Heating with saturated steam: condensation in a shell and tube unit

Where process heat is available as saturated steam, a shell and tube heat exchanger is the classic way to heat water or glycol circuits: the steam condenses on the tube side and gives up its enthalpy of vaporisation to the circuit.

Why is condensation so dense in duty?

In a phase change, steam releases not sensible heat over a temperature spread but its enthalpy of vaporisation - roughly 2000 to 2250 kJ/kg depending on pressure. For comparison: one kilogram of water over a 20 K spread delivers only about 84 kJ. That is why steam applications manage with small steam mass flows, and why the steam side calculates through the enthalpy difference rather than through m · cp · dT.

What is different about the sizing?

While the steam condenses, the steam side holds its temperature close to the saturation temperature - a temperature spread only arises once the condensate cools down further. Heat transfer is governed by the condensate film on the tube wall and depends on the inlet temperature and the temperature difference rather than on the mass flow. Our calculation core holds a dedicated condensation correlation for this and derives the steam mass flow from the enthalpy balance; in the configurator you select saturated steam as the hot medium and give the inlet and outlet temperature of the steam side. The inlet has to lie within the range the configurator shows for the medium.

What matters in operation?

Three points decide whether a steam application stays good. First, condensate removal: standing condensate floods tube area and takes duty away from the unit. Second, control: steam valves control via pressure and therefore via the saturation temperature - the design point should reflect the actual operating pressure. Third, materials: condensate is hot and low in oxygen, but aggressive towards unsuitable materials; the choice of material belongs in the quotation stage.

How do you start the sizing?

To begin with, the available steam pressure is enough (the saturation temperature follows from it), together with the condensate outlet temperature, the desired circuit temperatures and either the mass flow or the duty. The energy balance completes the rest - interactively in the configurator or step by step in the guide sizing a shell and tube heat exchanger.