Continuous production is a flow production method used to manufacture, produce, or process materials without interruption. Continuous production is called a continuous process or a continuous flow process because the materials, either dry bulk or fluids that are being processed are continuously in motion, undergoing chemical reactions or subject to mechanical or heat treatment. Continuous processing is contrasted with batch production. Continuous usually means operating 24 hours per day, seven days per week with infrequent maintenance shutdowns, such as semi-annual or annual. Some chemical plants can operate for more than one to two years without a shutdown. Blast furnaces can run from four to ten years without stopping. Common processes Some common continuous processes are the following: Oil refining Chemicals Synthetic fibers Fertilizers Pulp and paper Blast furnace (iron) Metal smelting Power stations Natural gas processing Sanitary waste water treatment Continuous casting of steel Strip processing lines: eg. pickling lines; tandem rolling mills; electrolytic tinning and galvanizing lines Rotary kilns for calcining lime or cement Float glass Dry pasta production Production workers in continuous production commonly work in rotating shifts. Processes are operated continuously for practical as well as economic reasons. Most of these industries are very capital intensive and the management is therefore very concerned about lost operating time. Shutting down and starting up many continuous processes typically results in off quality product that must be reprocessed or disposed of. Many tanks, vessels and pipes cannot be left full of materials because of unwanted chemical reactions, settling of suspended materials or crystallization or hardening of materials. Also, cycling temperatures and pressures from starting up and shutting down certain processes (line kilns, boilers, blast furnaces, pressure vessels, etc.) may cause metal fatigue or other wear from pressure or thermal cycling. In the more complex operations there are sequential shut down and start up procedures that must be carefully followed in order to protect personnel and equipment. Typically a start up or shut down will take several hours. Continuous processes use process control to automate and control operational variables such as flow rates, tank levels, pressures, temperatures and machine speeds. Semi-continuous processes Many processes such as assembly lines and light manufacturing that can be easily shut down and restarted are today considered semi-continuous. These can be operated for one or two shifts if necessary. History The oldest continuous flow processes is the blast furnace for producing pig iron. The blast furnace