A fire tube boiler is a boiler in which the hot gas travels inside the tubes and the water sits around them in a shell. A water tube boiler is the reverse arrangement: the water is inside the tubes and the gas passes around them. Pressure, size, response and the place the heat escapes all follow from that. All three machines we surveyed are fire tube.
Boilers/This page
| Fire tube (shell) | Water tube | |
|---|---|---|
| Inside the tubes | Flue gas | Water and steam |
| Around the tubes | Water | Flue gas |
| Water held | Large | Small |
| Response to load change | Slow, the mass rides it out | Fast, little stored water |
| Working pressure | Limited by shell diameter | Higher, the tubes take the pressure |
| Examples | Cochran, Lancashire, Cornish, package boilers | Babcock and Wilcox, Stirling, La Mont |
| Where it loses heat to the room | Shell ends, doors, manholes, mountings: large flat surfaces that are opened | Casing and headers |
The last row decides an insulation job. A shell boiler presents large flat surfaces that are opened on a cycle: the end plate at 130 °C over 5.32 m², the front door at 120 °C over the same area, the manhole, the economiser casing. Those are the surfaces where fixed lagging is cut away and never refitted.
| Machine | Type | Components | Bare kW | Panelled kW | Cut |
|---|---|---|---|---|---|
| Cochran package boiler | Fire tube, shell | 28 | 49.8 | 2.6 | −94.8% |
| Bosch UL-S | Fire tube, shell | 23 | 23.4 | 1.2 | −94.9% |
| Bosch ZFR | Fire tube, shell | 28 | 53.4 | 4.0 | −92.5% |
We have not surveyed a water tube boiler and publish no measured figures for one. The comparison above is of construction; the measured table is of the three shell boilers we have modelled and surveyed.
The eight readings behind those three machines.
| Surface °C | W / m² | Area surveyed m² | Where it was read |
|---|---|---|---|
| 96 | 1086 | 2.33 | Bosch UL-S front door |
| 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 |
| 146 | 2094 | 0.85 | Bosch burner flange |
| 180 | 2888 | 0.90 | Cochran check valve |
| 190 | 3178 | 0.45 | Bosch steam valves |
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.
Every figure on this page comes from the same chain: a radiometric camera reading of the surface, an area rebuilt in CAD from the surveyed machine, and an ISO 12241 calculation from those two. The instrument settings, the area allowance, the energy basis and the list of what was not measured are set out once, in full, on the method page.
In a fire tube boiler the flue gas is inside the tubes and the water is around them in a shell. In a water tube boiler the water is inside the tubes and the gas passes around them.
The fire tube boiler. The shell is full of water, which is why it rides out a load change slowly and why a water tube boiler responds faster.
The water tube boiler. In a shell boiler the pressure is limited by the diameter of the shell; in a water tube boiler the tubes carry the pressure.
Yes. It is a shell boiler with the gas inside the tubes, and so are the Bosch UL-S and ZFR we surveyed.
Yes. A shell boiler loses it through large flat surfaces that are opened on a cycle: end plate, front door, manhole, economiser casing, mountings. On the Cochran those come to 49.8 kW bare.
The surveyed view this page came from, with every element on it.Open
→/ 0228 components, 49.8 kW bare against 2.6 kW panelled, six surveyed views.Open
→/ 03Two more fire tube machines, 23.4 kW and 53.4 kW bare.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
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