Steam-to-Water
U-Tube Heat Exchangers

Steam-to-Water U-Tube Heat Exchangers

The Most Common Duty, and the Easiest to Install Badly

Steam condenses inside the tubes and releases latent heat to water flowing through the shell. Because the condensing coefficient is very high, the water side almost always controls the design, which means the exchanger is sized on water-side velocity and pressure drop rather than on anything happening in the steam.

Why the steam belongs in the tubes. Steam is the clean fluid, and a U-tube bundle cannot be mechanically cleaned through the bend, so the clean stream goes tube side by default. Containing steam pressure inside tubes is also cheaper than containing it in a shell. That leaves the water on the shell side, where the bundle can be withdrawn and the surface hydroblasted when plant water eventually lays a deposit down.

Two tube passes is normally right. A U-tube bundle is two-pass by geometry. Adding passes raises steam-side pressure drop, and pressure drop on the steam side lowers saturation temperature, which works directly against the duty. If a rating comes back wanting four or six passes on a steam heater, something else in the design needs looking at first.

Condensate has to leave as fast as it forms. This is the failure we are called about most. When condensate cannot drain it backs up from the bottom of the shell and progressively covers surface. Performance falls, the approach temperature widens, and every symptom points at a fouled exchanger. Before anyone pulls a bundle on a steam heater, check the trap, the back pressure and the drain leg.

Stall. When a modulating control valve throttles steam pressure down at low load, pressure inside the exchanger can fall below the pressure in the condensate return line. Condensate then physically cannot drain, the unit floods, and control becomes erratic. The remedies are well known — a pumping trap, a vented condensate arrangement, or a control scheme that keeps the unit out of the stall region — but they have to be designed in, because in service stall looks exactly like an undersized exchanger.

Steam hammer and startup. Admitting steam quickly into a cold exchanger holding condensate produces violent collapse and shock loading that damages tubes, joints and connected pipework. Drain first, warm through slowly, and size the trap for the startup condensate load, which is far larger than the running load.

Related pages: stock steam-to-liquid units, two-pass arrangements, CIP solution heating and gaskets and sealing.