Furnace efficiency moves to the front burner.

In Minimizing scrap has always been important to extruders. But with fuel costs rising dramatically, many have also started paying more attention to minimizing the waste not just of metal, but of the considerable dollars spent to heat it.

So it’s only logical that furnace manufacturers have developed technologies that capture heat that would otherwise be lost, and return as much of it as possible for re-use in heating the log or billet.

One goal, different approaches.

The goal, of course, is to maximize the actual heat obtained from every energy dollar. But different furnace manufacturers use different methods to achieve it.

Some furnace makers use strategically positioned fans to return exhaust gas to the pre-heat area, where it can be used to raise billet or log temperature at the preheat stage, prior to its entry into the actual combustion chamber. While effective, this places considerable stress on fans, since temperatures of exhaust gasses used in this configuration can easily exceed 1500°.

Another furnace configuration features a counter-flow design that collects exhaust gases and recirculates them through high-pressure piping to preheat billets. A pressurized entrance chamber prevents unwanted “ambient” air from entering the system. Granco Clark’s Hot Jet Furnace, the industry’s most popular furnace, uses this approach.

Lastly, some furnace configurations use recaptured heat to preheat combustion air, which should mean higher efficiency; less gas is needed with hotter combustion air. However, this method relies upon inefficient heat transfer from furnace exhaust, which only becomes available at meaningful temperatures after the furnace has been running for some time. Also, unless the this type of furnace uses air/fuel ratio controllers that throttle the amount of gas in accordance with the air temperature, furnace burners will not be operating efficiently and may even be polluting.

Efficiency—Buy the Numbers.

Getting solid furnace efficiency numbers from manufacturers can be tricky. Many will cite the peak efficiency of their furnace. While this might be interesting, it’s not very helpful; a furnace doesn’t always run at its peak efficiency. What you really want to know is its average efficiency. In the real world, it is average efficiency that determines your fuel cost.

If the average efficiency of the furnace isn’t stated, you can look to other clues—ambient air, for one. If you’re evaluating various furnace technologies, make sure you have an understanding of the amount of ambient air allowed into the furnace. It consumes the same amount of energy to heat one pound (about 3 cubic feet) of air as it does to heat one pound of aluminum. Make sure you’ll be heating billets, not air.

Exhaust gasses are another marker of furnace efficiency; the hotter the exhaust gasses, the poorer the efficiency. A lower temperature indicates higher efficiency. However, if you measure the temperature of exhaust gasses, make sure they are not diluted with cooler air before the point in the system at which you measure it.

Evaluating the efficiency of your furnace.

An efficient furnace can make a huge difference in the profitability of your operation. If you would like a simple formula you can use to evaluate the efficiency of your furnace, just contact Granco Clark by phone or through our Web site at www.grancoclark.com.