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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 55 OPENING THE DOOR The furnace door has to be opened for cleaning out the dross, agitation (stirring), charging and cleaning the walls. When the door is opened and the burners are turned down, radiant heat and hot gases pour out of the opening and cold air rushes in. The door refractory also loses some of its stored heat. In addition, the furnace is not productively employed for the duration. In our example, the door is 1.5m by 6m long. If the door is open for 15 minutes every hour, the losses are some 510 kWh. This is five times more than the conducted losses and much greater than losses from our example of a 6mm door leak. Consequently every effort is required to keep that door closed as much as possible. Measures include: ■ Fast and efficient charging (consider a charging machine) ■ Fast dross removal (consider a skimming machine) ■ Reducing the dross formation in the first place ■ Ease of furnace cleaning and non wetting refractories ■ Use other methods of stirring for alloying and reduction of temperature stratification. AIR FUEL RATIO At optimum air fuel ratio we don't heat any more air than we need, and we don't leave uncombusted fuel to be vented up the stack. The table shown earlier (Table 2) indicates the substantial increase in carbon equivalent of such unburned fuel. The reality is that while we want the ratio to be perfect, generally simple (cheaper) fixed ratio controllers have inaccuracies that force us to err on the side of excess air. Consequently, by having good control of the ratios with good instrumentation, we can control the ratio close to the optimum for that burner in question. It is also possible to monitor the exhaust gas with an oxygen monitor to achieve this and comply with regulations. Operating the burners with unnecessarily high excess air increases fuel consumption. Maintenance is required to ensure the correct settings are maintained and this should pay for itself. By way of example: Assuming in our example that instead of running at our desired set point of 10% excess air (approx 2% excess oxygen) the adjustment is incorrectly set to 20% excess air, this will result in an increase in fuel requirement of approximately 5%. If not monitored it may be a year or two before the resulting 5% increase in the fuel bill is noticed. In the process we generate an extra 225 ton of CO2 per year. As with the choice of refractories, it may be possible to buy a cheaper burner control, but that may not be a smart move. In addition, if you never monitor, you may never know. Not all burners can be turned down sufficiently for holding power. Sometimes designers will use excess air to compensate. This is intentionally making the burner inefficient in order to be able to reduce the heat input. It does not reduce the fuel input. The door may as well be left open. This is of course a waste of fuel, although it is under a low fire condition. In order to get around this problem, our example furnace has two large burners for melting and a smaller burner with sufficient turn down for holding. Consequently no excess air is required for control. COMPOUNDING LOSSES If we are not careful, these effects are compounded. Assuming we have to use 60% excess air in order to hold temperature due to insufficient turn down on our earlier example with a 6mm door leak, our original conduction loss of 102 kW has an additional 170 kW door leak, and we are maintaining those losses with a burner efficiency that has dropped from some 46% to 30% due to the excess air. Overall the holding consumption is up from 221 kW to 906 kW, an unnecessary