral components; they are flow restrictions that can either preserve system efficiency or erode it over time. The wrong choice can translate into 15%–25% higher fan energy costs across the system’s lifecycle.

Zepco LLC has seen this performance gap firsthand. In a 100,000 CFM system, the difference between properly optimized and poorly selected ductwork expansion joints can cost upwards of $47,000 annually. This isn’t just about component compatibility—it’s about airflow efficiency, energy savings, and the long-term sustainability of the facility.

Why Shape Matters More Than Tradition

Most engineers still pick expansion joints based solely on duct shape. Rectangular duct? Rectangular joint. Circular duct? Round joint. But the aerodynamics of airflow don’t play by those rules. Expansion joints affect continuity, turbulence, boundary layer development, and pressure recovery in ways that ripple downstream, impacting fans, filters, and energy bills.

In fluid dynamics, shape dictates efficiency. Rectangular geometries introduce sharp-corner turbulence, uneven velocity profiles, and extended pressure recovery distances. By contrast, round geometries offer streamlined flow, lower pressure drop, and rapid stabilization. Computational Fluid Dynamics (CFD) studies show round ductwork expansion joints consistently deliver 30–55% lower pressure drops than rectangular at equivalent velocities.

The Rectangular Reality: Efficiency in Tight Spaces

Rectangular ductwork expansion joints do offer one clear advantage—space efficiency. In buildings where ducts need to tuck between structural beams or integrate with rectangular floor plans, rectangular joints simplify integration. They are also easier to manufacture at scale and cheaper to install.

But engineers need to understand the aerodynamic penalties. At a typical 2,000 FPM face velocity, rectangular joints generate 0.15–0.35″ WC of pressure drop. That doesn’t sound like much until you calculate the fan energy required to overcome that resistance year after year. Add turbulence, uneven velocity distribution, and dead zones at corners, and the system begins working harder than it should.

Design optimizations—like corner radiusing, smoother surface finishes, or flow-straightening baffles—can help. But they rarely close the performance gap with round geometries.

The Round Advantage: Natural Aerodynamics

Round ductwork expansion joints are the aerodynamic equivalent of drafting behind a sports car. With no sharp corners, air maintains attached flow, turbulence intensity drops, and velocity profiles stay uniform across the cross-section. The result: pressure drops as low as 0.08–0.20″ WC at the same 2,000 FPM velocity.

That efficiency compounds. Lower resistance means fans consume 8–15% less power. In a large system, that’s $12,000–$35,000 in annual energy savings—paying back any additional installation costs within two to three years. On top of that, round joints recover pressure faster, minimizing downstream effects on filters and coils.

Engineers aiming to meet modern energy codes and sustainability targets should see round expansion joints as the aerodynamic baseline, not the exception.

Engineering Comparisons That Matter

When comparing rectangular and round ductwork expansion joints, three data points should drive engineering decisions:

  • Pressure Drop: Round joints average 30–55% lower drops than rectangular across flow velocities.
  • Energy Efficiency: Fan power reduction of 8–15% translates into tens of thousands in savings over the system lifecycle.
  • Flow Quality: Round geometries deliver 25–40% more uniform velocity distribution and up to 80% less turbulence intensity.

So, is rectangular ever the right choice? Yes—in highly space-constrained retrofits or when integration simplicity outweighs long-term efficiency. But in energy-critical applications, round is almost always the superior performer.

Zepco’s Role: Turning Aerodynamic Data Into Real-World Savings

This is where Zepco LLC steps in. As specialists in ductwork expansion joints, Zepco doesn’t just manufacture components—they engineer airflow solutions. Their team uses CFD analysis, laboratory testing, and field validation to quantify performance differences and recommend the best geometry for each system. Whether it’s optimizing rectangular joints with advanced corner treatments or designing custom round transitions for maximum efficiency, Zepco ensures system performance doesn’t get left to tradition.

The Engineering Decision Framework

For engineers weighing their next expansion joint specification, the takeaway is clear: don’t default to shape. Evaluate based on pressure drop budgets, fan power requirements, and lifecycle costs. The right ductwork expansion joints are not just duct accessories—they’re aerodynamic components that determine whether your system runs lean or wastes energy.

And when efficiency is tied to both operating budgets and carbon reduction goals, that decision becomes as critical as any fan, coil, or control system.

Zepco LLC is the partner helping engineers make those decisions based on data, not assumptions.

Ductwork Expansion Joints: Ensuring Flexibility & Longevity in HVAC & Industrial Systems

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In the fast-paced world of HVAC systems and industrial operations, ductwork expansion joints are more than just an afterthought—they’re an essential component ensuring system flexibility, longevity, and optimal performance. 

