In this module we will discuss how batch plants work, how gradation and asphalt binder content is controlled in a batch plant, operational principles and best management practices for batch plants, and using RAP or recycle production with a batch plant.
Batch type facilities, as shown here, have been common since the turn of the century and have changed little in overall concept. They produce hot-mix asphalt a “batch” at a time. This means ingredients are weighed up individually, then mixed together and dispensed into a truck or storage equipment on a “batch” basis, one load at a time. The following will describe the basic process by which material flows through a batch plant. The aggregate is first stockpiled and then moved to the cold feed bins (1) which hold each of the aggregate sizes and from which the aggregate is proportioned for feeding into the dryer (4). The asphalt binder is stored is either horizontal or vertical storage tanks (2). The aggregate is feed up the cold feed conveyor (3) into the dryer (4) where is dried and heated to the proper temperature (about 300 o F or 175 o C). The aggregate is feed into the hot elevator (5) and then into a screen deck (6) where it is separated into the various sizes that will be used in the production into HMA. The various sizes of aggregates are stored in hot bins (7). They are combined with the asphalt binder and mixed in the pugmill (9). After they are mixed they can be feed up a conveyor belt (10) to the hot storage silos (or bins) (11). If Recycled Asphalt Pavement is used it is added through a separate feeder (8). The completed HMA mixture is loaded into delivery trucks at area below the storage silos (12) or it can be loaded into trucks immediately below the pugmill (15). The plant operation is controlled from a control house (13). The plant is also provided with a air pollution control system (14) which in this diagram is a baghouse.
This illustration shows materials separated adequately by bulkhead.
This illustration shows a proper set of bin wall dividers. Notice there are no bin extensions on the back sides of the cold feed bins. This reduces the possibility of the loader operator overfilling the bins and materials overflowing from one bin to the other.
The aggregate is feed from the cold feed bins to the cold feed conveyor. The proportioning of the aggregate is controlled by the use of variable speed conveyors.
The aggregate dryer is one of the key pieces of equipment at any hot mix plant. Regardless of the type of plant, drying is the operation that governs the production rate of the plant facility. With a drum-mixer the production of the plant is the production of the dryer. With a batch plant, regardless of the size of the hot bins and the weigh hopper, and regardless of hot many silos might be installed on the plant, the plant output per hour is ultimately limited to the dryer capacity. It is the dryers responsibility to dry the aggregate and heat the aggregate to the required mix temperature. In some location high moisture conditions and the physical properties of the aggregate may create a bottleneck in plant operations because of the increased drying time for the aggregates.
There are primarily two types of aggregate dryers; counter-flow dryers, and parallel-flow dryers. This distinction relates to the direction of the aggregate flow relative to the gas flow. In a counter-flow dryer the aggregate flows into the burner. In a parallel-flow dryer the aggregate flows away from the burner. The parallel-flow dryer will be discussed in the drum-dryer block.
In a counter-flow dryer, aggregate flows toward the burner, and the hot production gases flow against the flow of the aggregate - hence the term “counter-flow”. This illustration shows the combustion, heating and drying areas in the dryer. The aggregate is heated as it flows toward the burner area.
This illustration shows a typical counter-flow dryer in the field.
This illustration highlights the general flow of material through a batch type facility. Once aggregate is dried it is conveyed to the top of the screening section with the aid of a bucket elevator. At the screening unit, the aggregate flow passes over different screens that separates the aggregate into different sizes. Sized aggregates are then stored in the “hot bins”, so called because they contain the hot, dried aggregate, which is waiting to be dispensed into the aggregate weigh hopper. The gates below these bins are typically referred to as “supply gates” or “hot bin gates”. The plant operator either manually or automatically “draws” material from each hot bin to match the job-mix formula and weighs the aggregate in the aggregate “weigh hopper,” which is positioned directly below the hot-bin gates. Asphalt binder is pumped into the asphalt binder “weigh bucket,” where it is weighed to the required amount. In modern, automated plants, the aggregate weighing and asphalt binder weighing is done simultaneously to shorten the batch cycle. The aggregate is added to the aggregate weigh hopper into the “pugmill”, where it is mixed for a brief period of time without asphalt binder to thoroughly mix the aggregate from each supply bin. This is called the “dry-mix cycle.” After the dry-mix cycle, the asphalt binder is discharged in the pugmill, where it is mixed with the blended aggregate in a “wet-mix cycle.” The HMA is then dispensed into a waiting vehicle or into transfer equipment for storage in a silo.
