Before you start

Transport is how momentum (moving fluid), heat and molecules get from one place to another. Pipes, pumps, heat exchangers, dryers and columns are all sized with it. This lesson recaps what it uses from the Material and energy balances course, so you can start here.

Units are SI: metres, kilograms, seconds, kelvin, newtons (N, the force that speeds 1 kg up by 1 m/s every second), pascals (Pa = N/m²) and watts (W = J/s). Gravity, g, is 9.81 m/s². A temperature difference of 1 °C is the same as 1 K. Flows are per second: m³/s, kg/s.

Mass and energy are conserved: at steady state what goes in comes out. Heat to warm a stream is ṁ × cp × ΔT, with water's cp about 4.18 kJ/(kg·K) and its density about 1000 kg/m³.

One pattern runs through everything here: rate = driving force ÷ resistance, like current = voltage ÷ resistance in a circuit. Fluid flows from high pressure to low; heat from hot to cold; molecules from high concentration to low. Double the driving force and you double the rate; add resistance and it drops. Resistances one after the other add up.

A flux is a rate per unit area, like W/m² or mol/(m²·s). A gradient is how fast something changes with distance: a temperature dropping 20 K across a 0.1 m wall is a gradient of 200 K/m. The steeper the gradient, the faster the transport.