Steam Systems

On a clean steam condenser the condensate is a regulated fluid, so construction follows ASME BPE or 3-A practice: 316L product contact, electropolished surfaces, orbital welds, and a steam side that drains completely by gravity. WFI stills add double tubesheets so a tube-joint leak cannot reach the distillate. These features add cost that a utility condenser does not need, and specifying them where the condensate goes to drain is a common way to overspend.

A flash tank vent or turbine exhaust that plumes off a roof is losing treated water, treatment chemicals and the fuel spent heating them. A condenser on that vent, cooled by a water stream the plant can use, returns all three. The design is governed by backpressure: traps and turbines upstream were sized to discharge at a set pressure, and a condenser that adds more than they can tolerate backs condensate into equipment or cuts turbine output.

Feedwater and makeup preheaters take low-pressure steam that has little other use and put its latent heat into water headed for the boiler. A feedwater heater downstream of the feed pump sees full boiler feed pressure on the water side; a makeup heater upstream of the deaerator sees cold, oxygen-laden water at low pressure. The two need different pressure ratings and tube materials even though both are steam-to-water condensers.

Steam condensers fail quietly when the condensate cannot leave. A trap that cannot pass condensate at low steam pressure, a drain line that rises, or a level controller tuned for full load all let condensate back up into the shell, flooding tubes and reducing usable surface. Sizing the drain and trap for the minimum steam pressure, not only the design point, keeps the condenser performing across the turndown range where it spends most of its hours.

Steam carries air and carbon dioxide picked up in the boiler, the condensate return and through vacuum breakers. Those gases collect where steam velocity falls, usually at the end of the shell away from the inlet, and form a stagnant layer that sharply reduces heat transfer. A thermostatic air vent at that point, and baffling that sweeps gas toward it, restores the surface. Carbon dioxide left in contact with condensate also forms carbonic acid that pits carbon steel.