Chris Mason of Western Technologies dispels some myths around the topic.
Westech uses a computer simulation to calculate the maximum production rate of their galvanizing furnaces. There are much simpler approaches, but how meaningful are they?
For example this kind of formula has been used:
p = (qw (heated wall area) – qs (surface area))
p = maximum production rate
qw = average rate of heat transfer to the kettle wall
qs = average rate of heat transfer to zinc surface
h = heat input to metal per unit production
qs and h may vary between furnaces depending on the extent of enclosure and ventilation rates, % of solid zinc addition during dipping and degree of steel preheat. No case heat loss is included.
However when assessing such a formula, the factor of most interest is qw.
A typical value given for qw is around 30 kW/m2.
But the decisive factor in determining the balance between production rate and kettle lifetime is not average heat transfer rate but maximum heat transfer rate at any point on the kettle wall, qz - usually at a point a short distance downstream of the burners for an end-fired furnace.
And wall heat transfer factors must also be given with reference to a particular molten zinc temperature - higher zinc temperatures require lower values of qz.
A guide for qz is approx. 40 kW/m2 when molten zinc is at 450°C.
We can consider qw relative to qz in terms of a ratio r of maximum to average heat transfer, ie: r = qz / qw
How does r vary between kettles and furnaces?
Comparative studies of galvanizing furnaces can be complicated by the large number of variables, which make it difficult to compare the production rates of kettles of different sizes.
Westech has taken a new approach in a recent study.
We have carried out a theoretical study where qz is held constant at 40 kW/m2 while variations in the ratio r are explored as changes are made to the kettle dimensions and the size of burners. The molten zinc temperature was taken to be 450°C.
For the standard burner heat output used in Westech furnace design, we found that r is in a surprisingly narrow range of 1.35 to 1.45 for a wide variety of kettle sizes covering almost the entire range of the 150+ furnaces supplied by Westech to date. This corresponds to a range of values of qw from 27.6 to 29.6 kW/m2.
However, less surprisingly, using fewer, larger burners with the same total heat output gives a higher value of r. For example based on a kettle 12.5 m long x 1.5 m wide x 2.6 m deep, moving from 6 to 4 burners increases r from 1.38 to 1.57. This corresponds to a 15 % decrease in maximum production rate under typical conditions.
This is because using fewer more powerful burners causes more intense local heating, and the output of each burner has to be scaled down to meet the target value of qz.
Perhaps the key shortcoming of many simple formulas is that they do not take into account the way in which heat is added to the furnace, and in particular the output per burner.
So the type of formula cited above could over-estimate maximum production, or justify a high maximum production at the expense of kettle lifetime.
An approach based on a fixed qz may also facilitate comparison to be made between different types of furnace, for example, end-fired and side-fired (flat-flame burners).
More detailed findings on the relationship between average and maximum heat transfer rates in end-fired furnaces will soon be published in a paper on the subject.
In the meantime, for more information on this and many other aspects of design for galvanizing please visit www.westechgalv.com.