Blowdown is water taken out of the boiler on purpose. Steam leaves the shell pure and the dissolved solids that came in with the feed water stay behind, so their concentration climbs until some water is let out and replaced. The two blowdown flanges on the machine we surveyed read 180 °C.
Boilers/Cochran/Feedwater set/Feed water/This page
Bottom, or intermittent, blowdown. A large valve at the lowest point of the shell is opened briefly on a set interval. It carries out sludge that has settled, which surface blowdown cannot reach.
Continuous, or surface, blowdown. A small flow is taken from just below the water line, where dissolved solids concentrate, and is often passed through a heat recovery vessel before it goes to drain.
Both leave the boiler at shell temperature, which is why the flanges on the machine we surveyed read the same 180 °C as the steam mountings on the other side.
The rate follows from a solids balance. Solids enter with the feed water and leave only with the blowdown, so at steady state:
Blowdown, as a fraction of steam produced = feed water TDS ÷ (boiler water TDS − feed water TDS)
Feed water TDS is the concentration coming in, boiler water TDS the concentration allowed to stand in the shell. The permitted boiler figure comes from BS EN 12953-10 and the water treatment regime for that machine.
We did not sample the water on the boiler we surveyed and quote no TDS figures for it. The balance above is the standard one and holds for any shell boiler at steady state.
| Element | Temp °C | Area m² | Bare W | Panelled W | Surface °C |
|---|---|---|---|---|---|
| Blowdown flanges ×2 | 180 | 0.52 | 1,501 | 76 | 39 |
| Total | 0.52 | 1,501 | 76 | ≤45 |
Temperatures are FLIR readings taken on a running boiler. Areas come from the CAD model of the same machine, uplifted for bolts and irregular geometry. Losses are ISO 12241:2022 steady state. The panelled column is capped at the 45 °C touch-safe design target, assessed to EN ISO 13732-1.
Two valves usually sit in series at the lowest point of the shell: the one nearer the boiler seals, the one further away throttles. Working them in the right order is what keeps the sealing seat in condition.
Inzonex did not operate the valves. What we measured is the two blowdown flanges on the boiler: 180 °C over 0.52 m², 1,501 W bare and 76 W panelled. The module comes off by hand, so valve travel and the handwheel are unaffected.
Water taken out of the shell on purpose so the dissolved solids left behind by evaporating steam do not build up past the permitted concentration.
Bottom blowdown opens a large valve at the lowest point briefly, to carry out settled sludge. Continuous blowdown takes a small steady flow from just below the water line, where dissolved solids concentrate.
Blowdown as a fraction of steam produced equals feed water TDS divided by the difference between boiler water TDS and feed water TDS. It is a solids balance at steady state.
The two blowdown flanges on the machine we surveyed read 180 °C on the FLIR, the same as the steam mountings. Water leaves at shell temperature.
1,501 W over 0.52 m² on this machine. Panelled they shed 76 W with the outside at 39 °C, and the valve keeps its travel.
The surveyed view this page came from, with every element on it.Open
→/ 02The other side of the same balance: what comes in, at 100 °C over 4.22 m².Open
→/ 03Where the blowdown valve sits in the list of mountings, and the five others measured at 180 °C.Open
→| Survey | FLIR S62 Pro, emissivity 0.90, reflected 25 °C, 3 m, full radiometric correction. UK commercial boiler house. |
| Geometry | Rebuilt in CAD from the surveyed machine. Area allowances are stated on the page they are used on and are allowances, not measured areas. |
| Calculation | ISO 12241 steady state. Heat loss is calculated from the measured surface temperature and the modelled area — it is not metered. Full method, basis and limits. |
| Regulation | The Pressure Systems Safety Regulations 2000 · HSE L122, Safety of pressure systems · HSE INDG261, Pressure systems |
Where surface loss sits inside the indirect method, and what 49.8 kW is against the fuel input.Open
→/ 02What has to be opened, how often, and who signs it off under PSSR 2000.Open
→/ 03Which machine this is, and why the losing surfaces differ between the two.Open
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