The Importance of Internal Flights (Liners) in a Dryer Drum of an Asphalt Hot Mix Plant

In an asphalt hot mix plant, the dryer drum plays a critical role in determining the quality, consistency, and efficiency of asphalt production. While burners and fuel systems often receive the spotlight, one of the most underrated yet vital components inside the drum is the internal flights, also commonly known as internal liners.

These internal flights are responsible for how aggregates are lifted, cascaded, and exposed to heat inside the rotating dryer drum. Their design, material selection, and precision of installation directly impact drying efficiency, fuel consumption, and final mix quality.

What Are Internal Flights / Liners?

Internal flights are fabricated steel plates mounted inside the dryer drum shell. As the drum rotates, these flights lift the aggregates and create a uniform curtain of material across the flame and hot gas stream, ensuring efficient heat transfer.

Their primary function is not just to move aggregates—but to control how aggregates interact with heat.

Why Internal Flights Are Crucial for Efficient Drying

1. Uniform Heat Exposure

Properly designed flights create a consistent aggregate curtain, allowing hot gases to pass evenly through the material. This ensures:

• Uniform moisture evaporation

• Prevention of cold spots

• Consistent aggregate temperature before mixing

Without optimized flights, aggregates tend to slide or roll instead of cascading, leading to inefficient drying.

2. Faster Moisture Removal

Well-engineered internal liners increase aggregate surface area exposure to hot air. This accelerates moisture removal, enabling the drum to achieve:

• Higher output capacity

• Lower residence time

• Stable aggregate discharge temperatures

This becomes especially important in high-capacity plants and in regions with high aggregate moisture content.

3. Achieving Required Aggregate Temperatures

To produce quality asphalt, aggregates must reach 160–180°C (or higher) depending on mix design. Precision flight geometry ensures:

• Correct material lift angle

• Adequate drop height

• Balanced heat absorption

Incorrect flight design can lead to underheated aggregates or overheating, both of which negatively affect bitumen coating and asphalt durability.

4. Improved Fuel Efficiency

Optimized internal flights allow maximum heat utilization, which directly reduces:

• Burner fuel consumption

• Heat loss through exhaust gases

• Load on pollution control systems

A poorly designed flight system forces operators to increase burner output to compensate for heat inefficiency.

5. Reduced Aggregate Degradation

Precision-cut liners ensure smooth lifting and controlled dropping of aggregates, minimizing:

• Aggregate breakdown

• Excess fines generation

• Uneven gradation

This protects the aggregate structure and ensures compliance with mix design specifications.

Importance of Precision in Flight Design & Installation

6. Flight Pattern Matters

Different zones of the dryer drum require different flight profiles:

• Charging zone: Aggressive lifting for wet aggregates

• Mid-drying zone: Uniform cascading for maximum heat transfer

• Discharge zone: Reduced lift to stabilize material flow

A one-pattern-fits-all approach leads to inefficiencies.

7. Correct Material & Thickness Selection

Internal liners are exposed to:

• High temperatures

• Abrasive aggregates

• Continuous impact

Using the correct steel grade and thickness improves:

• Wear resistance

• Service life

• Plant uptime

8. Balanced Drum Performance

Precision alignment and symmetric flight installation ensure:

• Smooth drum rotation

• Reduced vibration

• Lower load on trunnions and drive systems

Even small deviations in liner positioning can affect mechanical stability.

Impact on Overall Plant Performance

A well-designed internal flight system contributes to:

• Higher TPH output

• Stable aggregate temperatures

• Lower emissions

• Improved asphalt quality

• Reduced operating cost per ton

In high-capacity plants, the internal liner system often defines whether the plant operates at rated capacity or below potential.