Understanding mass flow rate
Kilograms per second, multiplied out.
Where mass flow shows up in process engineering, the units that vary by industry, and why mass beats volume for compressible fluids.
Mass flow = mass per time.
The SI unit is kilograms per second (kg/s). In practice, processes report in whatever unit fits their scale: kg/h for chemical plants, t/h (tonnes/hour) for grain handling and bulk solids, lb/min and lb/h for industrial workflows that still operate in imperial. All express the same physical quantity; the conversions are pure scaling.
ṁ = mass ÷ time
Why not volumetric flow.
Volumetric flow (m³/s, L/min, gpm) is more familiar — "the tap is doing 5 litres per minute". For incompressible liquids at constant temperature, volumetric and mass flow are interchangeable via density. For gases, they're not: a gas's density changes with pressure and temperature, so the same volumetric flow can carry very different mass per second depending on conditions. Process engineering for compressible fluids (air, steam, natural gas) standardises on mass flow because it doesn't require specifying T and P alongside.
A worked conversion.
A chemical plant reports a reactor feed rate of 5 t/h. Converting to SI: 5 t/h × 1000 kg/t × (1 h / 3600 s) = 1.389 kg/s. To lb/min: 5 t/h × 2204.6 lb/t × (1 h / 60 min) = 183.7 lb/min. Same flow, three notations; pick whichever matches the spec sheet you're reading from.
5 t/h in SI and imperial
t/h ↔ kg/s ↔ lb/min
Multiply by mass conversion, divide by time conversion.
5 × 1000 / 3600 = 1.389 ; 5 × 2204.6 / 60 = 183.7
= 1.39 kg/s ; 184 lb/min
Standard cubic vs actual cubic.
For gases, you'll see SCFM (standard cubic feet per minute) and ACFM (actual cubic feet per minute). SCFM specifies volume at standard conditions (60°F, 14.7 psi); ACFM is at operating conditions. Mass flow is the same in both because mass is conserved; volumetric flow differs. Most American compressor and pneumatic specs quote SCFM specifically to make it comparable across operating conditions — implicitly converting to mass flow underneath.
Common process scales.
Coffee grinder: ~0.001 kg/s. Garden hose: ~0.5 kg/s. Petrol pump nozzle: ~0.3 kg/s. Olympic-pool fill: ~1 kg/s averaged over hours. Industrial chemical reactor: 1-100 kg/s. Niagara Falls: ~2.5 × 10⁶ kg/s. The range spans nine orders of magnitude; understanding which scale you're at informs which units are convenient.