Units and Dimensions

Most of the physical properties determined in the laboratory are expressed in the cgs system, whereas the dimensions of the full-scale plant, its throughput, design, and operating characteristics appear either in some form of general engineering units or in special (often mixed) units, which have their origin in the history of the particular industry. This inconsistency is quite unavoidable and is a reflection of the fact that chemical engineering has in many cases developed as a synthesis of scientific knowledge and practical experience from diverse industrial applications. Standardisation of fundamental and derived units has been largely achieved with the introduction of International System of Units. This system is now in general use in Europe and is rapidly being adopted throughout the rest of the world. In this chapter the main systems of units are discussed and the importance of understanding dimensions is emphasised. It is also shown how dimensions can be used to formulate relationships between different parameters.