Hot Mix Asphalt (HMA) Facility The basic purpose of a HMA facility is to properly proportion, blend and heat aggregates and asphalt to produce a quality mixture of HMA, which meets the required specifications or job mix formula. The HMA mixture is used for paving Highways, Runways of airports, Parking lots, Race tracks etc. The two principal types of HMA facilities in use today are drum and batch facility. Brief description with typical layout of these facilities is described below; Drum Mix facility A schematic diagram of the components in a typical drum mix facility is shown in the figure below. The flow of material is basically from left to right. The aggregates are typically moved from the stockpile area to the cold feed bins via a front end loader which charges the cold feed bins. The metal cold feed bins have slanted sides with a vibrator attached (if gravity feed is inadequate) to keep a constant supply of material flowing out of the bottom of each cold feed bin. Typically an adjustable gate and a variable speed feeder are located at the bottom of each cold feed bin to proportion the material from each bin to meet the job mix formula (JMF) gradation. A gathering conveyor brings the material to the cold feed elevator, which moves the material to the dryer. An automatic weighing system on this elevator continuously weighs the amount of aggregate plus moisture going in to the drum mixer. In the control room a correction to the total weight is made for moisture content of the aggregates so the asphalt proportioning system can pump the proper amount of asphalt cement into the drum mixer. The moisture correction is made by physically determining the moisture content of the aggregates and providing this information to the computer.
A typical drum mixer is a parallel flow design in which the aggregates move in the same direction as (parallel) to the exhaust gases. Therefore the aggregate is fed into the drum at the burner end and then it is dried and heated as it moves down the drum toward the discharge end. The asphalt cement is pumped from the asphalt cement storage tank and enters the drum at a point about 1/3 the drum length from the discharge end. Some plants use a coater, which is similar to pugmill. The asphalt cement coats the aggregates as the materials move down the lower 1/3 of the drum. The HMA exits the drum through a discharge chute into a surge silo (or other type of storage bin) from which it is discharged into the trucks via an automatic scale system. It is then transported to the laydown site. Batch facility A schematic diagram of the components in a typical batch facility is shown in the figure below. A comparison of first two or three components in the batch and drum mix facilities shows that the cold feed proportioning system for both types are similar. The primary difference, up to the drying operation, is the automatic weighing system on the cold feed conveyor of the drum facility. The proportioned material is fed by the cold feed conveyor into the dryer. The dryer in a batch facility is typically of counter flow design in which the aggregates flow in the drum in opposition to (counter to) the flow of exhaust gases. The dryer has a burner which provides the heat energy for evaporating the moisture in the aggregate and then heating the aggregate up to a discharge temperature of typically 285 to 300 deg F. the heated aggregate is discharged from the dryer and lifted to the top of the tower plant via an enclosed hot elevator. The hot, dust laden exhaust gases from the dryer are passed through a dust collection system to
remove dust particles so that emission standards are met. The collected dust is returned to the hot material elevator or filler silo and is introduced into the HMA as required. After the aggregate exit the hot elevator at the top of the tower, they are discharged into vibrating screens that are designed to separate the material into a number of sizes. After screening the aggregates, they are stored in hot material bins. The stored aggregates are proportioned to produce the HMA. The control system then proportions the hot aggregates from the bins into the weigh box, which is mounted on a set of scales. These dry aggregates are discharged into the pugmill and the hot asphalt cement, which has been weighed and stored in a weight bucket, is sprayed into the pugmill after a few seconds of dry-mixing. The pugmill is typically a twin-shaft, counter-rotating mixer designed to coat the aggregate quickly with asphalt cement. After mixing, the HMA is discharged directly into a truck or transferred into a storage silo where trucks may be loaded for transport to the laydown site.
