In this module we will cover how gradation and asphalt binder is controlled with a drum-mixer, how that is different that with a batch plant. We will also review the different styles of drum-mixers, and how RAP is used and recycled mixes are produced.
With a drum-mixer the gradation is controlled at the cold feed, the total aggregate flow is measured with a belt scale device, then the asphalt binder is metered or proportioned to that aggregate flow. Mix is produced continuously, rather than a batch or load at a time as characterized by a batch plant. Surge and storage silos are required with drum-mixers so they can have a place to elevate, then store the finished mix prior to dispatch into a truck.
As in the batch plant the controlled gradations are deposited in the cold feed bins (1), from which they are feed onto the cold feed conveyor (2). An automatic weighing system monitors the weight of the aggregate flowing into the drum mixer for heating and drying. The weighing system is interlocked with the controls on the asphalt storage pump that draws the asphalt binder from a storage tank (3) and introduces it into the drum (4).The asphalt binder and the heated aggregates are mixed in the drum mixer (4). If recycled asphalt pavement (RAP) is included in the HMA mixture it is feed from a separate bin (5). From the drum mixer the hot mix asphalt is transported by conveyor (6) to a surge bin or storage silo (7), from which it is loaded into trucks and hauled to the paving site. The entire operation is controlled from a control house (8). A dust collection system captures the excess dust escaping from the drum (9).
With a drum plant, the composite gradation of the HMA is controlled at the cold feed by proportioning the material from the individual cold feed bins. It is important to note that the gradation and quality of the individual materials is controlled at the quarry, not at the hot plant. This is even more true with a drum plant than a batch plant. With a drum plant there is no opportunity for re-sizing or re-screening the cold feed materials what-so-ever. The only thing a drum plant can do is blend the individual materials together. The gradation must be correct coming from the aggregate source.
This photograph shows a typical cold feed bin for a drum-mix plant. Continuous belt type feeders are always used with drum plants. Vibrating pan type feeders, sometime found on batch plants, cannot control blending accurately enough to be used on a drum-mix plant.
Proportioning flow from each feeder on a drum plant is controlled two ways, by using a variable speed belt on the belt feeder, and adjusting the manual gate of the feeder opening to affect the minimum and maximum flow possible from the feeder.
Cold feed proportioning is done by calibrating the feeder so one knows the output at various belt speed. Different charts are created for different gate settings on the same feeder.
Notice how on this calibration chart the feeder speed on the x axis is plotted against the feeder output in tons per hour on the y axis. Lines 2, 3, and 4 are for gate position 2, 3, and 4 on the feeder. These could be 2”, 3” or 4”. Many state requirements insist that gates are clearly graduated, and it is better to mark the gates in inches. It is not advisable to run a feeder below 10% or above 90% of speed. Notice how the output of the feeder in non-linear below 10% and above 90% speed. Due to the nature of electro-mechanical devices, it is hard for any motor-controller package to be stable or perform consistently in these ranges. This is why manual strike-off gates are useful. In the example shown in this illustration, if one desired 30 tons per hour from this feeder, then it would be advisable to change the manual gate setting to “2”, and run the feeder at approximately 20% speed.
When it comes to actual feeder controls for drum-mixer plants many different styles are found in the field. They all can perform satisfactory blending of different cold feed materials.
This photograph shows another feeder system with digital readouts of the individual feeder output.
The first step in proportioning asphalt binder flow is to measure aggregate flow. This is typically done with a belt scale device.
This is a photograph of a belt scale on a drum-mix plant.
A belt scale has several parts. It uses a “weigh bridge” to measure the weight of the aggregate passing over the scale area. A separate piece of equipment measures belt speed. This is typically referred to as the “speed sensor”. Finally an “integrator” or “totalizer” calculates the flow rate based on these two signals. This flow rate in tons per hour is reported to the plant automation. The flow rate is measured wet.
This diagram shows how a belt scale works, and how each element contributes to the measurement process.
This photograph shows an “integrator” or “totalizer” in the control room. The term “totalizer” is often used because these devices also provide a reading of total tons over the scale, much like an odometer in a automobile.
The second step to proportioning asphalt binder is to control the flow of the asphalt binder to match the aggregate flow. The asphalt binder flow is controlled with the drum-mix pumping and metering unit.
This illustration shows one method of how the asphalt binder is controlled in a drum-mix plant. This type of system has become vary popular. A variable speed motor is installed on the asphalt binder pump, and the pump is sped up or slowed down to match the requirements of the flow. The meter installed in the unit measures the flow of asphalt binder and reports the flow rate to the control system.
This is a photograph of the same type of asphalt binder pumping and metering system shown in the previous diagram.
The asphalt binder proportioning system on a drum plant functions by reading the belt scale signal, adjusting it for the moisture the operator enters into the system to establish the dry flow rate of the aggregate, calculating the flow required for the asphalt binder based on what the operator has entered into the system, then driving the flow control device of the asphalt binder until the signal coming back from the meter is what the control system expects to see.
