The furnace equipment manufactures only guarantee on the power consumption per tone and melting rate for the furnace for melting down or charge under specific conditions, and also the power required for superheating of molten metal.
In practice in addition to the above additional power and also more time is required for the following operation, which brings down the melting rate and limits the out-put for the furnace.
Charging time –charging has to be done from time to time progressively and depends on the charging material also to a large extent. Solid charges such as blocks and pig iron take considerably less time than loose material such as steel/C.I. turnings/ borings to melt.
Slagging time – From time to time, as the level of metal builds-up slag is to be removed, as and when it accumulates, before further charging to avoid bridging. The time is considerable when light scraps or turnings and borings are used.
Composition adjustment time – even-though pre-calculated charges are made into the furnace, it is necessary to check the molten metal composition and make suitable additions for adjustments before pouring. Foundries having instant analysis facilities such as carbon equivalent meter or spectrograph can save on this time to a great extent, while those adopting chemical analysis by normal laboratory methods need longer time and in the intervening period, the furnace is kept ‘on’ leading power to additional power consumption and delays in pouring.
This composition adjustment time can be minimized with a tap-and-charge system of furnace operation, described elsewhere in this paper but calls for unit metal composition being used.
Tapping time – This is unavoidable, but can be kept at the barest minimum practicable by tapping the maximum possible quantity of molten metal at a time instead of small quantities.
Waiting time – For ensuring the optimal furnace out-put capacity, it is preferable to keep the molds waiting for molten metal instead of the furnace waiting, as this means higher power consumption and reduced out-put rates.