Units, flows and a basis

Engineers work with flows: how much passes a point per unit time. Mass flow is in kg/s or kg/h, molar flow in kmol/h, volume flow in m³/h or L/min. They're linked by density (mass flow = density × volume flow) and molar mass (molar flow = mass flow ÷ molar mass). Water at room temperature is about 1000 kg/m³: 3 m³/h of it is 3000 kg/h, or 3000 ÷ 18 = 167 kmol/h.

Unit slips cause more wrong answers than hard maths. Write the units with every number and cancel them like fractions: 72 km/h × 1000 m/km × 1 h/3600 s = 20 m/s. Keep to SI (kg, m, s, K, mol, J, Pa) where you can; American plants still use pounds, gallons, °F and psi, so you'll convert often. 1 lb = 0.4536 kg, 1 US gallon = 3.785 L, 1 psi = 6.895 kPa.

A process is steady state when nothing inside changes with time: flows, levels and temperatures hold constant. Most continuous plants run that way for weeks. A batch process, like brewing a tank of beer, goes through stages and isn't steady.

For a steady process you can pick any convenient amount to calculate with, called the basis, and scale at the end. If a feed is described by percentages, take 100 kg (or 100 mol) of feed: the percentages become kilograms. If you need the answer for 2500 kg/h, multiply everything by 25 afterwards.

Draw a flowchart first: a box for each unit, arrows for streams, and every known flow and composition written on its arrow. Label the unknowns with letters. Most of the work in a balance problem is this drawing.