Channel furnace start-up demands careful attention.

Proper equipment selection, monitoring and metal introduction can make the difference in the life of a furnace refractory lining.

No doubt about it - furnace relining and start-up are costly to a foundry in labor and lost production hours. But cutting corners or not paying close attention to proper start-up procedures only means it'll have to be done again a lot sooner than necessary - adding up to more downtime per year and increased refractory costs.

In the long term, sound start-up practice can be the difference between three months and nine months of refractory life. In the short term, careless start-up can mean refractory failure and even runout.

Simply, furnace start-up consists of heating the new refractories to a specific temperature for a prescribed period of time (curing), followed by careful control of the first heat. All variables in the procedure should be approached with as much care as those in any other foundry process.

This article provides some guidelines for channel induction furnace start-up. They are presented as comparisons of different start-up procedures and are not suggested as the exact program for any particular furnace. The refractory supplier and furnace manufacturer should be con-suited before any furnace start-up.

Choosing a Burner

Curing requires that the refractory be heated from a source outside the furnace, usually some type of gas burner [ILLUSTRATION FOR FIGURE 1 OMITTED]. The type of refractories installed and the size of the furnace play a part in selecting the proper burner. For example, while a larger furnace requires a higher BTU level from the burner, if the refractory is a high-moisture castable, you need a burner with excellent low-temperature control to avoid damaging the refractory.

Whichever burner is chosen, constant care and monitoring are necessary during curing, or there could be disastrous consequences later - when the furnace is full of molten metal. With dry refractories in particular, if something goes wrong with the heating apparatus in the middle of the process, the sudden chilling can cause the already hardened refractory surface to begin buckling away from the loose material behind it.

The Gas Torch - The earliest burner used to heat and cure refractory construction in channel furnaces was nothing more than a gas torch with combustion air ingressed into the furnace.

This was followed by another system using a gas pipe within an air pipe. The flow of gas and air is controlled by individual valves, making it difficult to achieve exact temperature requirements. The combustion air is usually supplied by a compressor, which makes the entire process expensive by today's standards.

Inspirator - This burner uses the gas flow to induce primary air into the system in proportion to the gas flow [ILLUSTRATION FOR FIGURE 2 OMITTED]. The burner draws air into the system, requiring no blower or compressed air. Usually, this system does not have the necessary turndown ratio or the gas flow to achieve the temperatures required. Accurate fuel air ratios are hard to control, so it is difficult to follow specific firing schedules.

Aspirator - This burner causes low-pressure air, usually supplied by a blower, to flow through a Venturi, which induces gas into the air stream in proportion to the air flow. A single air valve controls both the air flow and the gas flow, giving adequate proportions.

For the aspirator system to work properly, it must be sized for the correct ratio between the mixer throat and burner orifice. This burner, along with the inspirator burner, is subject to flashbacks.

Convection - This unit is a nozzle mix burner that eliminates problems associated with the inspirator and aspirator type burners. The convection burner system injects large volumes of combustion gases into the furnace at a velocity of more than 400 miles per hour [ILLUSTRATION FOR FIGURE 3 OMITTED].

The high volume of air wipes the refractory lining, removing moisture and cooler air while giving excellent temperature uniformity. By recirculating the products of combustion within the furnace, less fuel is required.

Once ignition is obtained, the air flow from the combustion blower is adjusted based on the furnace size. The temperature in the furnace can be accurately controlled by changing the flow of gas to the burner. The convection burner is the safest method and is the optimum system for curing channel furnace linings, yielding faster turnaround, increased refractory life and less fuel consumed.

Thermocouple Placements

Obviously, in any application requiring exact temperature control, monitoring is important. During start-up, it is necessary to receive temperature data from various points within the furnace so that optimal curing can be achieved throughout the unit [ILLUSTRATION FOR FIGURE 4 OMITTED]. All temperatures should be recorded on a strip chart and documented with an hourly and daily log sheet. These permanent recordings are an important quality control resource for consistency from one start-up to the next.

Quantity of Thermocouples - This number depends on the overall size of the channel furnace. A minimum number is five, with more added if two inductors are on the furnace.

Thermocouple Placement - Two thermocouples should be placed on the floor. One of the two will be placed to read the burner's temperature. The second is placed directly opposite the first to measure actual hot face refractory temperature under firing. Place another thermocouple to measure the exhaust temperature leaving the pouring spout. Additional thermocouples can be placed to measure temperatures of the receiver, shell and cooling water.

Inductor Thermocouples - If a solid steel form is used, two thermocouples should be attached to the top of the form (shorting bar). If the inductor has a hollow loop, install one thermocouple at the top of the inductor to measure the temperature going into the loop. Install another thermocouple so the tip is at the bottom of the loop. This area will be the lowest temperature point of the tnrhace.

