# Presentation: 806649

## HMA PLANT OPERATION

**Notes:**
The block of instruction will cover the production of hot mix asphalt.  At the end of this block the student will:
        Understand the function of an asphalt plant.
        Know the two basic types of plants and the major components of each

This material is covered in detail in Chapter 6, “Equipment and Construction”  in the recommended text book.

## Types Of Plants

**Types Of Plants**

**BATCH**

**DRUM  MIXER**

**Notes:**
There are two types of plants - the batch plant and the drum mixer plant. Both batch plants and drum mix plants are designed to accomplish this purpose.  The difference between the two plants is that the batch plants dry and heat the aggregate and then in a separate mixer blend the aggregate and asphalt one batch at a time; drum mix plants dry the aggregate and blend it with asphalt in a continuous process and in the same piece of equipment.

## Basic Purpose Of HMA Plants

**To produce a ****quality**** hot asphalt mixture that contains the desired proportions of asphalt and aggregate and meets all specifications. **

**Notes:**
Regardless of the type of plant - the basic purpose is the same.  It is as shown on this slide.  

## Batch planttypical layout

**Storage Silo**

**Batch Tower**

**Dryer**

**Conveyor Belt**

**Aggregate Bins**

**Burner**

**Asphalt Storage**

**Hot elevator**

**Notes:**
This is the typical layout for the batch plant.

## Drum Mixer Planttypical layout

**Storage Silo**

**Dryer**

**Conveyor Belts**

**Aggregate Bins**

**Burner**

**Asphalt Storage**

**Notes:**
This is a typical layout for a Drum mixer plant.

## Common Plant Operations

**Cold aggregate storage and feeding**
**Dust control and collection**
**Mix storage**
**Weighing and handling**

**Notes:**
Certain plant operations are common to both the batch plants and drum mix plants.  They  are noted here.   The next few slides will take the student through each of these areas.

## Common Plant Operations

**Cold aggregate storage and feeding**
**Dust control and collection**
**Mix storage**
**Weighing and handling**

**Notes:**
The first step in the manufacture of a HMA mix is to obtain aggregates and properly store them to insure that they are not contaminated with dirt or co-rmingled with other materials.  The aggregate storage and feeding systems move the cold (unheated) aggregate from the aggregate processing areas to the plant.

## Aggregate Stockpile

**Aggregate Stockpile**

**Notes:**
The stockpiles must be managed to prevent contamination.

## Stockpiling Operations

**Stockpiling Operations**

**Notes:**
It is easy when the piles are of different colors - but, many times that is not the case.

## Cold Feed 

**BINS**

**Conveyor Belts **

**Notes:**
After the aggregate is processed it is feed into cold feed bins where the exact proportions of the material are controlled.

## Common Plant Operations

**Cold aggregate storage and feeding**
**Dust control and collection**
**Mix storage**
**Weighing and handling**

**Notes:**
Another component of the plant is a dust control system.  All HMA plant s have an integrated dust control system.   

## Hma Mixing Plants Create Dust

**Hma Mixing Plants Create Dust**

**Notes:**
Dust is a constant problem around HMA plants - it must be controlled.

## Centrifugal Dust Collector

**Notes:**
In a centrifugal collector the dirty gas from the top of the dryer sucks the smoke and fine particles into the cyclone where it is spiraled around.  The large particles hit the outside wall and drop to the bottom of the cyclone.  The dust and smoke are discharged through the top of the collector.  The fines collected at the bottom of the cyclone are picked up by a dust-return auger and may be returned to the plant or wasted.  Many times the cyclone is the first step  in the removal process.  The gas will go from the cyclone to a baghouse. 

## Baghouse Dust Collector

**Notes:**
A baghouse is a large metal housing containing hundreds of  synthetic, heat-resistant fabric bags, usually silicone treated to increase their ability to collect very fine particles of dust.   A baghouse functions in the same manner as a vacuum cleaner.   A large vacuum fan creates a suction within the housing, which draws in dirty air and filters it through the fabric of the bags.  The air carrying the dust particles passes through the fabric of the filter bags depositing the dust on the surface of the bag.  The clean air then continues out of the chamber.    Eventually,  the dust accumulates into a “dust cake” which must be removed before it stops the flow of air through the bag.  The bags are flexed and the dust drops to an auger at the bottom of the baghouse and is generally returned to the plant or wasted.

## Collector Efficiency

**74 micron**

**30 micron**

**10 micron**

**  ****5 micron**

**  ****1 micron**

**Cylone (80-90%)**

**Multicone (80-90%)**

**Wet collector (90-96%)**

**Baghouse**

**Notes:**
The different efficiencies of the various collector types.

## Common Plant Operations

**Cold aggregate storage and feeding**
**Dust control and collection**
**Mix storage**
**Weighing and handling**

**Notes:**
To prevent plant shutdowns due to temporary interruptions of paving operations or shortages of trucks to haul material from the plant to paving site, most HMA plants are equipped with storage silos (surge bins) for temporary storage of HMA.  