Case study with Heatec Inc. USA Introduction to Heatec Inc. HEATEC 5200 Wilson Road Chattanooga, TN 37410 Phone 423-821-5200 and Fax 423-821-7673 Heatec Inc. in Chattanooga, USA produces broad line of liquid Asphalt cement (AC) heating and storage equipment for the HMA industry, including direct-fired and helical coil heaters for liquid AC. These heaters are typically used in the company s other products: horizontal & vertical storage tanks with both portable & stationary designs. Heatec storage-tank capacities ranges from 10,000 gallons (38,000 liters) up to 35,000 gallons (132,000 liters). Astec Industries is the parent company of Heatec Inc. Therefore, the main objective of Heatec has been to build heaters and storage tanks to support the Astec product line of hot-mix batch plants and drum mix plants. Today Heatec has been successfully diversified into new markets totally unrelated to HMA industry. You will find Heatec products in roofing industry, chemical plants, oil and gas refineries, textile factories and pharmaceutical companies. 50% of Heatec s production is for industrial applications outside HMA industry worldwide. Heatec Storage tanks at one of the HMA Facilities in USA
Level Measurement in Liquid AC storage tanks The tank level measuring device is an essential part of every storage tank manufactured. Heatec is using float and scale type continuous level indicator (Varec make) and has simple micro switches on the scale for hi & low level alarms to monitor liquid asphalt level and provide tank empty condition alarm (low level) and over flow protection alarm (Hi level). The direct purchase cost of these level indicators is very low. However, the installation and calibration are very costly as it is very labor intensive and time consuming. Also, these systems were prone to damage in transportation as they are transported after mounting on the tanks and rectification is very expensive. Apart from these problems, in operation this system are demanding lot of maintenance and causes frequent production loss because of following reasons; 1. The asphalt tank is at around 320 deg F (160 deg C) and asphalt vapors form deposits on the floats and other parts of the system coming in contact. This causes float to get stuck and stop functioning till deposits are removed. Therefore, no level reading is available. 2. The above situation leads to non availability of the level indication and some time causing overflow of liquid asphalt due to this, which is very expensive to clean up. 3. Also, if float wire or tape gets broken the float starts floating on the surface. Customer has to empty the tank and go inside the tank to mount the float again or replace if it is damaged. 4. As this system is mechanical system, it has no output for level inside the tank. 5. Once customer has to rotate the tank to see the scale for the level indication as during the initial installation of the tank, the scale was not visible. CON AG show in USA in March, 1999 Milltronics participated in this huge show for aggregate industry and displayed IQ 160 Radar level gauge. Mr. John Clayton (Director Of Engineering) of Heatec Inc. visited our stall and discussed his problem of asphalt level monitoring with Mike Gavan (Application Manager of Milltronics USA) who has shown him IQ 160 and explained the operation and features of Microwave technology. John was excited to use the IQ 160 on his asphalt tanks but Mike was hesitant because IQ 160 may fail due to high temperature of the liquid asphalt in the storage tank. Also, Ultrasonic is not an ideal solution due to vapors present and temp. gradient present inside the tank in different conditions. He contacted Rolf Panzke (Director Radar Technology, Milltronics) for this application. Both Rolf & Mike were convinced that if IQ 160 can solve the problem reliably then there is a huge potential lying in this application which involves hundreds of tanks in USA. Therefore, they decided to visit Heatec manufacturing plant in Chattanooga, USA with Mike Gavan. Rolf & Mike visited Heatec with one IQ 160 in April, 1999 and met John Clayton for offering trial on one of the asphalt tanks. John offered the one storage tank at their site at Vulcan Materials who are one of the largest HMA producers in USA. The tank was a vertical storage tank with 39 feet height and 16 feet diameter. The tank had 3 blade agitator and pipe coils at tank bottom, which were carrying the thermal fluid to heat the asphalt. The pressure in the tank was atmospheric and temperature was about 350 deg F (175 deg C), the vapors were present in large amount above the liquid surface and lots of deposits. The dielectric of asphalt is less then 4. IQ 160 with 2 mounting flange was installed at about 18 from the tank wall on a stand pipe with 4 height and power supply was of 110 Vac was applied. The initial trial gone very well and IQ 160 was tracking the asphalt level well. John offered second similar storage tank for trial at the same site and another IQ 160 was installed on this tank by Mike in May 1999. Both these trials are successful and both units are tracking the level of liquid asphalt very well till date at Vulcan materials.
IQ 160 Radar installed on Liquid Asphalt tank of Heatec Inc. Pipes carrying Thermal fluid to heat asphalt and agitator inside the tank.
