An economiser in a boiler, spelled economizer in American usage, is a heat exchanger standing in the flue path. Gas leaving the furnace passes over its tubes and gives up heat to the feed water before that water enters the shell. On the Cochran package boiler we surveyed it is the block behind the end plate. Its casing read 126 °C on the camera.
Boilers/Cochran/End plate and economiser/This page
Flue gas leaves the furnace of a shell boiler still carrying usable heat. The economiser stands in that path so the gas gives some of it back to the feed water on the way to the chimney. The water arrives warmer, the shell has less work to do, and the gas reaches the chimney cooler.
On the Cochran we surveyed the economiser sits directly behind the end plate, and the two are read as one view. The economiser accounts for 18,327 W of the 27,390 W measured on that view, which is sixty-seven per cent of it. The end plate carries the other 9,063 W at 130 °C.
The casing is the part that loses heat to the boiler house. At 126 °C over 11.31 m² it is the largest uninsulated surface on the machine. It is also among the most often left bare: the economiser is opened for tube cleaning, and lagging wrapped over it is cut away at the first clean.
| Element | Temp °C | Area m² | Bare W | Panelled W | Surface °C |
|---|---|---|---|---|---|
| Economiser | 126 | 11.31 | 18,327 | 954 | 34 |
| Total | 11.31 | 18,327 | 954 | ≤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.
Eight readings from our own surveys, so a surface at a known temperature can be sized without a visit.
| Surface °C | W / m² | Area surveyed m² | Where it was read |
|---|---|---|---|
| 100 | 1117 | 1.49 | Cochran feed pump |
| 120 | 1499 | 5.32 | Cochran front door |
| 126 | 1620 | 11.31 | Cochran economiser |
| 130 | 1704 | 5.32 | Cochran end plate |
| 180 | 2888 | 0.90 | Cochran check valve |
Each row is one of our own surveys: the measured loss divided by the CAD area of the same surface. Nothing is interpolated. ISO 12241:2022 steady state, still air, emissivity from the FLIR survey.
The economiser is opened on a cycle for gas-side cleaning, and that is the reason it is so often found bare: fixed lagging is cut at the first clean and never refitted. The sequence below is what a removable module has to survive.
The 18,327 W on this page is the casing loss measured while the boiler was running, over 11.31 m² of CAD area. Gas-side fouling, flue temperature and recovered heat are separate quantities and we publish no figures for them.
A heat exchanger in the flue path that preheats boiler feed water with the heat still left in the gas leaving the furnace. On the Cochran we surveyed it stands behind the end plate.
It moves heat from flue gas into feed water. The water enters the shell warmer and the gas reaches the chimney cooler.
On the machine we surveyed the casing read 126 °C on the FLIR. Over 11.31 m² of CAD area that is 18,327 W by ISO 12241.
Yes. Every module comes off by hand and refits onto the same geometry, so access doors and cleaning ports keep their clearance. Panelled, the outside sits at 34 °C.
An accessory. Mountings are the fittings required on the pressure vessel for it to be operated; accessories are added to raise efficiency, and the economiser is the clearest example.
The surveyed view this page came from, with every element on it.Open
→/ 02Where the economiser falls in the mountings-and-accessories split, and what the mountings lose at 180 °C.Open
→/ 03The other end of the same water path: pump, valves, strainer and piping at 100 °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. |
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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