Understanding their function, and types, and how to select the right ductwork expansion joint can make all the difference in avoiding costly maintenance and prolonging system life. Here’s an in-depth look at everything you need to know, brought to you by Zepco, the industry leader in providing reliable expansion joint solutions.

The Role of Ductwork Expansion Joints

When engineers and facility managers design duct systems, they often focus on efficiency, airflow, and durability. But what about the flexibility of the system? Ductwork expansion joints are designed specifically to address the thermal expansion and contraction that naturally occurs within these systems. These joints absorb movement and stress, preventing the entire system from experiencing damage or degradation due to fluctuating temperatures and pressure changes.

Without a properly selected ductwork expansion joint, your duct system can face severe issues—misalignment, cracks, or even rupture. The flexibility provided by a well-chosen expansion joint not only ensures smooth operation but also prevents the need for costly repairs or replacements down the line.

Types of Ductwork Expansion Joints

Ductwork expansion joints come in several types, each with its own set of advantages based on the unique needs of your HVAC or industrial system. Here’s a breakdown of the most commonly used types:

Elastomeric Expansion Joints

Made from rubber or synthetic materials, elastomeric joints are perfect for systems with moderate movement. They’re cost-effective and ideal for smaller duct systems where flexibility is still required but with minimal movement.

Metallic Expansion Joints

These joints are crafted from metals such as stainless steel and are ideal for systems that deal with high temperatures or aggressive environments. They can handle larger movements and extreme conditions, making them suitable for heavy-duty industrial applications.

Fabric Expansion Joints

Fabric joints are flexible and durable, making them excellent for applications requiring a high degree of flexibility and movement. They’re commonly used in systems where space constraints exist and can handle varying pressures and temperatures with ease.

Gasketed Expansion Joints

Offering a high level of sealing performance, these joints are used to prevent air or fluid leakage in systems with high-pressure needs. They provide flexibility while ensuring tight seals to maintain system efficiency.

Why Ductwork Expansion Joints Matter

The value of a quality ductwork expansion joint cannot be overstated. These components play a pivotal role in maintaining the structural integrity of duct systems by absorbing thermal and mechanical stress, which could otherwise lead to damage. 

Here’s why they matter:

  • Preventing System Failure: The constant expansion and contraction due to temperature fluctuations can weaken your ducts. A ductwork expansion joint absorbs these stresses, preventing the system from breaking down.
  • Extending System Life: By reducing the strain on your HVAC or industrial system, expansion joints prolong the lifespan of ducts and other critical components.
  • Enhancing Efficiency: Properly selected expansion joints help maintain airflow and pressure levels, ensuring the system works efficiently and reduces energy consumption.
  • Cost Savings: While installing ductwork expansion joints may require an upfront investment, they ultimately save money by reducing the need for frequent repairs or replacements.

How to Choose the Right Ductwork Expansion Joint

Choosing the right ductwork expansion joint is a crucial part of ensuring the long-term functionality of your system. Here are a few factors to consider when making your selection:

  • Movement Type: Consider the amount and type of movement your system will experience. Will there be axial, lateral, or angular movements? The type of movement affects the kind of expansion joint you need.
  • Temperature and Pressure Conditions: Ensure the expansion joint you select can withstand the temperature and pressure levels within your system. High-temperature systems will need robust materials like metal or fabric.
  • Material Compatibility: The material of the expansion joint should be compatible with the materials used in your duct system. This ensures durability and prevents premature wear.
  • Size and Space Constraints: Some systems may have limited space for expansion joints, requiring a more compact design. Always account for the physical space available.

Zepco’s wide range of ductwork expansion joints are designed to meet the most demanding specifications, providing high-performance solutions tailored to your needs.

Applications of Ductwork Expansion Joints

Ductwork expansion joints are indispensable in a variety of industries, from HVAC systems in commercial buildings to large-scale industrial applications. Their role in managing movement is especially crucial in environments where temperature fluctuations or pressure changes are significant. Some common applications include:

  • HVAC Systems: To maintain system integrity in heating, ventilation, and air conditioning setups, ensuring proper airflow and minimal leakage.
  • Industrial Ventilation: In industrial plants where high temperatures or pressures are common, ductwork expansion joints maintain efficiency and reduce the risk of system breakdown.
  • Power Plants: Where duct systems must withstand extreme environmental conditions, expansion joints ensure that pressure and temperature variations don’t cause costly damage.

Conclusion

Ductwork expansion joints are more than just a part of the system—they are a cornerstone of ensuring the reliability and longevity of your duct systems. 

By understanding their function, and types, and how to select the right one for your needs, engineers and facility managers can optimize their HVAC and industrial systems for maximum performance. Zepco’s industry-leading solutions offer the durability and flexibility your systems need to thrive.

By investing in high-quality ductwork expansion joints, you’re investing in the long-term success of your system, minimizing maintenance costs and maximizing operational efficiency.