RTO Duct Pressure Loss Calculation for Buyers

Direct answer: For a constant circular duct section, estimate straight-duct loss with Δp = fD × (L/D) × ρv²/2 and fitting loss with Δp = K × ρv²/2. Use actual gas velocity, local density and coefficients defined for the geometry. Add losses along the governing flow path, then reconcile equipment duties and pressure conventions.

An RTO duct pressure loss calculation turns an exhaust layout into a measurable fan pressure requirement. It helps buyers identify whether two offers include the same collection branches, operating cases and downstream equipment. A single pressure allowance without a boundary can conceal missing ductwork or count the same resistance twice.

This guide covers a preliminary steady-flow budget for low-pressure industrial exhaust. Its worked numbers are hypothetical, not Yuehua test results or universal sizing limits. Final design requires verified process conditions, geometry, material compatibility and an engineering review of capture and operating constraints.

Set the pressure boundary before calculating

Draw each complete flow path

Mark the source hood, branch duct, common header, pretreatment, RTO, fan and discharge. Record where each supplier starts and ends its pressure allowance. A fan downstream of the oxidizer handles a different temperature and volume from a fan upstream, even though the connected treatment train serves the same process.

Separate irreversible total-pressure loss from static pressure readings. Static pressure can change when velocity changes, including through a transition, without that change equaling the total loss. Ask whether a quotation states fan static or total pressure and identify inlet and outlet reference planes before comparing it with the system calculation.

List the operating combinations

Include normal production, the maximum simultaneous exhaust case, reduced load and relevant standby conditions. Individual branch flow is not always the total header flow. When another line starts, header velocity, balancing position and the fan operating point may all change. Name each case instead of attaching one unexplained safety factor to a normal duty.

Calculate velocity and density on the actual gas basis

For a circular duct, area A = πD²/4 and mean velocity v = QActual/(3,600A), with diameter in metres and actual flow in m³/h. The divisor converts hours to seconds. For other shapes, calculate the true flow area and use an appropriate hydraulic diameter for the friction method, while retaining geometry-specific fitting data.

Reference flow must first be converted to local actual conditions. For approximately ideal gas of unchanged composition and moisture basis, QActual = QRef × (TActual/TRef) × (PRef/PActual). Temperatures are kelvin and pressures are absolute. Use the RTO wet and dry gas flow conversion guide when moisture bases differ; a dry reference volume cannot simply be assigned to a wet duct.

Calculate or obtain gas density at the same local conditions. The ideal-gas expression ρ = PM/(RT) requires compatible units and the mixture molar mass. Air density is only an approximation for a dilute air-based stream. High moisture, temperature changes or a materially different gas composition require a documented correction.

Separate straight-duct friction from fitting losses

Use the Darcy friction factor consistently

In the Darcy-Weisbach equation, fD is dimensionless, L is straight length and D is internal diameter. Dynamic pressure ρv²/2 is in pascals when density is kg/m³ and velocity is m/s. Determine fD from Reynolds number and relative roughness using a suitable established correlation or chart, and record the selected method.

The Fanning friction factor uses a different convention: fD = 4fF. Confusing the two produces a fourfold error in the straight-section term. Deposits can change both effective roughness and internal area. A clean smooth-duct factor therefore does not establish the future pressure loss of a sticky or particulate-laden exhaust system.

Keep fitting coefficients tied to geometry

For an elbow, tee, transition or other fitting, use Δp = Kρv²/2 with the velocity reference specified by the coefficient source. An elbow radius, tee flow split or transition angle can materially change K. Never add coefficients referenced to different section velocities and multiply the sum by an arbitrary header velocity.

Equivalent length is another way to represent fitting resistance within a declared method. Use either its equivalent-length contribution or its K contribution for the same fitting. Adding both double-counts the loss. Closely spaced fittings and disturbed inlet flow may also make isolated-fitting coefficients an imperfect representation of the installed arrangement.

Work through one hypothetical duct section

Assume an actual flow of 18,000 m³/h, a circular internal diameter of 0.80 m, a 40 m straight length and local gas density of 1.00 kg/m³. Assume a Darcy factor of 0.020 and a combined fitting K of 3.0 referenced to this section velocity. These factors are illustrative inputs, not recommended values for an unmeasured installation.

The area is 0.503 m² and actual flow is 5.00 m³/s, giving a mean velocity of approximately 9.95 m/s. Dynamic pressure is approximately 49.5 Pa. Straight loss is 0.020 × (40/0.80) × 49.5 = 49.5 Pa. Fittings contribute 3.0 × 49.5 = 148.5 Pa, giving about 198 Pa for this section.

This subtotal excludes hoods, other branches, filters, the oxidizer, stack losses and fan connection effects. If actual flow increases by 20% while density and coefficients stay fixed, the calculated section loss rises by approximately 44%. That square-law comparison is a sensitivity check; a real network may change its flow distribution and friction factor.

Assemble the governing path and equipment budget

Calculate each branch-to-discharge path using the appropriate branch flow and shared-header flow. Parallel branches are not serial resistances, so summing every branch loss overstates the required pressure. The network must satisfy continuity and pressure balance, with balancing devices considered where shorter paths otherwise draw excessive flow.

Add vendor equipment losses at the declared duty. Keep clean and design-loaded pretreatment conditions separate. The RTO inlet dust loading guide supports the material basis for that review, but dust mass alone cannot predict a filter pressure curve. Obtain pressure-versus-flow information and the maintenance boundary from the collector supplier.

Reconcile fan pressure convention, system effects and discharge conditions with the fan supplier. Avoid adding an outlet velocity term twice when it is already included in a stated boundary. Use the RTO fan power calculation guide to translate a verified pressure duty into electricity with matched flow and efficiency definitions.

Verify the estimate against operating evidence

For commissioning, agree measurement planes and record flow, temperature, absolute pressure, fan speed, damper positions and production state together. Static taps near elbows or transitions may be unrepresentative. Use a suitable duct-flow traverse and pressure measurement arrangement, with instrument ranges and uncertainties declared in the test plan.

Compare comparable conditions before attributing extra pressure to fouling. Leakage, changed line combinations, a closed damper or a shifted fan duty can alter the result. Keep switching transients separate with the RTO valve switching pressure diagnostic guide. A steady resistance budget does not model rapid valve events.

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For a verified project example, retain the duct drawing, gas basis, coefficient sources, pressure schedule and commissioning results. Report actual improvements only when matched before-and-after records support them.

If downstream flow exceeds the process balance, investigate intentional air additions and potential ingress with the RTO air leakage estimate from dry oxygen data. Use a nonreacting duct boundary and uncertainty checks before attributing extra fan duty to leaks.

Frequently asked questions