12 13
Insula. ted cover
Kettle
Zinc blocks
4.4 Heating up
Heating a new furnace should always be carried out in accordance with the instructions of the manufacturer and should always be carried out slowly and uniformly. The relatively higher energy cost'for this is still much lower than the high costs of a kettle failure combined with production losses. During the heating up process a temperature equilibrium must be maintained in the whole kettle, it is important that the temperature inside the kettle wall must not be higher than 480 deg C, and the temperature difference between wall and base must not be more than 100 deg c and through the wall itself the differential temperature not more
than 50 deg c.
The necessary heating time is mainly determined by the size and the geometry of the kettle, big kettles need longer heating times than small ones because of the required temperature equilibrium.
Figure 2: Filling the kettle w;th zinc
u 500
Ol
-"o' 400
E
"'-' 300
-P
d
The zinc blocks lying nearest the kettle wall melt first and
'-
"0'.
E
200
cause the format on of a protective Fe-Zn layer. To avoid a too high pressure on the kettle walls a gap of about 100 mm must be k pt free in he middle. The expansion of zinc is about three t mes that of ron. To stabalize the zinc blocks some soft wooden b7ams can be put. in the gap. They are compressed by the expanding ZJ.nc blocks and -come up or are broken they then char in the liquid zinc and float to the surface. '
-"P' 100
Dura tlon of hea t1ng in days
Figure 3; Heat up curve for go.lvo.nlzlng kettles