As used in this chapter, unless the context otherwise requires:

1.    “Boiler” means a closed vessel in which water is heated, steam is generated, steam is superheated, or any combination thereof, under pressure or vacuum for use externally to the boiler by the direct application of heat from the combustion of fuels, or from electricity or nuclear energy. The term includes fired units for vaporizing liquids other than water when these units are separate from processing systems and are complete within themselves.

2.    “Department” means the department of environmental quality.