A field of engineering dealing with ways and means by which conditions of processes are brought to and maintained at desired values, and undesirable conditions are avoided as much as possible. In general, a process is understood to mean any system where material and energy streams are made to interact and to transform each other. Examples are the generation of steam in a boiler; the separation of crude oil by fractional distillation into gas, gasoline, kerosine, gas-oil, and residue; the sintering of iron ore particles into pellets; the polymerization of propylene molecules for the manufacture of polypropylene; the incineration of waste; and the roasting of green coffee beans.