Hot Mix Asphalt Plant Tour

 

Types of Plants

There are two basic types of plants used to manufacture hot-mix asphalt - batch plants and drum plants.
Both plants are capable of producing quality HMA
    
                                 Batch Plant                                                                                         Drum Plant

Common Components of Different Plants

There are many components common to batch and drum plants as can be seen below.These include:

- Cold feed bins
- Asphalt binder strorage
- Dryer
- Emission controls
- Storage silos

 
                                     Drum Plant                                                                                        Batch Plant

Aggregate Storage

HMA is a mixture of aggregate (crushed stone, gravel, and sand), and asphalt binder.

Aggregates are usually stored in stockpiles as shown. Stockpiles may be covered to prevent rain from soaking the aggregate or aggregates may be stored in large silos or bunkers. When storage silos or bunkers are used they often act as cold feed bins to meter the aggregates into the plant.

 
                       Horizontal Stockpile                                                                                        Bunker

Aggregate Silos

Cold Feed Bins

Aggregates are typically moved from the stockpiles to the cold feed bin using a front-end loader. Cold feed bins are used to accurately meter the different aggregates used in the mix to the drying drum. The amount of each aggregate is controlled by a combination of the gate opening at the bottom of the bin and conveyor belt speed.
   

Aggregate Feed System

A conveyor below the cold feed bins transfers the aggregate to the dryer where the aggregate is dried and heated.Because drum plants produce mix continuously, a weigh scale is used to weigh the aggregate before it enters the dryer so the amount of asphalt binder can be accurately added.

 

Types of Dryers

Aggregates are dried and heated in a dryer drum as they are tumbled through a hot air stream. The hot air is created by a burner and is pulled through the drum by a fan.

The direction of air flow relative to the direction of aggregate flow describes the two types of dryers-parallel-flow and counterflow.

In a parallel-flow drum, aggregates move in the same direction as the hot air. For a counterflow plant, the aggregates move in the opposite direction.

                                 Parallel Flow                                                                                     Counter Flow

Emission Control System

As hot air passes through the aggregate it picks up some fine sand and dust particles. These particles are removed by the emissions control system before the air goes into the atmosphere. Most plants have primary and secondary collectors to remove these particles. The fine sand and some dust is collected in the primary collector and returned to the mix. The secondary collector removes the remaining fine dust. In most cases this dust can also be returned to the mix.
 

Batch Plant

After the aggregate has been dried and heated the difference between batch and drum plants becomes clear.

In batch plants the dried hot aggregates exit the dryer and travel up a bucket elevator to the batch tower, where they are separated by size over screens and stored in hot bins.The amount of each aggregate is accurately proportioned by weight in the aggregate weigh hopper.

                              
After the aggregates have been weighed they are dropped into the pugmill where they are mixed with the asphalt binder.

The amount of asphalt added to the aggregate is measured either by a weigh bucket or by a pump and meter system. After the asphalt and aggregate are mixed they are either deposited in a truck or in a storage silo.

Drum Plant

The mix is made continuously in a drum plant.

Asphalt and aggregate are mixed in the drum after the aggregate has been dried and heated. The asphalt is added to the aggregate where the air stream is cooler.

After the asphalt and aggregate are mixed, they are moved to a stoage silo where the mix is loaded into trucks.
 
                            Parallel Flow Drum Mixer                                             Counter Flow Drum Mixer

Storage Silos

Drum mix plants must have storage silos, since they produce mix continuously. Batch plants do not require a storage silo, but they often do have them to increase the plant's production capacity.

Storage silos are insulated and may be heated to prevent heat loss. A mix may be stored in a silo for days.

Mix is discharged from the silo into trucks for transportation to the paving site.
 
                               Multiple Silos                                                                                 Portable Silo

Control

Most hot-mix asphalt (HMA) plants control their production using computers and automation. When necessary, plants may still be operated manually.

Conclusion

Both batch and drum mix plants are capable of producing quality hot mix asphalt for use on streets, highways, parking lots, driveways, bike trails, and other structures. The type of plant used by a contractor is a business decision dependent on cost and market.