plants for the recovery of chemicals in pulp and paper manufacturing proceses featuring control apparatus and instrumentation for lighting, heating, steam generation, heat treatment, refrigeration, drying and ventilation, sold as a unit; pollution control plants, namely waste water and air purification units for industrial use; nuclear power plants, sold as a unit; gas producing installations comprised of gasifiers, air compressors, air blowers, heat exchangers, burners, boilers, scrubbers, gas and steam turbines, filters, filter presses, mixing machines, feeders, electrical controllers and instruments, sold as a unit for the production of combustible gases and for monitoring such processes