About the Film Coefficient & Pressure Drop Calculator
Use this tool to estimate internal convection and friction losses for flow through a rectangular channel. It is useful for early heat exchanger, cold plate, duct, and channel-flow design checks where Reynolds number, Nusselt number, film coefficient, and straight-channel pressure drop are needed.
How to use this calculator
- Select a preset fluid or choose custom properties.
- Enter the channel width and height in millimeters, channel length in meters, flow velocity in m/sec, and absolute roughness in millimeters.
- Review or edit density, dynamic viscosity, thermal conductivity, and specific heat.
- Optionally enter heat transfer area and temperature difference to estimate heat rate.
- Click Calculate to view the film coefficient, pressure drop, flow regime, and supporting values.
- Click Reset to restore the default water example.
How it works
The calculator first converts the rectangular channel dimensions from millimeters to meters. It computes flow area, wetted perimeter, and hydraulic diameter using Dh = 4A/P, where A is the open flow area and P is the wetted perimeter.
Reynolds number is calculated as Re = ρVDh/μ, and Prandtl number as Pr = Cpμ/k. The flow is classified as laminar below Re 2300, transitional from 2300 to 4000, and turbulent above 4000. Laminar Nusselt number is taken as 3.66, while turbulent flow uses the Gnielinski correlation; the transition range is interpolated.
The convective film coefficient is found from h = Nu × k / Dh. Pressure drop uses the Darcy-Weisbach straight-channel friction loss, ΔP = f(L/Dh)(ρV²/2), with laminar friction factor 64/Re and a turbulent roughness-based approximation, then converts pascals to bar using 1 bar = 100,000 Pa.
The optional heat rate is Q = h × A × ΔT. Results are engineering estimates for straight-channel flow only and do not include entrances, exits, fittings, bends, manifolds, elevation changes, or acceleration losses.
Example calculation
For the default water case with width 20 mm, height 5 mm, length 1.2 m, velocity 1.5 m/sec, and roughness 0.0015 mm, the hydraulic diameter is 8.0 mm. Using water properties at 25 °C, Re is about 13,442, so the flow is turbulent; Pr is about 6.13, Nu is about 92.4, and h = Nu × k / Dh is about 7,010 W/m²·K. The straight-channel pressure drop is about 0.049 bar, and with area 0.08 m² and ΔT 15 K, the estimated heat rate is about 8,410 W.
Frequently asked questions
What is hydraulic diameter for a rectangular channel?
Hydraulic diameter is Dh = 4A/P. For a rectangle, this simplifies to Dh = 2wh/(w + h), using the channel width and height.
Does the pressure drop include bends or fittings?
No. The calculator estimates straight-channel friction loss only. Entrance effects, exits, elbows, manifolds, fittings, and other minor losses must be added separately.
When is the Gnielinski correlation used?
The calculator uses a fixed Nu of 3.66 for laminar flow, interpolates in the transition range, and applies the Gnielinski correlation for turbulent flow at Re 4000 and above.
What does the optional heat transfer area do?
If area and temperature difference are entered, the calculator estimates heat duty using Q = hAΔT. It does not change the Reynolds number, Nusselt number, film coefficient, or pressure drop.
Are the preset fluid properties exact?
No. The preset properties are approximate values at the stated temperatures. For detailed design, use verified fluid properties at the actual operating temperature and pressure.