John Clayton the designer and manufacturer of asphalt storage tanks sees the following advantages in monitoring asphalt level with Milltronic IQ 160 Radar ; 1. Increase in Heatec s manufacturing capacity. As the IQ 160 is small, compact & low weight the installation is very easy and ver fast, just bolt in place on a stand pipe. This leads to time saving in installation in comparison to float & tape type level indicator. Therefore, Heatec can increase the production from 85tanks per year to 105 tanks. 2. IQ 160 is a non contact type instrument therefore no fauling like in float systems due to deposits of asphalt on contact parts. Therefore, level monitoring is available all the times. 3. The commissioning and trouble shooting is simple. No need of filling or emptying the tanks to calibrate the system or empty the tank for removing the floats. 4. The continuous level output is available in control room and signals can be easily integrated with their control system. 5. Once commissioned, IQ 160 is practically maintenance free. Low cost of ownership. John Clayton is very happy to resolve the asphalt level measuring problems and going to order more IQ 160 for the tanks which Heatec manufactures and will also recommend his clients to replace existing float & scale type level indicators with Milltronics IQ 160 Radar level gauge. The application details and echo profiles for the installations on Heatec tanks are attached. Photograph of other tanks at site We have other successful sites listed below where Milltronic IQ 160 Radar level gauge is tracking the level in asphalt storage tanks; 1. Rid Glass Fresno, USA 2. Kewit Pacific Beuver Ville, California, USA 3. Tamco Roofing, Joplin, USA 4. Heidelberger Dummstoffe Germany 5. Lafarge Redland, U.K. 6. Przeda Poland
Outstanding features of Milltronics IQ 160 Radar in monitoring liquid asphalt level. Microwave technology used by IQ 160 is virtually uninfluenced by atmospheric or temp. conditions above the liquid surface. It effectively overcomes the level measurement problems associated with asphalt like vapors, steam, deposits, turbulence and agitation. Because of the low 5.8 GHz emission frequency microwaves of IQ 160, the unit is insensitive to material build-up and encrustation. The IQ 160 is small, compact and low weight. Therefore, Installation is simple and economical compared to any other technology. No special tools are required, just position the unit on a stand pipe bolt it in place and connect it to the power supply. No need of filling or emptying the tank for calibration purpose. The programming of IQ 160 is very easy and user friendly through IR programmer or remotely through PC over RS 485 link. It needs to set the values for basic seven parameters only for start-up of the unit. The measuring range and ma output can be adjusted to any span with in the specified range of IQ 160. Milltronics Sonic intelligence is a proven echo processing technique in the field, which forms the basis of exceptional echo analysis performance of IQ 160. Sonic intelligence differentiates between the real echo from the liquid asphalt surface and false echoes coming from agitators, pipes, tank walls, deposits etc. It is also capable of evaluating weak echoes in extremely hostile conditions, where hot, cold, steamy or vaporous conditions prevail. Very low maintenance cost as the IQ 160 is practically maintenance free because of NEMA 6 epoxy coated aluminum housing, a chemically resistant PTFE antenna, proven Milltronic Sonic intelligence signal processing and built-in diagnostics for quick trouble shooting. This brings very low cost of ownership to the users. With above features Milltronics patented pulse-based microwave technology delivers advanced continuous level detection on liquids and slurries at an unmatched price / performance ratio. Limitations of other level Technologies in liquid asphalt storage tanks. Float & Scale (Mechanical) type level gauging system. Customer has to specify exact measuring range at the time of purchase as you cannot increase the length any more. You have to specify the Sp. Gravity correctly to select the correct float as it is working on buoyancy principle. Because of above two reasons it is not possible to interchange the units for applications even at manufacturing stage. As each float is made for a particular application. Expensive liquid seals are applied to prevent vapors escaping into atmosphere or damaging the moving parts. Installation is very labor intensive and time consuming, therefore expensive. The tank has to be made empty to install the gauge. For proper functioning of the float and preventing frequent damage to the float it needs to be guided over guide wires. These becomes extra parts inside the tank on which asphalt vapors can get easily deposited and obstruct the free movement of the float. Needs frequent cleaning. Liquid seals need refilling at regular intervals. Increased maintenance cost. If float breaks or gets jammed customer has to empty the tank and mount the float again. This results in production loss due to increased down time period. Mechanical system, no level proportional signal output available. Therefore, it is an outdated technology.
The metal scale can get corroded in corrosive atmosphere. Difficult to transport. Hydrostatic Technology This is also contact type technology as it is mounted from side or lowered from top inside the tank up to 0% level mark. The asphalt deposits on the diaphragm needs cleaning otherwise performance is affected. Frequent cleaning can cause high maintenance cost and loss of production. If built-up hardens on the surface e.g. during shut down periods then the cell can get damaged during cleaning and instrument will fail immediately. High temperature of around 175 deg C can damage the hydrostatic cell as it is in direct contact with the asphalt. Therefore, the life is reduced. Also, temperature gradients in the tank will cause layers of different densities which will make the instrument inaccurate and unreliable as this layers are not constant and will cause frequent zero and span shifting, which needs to be corrected. For calibration you have to fill & empty the tanks at least at two points and preferably over entire measuring range. Ultrasonic Technology (to be checked by actual comparing in the application) The asphalt vapor causes unreliability of measurement as the sound velocity changes as per the concentration of the vapors and this concentration keeps changing. The temperature gradient also causes sound velocity change and therefore the unreliable performance in measurement. The temp. at the sensor surface should not exceed 145 deg C and special care to be taken that sensor is not in contact with the medium. However, in asphalt storage tanks in HMA industry the product comes up to the tank top and that time the sensor can get damaged. Also, with hi-temp. sensors ultrasonic is limited on the range and effect can be seen that echo attenuation increases as the target goes further from the sensor. Because of above ultrasonic cannot be applied with same confidence like Microwave technology, which is not affected by vapors, product temp. and temp. gradients above the liquid surface.