Everything so far delivered steam to the process door. This faculty walks through it — and the first room is indirect heating: coils, jackets and shell-and-tube exchangers, where steam gives up its latent heat across a wall (lessons A6 and A11 built the theory; this lesson runs the equipment). It ends with the stall phenomenon — the single most misdiagnosed fault in steam heating, responsible for more "bad trap" accusations than every actual bad trap combined.
Choosing the surface
Jackets wrap the vessel — simple, cleanable on the product side, modest in area (the practice plant's 9.5 m² jacket from lesson A11 is typical), which caps their heating speed. Coils put the surface inside the product: several times the area of a jacket in the same vessel, faster response, at the price of cleaning access and a wetted obstruction. Shell-and-tube exchangers heat flowing product in-line — steam in the shell, product in tubes — compact and continuous; their close cousin, the plate heat exchanger, trades higher performance for gasket temperature limits. The selection logic runs on lesson A11's equation: the duty needs U × A × LMTD, and each geometry offers a different bargain of area, U and cleanability. What never changes: the steam side must shed condensate and air continuously, or the surface you paid for stops being steam-heated surface.
Piping the steam side right
Every indirect heater follows one pattern: steam enters at the top of the space (steam is lighter than the condensate it becomes; top entry lets the space fill downward), condensate leaves at the bottom lowest point through its trap (lesson D5), and the air vent sits far-top-corner (lesson C8). A control valve meters steam in response to product temperature (lesson D3). Simple — and behind that simplicity waits the trap-drainage problem this lesson exists to explain.
Stall: when the pressure disappears
A trap passes condensate on pressure difference: space pressure minus return pressure. Here is what the control valve does to that difference. On a mild day, light load: the valve throttles down, and the pressure in the jacket falls — that is how temperature control works, because lower pressure means lower saturation temperature (lesson A3, run backwards). Throttle far enough and the space pressure drops to — and below — the pressure in the condensate return line. The differential across the trap reaches zero. The condensate stops leaving — not because the trap failed, but because nothing pushes it. That is stall. The condensate backs up into the jacket, drowning heating surface; temperature control goes sluggish, then cyclic — the drowned surface under-heats, the controller opens the valve, pressure returns and slams the backed-up condensate out, the surface over-heats, the valve closes, and the cycle repeats. Symptoms at site: hammering in the equipment at part load (lesson C5's thermal shock, made hourly), rusty streaked condensate (waterlogged surfaces corrode), product temperature see-sawing at exactly the loads where everything should be gentle.
Getting out of stall
Three honest exits, in rising order of cost. Drain by gravity to atmosphere: if the trap can discharge to an atmospheric receiver below the equipment, the differential never quite dies — the classic fix for stand-alone vessels, at the price of pumping the condensate onward (lesson D6). Lift the back pressure problem away: re-route the trap discharge out of a high, flooded or pressurised common return into a local low-level receiver. The pumping trap: a trap combined with a small steam-powered pump in one body — drains normally when differential exists, pumps when it doesn't. The engineered answer for temperature-controlled equipment that lives at part load, and the standard end to the see-saw story above. What is never the answer: a bigger trap. Zero differential across a bigger orifice is still zero.
- Any temperature-controlled steam equipment that hammers or hunts at part load: suspect stall before the trap. Check the space pressure at light load against the return pressure — two gauges settle it.
- Sketch the stall chart for your equipment once: design pressure, return pressure, load range. The crossing tells you whether you need the pumping trap now or never.
- Steam in at top, trap at true bottom, vent far corner — walk each heater against the pattern; retrofit plumbing sins are cheap to fix cold.
- Rusty condensate from one heater = waterlogging there. Follow it back before it becomes tube failure.
- Indirect heating = A11's equation with plumbing: area × U × LMTD, kept drained and vented.
- Temperature control works by lowering space pressure — which is exactly what kills trap differential at part load.
- Stall = condensate with nothing pushing it. The trap is innocent; the physics is guilty.
- Exits: gravity to atmosphere, local receiver, or a pumping trap. Never just a bigger trap.
Stall earned its textbook status in the laundry and food industries, where hundreds of small temperature-controlled machines — ironers, kettles, tanks — all ran at part load most of the day. Whole maintenance departments changed traps on a rota, and the hammering continued regardless; the pumping trap, developed in the 1980s–90s precisely for this duty, ended a decades-long argument between operators ("the trap is bad") and vendors ("the trap tests fine") in which both sides were right — the trap was fine, and it wasn't draining. The lesson outlived the laundries: control the temperature, and you have signed up to manage the differential.