Achieving High RAP Production in Asphalt Plants Through Advanced Temperature Control

Producing asphalt mixes with high percentages of reclaimed asphalt pavement (RAP) presents a persistent challenge for plant operators: maintaining proper temperature control without damaging baghouse equipment. When RAP content exceeds 30 percent, the dynamics inside the dryer drum change significantly, often leading to overheated exhaust gases that can damage baghouse bags or, conversely, moisture condensation that causes mud buildup. Understanding how modern temperature control systems address this problem is essential for any producer looking to maximize RAP usage while protecting their equipment. For contractors exploring how these methods apply to large-scale projects, Rap Recycling and High Production Milling On Interstate offers practical insight into real-world implementation.

This article examines the engineering behind high-RAP temperature management, focusing on the V-Pack Stack Temperature Control System developed by Astec and the V-flight technology that makes consistent high-RAP production possible. It covers the fundamental problem, the mechanical solution, operational results, and key considerations for plant upgrades.

The Temperature Challenge in High RAP Production

Why High RAP Overheats Baghouses

The core problem with high RAP mixes lies in how heat is transferred inside the drum. In standard operation, virgin aggregate is heated directly by the burner flame and hot exhaust gases. RAP is heated indirectly by mixing it with superheated virgin aggregate. When RAP content reaches 50 percent or more, there is significantly less virgin aggregate available to carry heat into the mix.

Inside the drum, flights (metal pocket-like devices attached to the drum wall) lift virgin aggregate and shower it through the hot exhaust gases, creating what operators call a veil. When the volume of virgin aggregate drops, the veil becomes less dense, creating gaps through which hot exhaust gases pass directly into the baghouse without transferring heat to the aggregate.

The Tradeoff Between Virgin and High RAP Mixes

A seemingly straightforward fix is to add more flights to the drum, increasing the density of the veil. This works for high RAP mixes by capturing more heat before it reaches the baghouse. However, it creates a different problem when the same plant switches to a 100 percent virgin aggregate mix:

• Virgin mixes become too cold. The dense veil traps too much heat, and not enough escapes to keep the baghouse at operating temperature.
• Moisture condenses inside the baghouse. When exhaust gas temperatures drop too low, moisture from combustion condenses on filter bags.
• Mud deposits accumulate. Condensed moisture mixes with fine dust to form mud on bags, ductwork, and dryer components.
• Production slows or stops. Mud buildup restricts airflow and eventually forces a shutdown for cleaning.

Operators face an inherent tradeoff: flight configurations that work well for high RAP mixes perform poorly for virgin mixes, and vice versa. Before variable-flight technology, producers had to compromise on one or the other. The challenge of balancing these competing demands is explored further in Meeting the Challenges of Rap Ras and Wma, which discusses the broader technical landscape of recycled asphalt production.

V-Flight Technology and Active Temperature Control

How V-Shaped Flights Create a Uniform Veil

The key innovation in the V-Pack system is the V-flight, a drum flight with a deep V-shaped cross section. Unlike conventional flights, which depend on being filled to a certain level to function properly, V-flights produce a consistent shower of aggregate across the drum regardless of how much material is present. This uniform veil is maintained whether the plant is running 100 percent virgin aggregate or a mix with 60 percent RAP.

The design accomplishes three important goals:

1. Consistent heat transfer. Because the veil has no gaps, exhaust gases always pass through a curtain of aggregate before reaching the baghouse.
2. No manual flight adjustment. Operators no longer need to reconfigure flights when switching between mix types.
3. Wider operating range. The same drum configuration handles high RAP, intermediate blends, and all-virgin mixes effectively.

The V-Pack Control System: Variable Drum Speed

While V-flights solve the mechanical problem of veil uniformity, the V-Pack system adds an active control loop that adjusts drum rotation speed based on real-time exhaust temperature readings at the baghouse inlet. The control logic operates as follows:

1. The system measures exhaust gas temperature at the baghouse inlet.
2. It compares the measured temperature against a user-defined set point (typically 250 degrees Fahrenheit).
3. If the temperature exceeds the set point, the control system increases drum rotational speed.
4. Faster rotation lifts more aggregate per minute, thickening the veil and capturing more heat from exhaust gases.
5. As the temperature drops toward the set point, the speed stabilizes.

Drum speed can vary continuously from approximately 7 to 12 rpm, compared to the standard fixed speed of 8 rpm on typical Astec drums. This range gives the control system enough authority to respond to changing conditions without exceeding mechanical limits.

Temperature Performance Comparison

Holding a consistent 250 degrees F at the baghouse inlet across all mix types eliminates both the overheating problem in high RAP mixes and the condensation problem in virgin mixes. This stability also translates directly into higher production rates.

