Standard polymer asphalt blending systems are designed for stationary installation at facilities such as asphalt terminals. The systems blend SBS polymer with virgin asphalt cement, or bitumen, to make polymer-modifed asphalt cement (PMAC). These stationary systems include box dumper, polymer hopper, auger (screw conveyor), two mixing tanks, mill and let-down tank.
The hydraulically-driven box dumper dumps full 2500-pound Gaylord boxes of SBS polymer into the polymer hopper. The box dumper is activated by an on/off switch and can be raised or lowered to any position up to a full 135 degree rotation. An automatic safety overload stop is provided. Heavy-duty pivot bearings allow for long trouble-free service. Box dimension is 62 x 48 inches.
The polymer hopper feeds SBS polymer into the auger (screw conveyor). The hopper has a capacity of 150 cubic feet to feed the system with an adequate supply of polymer. Bin walls are at a 60° angle for feeding polymer into the auger. The bottom of the hopper has a manually-adjusted slide gate that allows proper flow of the polymer into the auger (screw conveyor). Steel grating at the top of the hopper breaks up any clumped material.
The auger (screw conveyor) transfers polymer from the hopper to the two mixing tanks. The auger is a 14-inch diameter inclined screw conveyor with a 45° incline. It features a 2-way diverter chute with automatic tank selection for feeding polymer into the two mix tanks. Diverting is completed by two power-actuated valves which are controlled from the main control panel.
The screw conveyor is designed for 1200 cubic feet per hour, based on a weight of 20 pounds per cubic foot. Speed of the unit is 100 rpm. It is operated by a 10 horsepower motor.
Each mixing tank has 4,000 gallon capacity. Each tank has a mixer with three sets of 4-blade impellers. Size and pitch of the impellers is designed for optimum agitation and blending of the polymer-asphalt mixture. Four vertical baffles inside the tank are placed for optimum mixing and agitation. The mixer shaft and impeller assembly is driven at optimum output speed of 87 rpm by a heavy-duty 25 horsepower motor. The motor is of totally-enclosed fan-cooled (TEFC) design.
Each tank is mounted on 3 load cells for weighing the polymer asphalt mixture. Load cells are connected to a digital indicator with set point functions mounted on the system main control panel.
Tanks are externally heated with hot oil channels on the tank bottom and sides. Tank walls are insulated with 3-inch fiberglass insulation. Top heads are insulated with 6-inch fiberlass insulation.
Mixing tanks are built according to UL standards. This gives you the assurance that the tanks are safe. Safety features include a high-limit switch that is designed to shut off the asphalt unloading pump. If the tank high-limit switch is tripped, the unloading pump shuts off automatically to prevent overfilling the tank and spilling asphalt onto the ground.
A thermometer is installed in the tank shell for temperature indication. If your asphalt falls to a predetermined temperature, the agitator will not operate. This prevents burning up the mixer motor or breaking the mixing shaft and impellers.
Each tank has a 24-inch quick-access manhole on the top. The tanks have a dished bottom with skirt and a second manhole to allow access to the underside of the tanks. Each tank includes an OSHA-approved caged ladder and OSHA-approved handrails on the top of the tank. A 1-1/2 inch, self-cleaning sample valve is also included.
The Siefer mill shears and re-shears the SBS polymer into tiny particles to help blend it with the liquid asphalt (bitumen). The concentrated liquid mixture of asphalt and polymer makes multiple passes through the Siefer mill to help attain the best possible blending. The Siefer mill is driven by a 300 horsepower TEFC motor.
In the let-down tank, concentrated polymer-asphalt liquid is blended with unmodified (virgin) asphalt to attain the desired mixture of finished PMAC (polymer modified asphalt cement).
The let-down tank is a Heatec Thermo-Guard® vertical asphalt storage tank. Many sizes are available. The tank includes a mixer with impellers to keep the polymer-modifed asphalt cement in proper suspension. The let-down tank is heated by thermal fluid from a hot oil heater (not included). Heated fluid flows through serpentine heating coils in the bottom of the tank, transferring heat to the material stored inside the tank.
All tank surfaces of Thermo-Guard® asphalt tanks are fully insulated. The top bulkhead has a double-wall that encloses 6 inches of fiberglass insulation. Outer surfaces of the side walls also have 6 inches of fiberglass insulation. The tank bottom is insulated by 3-inch foam glass insulation that is protected by a metal cover.