Many years of experience in the design, construction, installation, operation and repair of industrial heat treating equipment has made it clear to the author that furnace performance can be maintained at optimum levels by creating a sound maintenance program. The author has developed a three-step program consisting of some simple, straightforward steps to help keep heat treating equipment at peak performance. The program involves determining existing problems and making immediate adjustments, setting up a semiannual preventative maintenance program and rebuilding the furnace and performing annual preventative maintenance. The following application example shows how this program can bring equipment with declining performance back up to high efficiency.
Roller hearth furnace case study
A captive heat treater was experiencing problems with a roller hearth furnace used to heat treat AISI 52100 (UNS G52986) steel bearings. The bearings had scale and oxidation on one side and had a light case on the other.
Step one: Determine existing problems and make necessary adjustments. Following an inspection and various tests, it was determined that the CO was too low and the CO2, CH4 and dew point were too high. Further inspection revealed that the furnace was sucking air from various sources including burners, packing glands, outer and inner doors, atmosphere test ports, burn-out ports, site glasses, probes and all bolts and gaskets, which created problematic atmosphere readings.
For example, a three-gas analyzer was connected to the charge end (four control zones/two atmosphere zones), and when the discharge door opened, the dew point jumped ten points and the CO decreased by five points. The door travel was 24 in. (609 mm) for a 10 in. (254 mm) part and the furnace had no insulation in the front end. As a result, air entered into the furnace every time the door opened.
The first adjustment necessary was to reduce the door travel to 12 in. (305 mm) to eliminate the excessive opening above the load. To bring the atmosphere within the desired levels in the main body of the furnace, the nitrogen flow was increased to 650 cfh (18 m3/h) and the methanol increased to 6.0 gpm (23 litre/min). (The total Endo equivalent flow was 2,000 cfh, or 57 m3/h). These adjustments resolved the scale and oxidation problems so that production could continue without experiencing poor metallurgical results. The furnace was at temperature and atmosphere was barely within an acceptable range.
The only way to increase the atmosphere to the desired composition would be to shut the furnace down to room temperature and remove, clean, pressure check and reseal all mechanical components, including rolls, burners, fan, inner and outer doors, packing glands, cover plates, TLCs, atmosphere ports, etc. All of these tasks would become part of the preventative maintenance checklist.
When the furnace charge and discharge door cycled, it was noticeable that the door jambs and side walls had refractory brick missing and the furnace casing had hot spots. It was apparent that the furnace would need a refractory reline. At a length of over 40 ft (12 m), the furnace needed an additional fan and additional zone baffels to increase atmosphere uniformity.
Step two: Set-up semiannual preventative maintenance. To set up an appropriate maintenance program, it was necessary to document operating conditions in the furnace. Based on this information, preventative maintenance corrective actions were performed in the heating system, refractory, heating chamber, charge and discharge vestibules and mechanical connections. The actions are shown in Table 1.
The furnace was smoke tested following the corrective actions to ensure that all air leaks were stopped. Smoke testing the furnace is a critical part of preventative maintenance and should always be performed prior to start-up. Table 2 shows the atmosphere readings in the furnace prior to shut down and after semiannual preventative maintenance corrective actions.
Step three: Furnace rebuild and annual preventative maintenance. The existing refractory was in poor condition, such as loose bricks on sidewalls, which were pulling away from casing and spalled in many places showing significant cracks. In addition, rub rails were out of alignment due to sidewall shifting, and all flat arches and door jambs were loose and bricks were missing.
Therefore, the refractory furnace insulation was totally removed and rebuilt, and two zone walls were added to separate heating and atmosphere control zones. An additional recirculating fan was added, which would increase heat-up rate by 10% and improve atmosphere control.
The furnace atmosphere piping was modified to accept a new three-zone Thermax Inc. (S. Dartmouth, Mass.; www.thermaxinc.com) methanol vaporizer which has three controllers and allows the temperatures and methanol flow to be controlled individually. This new vaporizer increased the efficiency as it did on a batch furnace that was modified with this system.
The cast blocks spanning the burner opening were changed to a 2800°F (1540°C) large #2 wedge insulating firebrick flat arch. A 0.125 in. diameter by 4 in. long (~3 by 100 mm) RA304 (Rolled Alloys, Temperance, Mich., www.rolledalloys.com) stainless steel bar window was welded around burner opening in the casing.
The furnace door and zone wall flat arch 2300°F (1260°C) small #2 wedge insulating firebrick was changed to a 2800°F large #2 wedge insulating firebrick. The furnace doorjamb was changed from a 2300°F insulating firebrick to a 2800°F insulating firebrick.
Stainless steel brick tieback anchors were installed in the sidewalls to help prevent wall deformation over time. The main arch at the charge end of the furnace zones 1, 2 and 3 was changed to a 12 in. °- 12 in. °- 12 in. (305 °- 305 °- 305 mm) high-density pyro-fold module. The secondary arch in zone 4 consists of 2300°F wedge brick and is backed with ceramic fiber.
The tray guide through the furnace, a 24 in. (609 mm) long castable slab, was changed to Harbison Walker KX-99 (large) super-duty firebricks (9 in. °- 6.75 in. °- 3 in., or ~228 °- 171 °- 76 mm) along both sides of furnace, running the length of the furnace. (The KX-99 can be readily replaced).
The inner hot face lining (brick and fiber) was sprayed using International Technical Ceramics #ITC100 ceramic coating. This resists carbon penetration and decreases cold-face temperature, which saves energy. Virginia Carolina Refractory (Denver, N.C.) relined the furnace. Table 3 compares the furnace shell temperatures before and after the rebuild.
Following the rebuild, production in the roller hearth furnace increased by more than 35%, and the efficiency of the furnace was increased by more than 30%. In addition, the performance of the drive system was improved from running at 40 Hz to 60 Hz.
Annual preventative maintenance
Prior to the scheduled shutdown for annual maintenance, nitrogen and methanol gas flow and the percentage of CO, CO2 and CH4 were determined in all three zones with the unit at operating temperature and nitrogen-methanol-natural gas atmosphere gas present. Based on this information, preventative maintenance corrective actions were performed in the heating system, refractory, heating chamber, charge and discharge vestibules and mechanical connections. The actions are shown in Table 1.
The furnace was smoke tested following the corrective actions to ensure that all air leaks were stopped. Table 4 lists furnace operating conditions before shut down and before installation of a vaporizer and after maintenance and vaporizer installation. Burner settings (%O2) after start-up and fine tuning are shown in Table 5. IH
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