Frequently Asked Question

Loss mechanisms

Internal-flow pressure loss is the combined effect of wall shear, area change, separation, fittings, porous resistance, elevation, and machine work. Hydraulic diameter is an approximation whose suitability depends on cross-section and flow regime. Local-loss coefficients must use the velocity and area to which they were defined. Pressure planes must be selected consistently so that static pressure, total pressure, and elevation are not mixed.

f is friction factor, L is length, D_h is hydraulic diameter, U is reference velocity, and K represents local loss.

Worked example

For f = 0.02, L/D_h = 50, ρ = 1000 kg/m³, U = 2 m/s, and ΣK = 1.5, the estimate is [0.02×50+1.5](1000×2²/2) = 2,750 Pa.

Check: compare integrated pressure and momentum balances and keep pump or fan work separate from passive loss.

Engineering check

For Pressure Drop and Hydraulic Diameter, maintain traceability from requirement to risk, design output, evidence, and approval. Record the configuration, acceptance criterion, test or analysis conditions, open actions, and residual risk. A method is not complete when the document is filled in; it is complete when the evidence supports the decision and affected controls are updated.

Engineering note

For Pressure Drop and Hydraulic Diameter, state the intended use, input range, dominant mechanism, units, boundary conditions, acceptance criterion, and evidence owner. Use an independent balance, limiting case, repeat measurement, or sensitivity check to challenge the result. Document the configuration and uncertainty so another engineer can reproduce the reasoning and determine whether the result remains valid after a design, material, boundary, or process change.