A group of unfired pressure vessels operating as a single unit such as the vessels in a refrigeration system, evaporators, ironers and paper machines shall have an individual state serial number marked on each boiler or unfired pressure vessel. The marking should be conspicuously located and as close as possible to the boiler or unfired pressure vessel nameplate. The certificate of inspection fee shall be as outlined in WAC 296-104-700, for each vessel of the system.