The La Mont boiler (also written Lamont or LaMont) is a high-pressure, forced-circulation, water-tube boiler in which a centrifugal pump drives water through the evaporator tubes at about 8 to 10 times the rate at which steam is actually produced. Because the tubes always carry far more water than is boiled off, their inner walls stay wet and cool even under intense furnace heat. The design is credited to Walter Douglas La Mont and is usually dated to 1925. Textbooks quote La Mont boilers generating about 45-50 tonnes of superheated steam per hour at around 130 bar (some sources up to 170 bar) and 500 °C.
Within the classification of boilers, La Mont is a water-tube, externally fired, forced-circulation boiler with a drum. That drum and its circulating pump are what set it apart from the once-through Benson boiler and the steam-evaporating Loeffler boiler.
Why forced circulation?
In a natural-circulation boiler such as the Babcock and Wilcox boiler, water circulates because the hot steam-water mixture in the risers is lighter than the cooler water in the downcomers. As pressure rises towards the critical pressure (about 221 bar), the density of water and steam get closer, the driving force weakens, and circulation becomes sluggish. Poor circulation lets steam blanket a tube wall, and a dry tube in a furnace overheats and fails.
La Mont’s answer was to stop relying on density difference and pump the water round instead. The pump sets the flow, so the boiler can use small-diameter tubes, run at higher pressure, and be arranged in any layout, including water walls lining the furnace.
Parts of the La Mont boiler, in flow order
Following the water and steam from the feed pump to the turbine:
| No. | Part | What it does |
|---|---|---|
| 1 | Feed pump | Pumps treated feed water from the hotwell up to boiler pressure |
| 2 | Economiser | Preheats feed water using flue gas leaving the boiler |
| 3 | Steam separating drum | Receives feed water and the steam-water mixture from the evaporator; steam separates at the top, water collects at the bottom. It sits outside the furnace |
| 4 | Centrifugal circulating pump | Draws water from the drum and forces it through the evaporator at 8-10 times the steaming rate |
| 5 | Distribution header with nozzles | Divides pump flow among the evaporator tubes; a nozzle (orifice) at each tube inlet meters the flow so every tube gets its share |
| 6 | Evaporator (radiant) tubes | Tubes lining the furnace absorb radiant heat; part of the water turns to steam and the mixture returns to the drum |
| 7 | Convection superheater | Takes dry saturated steam from the drum and superheats it in the flue-gas path |
| 8 | Air preheater | Recovers more heat from the flue gas to preheat combustion air, before the gas goes to the chimney |
The flue gas travels the opposite way: furnace (radiant evaporator), then convection superheater, then economiser, then air preheater, then chimney.
La Mont boiler working principle, step by step
- Feed. The feed pump delivers water through the economiser, where it is heated by the outgoing flue gas, into the steam separating drum.
- Circulation. The centrifugal circulating pump draws water from the bottom of the drum and delivers it to the distribution header.
- Evaporation. The header nozzles share the flow among the evaporator tubes around the furnace. As the water passes through them it absorbs radiant heat and a small fraction, roughly one-eighth to one-tenth, turns into steam.
- Separation. The steam-water mixture returns to the drum. Steam rises out of the water; the unevaporated water drops back to be pumped round again.
- Superheating. Saturated steam from the top of the drum passes through the convection superheater and leaves at high temperature for the turbine or process.
- Make-up. The feed pump replaces the water that left as steam, keeping the drum level constant.
The key idea: circulation ratio
The circulation ratio is the mass of water pumped through the evaporator divided by the mass of steam generated in the same time. For the La Mont boiler it is usually quoted as 8 to 10.
It fixes the dryness fraction of the mixture leaving the tubes: dryness at tube exit = 1 / circulation ratio. At a ratio of 8, the mixture leaving each tube is only 1/8 = 0.125 steam by mass, or 12.5 percent. The other 87.5 percent is liquid water washing the tube wall. That film of water is what keeps the tube metal close to the saturation temperature instead of hundreds of degrees hotter.
Worked example: circulating pump duty
A La Mont boiler evaporates 50 t/h of steam at a drum pressure of 130 bar with a circulation ratio of 8. The circulating pump develops a pressure rise of 2.5 bar to overcome the resistance of the nozzles and tubes, and has an efficiency of 70 percent. Find the pump mass flow, volume flow and power.
