What factors determine the size of a process vessel?

Vessel sizing depends on function and process requirements. Key factors: residence time (volume = flow rate x time), phase separation (settling velocity and area requirement), surge capacity (buffer for downstream upset), liquid level ranges (normal, high, low), vapor space (for disengagement), and internals requirements (distributors, demisters). Design considerations: L/D ratio (typically 2-4 for horizontal, 2-5 for vertical), nozzle sizes (velocity limits), mechanical constraints (thickness, weight), and access for maintenance. Process engineer provides functional requirements; mechanical engineer completes detailed design. Always document design basis for future reference.