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Equipment and Parts
GTI Engineering - Furnace Efficiency Comparison

Endfire002a.jpg (32862 bytes)Furnace design & kettle life are closely related, and this comparison will outline what GTI recommends & why we make these recommendations.  We have been working in the industry for twenty five years or so, and our experience has taught us that several furnace designs are useful.

Kettle life depends entirely on the amount of heat transferred through the kettle wall and the design of the furnace and it’s ability to transfer the heat efficiently with minimum damage to the kettle.  Without alot of boring detail, the furnace that is most efficient & produces the best kettle life is the one that produces the most even radiation of heat onto the outer surface of the kettle wall.  In this way, the inner kettle wall is kept at the lowest most uniform temperature, and erosion of the wall is minimized.  This is elementary heat transfer theory, but I am constantly amazed at how often it is ignored.  Historically, the industry has undersized and over fired the galvanizing furnace, and as a result, short kettle life has been the rule.  GTI builds all commonly used furnace designs, so we have no particular ax to grind, and our advice is based on our desire to reduce your overall cost as much as possible.

End Fired Furnaces

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End fired furnaces have been in use for many years.  They are excellent choices  for specific plant applications with moderate production rates, and they are easy to run and maintain.  They are also less expensive to build.  However, end fired furnaces must be carefully designed and the kettle wall must be protected with insulation for the first few feet from the burner or rapid erosion of the kettle inner wall will occur.  The major portion of heat transfer through the kettle wall occurs in the next several feet (where the flame is radiant).  This can easily be seen during meltdown of longer end fired furnaces when the zinc in this region of the kettle is the first to melt.                                                                                                          

Even though there has been a lot of conversation about the higher efficiency of the burner itself, it is the overall furnace efficiency that we are interested in. There is also a current myth about the amount of heat transferred to the kettle by the circulating hot gases.  All that is needed to expose this myth is a reading of any basic thermodynamics text.  It is particularly important that this type of furnace not be over fired, as several of our clients have found at some expense.  GTI is careful to choose the proper size burner so that the furnace can not be damaged by increasing the heat output from the burner as has been the case with some designs.

Flat Flame Furnaces

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For the highest production rate applications, a well designed side fired "flat flame" furnace is a good choice.  No other furnace design, excepting electric furnaces, allows the even radiation of heat through the kettle sidewall.  Each burner is positioned at the proper spacing from its neighbor so that the kettle will not be overheated, and if the kettle is properly sized for the desired production rate, kettle life will be much improved over industry average.

In summary, both furnace designs have their place in the industry.  All modern burner designs will burn fuel efficiently, but burner combustion efficiency has little to do with the efficient transfer of heat through the kettle wall.  It is the job of the furnace designer to evaluate the needs of the process, and help the operator to choose what is best for his company.

 

Naturally Aspirated or "Inspirator" Furnace

A third furnace design that is much less expensive uses multiple small burners along both sides of the furnace near the kettle bottom.  These small burners are normally aspirated (drawing their own air supply through a venturi) and do not require a combustion blower.  The control system is also quitesimple, but electronic flame safeguards are not practical because of the large number of burners. Another advantage of this furnace system is that it will continue to run without electrical power.  The fuel efficiency of this type furnace is about 7 - 10% less than the powered furnaces described above.

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Electric Furnaces

Electric furnaces are the most reliable and maintenance free of all, and the cost is about the same as a flat flame furnace.  However, the cost of electricity usually makes this furnace economically unattractive.

                                                                                                                                             

I hope that I have shed some light on the subject of furnace design, and if you would like to study a little further, just pull out your old thermodynamics text and wade in.

J.C. Birdsall, P.E.

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