The aluminum extrusion industry uses billet-heating furnaces to bring billets, or logs, up to the required hot-working temperature before loading the billets into the extrusion press (Fig.1). A global supplier and leading equipment manufacturer for use in the aluminum extrusion industry is Granco Clark Inc., Belding, Mich., who is one of the major suppliers of billet-heating furnaces to the industry. The Granco Clark furnace uses direct flame impingement and exhaust gas recuperative air jet convection to heat the billets (Fig. 2). The company recently introduced changes to its aluminum billet-heating furnaces to increase the furnace energy efficiency and improve combustion control.
Furnace characteristics
The preheat section of the Granco Clark furnace is said to be relatively unique in that high-pressure jet pipes use the exhaust gases exiting the combustion zones of the furnace to scrub the incoming billets. When comparing the fuel consumed versus output, the use of this technique results in the Granco Clark furnace typically exceeding 55% thermal efficiency versus typically 35% efficiency found with competitive furnaces. Further, Granco Clark has simplified the air-fuel controls on the zonal gas trains (Fig. 3), improving combustion control, de-creasing the space required by the gas train and making the system easier to assemble, install and maintain.
The furnace manufacturer uses Siemens' SKP70 gas valves to achieve the desired air-fuel ratio control. It replaces the blocking gas shut-off valve, ratio regulator and air-gas mixing tube with a single component. This eliminates the need for mechanical cams and linkages while improving burner performance. Three functions performed by SKP70 electrohydraulic actuators are safety shutoff, gas pressure regulation and air/gas ratio control.
Installation
The combustion air sensing line is installed from the air port on the actuator to downstream of the air butterfly valve in the combustion air line. The combustion gas sensing line is installed in a straight section of pipe downstream of the gas valve (Fig.4). As the air butterfly valve controls the amount of combustion air to the burners, the SKP70 modulates the corresponding amount of gas. The actuator compensates for changes in backpressure and other conditions to maintain the set air-gas ratios.
The set-up with the SKP70 valves requires only two adjustments, which simplifies start-up considerably. The first setting adjusts the air-gas ratio in relation to the pressure readings monitored in the sensing lines. The air/gas ratio is adjustable between a range of 9:1 to 0.4:1 gas to air pressure readings. The second adjustment changes the amount of gas bias at low fire, to run with up to 0.4 inch water column air or gas bias. Without an air-gas fuel ratio controller, the combustion system may be out of ratio at many times during furnace operation, at the cost of optimal efficiency.
Siemens gas valves are subjected to zero-leakage test procedures, which ensure safety and quality in accordance with all applicable North American 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.
A typical maintenance issue on furnaces is the control motor connected to a valve by means of a linkage assembly. Coarse motor resolution, as well as "slop" in improperly adjusted linkages, results in poor burner control. This condition is especially noticeable at low fire.
The Granco Clark furnace now uses direct-connect control motor technology (Fig. 5). The direct couple system has virtually no play between the motor and valve. Installation and adjustment were made by using the control motors auto-manual switch and easy adjustment of the switches on the actuator.
Another furnace feature that differentiates Granco Clark's furnace from other furnaces is its dynamic temperature sensor control system. The thermocouple probes and burners run along the horizontal centerline of the furnace. The probes are retracted and advanced to the billets using an algorithm, which sets the timing of the probe cycle for each zone. When the billets are cold or at a temperature far from set point, the thermocouples remain out of the furnace longer as the burners remain at high fire to get the billets quickly up to temperature. By using this technique, the probes are not subjected to direct impingement of burner flames for long periods of time. As the billet temperature gets closer to the set point, the probes remain in contact with the billets longer. This decreases thermocouple maintenance while improving temperature control and accuracy.
The furnace also incorporates Granco Clark's Supervisory Computer System (SCS Extrude) to enhance efficiency by developing schedules, loading equipment parameters, tracking faults and logging production and downtime data. These modifications represent some of the latest developments in furnace technology and economical combustion control systems. IH
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