Returning a tired heat treat furnace to original condition is one rebuild option. Another option is to try something new after evaluating available combustion technology and after updating manufacturing goals and objectives. A recent rebuild project allowed Kromschroder Inc. (Hudson, Ohio) to demonstrate the advantages gained in furnace performance by taking the latter approach, according to the company. Working with its distributor, Custom Electric Manufacturing Co. (Wixom, Mich.), Kromschroder rebuilt a batch heat treating furnace for a large, Minnesota commercial heat treater.

The following specific goals were established by the customer before initiating the rebuild project:

  • Improve product quality
  • Decrease furnace operating costs
  • Expand product potential through tighter temperature specifications
  • Provide a cooling cycle for added flexibility
  • Eliminate the need to preheat products prior to heat treating
  • Shorten furnace cycle times
  • Improve radiant tube life

The heat treating furnace was out of service for six weeks to rebuild, including the time necessary to dry out the new refractory, and four days to recertify the furnace. When the furnace was returned to service, operation was closely monitored and documented. Table 1 compares furnace performance before and after the rebuild.

Fig 1 Comparison of minimum cycle time before and after furnace rebuild Fig 2 Comparison of fuel consumption before and after furnace rebuild Fig 3 Thermal efficiency improvement after rebuild Fig 4 Improvement in productivity after rebuild
According the furnace rebuilder, all performance goals established by the customer were achieved, with results even better than expected. Temperature uniformity improved, cycle times improved, and the quality of the heat-treated parts improved. In addition, operating costs dropped, and the addition of a cooling cycle expanded processing capabilities. Recovery times for the rebuilt furnace were reduced more than 25% (Fig. 1), fuel costs dropped 59% (Fig. 2), thermal efficiency jumped 48% (Fig. 3), and furnace productivity increased 11% (Fig. 4).

Before the rebuild, the furnace was heated using four 220,000 Btu/h burners fired into two trident-style radiant tubes. Each tube had a central recuperator to provide preheated air for each burner. The burner control system was a cross-connected ratio system consisting of a single ratio regulator and air butterfly valve, and furnace process temperature was achieved by varying the burner input between low and high fire. The air butterfly valve was controlled by means of a time-proportioning temperature controller, while an impulse connection to the gas ratio regulator from the combustion air butterfly valve controlled gas flow. The maximum capacity of the combustion system was 880,000 Btu/h with a net input of 420,000 Btu/h. The system had a thermal efficiency of approximately 50%. While this was the traditional system used at the facility, the company was looking for something new.

Fig 5 Comparison of thermal input before and after furnace rebuild

The furnace rebuilder recommended a system that included a new gas train, combustion air train, burners and components, and control panel. The new combustion and control system was designed to provide a maximum furnace input of 880,000 Btu/h and higher thermal efficiency at 1,700F, or 930C (Fig. 5).

The combustion system consists of six single-ended, self-recuperative burners (each rated at 136,000 Btu/h). The burners are capable of operating at a 100 Btu/in.2 heat flux at 1,700F, and can withstand extreme exhaust gas temperatures.

Fig 6 Expected improvement in radiant-tube life after furnace rebuild.

In the past, burners equipped with silicon-carbide outer tubes would have been part of the rebuild system. In this case, however, APM alloy radiant tubes (Kanthal Corp., Sweden) were selected from a number of available options. APM tubes are extruded from a Fe-Cr-Al powder-metallurgy alloy. According to Custom Electric, who supplied the tubes, the price and lead-time for APM tubes falls between those for nickel-chrome tubes and silicon-carbide tubes. Also, APM tubes have the same heat flux capability as that of silicon carbide tubes, but have better resistance to damage from impact loading, and they have a service life up to four times greater than that of nickel-chrome tubes (Fig. 6). The tube size used for the furnace rebuild was a 6-in. outside diameter by a 63-in. effective heating length. This provides a total tube surface area of 1,130 in.2 per tube and 84 Btu/in.2 heat flux per tube.

Other components used in the rebuild included burners controlled by a Kromschroder PF-1 9 flame-management and pulse-control system-a prepackaged unit having an integrated hardware pulse controller. The MPT pulse controller interfaces with a Honeywell UDC 3300 temperature controller having a current output of 4 to 20 mA. The output signal is fed to the pulse controller. The unit does not require an external interface or complicated programming methods for configuration, which makes it easy for plant maintenance personnel to understand and maintain. It also is said to be the most economical means of applying true pulse firing to the furnace.

The MPT pulse controller interfaces with each flame card to provide the input signal to turn burners on and off, adjust the time the burner is on, and adjust minimum off time. A microprocessor determines maximum off time, which is related to the input requirements of the furnace. This allows the firing patterns of the burner system to be optimized. It also allowed a cooling cycle to be incorporated into the operation to facilitate broader product processing capability.

For more information: Brian Hall is sales manager and combustion specialist, Kromshroder Inc., 1691-H Georgetown Rd., Hudson, OH 44236; tel: 330-342-0595; fax: 330-342-0596; Internet: www.kromshroder. com. Robert Edwards is president, Custom Electric Manufacturing Co., 48941 West Rd., Wixom, MI 48393-3555; tel: 248-305-7700; fax: 248-305-7705; Internet: www.custom-electric.com