Operational Results and Warm Mix Integration

Case Study: Willets Point Asphalt Corp.

Willets Point Asphalt Corp. in Flushing, New York, operates an Astec Double Barrel plant with a warm mix foaming system. Owner Ken Tully runs both virgin mixes and mixes with 40 to 45 percent RAP, and sometimes higher. Before the V-Pack system was installed, high RAP mixes produced stack gas temperatures around 350 degrees F, while virgin mixes struggled to stay above 220 degrees F. The low temperatures on virgin runs caused mudding of the baghouse bags.

After retrofitting with the V-Pack system, stainless steel combustion flights, and modified binder injection locations, the plant achieved the following results:

• Stack temperature stabilized at 250 degrees F regardless of RAP percentage.
• RAP capability increased above 60 percent.
• Virgin mix production rose from 300 to 375 tph.
• Baghouse mudding problems were eliminated entirely.
• High RAP mix production improved without sacrificing baghouse protection.

Stainless steel combustion zone flights proved essential for this upgrade. Standard mild steel flights in the burner zone deformed under high heat during high-RAP production, so replacing them with stainless steel eliminated heat warping and maintained proper aggregate lifting over the life of the drum.

Case Study: C.R. Jackson Inc.

C.R. Jackson Inc. in Columbia, South Carolina, runs an even wider variety of mixes, including traditional hot mix, warm mix, mixes with and without RAP, and open-graded friction course (OGFC) mixtures. Before the V-Pack system was installed on its Double Barrel plant, the facility could produce only 150 tph of OGFC for an Interstate highway project. After the retrofit, the same plant produced 350 tph of the same mix. Astec President Malcolm Swanson noted that the OGFC application was the first clear demonstration of the system’s ability to handle demanding specialty mixes at high production rates.

Warm Mix Integration With High RAP

Combining warm mix technology with high RAP content offers additional benefits that complement the V-Pack system. At Willets Point, Tully heats the asphalt to 325 degrees F using the foaming attachment, which injects a small amount of water into the liquid binder to create foam. This approach delivers:

• Extended workability. Conventional hot mix becomes difficult to work with below 250 degrees F. Warm mix remains workable down to 220-230 degrees F, giving crews up to 90 minutes extra placement time.
• Same production rates. The plant achieves 375 tph with warm mix and RAP, matching virgin mix production.
• Lower fuel consumption. Heating asphalt to 325 degrees F instead of 350-plus degrees F reduces burner fuel usage.
• Reduced emissions. Lower production temperatures decrease visible emissions around the paver.

The combination of V-flight technology, active temperature control, and warm mix foaming allows plants to push RAP content beyond 60 percent while maintaining high production rates. For contractors evaluating similar upgrades, Maximizing Rap Production in Asphalt Plants What Contractors provides additional data on what state agencies have demonstrated is achievable.

Key Considerations for Plant Upgrades

Producers evaluating V-flight and active temperature control technology should consider several factors before committing to a retrofit:

1. Drum compatibility. The V-Pack system is designed for Astec Double Barrel plants. Check with the manufacturer for compatibility with other drum configurations.
2. Binder injection changes. The system may require repositioning binder injection points to optimize heat transfer through the drum.
3. Combustion zone materials. Stainless steel flights in the combustion zone are recommended alongside V-flights to prevent heat deformation over time.
4. Control system needs. The system requires sensors at the baghouse inlet and a variable-speed drum drive. Confirm existing electrical infrastructure supports these additions.
5. Operator training. While the system automates temperature control, operators need to understand the set point logic and recognize when manual intervention is needed during startup.

Return on Investment

The documented results from Willets Point and C.R. Jackson demonstrate that the V-Pack system can pay for itself through increased production alone within a single construction season, with additional savings from reduced maintenance, lower material costs from higher RAP usage, and fuel savings from warm mix integration. For plant managers designing new facilities or upgrading existing ones, holistic production system design is essential. The principles discussed in Design Best Practices for Luxury Production Homes Creating offer a useful framework for thinking about how individual components must work together rather than being optimized for a single operating point.

As state DOTs and contractors push toward higher recycled content in asphalt pavements, temperature control systems that handle wide swings in RAP percentage will become increasingly important. The V-Pack system replaces the fixed-flight approach with a variable configuration that adapts to the mix being produced. For producers who want to remain competitive in a market that increasingly demands high recycled content without sacrificing production rates, investing in active temperature control technology is a strategic necessity. The ability to produce 60 percent RAP mixes at 375 tons per hour with consistent quality and no baghouse problems gives plants a distinct competitive advantage in bidding for both public and private projects.