Managing Vertical Up Airflow in Expansion Joints

Expansion joints in air handling systems are critical for maintaining reliable performance, particularly in vertical up airflow applications. While upward airflow might seem straightforward, the real challenge occurs when the system is shut down. Instead of “up, up and away,” the situation becomes one of “back at you,” as airflow reverses and particulate debris falls back into the ductwork. This backflow creates the risk of clogging within expansion joints and reducing their designed movement capabilities.

To extend the operational life of expansion joints, it is essential to understand the mechanics of backflow, the role of deflectors, and how installation strategies can minimize debris accumulation.

The Challenge of Backflow

When an air handling system shuts down, any particulate matter previously carried upward can settle downward through the system. Without protective measures, expansion joints act like scoops, collecting debris. Over time, these cavities fill, impairing the joint’s ability to accommodate thermal expansion, lateral offset, or axial compression.

One of the most effective solutions to this problem is the installation of a properly designed deflector. By redirecting falling debris toward the center of the airflow and away from the liner’s end-gap opening, deflectors reduce buildup inside the joint.

The Role of the Deflector

The deflector serves as the first line of defense against particulate accumulation. A V-shaped design ensures debris is not collected regardless of the airflow direction. Both ends of the “V” are welded inside the duct, and its peak is designed to match the liner’s internal face during lateral movement. This alignment is critical because it prevents debris from settling in the liner gap.

Although highly effective, the deflector is not perfect. It disrupts airflow, creating turbulence and a low-pressure zone behind it. While most debris is swept back into the main flow, some particles can settle in this zone. To address this, careful consideration of installation direction is required to determine where debris is most likely to collect.

Importance of Operating Conditions

A key factor in effective airflow management is how frequently the system cycles. Expansion joint design must account for:
1. Cycle Frequency – Systems operating continuously for long durations benefit from one orientation of installation, while frequently cycled systems require another.
2. Axial Compression – Proper liner length must be calculated so that in hot operating conditions, the liner gap is minimized.
3. Lateral Movement – Deflector height must be designed to cover the liner end-gap under maximum offset, ensuring debris is redirected past the cavity.

By incorporating these three considerations into planning, the risk of particulate buildup is significantly reduced, and the service life of the joint is extended.

Long-Term vs. Frequent Cycling

Different operational environments require different installation strategies. For systems that operate for extended periods of time and only experience occasional shutdowns, whether quarterly, semi-annual, or annual, the deflector is most effective when it is positioned to counteract debris buildup during rare backflow events. In these conditions, debris accumulation during operation is minimal, and the deflector’s impact on upward flow is negligible.

Conversely, in systems where shutdowns occur daily or weekly, the predominant concern is repeated backflow. Here, designing the liner and deflector in a way that prioritizes backflow management ensures debris bypasses the joint cavity entirely. This approach prevents clogging even in high-cycle environments.

Extending Service Life Through Planning

The challenge of clogged expansion joints caused by debris-filled vertical up airflow can be effectively managed through thoughtful design. By considering three key factors, which include cycle frequency, axial compression, and lateral movement, engineers can optimize the use of deflectors and the overall installation strategy. This approach results in longer-lasting joints that continue to perform their intended function without costly interruptions.

In summary, vertical up airflow management is not just about directing flow but about anticipating shutdowns, understanding particulate behavior, and engineering solutions that protect system integrity. With careful planning, deflectors provide a simple yet effective method for extending the operational life of expansion joints in demanding environments.

For those interested in reading the full Sealing Sense Article “Up, Up & Away: Managing Vertical Up Airflow” click here.