In a batch facility aggregate gradation is controlled at the aggregate “weigh hopper” by drawing different quantities of material from each hot bin. Asphalt binder is controlled in the asphalt binder “weigh bucket” by weighing up the desired quantity of asphalt binder for each batch.
Here material passes over several different sized screens, referred to as “decks”. As material successfully passes down through the screen “cloth” on each “deck”, it is directed into one of the hot bins in the batch tower. Material that passes through all the decks is deposited into the hot bin closest to the hot elevator. This is commonly referred to as the “fines bin” or No. 1 bin. This bin typically has -#10 material in it, and is the fine aggregate bin. Material retained on the deck over the fines bin, but passing through all other screen decks ends up in the No. 2 bin. Bins are numbered from the hot elevator. Most plants have four hot bins, but some have five. Some plants were constructed with three hot bins, and some with six, but these are rare to find in the field. Material that is retained on the screen sizing for the No. 2 bin, but passing all other screens is directed to the No. 3 bin. This type of sizing process continues to fill all the hot bins on the plant. Material that is retained on all screens is rejected from the tower through the overflow chute. We will see an illustration and photograph of this device later.
This photograph shows a screen deck on a batch tower. The pipe coming off the deck in the upper left is for fugitive dust control. This pipe is ducted to the dust handling system.
Once the material is sized by the screens it is deposited into the hot storage bins. Most plants have four or five hot bins. Some plants were made with three hot bins, and a few were made with six. Hot bin capacity in a batch plant is typically about 40-50% dedicated to the No. 1 of fines bin. The rest of the capacity is spread between the coarse material bins. Some plants have substantial hot bin capacity. Batch plants with up to 350 tons of hot rock storage have been manufactured. Most plants have 40-80 tons of storage depending on the size of the plants, with portable batch plants having the least amount of storage. With a portable batch plant, or a batch plant with a small amount of hot storage, balancing the cold feed flow to match the hot bin pulls is critical.
Overflow chutes are chutes located at the top of each hot bin to keep the bin from overflowing. If material does back up in the bin, the idea is that this built-up material does not flow over to the next bin corrupting the gradation in the adjacent bin, or back up into the screen. If the material backs up into the screen, the screen can be damaged, or the screen vibration will change which affects the screening capability of the unit, or it can cause a deck to blind over. Besides the obvious damage to the screen deck itself, and the cost associated with it, if the screening unit does not function effectively it has the same net effect as blinding the screen deck; small material is carried over into the larger aggregate bins, and the aggregates are not fractionalized or sized properly.
This illustration is a good depiction of not only the screening operation, but also of the way overflow chutes are built into the hot bins. While this illustration shows the concept clearly, it is deceptive in that it shows the chutes lower in the bin than they actually are. The artist did this to depict the screening operation more clearly. In reality the inlets to the overflow chutes are high in the hot bins near the top. Notice also the directional slope of the wall dividing the hot bins. This will be of interest to us shortly when we talk of material contamination caused by bin wall wear.
This photograph shows the way the overflow chutes function on the outside of the batch tower. The material from all the hot bins overflows into the chute on the right. The chute on the left is used for scalping purposes only. Material that is retained on all the screen decks flows down this chute. The gates on the bottom of the chutes are for safety. By installing air operated gates on these chutes, people on the ground are not in danger of being hit by stones coming down the chutes. These gates suggest a management item; if the chutes are never empty they can fill up and negate the reason for having overflow chutes on the batch tower in the first place.
Rotary bin indicators, called hot bin indicators, help the operator manage his plant operation by informing him if the bins are too full or too empty. This helps the operator balance his cold feed flow to his hot bin pulls. Balancing cold feed flow to hot bin pulls is critical to producing a consistent and uniform mix. This ensures that there is sufficient quantity of material in each hot bin for each batch of the mix formula.
The gates located below the hot bins are referred to as the hot bin gates, or the supply gates.
The gates located below the hot bins are referred to as the hot bin gates, or the supply gates. They are typically operated with electrically actuated air or hydraulic cylinders that are connected directly to the gates.