Ferrous Inductor Forms - When using a dry vibratable inductor material, the thermocouples are secured to the outside of the form extending through the refractory to the shell. Added suggestions are to use two thermocouples under the shorting bar, two about halfway down and two at the bottom of the loop.

Exhaust Points

It is crucial to seal any furnace openings before curing to prevent damage to the steel, piping and hosing, as well as to maximize the efficiency of the burners.

Slag Door - This area is usually sealed with a ceramic fiber between the door and furnace shell. At times it is necessary to use a plastic refractory to seal the larger gaps.

Receiver - A ceramic fiber blanket is laid over the top. Large receivers should have the blanket supported by a steel frame.

Pour Spout - This point will use a ceramic blanket cover and at times needs adjustment to the opening to increase or decrease exhaust gases.

Inductor Form Options

Relining the inductor on a channel induction furnace requires special care. Since the form, which makes the inductor channel around which the refractories are set, cannot in most cases be removed from the furnace, its material must be carefully considered.

Also, the form's physical characteristics determine the width of the channel and will affect furnace performance over time. If the cavity the form leaves is too narrow, slag blockage will quickly become a problem, leading to poor metal circulation and uneven heat distribution.

Refractory Firing Schedule

It is imperative to consult with the refractory supplier for the
exact timetable needed for the particular furnace size and design.

Dry Vibratable

Ambient to 1000F at 400F per hour                            3 hours
Hold at 1000F                                                8 hours
For dry vibratables with removal form options
The form is usually pulled after the 8 hour hold
600F to 800F increase to 2200F at 400F per hour              4 hours
Hold at 2200F                                               14 hours
Start to cycle the inductor four hours into hold

Dry Vibratable Inductors with Brick or Rammed Alumina in the Throat
and Upper Case

Ambient to 250F                                              2 hours
250F to 1200F at 25F per hour                               38 hours
1200F to 1800F at 40F per hour                              15 hours
1800F to 2400F at 75F per hour                               8 hours
Hold at 2400F                                                9 hours

Low-Cement Castables

Ambient to 300F                                              2 hours
Hold at 300F                                                 8 hours
300F to 800F at 20F per hour                                25 hours
Hold at 800F                                                 8 hours
800F to 1800F at 40F per hour                               25 hours
1800F to 2400F at 60F per hour                              10 hours
Hold at 2400F                                                6 hours
The two holds allow for phase changes at 390F and 810F

All firing schedules performed with the use of convection burners.

Hollow Loop - Normally cast out of steel and to the exact dimensions of the inductor cavity, it is usually manufactured in-house. It is melted when inductor power is applied and becomes part of the first heat.

Fabricated Loop Form (Steel Hollow Form) - Purchased from an outside source, this form is designed to the exact inductor cavity dimensions and is the most common of all forms used for inductor installations.

Woodloop - This form is usually manufactured by an outside pattern shop. Consult the furnace manufacturer regarding all pattern specifications. The wood loop is bu reed out prior to metal introduction.

Throat Forms - These are usually supplied by the furnace manufacturer. Unlike the other forms, they are not melted or burned out, but can be removed. The forms are 1/2-in. steel constructed and angle-reinforced to withstand installation and removal after precuring of the refractory.

Cooling of the Inductor

The inductor's cooling system must be engaged before iron is introduced, and it is very important to follow the manufacturer's instructions to be sure it is working properly. The brittle nature of refractories cannot handle sudden sharp variations from the established temperature gradient. Curing may have already caused stress in the refractories and thermal shock can lead to failure, and eventually to runout.

Proper maintenance checks are mandatory to ensure correct water flow and temperature. Inlet and outlet temperatures should be monitored to ensure the cooling system is performing to the furnace's preset standards. If, for instance, the water heaterwere to fail and regular tap-temperature water suddenly introduced, the inductor could crack.

Inductor Metal Introduction

The first heat after feline should be carefully controlled to minimize damage to the new refractories [ILLUSTRATION FOR FIGURE 5 OMITTED]. Both temperature and metallurgy are key factors in an effective start-up.

Metal Temperature - Metal should be as cold as possible to prevent thermal shocking of the refractory lining in the upper case, throat and inductor.

Metal Temperature Gradation - Bring the iron up to operating temperature at a controlled rate of 25-50F per hour after hold. This is done under electrical inductor power.

Carbon and Silicon Content - These constituent levels should be high as possible to provide a homogenous flow for refractory grade and temperature introduction.

Hold Time on Metal Introduction - Hold the metal at 2400F for 72-96 hours and possibly longer, if the overall production of the furnace permits. Once again, cutting corners now will only shorten refractory life and lead to more frequent reline and start-up.