Purpose The purpose of the IGG and SCTR cleaning furnaces is to clean plastic residues from metal parts, breaker plates, dies, filter screen packs, screws, spinnerettes, polymer pumps and numerous other metal parts. SCTR models are designed specifically to clean extruder screws from extrusion processes or injection moulding machines. The long skinny shape of the furnace has been designed to accommodate screws along with other smaller metal parts such as breaker plates, dies, moulds, etc. For most engineering plastics such as polycarbonates, polyacetals, polyesters, polyfins etc a temperature of 430°c and limited oxygen is sufficient to remove these plastic residues from the metal parts. For some of the new high temperature plastics, higher temperatures and higher oxygen levels are necessary to remove the charred residues of the pyrolysed plastic from the metal parts. One of the more difficult engineering plastics to remove is the material known as PEEK (polyether ether ketone). Temperatures in the range of 510°c to 530°c are necessary to remove the PEEK from metal processing parts such as screws and dies. A higher level of oxygen in the furnace is needed than would normally be present to aid oxidation of the plastic residue, thus a special design feature has been included in the IGG/SCTR range of furnaces to aid oxidation (See oxidation control Mode)
Process Description SCTR and IGG cleaning furnaces use heat at 430°c to 480°c to decompose (pyrolyze) the plastic residues to smoke that is consumed in a secondary combustion chamber. This temperature is sufficient to remove most polymers without harming metal parts. Certain high temperature polymers require cleaning temperatures from 495°c to 520°c. The plastic coated items to be cleaned are placed in the cold furnace. The furnace is sealed and the main chamber temperature is gently raised towards its set operating temperature of 450° to 480°c, the duration of the cleaning cycle having been set by the operator.
The primary burner that is heating the furnace main chamber is simultaneously reducing the main chamber's oxygen content below the critical limit at which combustion can be supported. When the main chamber has reached approximately 350°c, the organic components of the plastic, prevented by the lack of oxygen from burning, begin to break down (pyrolysis) and to turn into smoke and fumes. This pyrolysis gas is drawn into the secondary burner system where it is incinerated at 850°c to 900°c. When the rate of pyrolysis gas production approaches the incineration capacity of the secondary burner system, water spray is injected into the main furnace body to control the pyrolysis process. Any carbonised residues left in the furnace are oxidised by the low levels of oxygen in the furnace chamber, leaving the parts free of any organic material. Only inorganic ashes, pigments, or dirt remain which can normally be easily removed with compressed air or water.
The IGG/SCTR Furnace as a System The furnace can be viewed as a system based on two fundamental processes, pyrolysis and incineration, linked by natural draught. The functional subsystems of the furnace are the primary burner system, the furnace main chamber, the secondary burner system, and the control system with its adjunct, the water spray system.
Primary Burner System: The lower burner fires through the furnace wall into an insulated combustion chamber within the furnace main chamber. The burner is adjusted to blow a mixture of excess air and hot gas into the combustion chamber, which ducts it into the furnace main chamber to heat it and its contents. The burner runs at high-fire until the control system switches it to low-fire when the furnace main chamber reaches its first set point (430°c).
Furnace Main Chamber: The hot air ducted from the primary burner system circulates within the furnace main chamber, gradually heating its whole volume and the parts to be cleaned and simultaneously establishing an oxygen content below the critical limit that supports burning. Eventually a temperature is reached where the pyrolysis of the organic components of the coating sets in and pyrolysis gas is driven off the parts to be cleaned. The pyrolysis gas is drawn into the secondary burner system.
Secondary Burner System:
The upper burner fires through the furnace wall into an insulated combustion chamber within the furnace main chamber. The burner is adjusted to blow a mixture of excess air and hot gas into the combustion chamber. The pyrolysis gas inlet hole in the combustion chamber draws in the pyrolysis gas and air from the furnace main chamber into the secondary burner flame, where incineration takes place, and the products of incineration then pass to the furnace stack and the atmosphere by natural draught.
Control System: The system senses the temperatures in the furnace main chamber and in the secondary burner system and compares the data with previously programmed set points. When the furnace main chamber temperature is below its first set point, the primary burner runs at high fire, and at or above that set point it runs at low fire. The furnace controller's second set point (usually at 480°c) is intended, as protection against over temperature through overloading or through accidental loading of substances the furnace is not designed to cater for. At this set point, the water spray system is turned on. When a temperature is reached in the secondary burner system (to be precise a point in the lower chimney section) 60 degrees above the stack set point (850°c to 950°c), the water spray system is turned on.
Water Spray System: The control system opens a solenoid valve to allow water to pass through the nozzles within the furnace main chamber. The fine water mist cools the main chamber and reduces the rate of pyrolysis. The water spray system is activated only when the second furnace set point or the stack set-point is reached and it is deactivated when these conditions no longer prevail.
TWO SPECIAL FEATURES OF THE IGG/SCTR FURNACE
WATER SPRAY PULSE TIMER SYSTEM A special feature of the water spray system which controls the processing temperatures of the IGG and SCTR Cleaning furnaces is a Water Pulse Timer relay. This relay initiates a 2-3 second pulse of water mist (from spray system No 1 only) every 90-100 seconds once the relay has been energised. The Alarm is energised at the factory default setting of 260°c.
The 2-3 second pulse of water mist every 90-100 seconds helps "tumble" the furnace atmosphere as pyrolysis is occurring and aids in smoother control of the furnace and stack processing temperature. The water pulses occur as long as the furnace temperature is above 260 degrees centigrade, regardless of the controlling action of the temperature controllers. Once the cleaning cycle is over and the furnace temperature drops below 260°c the water pulses stop.
OXIDATION CONTROL MODE (OPTIONAL) High levels of oxygen help to speed up the oxidation of the char off metal parts. However due to the thermal nature of the pyrolytic cleaning process, this dictates that for safety reasons the furnace atmosphere is starved of oxygen during the first part of cycle when the plastic is being vaporised into combustible smoke and pyrolysis gases. This prevents ignition of the smoke and fires inside the furnace chamber during the cleaning process. Yet after most plastics have been vaporised into smoke by the furnace heat, residues of carbon and char are left on the metal parts. The char can be removed only by the surface oxidation of the char ultimately to carbon dioxide. The rate of char oxidation is surface limited and is accelerated by higher levels of oxygen. In fact, if the furnace atmosphere were totally inerted, the metal parts would never come completely clean. Although the bulk of the plastic may have been vaporised into smoke, the carbon residues produced during the pyrolysis stage of the cleaning process can only be removed by oxidation. To solve this problem, SCTR and IGG furnaces use a special control system which, will, after all the smoke and gas have been drawn off activate a timer that puts the bottom burner into an high/low cycling mode. The high/low cycling of the primary burner(s) allows air to leak into the furnace while the burner(s) are on low fire, thereby raising the oxygen level and thus increasing the oxidation of any material making it easier to remove. In time all of the char can be oxidised away as gaseous carbon dioxide at the high temperature.
Registered Address - Unit 11 Mid Sussex Business Park, Folders Lane, Ditchling Common, East Sussex, BN6 8SE, U.K. Company Reg No 05821449 : VAT No 883789941.
