Process Control & Instrumentation: Galvanizer Invests in Energy-Saving Techniques
The ever-rising natural-gas prices require innovative ways to save energy costs for heat treaters. This was the situation for GalvTech, a major manufacturer of galvanized steel strip. GalvTech is part of The Techs group located in the Pittsburgh, Pa. area. Two other group facilities are NexTech and MetalTech. GalvTech is a state-of-the-art hot-dip galvanizing facility that has been in business since 1996. The company produces galvanized strip in thicknesses ranging from 0.015 to 0.045 in. (~0.4 to 1.14 mm) and up to 61 in. (1,550 mm) wide. GalvTech purchased its furnace equipment from Electric Furnace Co. in 1996 and upgraded it to increase production in 1999, and was looking for ways to beat the ever-increasing price of natural gas.
Electric Furnace Co. entered into an informal working relationship with GalvTech in July 2003 to mutually benefit both companies by improving the control capability of GalvTech's equipment and allowing Electric Furnace Co. to investigate future innovations in process-control technology, innovations which could potentially be incorporated into Galv-Tech's equipment in the future. In December of 2004, GalvTech added the energy saving recuperation addition to its direct gas fired preheat furnace. While the company knew about recuperating technology for many years, the sustained increase in natural-gas prices recently justified installation of recuperation on its line. The company reports that fuel savings at full production conditions after the modification exceed 20%, which translates into substantial energy and cost savings. The following example illustrates how incorporating recuperation in a preheat furnace with an annual production up-time of 7,560 hr (12-hr shift; 2 shifts/day; 315 days/year) translates into substantial energy and cost savings:
- Gas use (full fire) before recuperation: 82,800 scfh
- Fuel saved after adding 20% (x 82,800) recuperation: = 16,560 scfh
- Total fuel/year saved: 125,193,600 scf (16,560 x 7,560)
- Btu/yr saved (fuel saved x 1000 Btu/ft3: 125,193,600,000
At a cost of $8.00/decatherm (1 decatherm = 1 million Btu), the fuel savings/yr at maximum production is over $1 million (125,193,600,000 Btu/1,000,000 Btu/ decatherm = 125,193.6 ¥ $8 = $1,001,548.80).
Current system modificationsGalvTech's equipment included computer controls supplied by Electric Furnace Co., enabling the collection of large amounts of process data, such as process variables and alarm and deviation data, during production of GalvTech's various products. The data were analyzed and merged with GalvTech's production reports to qualify how the control system was reacting to the current production scenario. The analysis indicated several areas that could be optimized in its control system. Because the preheat furnace control scheme had to be substantially revised, the optimization features were incorporated during the upgrade.
Recuperation system detailsOn GalvTech's furnace, the products of combustion (primarily H2 and CO) and atmosphere (H2 and N2) flow from the entry of the preheat furnace into the unfired entry tunnel on their way toward the flue. The gases preheat incoming strip by passing over it as it enters the furnace, which also saves energy; the strip temperature increases prior to entering the furnace. This is an energy savings benefit inherent in the original design of the furnace. In conjunction with the non-ox preheat furnace's unfired tunnel, additional fuel savings is obtained by using a recuperator to preheat the combustion air for the burners of all zones in the preheat furnace.
After leaving the preheat tunnel, the flue gases pass through an afterburner section to burn the combustibles present in the flue gases. The flue gases then pass through a water-cooled damper located in the flue to control furnace pressure.
The exhaust products are diluted with cold air as needed to control the exhaust gas temperature within the required limits of the recuperator. The flue gases then pass through the recuperator to preheat the combustion air using heat from the products of combustion. The recuperator contains stainless steel tubes and is enclosed by a refractory lined shell. The products of combustion pass through the recuperator by flowing around the tubes. Combustion air passes through the tubes and is heated before passing to the burners through insulated steel piping. A hot air bleed system is included to protect the burners against excessive combustion air temperature.
After passing through the recuperator, flue products are exhausted to the outside of the building through an exhaust system consisting of duct, exhaust fan and exhaust stack. The exhaust fan is driven by a variable frequency ac motor, and fan speed is automatically controlled to maintain a preset exhaust system pressure. An automatically operated cold air dilution valve is provided upstream of the exhaust fan to protect the fan against overheating.
All nonsafety control was accomplished via a programmable logic controller (PLC) provided by Electric Furnace Co. with the existing furnace equipment. This provided a seamless integration into the existing man-machine computerized interface system with which the operator was already familiar.
The project followed a strict timeline because field installation had to be completed during a plant holiday shut down, and the line had to return to normal production immediately after the holiday. To minimize work at GalvTech during the shutdown, the equipment was erected, piped and wired in Electric Furnace Co.'s shop as far as practicable prior to shipment to the site. Electrical controls were provided in prewired, pretested cabinets to minimize field revision of the existing cabinetry as much as possible. This approach minimized revision of existing wiring and reduced field troubleshooting. Electric Furnace also performed all site work that could be accomplished prior to shutdown. This approach paid off as the project started production on time, and the product coming off the line at start up was acceptable to fill customer orders.
Other energy-saving methodsA waste heat boiler is often also inserted into the recuperation system to further use the flue gases. Steam from the boiler can be used for various associated and nonassociated equipment.
Weld-ignore logic is provided to bypass the area of the strip around a weld. This is done because the preheat furnace is controlled by using strip temperature, and the strip around the weld often varies greatly in surface texture. This variation often affects the materials emissivity, causing the optical pyrometer sensing strip temperature to vary. Temporarily ignoring this area until the weld passes greatly improves control stability, which minimizes strip temperature variation, eliminates control upsets and saves fuel.
Dynamic self-tuning can also enhance energy conservation. The control loops can become de-tuned due to load or cycle changes during production. Dynamic tuning monitors the furnace during operation and constantly re-tunes the loops if an upset occurs. Instead of a loop cycling that uses more fuel than required, dynamic self-tuning can virtually eliminate this problem.
All radiant-tube strip furnaces can be fitted with convection preheater sections. A convection preheater is a jet impingement section used to preheat the strip. The preheater obtains fuel savings by impinging the strip with recirculated atmosphere gas that has been preheated by radiant tube exhaust products. Atmosphere recirculating fans draw a portion of furnace processing atmosphere from the convection furnace section and pass it through a recuperator to heat the atmosphere. The heated atmosphere gas then passes back into the chamber to preheat the strip by direct impingement.
Radiant-tube recuperation is another energy-saving method in which a heat exchanger is in the exhaust leg of a U-shaped radiant tube. The combustion air from the blower passes through the heat exchanger and crosses over into the burner after being preheated. Low-NOx versions of these units are also available.