Thank jmec87 for detailed explaination.
This means that,
Example, for PSL3 Tee of size 12"x12"x12", it is accpeteable to have QTC OD 5" or 4" SQ.for batch type furnace.
Whereas For continuous for furnace, the QTC should be OD 10" or 10" SQ.Minimum.
Please advice.