In chemical engineering and reactor engineering, space velocity refers to the quotient of the entering volumetric flow rate of the reactants divided by the reactor volume. It is commonly regarded as the reciprocal of the reactor space time. In industry, it can be further defined by the phase of the reactants at given conditions. Special values for this measurement exist for liquids and gases, and for systems that use solid catalysts.
By definition, space velocity can be expressed mathematically as SV ≡ υ0 / V. In this expression, υ0 represents the volumetric flow rate of the reactants entering the reactor and V represents the volume of the reactor itself. This expression is the reciprocal of the definition for the reactor space time, τ. The space time is measured at the conditions of the reactor entrance, however, and the space velocity is often measured at a set of standard conditions, so the reported velocity may be different from the reciprocal of the measured space time.
Liquid hourly space velocity (LHSV), is a method for relating the reactant liquid flow rate to the reactor volume at a standard temperature. Usually, this temperature ranges from 60° Fahrenheit to 75° Fahrenheit (15.6° Celsius to 23.9° Celsius). The volumetric flow rate is treated as a liquid at these conditions, even though the actual material may be a gas under normal operating conditions.
Gas hourly space velocity (GHSV) is a similar method for relating the reactant gas flow rate to the reactor volume. GHSV is usually measured at standard temperature and pressure. Different industries may have their own definitions for standard temperature and pressure and these conditions may be closer to ambient conditions than to the International Union of Pure and Applied Chemistry values of 32°F (0°C) and 1 bar (100 kPa). It is always important for an engineer to check the basis of calculation.
Weight hourly space velocity (WHSV) differs from LHSV and GHSV, because volume is not utilized. Mass, rather than volume, provides the basis for WHSV. This measurement is the quotient of the mass flow rate of the reactants divided by the mass of the catalyst in the reactor.
Calculations are straightforward when the reactor volume is known and the incoming reactant flow rate is known. For example, if 70 feet3/hour of a reactant enter a reactor with an internal volume of 250 feet3, the calculated space velocity is approximately 0.28 hour-1. This can be viewed as the number of reactor changes the system is undergoing in one hour.