This diagram shows the process of the plant control system in regulating asphalt binder flow to aggregate flow, using the flow control device in the asphalt binder skid to vary the flow, and the meter in the asphalt binder skid to measure the flow.
There are many types of automation systems found on drum-mix plants. Whether the plant has simple or complicated automation, mix of equal quality can be produced at the facility.
Some plants are now being manufactured with a “touch-pad” or “touch-screen” operator interface. All plant controls are available from this one terminal. The manual backup controls on this plant are located to the left of where the operator is sitting.
When calibrating a drum-mixer, the belt scale is first calibrated, then the feeders are calibrated. The asphalt binder meter is typically calibrated after the belt scale in case a simulated run is desired to ensure the asphalt binder is tracking the aggregate flow.
When calibrating a belt scale, the following steps are typically used: 1) Verify belt scale reads zero per manufacturers guidelines 2) Set the cold feed gates at an opening that will allow a good material feed 3) Start main conveyor belt 4) Run the variable speed belt feeder for the coarse aggregate bin - about 50% of desired production rate 5) Record the belt feeder speed and weight indicated on the belt scale. 6) Stop belt and remove a six foot section of material from the conveyor 7) Weigh the material removed 8) Repeat process to get an average of two weights. 9) Adjust instrument per guidelines so that instrument reads correctly
The following example shown in the illustrations will serve to illustrate the procedure.
Once the procedure is finished, it is important to adjust the scale so that it reads the corrected value. The test must be repeated and any required adjustments made until two consecutive readings are within acceptable tolerance. It should be noted that there are no published specifications for belt scale accuracy. Similarly, belt scales are not required to be checked every six months like batch plant scales. With batch plants it is possible that the scales are being used to not only weigh the individual materials, but to sell the mix to the department. No such situation exists with a drum plant. Drum-mix materials are sold through the silo.
This alternative for calibrating a belt scale is popular, as it eliminates much of the mathematics associated with calibration. It involves securing a truck, and requires the plant layout is sufficient to direct the material from the belt directly into the truck. With this method, start with a tared truck, and run material into the truck from the feed bin at a medium rate. Make sure the belt is empty before the test and after the test.
Record the total amount of material loaded into the truck, not the rate. Compare the total amount of material weighed on the belt scale with the net weight in the truck. (Make sure you use a certified truck scale for your test.) Adjust the instrument based on manufacturers guidelines, retest, and readjust until two consecutive tests are within tolerance.
The following example shown in the illustrations will serve to illustrate this alternative procedure. This is the type of test most frequently used in the field, but the other belt-cut method is handy if no trucks are available, or the plant equipment layout won’t allow loading a truck directly from a conveyor.
There are two areas of interests to us with asphalt binder distribution in drum-mix plants; understanding how flow requirements are calculated, and understanding how meters are calibrated.
Asphalt binder distribution is drum plants is typically accomplished automatically by the control system. The way this is established, however, is understood by running some simple math. The rate of asphalt binder required to be delivered in gallons per minute(gpm) is shown by the formula in the illustration.
Following our previous example of 200 ton per hour with 6% liquid asphalt binder, we arrive at the values shown for the formula.
Substituting the values in the formula creates the simple math problem shown.
Completing the math provides 47.71 gpm as the target asphalt binder flow in gallons per minute (gpm).
