What this calculator does
A condensate line is a gravity drain that has to carry two things, not one: the liquid condensate, and the flash steam released when hot condensate drops to atmospheric pressure. The flash is the part that gets forgotten, and it is what fills the line with vapour, slows the liquid and backs steam up into the equipment. A line sized on liquid volume alone works when it is cold and floods when it is hot.
How to use it
- Enter the condensate mass flow from the equipment.
- Enter the temperature and pressure of the condensate LEAVING THE TRAP, from your own steam data. This tool deliberately does not look up a saturated steam table, because the flash fraction is directly proportional to the sensible heat available.
- Enter the ambient temperature the line discharges into. The difference between the trap temperature and the ambient is what drives the flash.
- Read the flash fraction before the pipe size. A flash above about 10 % changes the trap arrangement and the venting, not just the line diameter.
Governing formula
f_flash = h_fg / (h_fg + c_p · ΔT) → load = m · (1 − f_flash) · (1 + S) Where:
-
h_fg= Latent heat of vaporisation [kJ/kg] -
c_p= Specific heat of the condensate [kJ/kg·K] -
ΔT= Trap temperature minus ambient [K] -
S= Vapour loading factor
Derived Equations:
A = πd²/8, P = πd/2, R = d/4 capacity = ρ(1/n)·A·R^(2/3)·S^(1/2) Worked example — 0.5 kg/s from 100 °C at 3 bar into a 40 mm line at 0.5 %
The figures below were produced by running this page's own engine with the example inputs (Condensate mass flow (kg/s), Pressure at the trap outlet (bar abs), Temperature at the trap outlet (°C), Ambient temperature (°C), Drain line internal diameter (mm), Slope (m/m), Vapour loading factor (–), Specific heat of water (kJ/kg·K), Latent heat of vaporisation (kJ/kg)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Fraction that flashes to vapour | 0.89 % |
|---|---|
| Effective liquid load on the line | 0.07 kg/s |
| Line carries the load | — |
| Liquid-only load | 0.06 kg/s |
| Flash steam load | 0.44 kg/s |
| Line capacity at this slope | 0.16 kg/s |
| Line utilisation | 47.02 % |
| Flash significant enough to vent | — |
| What this means | Cooling from 100 °C at the trap to 30 °C ambient gives up 292.6 kJ/kg of sensible heat, and against a latent heat of 2257 kJ/kg that flashes 88.5 % of the condensate to vapour. Of 0.50 kg/s arriving, 0.057 kg/s leaves as liquid and 0.443 kg/s as flash steam. Applying your vapour loading factor of 0.30 gives an effective liquid load of 0.075 kg/s. A 40 mm line at 5.0 mm/m carries about 0.159 kg/s, so the line runs at 47 % of capacity. At 89 % flash this line needs venting, or a trap and drip-leg arrangement that can pass the vapour. A liquid-only sizing would have said this pipe was fine. The capacity is a uniform-flow figure at half depth, which is the most efficient depth the line reaches; a line running near it will not actually flow at it. |
- The condensate drops 70 K on the way to ambient, giving up 4.18 × 70 = 293 kJ/kg of sensible heat.
- Against 2257 kJ/kg latent, that flashes 2257 / (2257 + 293) = 88.5 % — far more than a liquid-only sizing would ever see.
- Only 0.5 × (1 − 0.885) = 0.058 kg/s stays liquid; the rest is steam.
- With a vapour loading factor of 0.3 the effective load on the line is 0.075 kg/s, and a 40 mm line at 0.5 % carries several times that — but only because it is vented.
Assumptions & limitations
Engineering calculation. Isenthalpic flash fraction from the sensible heat available as the condensate drops to ambient, then the effective liquid load against the gravity capacity of the line at the slope entered.
- Trap pressure, trap temperature, ambient temperature, pipe diameter, slope, vapour loading factor and the water properties are ALL USER inputs. This tool holds no steam-table data, no pipe capacity table and no code, and looks nothing up.
- It will NOT derive the trap temperature or pressure from a saturated steam table. Those are thermodynamic data, and the flash fraction is directly proportional to the sensible heat available, so inventing them would produce a confidently wrong answer.
- The capacity is a uniform-flow Manning figure at half depth. Real condensate lines run partly full with a vapour fraction that changes along their length, and a line at capacity will not flow at capacity.
- Only the flash from the pressure drop at the trap is modelled. Vapour carried in from the equipment, non-condensable gases, and the effect of a flooded line backing up are not.
- Trap selection, venting arrangement, cleanouts, trap freezing and the minimum slope required by any local code are all outside what this tool checks.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 1285 — air handling and air conditioning, for condensate discharge provisions.
- ASME PTC 4 — industrial piping, for gravity drainage sizing practice.