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
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

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.
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.