A Study In Sintering-Furnace Performance Improvements
Furnace manufacturers are continually challenged to meet user demands for lower maintenance requirements and better energy efficiency. In the case of a sintering furnace, the cost benefits derived from meeting these demands can help powder-metallurgy companies maintain a competitive edge. User feedback played a major role in tackling sintering-furnace performance issues in the areas of control system and operator interfaces, improved flame supervision systems and operating energy costs.
Furnace insulation. The initial step in a sintering process is removing the binder material and lubricants from the green compacted parts in a "burn-out" zone prior to sintering to final part density. These burn-out materials are not only bothersome to handle, but also can coat the furnace refractory causing deterioration. For example, a carbon deposit on the insulation board on the interior walls of the rapid burn-off (RBO) unit caused insulation-board flaking, requiring periodic replacement of the insulation. Flaking also can damage parts being processed.
In addition, insulation board used by itself allows the furnace shell to get hot. Replacing the insulation board with a combination insulating fire brick (IFB) backed by a 1-in. insulation board lowered the shell temperature from 250F to 160F (120C to 70F). The IFB also provided a hard surface on which the hydrocarbons from the binder/lubricant can condense. The carbon adheres to the brick until it is scraped off or burned off.
RBO burners. Previously, if burners for the rapid burn-off unit were extinguished, it was necessary to purge the entire furnace muffle before re-igniting the burners. The need for process purging was eliminated by purging the burners using nitrogen.
Premix burners in the rapid burn-off unit burn rich, so there never is excess air (or oxygen) to combust with the hydrogen atmosphere from the furnace muffle. Final combustion occurs at the furnace entry and exit doors where the door pilots ignite the combustible gases.
Fuel-mixing system. Replacing a mechanical gas-mixing system with highly accurate combination air-gas ratio, safety shut-off valves increased performance. Multi-function SKP50 (Siemens) safety and control gas valves provided the functions of safety shut-off, constant pressure regulation, and proportional flow control. The valves eliminated several redundant components including pressure regulators and air/gas mixers, and provided an added benefit of shortening the length of the gas trains.
An independent testing company assembles and tests the gas trains. Valves are subjected to zero-leakage test procedures, which ensures safety and quality in accordance with all applicable NA and European standards. In addition, a steel mesh filter installed in each valve protects downstream devices against damage from foreign particles and eliminates the need for separately installed filters.
Leakage or inoperability of the vent valve is a typical problem on gas trains. Introducing an automated leak test control in place of the vent valve solved this problem. The leak detection unit (LDU) checks the sealing integrity of the gas shut-off valves prior to burner start-up and/or immediately after burner shut down. The LDU has an easy-to-read dial indicating the progress of the test program and has no inlet gas-pressure limitations. This provided an added level of equipment diagnostics and requires less maintenance for the furnace operator. The system is installed in the control panel, adding a significant degree of safety to the combustion system.
A typical maintenance and control issue on many sintering furnaces is the control motor connected to a control valve by means of a linkage assembly. Coarse motor resolution as well as "slop" in improperly adjusted linkages provided poor burner control. This condition is especially noticeable at low fire.
Mechanical linkages were eliminated using a Siemens SQM5 direct-coupled control motor with butterfly valve, which simplified maintenance for operators. The valve attaches directly to the motor shaft, eliminating the need for linkages and reducing the set-up time, as well as simplifying maintenance and reducing space requirements. The control motor senses a change in position of less than 1/3 degree compared with 2 degree adjustments with other motor controllers. This provided a six-fold improvement in valve position control (furnace control), minimizing controllers "hunting" for the set point and, thereby, providing improved product quality and consistency.
Furnace internal hardware. Porous silicon-carbide (SiC) tiles with Ni-base heat-resistant alloy strips on top over which the muffle slides are subject to oxidation over time. The oxide (SiO2) caused the tiles to expand and crack, requiring periodic replacement. Replacing the SiC tiles with Si/SiC square beams having cordierite capping between the beams and the muffle not only eliminated the cracking problem and the need for high alloy skid bars and pins, but also reduced the number of piers needed from 14 original brick piers to 6 precast piers. In addition, elimination of brick and mortar by replacing floor brick with 8 in. of insulation block and a layer of IFB laid loose significantly reduced bake out time.
With ever-increasing energy costs, a major concern of furnace users is the energy required for operation. Energy input requirements for furnace start-up and operation were reduced by replacing conventional refractory brick lining with thinner fiber-module lining. Furnace energy input was reduced by more than 20% by replacing 11-in. refractory brick walls with 8-in. thick insulation. In addition, casing wall temperature dropped from 250F to 185F (120C to 85C).
Precasting furnace flues made it possible to double the insulation thickness using vacuum-formed insulation blocks. The thicker insulation not only saved energy, but also reduced the heat escaping making the furnace shell cooler (surface temperature below 200F, or 95C) and more comfortable for personnel to work around.
Reduction of heat loss through the furnace casing allowed the use of smaller burners (except in the first zone). This provided more uniform heating and allowed reducing the combustion air blower size, filter and silencer, which translated to additional cost savings.