Preheating unfired preheat vestibules

Unfired preheat vestibules take many different forms, such as (1) an elongated conveyor though a furnace extension, (2) loading cold charges down the stack of a... 

Fig. 5.14. An unfired preheat vestibule is an inexpensive way to practice heat recovery. The only extra expenses are an insulated extension of the furnace (no burners), extension of the conveyor, and some floor space.

Fig. 5.15. Ceramic tunnel kiln (not to scale) with unfired preheat vestibule for heat recovery. Long, narrow kiln or furnace geometry minimizes the proportion of heat loss at the conveyor entrance and exit. Air-lock chambers are even better.

If an unfired preheat vestibule is selected as the vehicle for heat recovery, there may be a great temptation later to add burners to the preheat section for higher capacity. With any preheat section, unfired or fired, careful attention must be paid to gas flow patterns. 

Preheat zones of steel reheat furnaces were formerly unfired, in line with the unfired preheat vestibule philosophy (advocated earlier in this chapter) for recovering heat from the gases exiting the soak and heat zones. However, the regenerative burners are so effective at recovering heat that their final throwaway temperature is just as low with, or lower than, an unfired preheat zone. And the furnace now has much additional... 

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