In steam exchangers with control valves, as the load decreases, the valve throttles and the steam pressure inside the heat exchanger decreases. When this pressure falls below the back pressure in the condensate line, the condensate cannot pass through the trap and accumulates in the heat exchanger. This is called a stall ; the consequences are water hammer, corrosion, temperature fluctuation, and freezing.
Formula
Ts: steam saturation temperature at full load, Tb: saturation temperature at back pressure, Tm: average temperature of the heated fluid. If the heat exchanger is selected with excess surface, the stall point rises by a factor of (1 + excess surface ratio).
Example
The stall point is 64% for a heat exchanger that heats water from 15 °C to 65 °C, runs on 4 bar(g) steam at full load, and delivers condensate to a line with 0.5 bar(g) back pressure; it increases to 90% with 40% excess surface. That is, when the load drops below 90%, condensate begins to accumulate.
The solution is usually free drainage with a pump-trap (pumped condensate discharge) or vacuum breaker. To evaluate your steam system as a whole, you can take a look at our steam boiler guide and get a free quote for your project.