Plant Design & Economics

A process flow diagram shows the major equipment, streams and a balance-based stream table, while a P&ID adds every instrument, valve and safety device with tag numbers; the sequence is BFD → PFD → P&ID as design detail increases.

Plant location is chosen by weighting factors such as raw-material and market proximity, utilities, labour and regulations into a comparative score, while layout arranges units for safety, flow efficiency and maintenance access.

Capital-cost estimates use scaling laws (the six-tenths rule for capacity, cost indices for time) and factored methods (the Lang factor) to turn a known equipment cost into an installed plant cost at the required accuracy class.

Project profitability is judged by discounting future cash flows to the present: a positive net present value at the required rate of return means acceptance, and the internal rate of return is the discount rate that makes NPV zero.

Scale-up preserves the controlling dimensionless group rather than a single dimension, because geometric, mixing and heat-transfer requirements cannot all be held constant at once; the engineer decides which criterion (e.g. constant P/V) governs.