The Header Sizing Problem
A PTFE heat exchanger with 60 tubes requires headers to distribute steam to all tubes and collect condensate from all tubes. The header diameter affects three performance parameters: the uniformity of steam distribution among tubes, the pressure drop along the header, and the ability of the condensate header to drain without flooding.
A header that is too small has high steam velocity, which causes pressure drop along the header and non-uniform flow to tubes at different positions. The condensate header may flood if its cross-sectional area is insufficient for the two-phase flow of condensate and flash steam.
A header that is too large is expensive, takes up space, and has a large internal volume that increases the warm-up time and the mass of PTFE required.
The optimal diameter is the smallest diameter that maintains acceptable flow uniformity and drainage capacity.
The Steam Header Diameter Calculation
The steam header receives saturated steam at 4 barg (152°C saturation) and distributes it to 60 tubes, each 10mm OD × 8mm ID × 1.5m long. The total heat duty is 60 kW. The total steam mass flow is: m = 60 kW / 2,085 kJ/kg (latent heat at 4 barg) = 0.0288 kg/s = 104 kg/h.
The steam density at 4 barg is 2.67 kg/m³. The volumetric flow is: V = 0.0288 / 2.67 = 0.0108 m³/s.
The recommended maximum steam velocity in the header is 25 m/s (to limit pressure drop and erosion). The minimum header cross-sectional area is: A = V / v = 0.0108 / 25 = 0.00043 m² = 430 mm². The minimum header diameter is: D = √(4A/π) = √(4 × 430 / 3.14) = 23.4mm.
To provide a margin for flow distribution, a header diameter of 40mm (DN40) is recommended-approximately 1.7× the minimum. This provides a steam velocity of approximately 8.6 m/s at full flow, well below the erosion limit and with negligible pressure drop along the header.
| Header Sizing Parameter | Steam Header | Condensate Header |
|---|---|---|
| Design pressure | 4 barg | Atmospheric (0 barg) |
| Mass flow (kg/h) | 104 | 104 |
| Fluid density (kg/m³) | 2.67 (steam) | ~900 (condensate) + flash steam |
| Volumetric flow (m³/s) | 0.0108 | ~0.00003 (liquid) + ~0.005 (flash) |
| Minimum diameter (calculated) | 23mm | ~20mm (liquid only; larger for two-phase) |
| Recommended diameter | 40mm (DN40) | 32mm (DN32) minimum; 40mm preferred |
| Velocity at recommended diameter | 8.6 m/s | Two-phase; adequate drainage |
The Condensate Header Special Requirements
The condensate header must handle two-phase flow-liquid condensate plus flash steam generated when the condensate depressurizes through the steam trap. The flash steam fraction at 4 barg discharge to atmospheric is approximately 10% by mass but much larger by volume. The volumetric flow of flash steam is: V_flash = 0.1 × 104 kg/h × 1.69 m³/kg (specific volume of steam at atmospheric pressure) = 17.6 m³/h = 0.0049 m³/s.
The condensate header must be sized for this two-phase flow without excessive pressure drop or slugging. A minimum diameter of 32mm is recommended, with 40mm preferred for reliable drainage. The header must slope continuously downward toward the steam trap connection at a minimum of 1:100.
The Flow Distribution Verification
With a 40mm steam header and 60 tubes, the flow distribution is verified by calculating the pressure drop from the header inlet to the farthest tube. The pressure drop along the header at 8.6 m/s steam velocity is less than 50 Pa over the header length-negligible compared to the tube-side pressure drop of approximately 5,000-8,000 Pa. All tubes receive essentially the same driving pressure. The flow is uniform within 2-3%.
Summary
The optimal header diameter for a 60-tube, 60 kW PTFE heat exchanger at 4 barg steam is 40mm (DN40) for the steam header and 32-40mm for the condensate header. This provides uniform steam distribution, adequate condensate drainage, and velocities well below the erosion limit. The sizing method can be applied to any tube count and steam pressure by scaling the mass flow and volumetric flow accordingly.
Engineering support for PTFE heat exchanger header sizing is available upon submission of tube count and dimensions, steam pressure, heat duty, and header configuration.