What you will learn….• How gradation is controlled in a drum-mixer• How asphalt is controlled in a drum-mixer• Different styles of drum-mixers• Using RAP with a drum plantConstruction Drum Plants 2
Drum-Mix ProductionHot Mix Asphalt production characterized by: • Gradation Control at Cold Feed • Measure aggregate flow with belt scale • Meter asphalt binder to aggregate flow • Produce HMA continuouslyConstruction Drum Plants 3
Cold Feed Proportioning Composite gradation is controlled at cold feed by proportioning material from individual bins. Gradation and quality of the individual materials is controlled at the quarry.Construction Drum Plants 5
Cold Feed ProportioningVariation in material flow from individual bins: • Based on variable speed motor on belt • Adjustable manual gates help control minimum and maximum flowConstruction Drum Plants 7
Cold Feed Proportioning• Accomplished by calibrating the feeders• Material output is charted against belt speed in calibration charts• Different charts are created for different gate settings on the same feederConstruction Drum Plants 8
How a Belt Scale Works• Weigh bridge measures weight of aggregate.• Speed sensor measures speed of belt.• “Integrator / Totalizer” calculates aggregate flow in ton (tonnes) per hour• “Integrator / Totalizer” reports flow rate to plant control systemConstruction Drum Plants 14
How the Asphalt Binder Proportioning System Works• Controls system reads wet aggregate flow from belt scale.• Controls accepts moisture content entered by operator to establish dry flow rate of aggregate• Control system accepts target asphalt content entered by operator.• Control system drives pumping/metering unit flow control device.• Control system adjusts asphalt binder flow control system based on signal received back from meter Construction Drum Plants 20
Step 1 - Calibrating Belt ScalesConstruction Drum Plants 25
Step 1 - Calibrate Belt Scale• Verify belt scale reads zero per manufacturers guidelines• Set the cold feed gates at an opening that will allow a good material feed• Start main conveyor belt• Run the variable speed belt feeder for the coarse aggregate bin - about 50% of desired production rate• Record the belt feeder speed and weight indicated on the belt scale.Construction Drum Plants 26
Step 1 - Calibrate Belt Scale (cont.)• Stop belt and remove a six foot section of material from the conveyor• Weigh the material removed• Repeat process to get an average of two weights• Adjust instrument so that it reads correctlyConstruction Drum Plants 27
Example• Conveyor is run at 225 feet per minute• Run the belt feeder until the digital readout reads 90 tons per hour• Weight of material on a six ft section of belt = 76 lbs and 78 lbs for an average of 77 lbsConstruction Drum Plants 28
Example - cont.• Determine the aggregate weight in lbs per minute. WR Q= rwhere Q = weight of aggregate, lbs/min W= wt of agg measured, lbs R = speed of belt r = length of belt from which material was removedConstruction Drum Plants 29
Example - cont• Therefore the aggregate weight in lbs/min is: q = WR/r = 77 x 225 = 2887.5 lbs/min 6• To convert to tons per hour (q x 60)/2000 2887.5 x 60 = 86.63 tph < 90 tph 2000Construction Drum Plants 30
Example - cont• Adjust instrument per manufacturer’s guidelines so that next test it reads 86.63• Repeat test, adjust, until two consecutive readings are within acceptable tolerance (Unlike batch plant no department spec on this scale tolerance - because mix purchased from silo weigh tickets)Construction Drum Plants 31
Step 1 - Alternate Calibrating Belt Scale• Verify belt scale reads zero per manufacturers guidelines• Set the cold feed gates at an opening that will allow a good material feed• Start main conveyor belt• Run the variable speed belt feeder for the coarse aggregate bin - about 50% of desired production rate• Starting with empty belt run material in tared truck and stop material flow to end with empty beltConstruction Drum Plants 32
Step 1 - Alternate Calibrating Belt Scale• Record the total weight indicated on the belt scale.• Compare weight total on belt scale with actual weight on truck.• Following manufacturers guidelines, adjust belt scale instrument based on weight difference.• Repeat test, adjusting instrument, until two consecutive tests are within tolerance.Construction Drum Plants 33
Example• Start belt scale and verify reads zero• Tare empty truck at 22,375#• Record belt scale totalizer at 437897.65• Start feeder at approximately 50% flow• Fill truck and stop and empty belt• Record belt scale totalizer at 437906.87• Calculate tonnage on scale at 9.22 tons (437906.87 - 437897.65 = 9.22 tons)Construction Drum Plants 34
Example - cont.• Weigh truck at 41,401#• Calculate weight in truck at 9.51 ton (41,401- 22,375 = 19,026 ÷ 2,000 = 9.51 ton)• Recall “span” value of instrument at 897654• Adjust span value by 1.0314 (9.51 ÷ 9.22 = 1.0314)• New span value = 925,840 (897654 x 1.0314 = 925,840)• Re-test, re-adjust, until two tests passConstruction Drum Plants 35
Asphalt Binder Distribution in Drum-Mix PlantsConstruction Drum Plants 36
Asphalt Binder Distribution Calculating AC FlowNeed to know the number of gallons/minute of asphaltbinder that is to be delivered R = P x T x 2000 x 1 100 W 60Where R = asphalt binder to be delivered, gpm P= optimum asphalt binder T= production rate, tons per hour W= unit weight of asphalt binder (typically about 8.4 lb/gal)Construction Drum Plants 38
Example – Asphalt Binder Flow RequiredThis example, Asphalt Binder target is 6.0% of 200 tons per hour R = P x T x 2000 x 1 100 W 60 Where R = asphalt binder to be delivered, gpm P= 6 T= 200 W = 8.4Construction Drum Plants 39
Example – Asphalt Binder Flow RequiredThis example, Asphalt binder target is 6.0% of 200 tons per hour R = 6 x 200 x 2000 x 1 100 8.4 60 Where R = asphalt binder to be delivered, gpm P= 6 T= 200 W = 8.4Construction Drum Plants 40
Example – Asphalt Binder Flow RequiredThis example, Asphalt Binder target is 6.0% of 200 tons per hour R = .06 x 200 x 238.095 x .0167 R = 47.71 gpmConstruction Drum Plants 41