This photograph shows a sampling device positioned below the No. 2 hot bin. On this plant, the air cylinders operating the gates are horizontal in design and are attached to the bottom of the gate. When the cylinder is activated, the rod retracts into the cylinder and pulls the gate open toward the viewer. The sampling door is on the other side of the batch tower from the view shown.
Material from the hot bins discharges into the aggregate weigh hopper.
Material from the hot bins discharges into the aggregate weigh hopper. The aggregate weigh hopper is positioned directly below the hot bin gate area. It is designed to be a scale, and is attached to a mechanical scale linkage or is supported from load cells.
This photograph shows the position of the weigh hopper in the batch tower. It sits on the deck where the gate cylinders and hot bin sample doors are located.
In the plant the asphalt binder weigh bucket is located on the same deck as the weigh hopper While aggregate is being weighed up, the asphalt binder is being weighed up.
The asphalt binder weigh bucket is also enclosed and suspended from load cells. This ensures the asphalt binder is weighed correctly, and not affected by wind. There is typically a drain located below the weigh bucket so the asphalt binder can run down into the pugmill when discharged into the mixer.
An air or hydraulically actuated valve typically sits next to the asphalt binder weigh bucket. The control automation actuates this valve when asphalt binder is required, and directs the flow of asphalt into the weigh bucket until the target weight is reached. In a batch plant asphalt binder is kept in constant circulation back into the tank, until the operator or the plant automation calls for asphalt binder to be weighed for the batch.
The pugmill is the last equipment group in the batch tower. It is located below the aggregate weigh hopper and the asphalt binder weigh bucket. The pugmill is responsible for thorough mixing and coating. The shafts, arms, tips in the mixer blend the aggregates together in the dry mix cycle before the asphalt binder is introduced. After asphalt i binder s introduced, they are responsible for adequate coating in the wet-mix cycle.
The distance between the bottom of the pugmill and the tops of the tips as they rotate is referred to in the industry as the “live zone”. Generally specifications require that aggregate is not charged into the pugmill above the live zone.
If an aggregate mixture is allowed to fill above the tips on the shafts, inadequate mixing occurs. Whichever aggregate was weighed last in the weigh hopper is likely to be the aggregate that does not get mixed into the HMA product. In this example, the asphalt binder will also be higher above the pugmill arms because the asphalt binder that drains into the mixer will settle on top of the aggregate in the mixer and this aggregate cannot get pulled down adequately by the arms and tips of the pugmill. Inadequate mixing can also occur if too small of a batch is attempted. If the bed of material is too low in the pugmill, it is possible that segregation can occur to the middle of the mixer, and all aggregate particles may not be thoroughly blended.
This photograph shows the insides of a typical pugmill or mixer. The tips and liners are new. Notice the configuration of the arms and tips. Notice the clearance between the tips and liners. This is what ensures adequate mixing.
Virtually every plant in the field has some sort of automation. Older automation systems often focused on only setting the dry mix and wet mix cycles. The operator was responsible for weighing the individual aggregates. These were historically referred to as “semi-automatic” systems. “ Automatic” systems allowed the operator to enter a target for each hot bin pull, in addition to the cycle times, and thereby entering the job mix formula. This freed the operator to spend more time managing the operation rather than watching the scales.
Modern, computerized batch systems allow the operator to enter formulas into stored memory for later recall, cue up trucks for different mix formulas and batch sizes, watch target weight vs. actual weight tolerances, print individual weights on batch tickets, record the customer/project/mix number, provide accumulated tonnages for the job to-day and to-date, and even provide directions to the job site. Batch plants with silos often have two systems, as shown here, with shared database information. Regardless of whether the HMA is loaded from the silo or the batch tower, the project totals are kept current.
This photograph shows a typical batch automation screen. Notice the “required” and “actual” weight columns, and the “difference” column expressed as a percentage.
With computer automation becoming more compact, it is easy to automate any batch plant as long as there is room on a desk somewhere to place a computer. This table-top model has all the features of a full console unit.
Block 15 Batch Plants 13
Senior/Graduate HMA Course HMA Production BATCH PLANTSConstruction HMA Production Batch Plants - 1
What you will learn….• How gradation is controlled in a batch plant• How asphalt binder is controlled in a batch plant• Operational principles for batch plants• Using RAP & recycle production with a batch plantConstruction HMA Production Batch Plants - 2