Condensate pipes

Routing of pipework

It is recommended that condensate pipes are installed with a continuous slope of at least 1% in the flow direction to allow liquid condensate to drain off easily and heat exchangers and pipework to be emptied. This makes the heat exchanger start-up process easier and reduces the danger of corrosion.

Water pockets in particular should be avoided as these can lead to steam hammer, especially when starting up the heat exchanger.

Vertical sections in condensate pipes are possible. In addition to the hydrostatic pressure loss, higher flow pressure losses must also be taken into account in this case. Horizontal sections must still be sloped and suitable cold liquid condensate or start-up drainage outlets provided at the lowest points.

As the condensate normally does not accumulate at a sufficient height above the feed water vessel, it should be collected in condensate tanks and recirculated via condensate pumps/siphons.


Condensate pipes must on no account be dealt with in the same way as pipes that only carry water. As the volume increases significantly during re-evaporation, both the steam fraction and water fraction must be taken into account during sizing.

If the required cross-sectional area is significantly undercut, this can lead to water droplet erosion on valves and elbows due to the resulting high flow speed.

The area required for the steam fraction and water fraction are derived from the corresponding densities, mass flow rates and recommended speeds.


Water droplet erosion

Water droplet erosion, also known as droplet impingement, refers to erosive wear by liquid droplets. Water droplet erosion is microscopic water hammer.

This occurs when droplets strike a surface at high speed. Although water has a “soft” appearance, the droplets have an abrasive erosive effect due to their incompressibility, high impulse and inertia. This leads to wear of surfaces through continuous exposure.


Equation for calculation of required cross-sectional area of pipework

By rearranging the equation to make the diameter the subject, the following is obtained:


Equation for calculation of required nominal diameter of pipework



Nominal pipe diameter


Mass flow rate, condensate [kg/s]


Ratio of expansion steam when expanding to container pressure [kg/kg]


Saturated steam density at container pressure [kg/m³]


Boiling water density at container pressure [kg/m³]


Recommended speed of steam fraction [15 m/s]


Recommended speed of water fraction [2 m/s]


Required cross-sectional area of pipework [m²]


Required cross-sectional area of pipework, steam fraction


Required cross-sectional area of pipework, water fraction


TCo = 130 °C Condensate temperature before expansion

pCo-tank = 0.2 bar Pressure after expansion (container pressure)

xES = 5.0 % Calculated ratio of expansion steam

̇Co = 1,000 kg/h Condensate mass flow rate

Info on Expansion steam


Example calculation for determining the required cross-sectional area of the pipework

Areq,S = kgh % kg ⋅ 15 m s 1 h 3,600 s ⋅ ( 1,000 mm 1 m )² = 1,376 mm²

Areq,w = kgh ⋅ ( 1 – 5.0 % ) kg ⋅ 2 m s 1 h 3,600 s ⋅ ( 1,000 mm 1 m )² = 138 mm²

Example calculation for determining the required nominal diameter of the pipework

DN ≥
4 π ⋅ ( 1,376 + 138 ) mm²
= 43.9

→ DN 50