Drum Dryer for Asphalt Mixing Plant
1. Brief Introduction
Drying drum is used to dry and heat cold aggregate and it is the very key component of the asphalt mixing plant.
The drying quality degree of cold aggregate directly affects the quality of the finished product. The drying drum from Batch mixing plant adopts special counterflow heat method.The dried aggregate temperature is at 140 ℃ to 160 ℃.
(1) Working process
Feed→(aggregate is thrown into the drying drum at a certain speed by belt conveyor)→preheat aggregate→drying aggregate (form uniform curtains to exchange heat with gas when going through)→ burning aggregate→discharge (infrared thermometer to monitor the temperature)→the burner.
The drum is a long rotating cylindrical structure. Cylinder is rolled and welded with heat boiler steel. Drum wall generally has one or two big rounded. Cylinder sits on the roller based its weight and the whole support on the stents. The drum wall is covered by rock wool insulation layer and galvanized sheet to reduce heat loss. Usually the drum length is 4 times of the drum diameter.
Motors(SIEMENS motors), hard-teeth reduction gear, thermocouple thermometer, negative pressure detection system, feeding box, feeding ring, discharge box, burner, blower and so on.
Manganese steel for drum, boiler steel for import zone, galvanized sheet for outside shell
2. Main components
(1)Cold aggregate dry heating system
Cold aggregate dry heating system is composed by drying drum (or dry mixing drum) and heating device.
Drying drum is used to heat and dry cold aggregate.
Drying way: gear drive, chain drive and friction wheel drive.
Drying drum feed box is equipped with openings and install the feeding device and smoke box to facilitate feeding and gas emissions
Heating device is used to dry and heat aggregate to working temperature.Now heating equipment matched drying drum are mostly used liquid fuel (usually use heavy oil and diesel oil). Heating device is composed by the fuel tank, pump tube, burner, blower and fire box, etc. If using heavy oil as fuel, the fuel tank is added by heat pipe inside, and is equipped with heavy oil pre-heater in the fuel oil supply system.
The combustion nozzle is the core component in the burner.According to the different methods of liquid fuel, that can be divided into: high pressure type, medium atomization and rotor type.
(4)Introduction of Flights
Cold aggregate is thrown into from drying drum high-end, the burner is installed in the other end,lifting blade is installed on cylinder wall. When drum rotates, aggregates are lifted up and fall to form certain curtain under the help of lifting blades or flights or lifters. The roller blades have different shapes and requirements in different zones, such as in the burning area, the blade insures no material curtain and also make full use of the heat which as much as possible is transferred to aggregate. The other areas of the blade should be benefit for aggregate drying and heating.
Burning heat and aggregate transfer in cylinder in counterflow way, aggregate is transferred heat when the heat flow pass the curtain. The smaller density of the curtain, the faster hot air pass, which lead exhaust temperature to be higher. That effect the usage of the baghouse dust. The bigger the curtain density is, the slower hot air pass, which increases heat utilization rate but reduces the exhaust temperature, it is easy to make the condensation phenomenon in the bag.
The curtain density should can make full use of the heat energy and conform to the general requirements of the asphalt mixing plant and energy consumption.
3. Influence factors
(1) Drying ability
Drum size: The drum length affects heat exchange time of aggregate in the cylinder body and the diameter has direct influence on the area and density of the aggreate curtain. Therefore the drying ability is proportional to the drum size.
Material factors: It is related with the moisture content of aggregate and aggregate size. The higher the water content is, the lower the drying ability becomes, and the moisture content is usually required to be less than 5%. It is easier to heat coarse aggregates are than fine aggregates.
Environmental factors: The higher the altitude is, the rarer the air it contains, the lower the output is. The higher the environment temperature is, the higher the output is.
Temperature factor: The higher the required heating temperature is, the lower the output is. The higher the exhaust gas temperature is, the lower the output is.
(2) Heating efficiency
Effective heat: aggregate heating heat required + water vaporization heat required
noneffective heat: a small part of heat radiate to the external environment by a drying drum wall (wall radiation heat) +loss heat of emissions
Total heat: total heat burner release
Heating efficiency:the ratio of effectively heat and total calories
Heating efficiency is effected by the blade shape, leaf arrangement and materials.
4.The problem of common equipment
(1) Sealing problem
Cylinder seal is bad which causes the cold air outside into internal area because of negative pressure effect in the cylinder. Too much air leak will increase exhaust of the system.And air leak will change air pressure at the same time, reduce the flue gas velocity and then affects the heat efficiency.
Drying cylinder seal ways includes: end seal,ring seal,friction seal and Labyrinth seal etc.
The structure of end seal,ring seal,friction seal is relatively complex, and the sealing ring or flexible material are easy to loss or damage,which is not benefit for maintenance of the drying drum.Labyrinth seal adopts non-contact design, simple structure and do not need to maintain, reasonable design can achieve good sealing, This design is commonly used in the drying drum of asphalt mixing plant.
(2) Axial dynamic problem
Drying cylinder has certain axial dynamic problem due to relative supporting wheel .Axial dynamic will cause the gap change between drying drum and feed box and discharging box,effect cylinder seal, and then lower the thermal efficiency.
The sinking foundation when drying cylinder is installed will make several stents not on the same level.Destroy dynamic balance, make the wheel wear more severe.
The manufacturing materials of supporting wheel and running hoop, surface dust, oil, aggregate temperature, the capacity change of the drying drum in actual operation production and environmental wind also effect axial dynamic in some way.
Main advantages: SIEMENS motors, hard-teeth reduction gear, friction driving, drum rotating stably. Optimized lift blade design, make sure heating equably and high energy using ratio. With 50mm insulating layer and stainless steel skin, avoid heat be lost.