For the purposes of this Section, the following terms are defined:     

Combustion for indirect heating
"Combustion for indirect heating" is the burning of fuel in equipment, such as steam boilers, water or air heaters, stills, or brew kettles, where there is no contact between the products of combustion and the materials being heated.

Dust
"Dust" is solid particulate matter capable of being air- or gas-borne.      

Particulate matter
"Particulate matter" is any finely divided liquid or solid matter capable of being air- or gas-borne.     

Process weight
"Process weight" is the total weight of all materials used in any process which discharges dust into the atmosphere. Such materials shall include solid fuels, but not liquid or gaseous fuels or combustion air.   

Smoke
"Smoke" is any visible emission into the open air from any source, except emissions of an uncontaminated water vapor.   

Smoke unit
A "smoke unit" is a measure of the quantity of smoke being discharged and is the number obtained by multiplying the smoke density in a Standard Smoke Chart number by the time of emission in minutes. For example, the emission of Standard Smoke Chart number 1 for one minute equals one smoke unit.

Standard Smoke Chart numbers
"Standard Smoke Chart numbers" are the numbers on the Standard Smoke Chart of the Department of Air Pollution Control that coincide most nearly with the grids on the Standard Smoke Chart indicating graduations of light-obscuring capacity of smoke.