## Storage Stucture Typical Layout

**HOT MIX PLANT**

**Surge  Bin**

**Conveyor**

## SURGE SILO

**SURGE SILO**

**Notes:**
Typical surge silo.

## Common Plant Operations

**Cold aggregate storage and feeding**
**Dust control and collection**
**Mix storage**
**Weighing and handling**

**Notes:**
HMA is hauled to the paving site in trucks.  The type of equipment will vary from one contractor to another.  The quantity of HMA delivered to the paving site can be determined by either of two methods (1) weighing the trucks on scales or (2) using an HMA plant’s automatic recording system.   

## Plants are computer controlled

**Notes:**
Plants are computer controlled.

## TRUCK BEING LOADED FROM A DIRECTLY FROM A BATCH PLANT

**TRUCK BEING LOADED FROM A DIRECTLY FROM A BATCH PLANT**

**Notes:**
The block of instruction will cover the production of hot mix asphalt.  At the end of this block the student will:
        Understand the function of an asphalt plant.
        Know the two basic types of plants and the major components of each

This material is covered in detail in Chapter 6, “Equipment and Construction”  in the recommended text book.

## Truck Being Loaded From A Surge Silo

**Truck Being Loaded From A Surge Silo**

**Notes:**
There are two types of plants - the batch plant and the drum mixer plant. Both batch plants and drum mix plants are designed to accomplish this purpose.  The difference between the two plants is that the batch plants dry and heat the aggregate and then in a separate mixer blend the aggregate and asphalt one batch at a time; drum mix plants dry the aggregate and blend it with asphalt in a continuous process and in the same piece of equipment.

## Truck Loading from a Surge Bin

## BATCH PLANTtypical layout

**Storage Silo**

**Batch Tower**

**Dryer**

**Conveyor Belt**

**Aggregate Bins**

**Burner**

**Asphalt Storage**

**Hot elevator**

**Notes:**
Today’s  batch plant has changed little since 1940.

## Batch Plant

**Batch Tower**

**Pug Mill**

## Burner - Batch Plant

**Notes:**
The batch plant is a parallel flow plant.  The aggregate enters at the burner end of the dryer.

## Schematic of a batch plant

**Schematic of a batch plant**

**Notes:**
From the dryer the material moves up the hot elevator to the hot bins and then to weight box and then to the pug mill where the asphalt cement and the aggregates are mixed.

## Screen Deck

**HOT BINS**

**Notes:**
After the material is brought up the hot elevators it is re-screened and the separate sizes are placed in hot bins.

## Pug Mill

**Notes:**
The mixing unit for the batch plant.  The asphalt and aggregate will be mixed in the here.  The mixing time must be long enough to produce a homogeneous mixture of evenly distributed and uniformly coated aggregate particles.  If the mixing time is too long you can adversely affect the properties of the asphalt binder.  

## Drum Mixer Planttypical layout

**Storage Silo**

**Dryer**

**Conveyor Belts**

**Aggregate Bins**

**Burner**

**Asphalt Storage**

**Notes:**
Drum mix plants were first introduced about 1910.  More than 100 small drum mix plants were operated until the mid-1930s.  They were replaced by batch plants.  They were resurrected in a revised form in the late 1960s.  Introduced on a large scale in the 1970s about 70% of the plants now sold in the US are drum mix plants.  They represent about 50% of the plants in the US.

## Drum Dryer Mixer

**Drum Dryer Mixer**

**Notes:**
This is a schematic of the drum mix plant.

## Temperature Profile in Drum Mixer

**Notes:**
This is the temperature profile of the inside of the drum mixer plant.  

## Dryer For Drum Dryer Mix Plant

**Dryer For Drum Dryer Mix Plant**

**Notes:**
This is the working part of the plant - the drum mixer.  Note that it slopes down away from the burner.  The rate of the material flow through the mixer can be controlled somewhat by the slope of the drum.

## Dryer For A Drum Mixer Plant

**Dryer For A Drum Mixer Plant**

**Aggregate Entry Point**

**Notes:**
In  a drum mix plant the aggregate enters at the burner end of the plant.

## Controls

**Notes:**
Drum mix plants are computer controlled.  The controls are linked to a weigh belt.  As the material moves across this weigh bridge the weight of  the  material on the belt is determined and knowing the speed of the belt - the amount of material per unit of time can be determined.  This is then used to  control through the computer the amount of asphalt cement to be feed into the mix.

## Small Drum Mixer Plant

**Small Drum Mixer Plant**

**Notes:**
The advantage of the drum-mix plant is that a high production (200 tons per hour) plant can be moved and set up quickly.  This is a “six-pack”.  It can be moved in six units.

## Sometimes Things Go Wrong

**Sometimes Things Go Wrong**

## Questions - ?