Stack operating time
Stack operating time means the portion of a clock hour during which flue gases flow through a particular stack or duct while the associated unit are combusting fuel. The stack operating time, in hours, is expressed as a decimal fraction, with valid values ranging from 0.00 to 1.00.
40 CFR § 72.2
The terms used in this part, in parts 73, 74, 75, 76, 77 and 78 of this chapter shall have the meanings set forth in the Act, including sections 302 and 402 of the Act, and in this section as follows: