WELDING ENGINEERS LTD.
Rue Saint Léger 4 - CH-1205 GENEVA SWITZERLAND
The principle of operation of our dewatering extruders (SDU) and drying extruders (VCU) has not changed since the early days. The design has however been constantly developed to meet the industry's new challenges for higher performance, reduced emissions, lower maintenance and ever increasing online time.
In the following pages we present a general description of Welding Engineers' latest system for drying synthetic rubber. The word "latest" implies, of course, that we designed other system prior to this one. In point of fact, our first system went into operation as far back as in 1946.
Welding Engineers' rubber drying system accepts slurry containing as little as 5 percent solids, thus direct from the slurry tank, using no shaker screen for initial water separation, and delivers rubber dried to specification in pellet or crumb form.
The system comprises the following main equipments:
A. The Slurry Dewatering Unit (SDU)
The mechanical compression separates the water from the rubber which is discharged in the form of large chunks to feed the VCU.
B. Volatiles Control Unit (VCU)
Rubber temperature is increased by shear at controlled conditions.
Fixed at the end of the VCU permits to adjust the pressure and temperature in the VCU. It consists of a variable die (adjustable during operation), a pelletizer and an airveying system. Moisture is flashed, rubber is cut in particles of optimum size and airveyed to the TCU.
The particles of rubber are fluidised and conveyed on a cushion of hot, dry air which removes the residual moisture. It can also include a cooling zone to reduce the rubber temperature to optimum baling conditions.
Slurry Dewatering Unit (SDU)
The aqueous slurry coming from the slurry tank, is admitted through a pipe into a feed tank from which it flows uniformly over a static wedge bar screen . The crumbs, thus separated from the free water, fall into the feed section of the dewatering screw.
The screw, mounted at an angle, is designed to accept the crumb, forward it into the compression section of the screw and discharge it through a variable die. Cutting is accomplished by knives turning with the screw. The variable die allows to optimise the pressure conditions and thus the dewatering during operation.
The water expressed from the rubber flows backwards inside the barrel towards the feed zone. The water is removed through drain openings located in the conical and feed barrels.
The extruder (feed sections and barrels) is entirely closed. No water on the floor and no steam inside the plant building. Two flanges, permit evacuation of vapour. Water drains are directly connected to customer recycle water system. There are no slots or other drain openings that would plug, interrupting the process or causing large variations in the degree of dewatering. Barrels are jacketed for preheating.
The screw can be supported by bearings at both ends, with no contact between screw and barrel.
This is a specially designed extruder for controlling the residual volatiles in the product. The feed section with large volume assures good acceptance of the product from the SDU. A drain permits to evacuate any residual water expressed during the initial compression stage. The cylindrical section generates the pressure and temperature required to bring the product to the die at optimum conditions. Barrels are jacketed for preheating. The barrel sections are entirely closed. The screw is of the floating type. In order to minimize wear, screw flights are stellited and barrels are made from bimetallic tubes.
The die is incorporated in a special device, the TURBULATOR, patented by Welding Engineers. The die holes are drilled through two concentric sleeves. The moist rubber driven under pressure through the die holes immediately
undergoes decompression, causing it to separate into two distinct phases of dry rubber and steam. A cutting tool, resembling a conventional milling tool, rotates inside the inner sleeve, so as to transform the dry rubber into particles of optimum size. The chamber formed by the sleeves is connected to an air source to transport the rubber through an airveying conduit at high velocity to a cyclone. The die surface can be constantly varied during operation, thus varying the die pressure and temperature. The Turbulator offers a positive way of regulating the amount of energy put into the polymer. Consequently, optimum conditions can be obtained in the drying extruder for each type and grade of rubber.
The Turbulator can be equipped with a quick opening device.
Temperature Control Unit
This is a vibrating conveyor with a perforated deck. Air is blown from the plenums beneath the deck and through the product. The system comprises blowers and steam batteries with control for air flow at desired temperature and velocity for the particular operation. The particles of rubber from the Turbulator are thus conveyed in a fluidised stream, each particles surrounded by air, offering maximum efficiency by initially heating and then if desired, gradually reducing the product temperature to the required level for baling or bagging.
An exhaust hood to remove all the process vapours is mounted over the conveyor.