A combination cabinet includes at least one pair of door frames, at least one separation bar mounted between the door frames, a pair of fixing seats mounted on the separation bar, and a plurality of pivot pieces each mounted in a respective one of the door frames and each pivotally connected with a respective one of the fixing seats. A second pillar is mounted on the platform defining a third and fourth load supporting surfaces. A manufacturing method includes fabricating the air-cans out of one material and fabricating the open port out of another material, and in particular making the open port from a material that is more corrosion resistant than the material the air-can is fabricated from. A reactor model relates system operating cost to system operating parameters, such as electrical power consumption andor rate of oxidant addition, where applicable.