- Pump mass flow = 8 × 50 = 400 t/h = 400,000 / 3600 = 111.1 kg/s
- Density of saturated water at 130 bar (from steam tables, saturation temperature about 331 °C) is about 638 kg/m3
- Volume flow = 111.1 / 638 = 0.174 m3/s, or about 627 m3/h
- Hydraulic power = volume flow × pressure rise = 0.174 × 2.5 × 105 = 43.5 kW
- Shaft power = 43.5 / 0.70 = about 62 kW
- Dryness fraction at evaporator exit = 1/8 = 0.125
Note the water density: at 130 bar saturated water is about a third lighter than cold water, so a student who uses 1000 kg/m3 will underestimate the volume flow by about 36 percent. The pressure rise here is an assumed design figure for the example; a real value depends on tube length, nozzle size and layout.
How to draw the La Mont boiler diagram
The La Mont boiler diagram is a flow sketch, not a scale drawing. Draw it in this order:
- Draw a tall rectangle for the furnace with a burner at the bottom. Draw vertical evaporator tubes lining its walls.
- Draw a horizontal cylinder, the steam separating drum, outside and above the furnace. Show a water level line inside it.
- From the bottom of the drum, draw a pipe down to a circle marked circulating pump, then on to a horizontal distribution header at the bottom of the evaporator tubes. Mark small nozzles at each tube inlet.
- Draw the tops of the evaporator tubes returning into the drum (the steam-water mixture).
- In the gas outlet above the furnace, draw a coil for the convection superheater, fed from the top of the drum, with an arrow “to turbine”.
- Below it in the gas path, draw the economiser coil, fed by the feed pump and discharging into the drum. Add an air preheater and an arrow “to chimney”.
Use arrows for every flow and label all parts. The mark-winning details are the pump between drum and header, and the nozzles at the tube inlets.
Advantages of the La Mont boiler
- High, controlled circulation keeps tubes cool, so small-diameter tubes can be used at high pressure.
- High heat-transfer rates, so a compact boiler raises a lot of steam.
- Quick start-up and fast response to load changes, because the water content is small.
- Free choice of tube layout: tubes can line the furnace walls or run in any direction, since circulation does not depend on height.
- Even heating of all tubes reduces thermal stress.
Limitations of the La Mont boiler
- Bubble formation and deposits: steam bubbles form and cling to the inner tube wall. They reduce heat transfer, and as water evaporates at the wall, dissolved salts deposit there. This scaling was the main weakness the Loeffler boiler set out to fix.
- Tube overheating at low circulation: if the pump trips or flow falls, the tubes can dry out in seconds, so pump reliability and protection are critical.
- Pump power and cost: the pump runs continuously and must handle water near saturation at full boiler pressure, which needs special seals and enough suction head to avoid cavitation.
- Feed-water treatment: high heat flux in small tubes demands very clean feed water.
La Mont vs Benson vs Loeffler boiler
| Point | La Mont | Benson | Loeffler |
|---|---|---|---|
| Circulation | Forced, by a water circulating pump | Once-through, by the feed pump | Forced, by a steam circulating pump |
| Drum | Steam separating drum | None (drumless) | Evaporating drum outside the furnace |
| What flows in the furnace tubes | Water and steam-water mixture | Water turning to steam in a single pass | Superheated steam only |
| How water is evaporated | In the heated tubes | In the heated tubes | By mixing with superheated steam in the drum |
| Pressure | High (textbook about 130-170 bar) | High to supercritical (above about 221 bar) | High (textbook about 140 bar) |
| Main weakness | Deposits and bubbles in tubes | Salt deposits in the transition zone; needs very pure water | Steam pump and large steam volumes |
For the steam-mixing design, see the Loeffler boiler. The drumless alternative is the Benson boiler.
High-pressure boilers are part of the thermal engineering course in the AICTE model curriculum, and NPTEL has lectures on steam generators.
FAQs
What is a La Mont boiler?
It is a high-pressure, forced-circulation, water-tube boiler with a steam drum, in which a centrifugal pump circulates water through the evaporator tubes at about 8 to 10 times the rate of steam generation.
What is the working principle of the La Mont boiler?
A circulating pump forces water from the steam drum through nozzles into furnace tubes, where part of it evaporates. The steam-water mixture returns to the drum, steam separates and is superheated, and the remaining water is pumped round again.
What is the circulation ratio in a La Mont boiler?
About 8 to 10. The pump circulates 8 to 10 kg of water for every 1 kg of steam produced, so the mixture leaving the tubes is only about 10 to 12.5 percent steam.
What is the function of the nozzles in a La Mont boiler?
They sit at the inlet of each evaporator tube and meter the water from the distribution header, so that every tube receives an even share of the flow and none runs short of water.
What is the main disadvantage of the La Mont boiler?
Steam bubbles form and cling to the inner tube surfaces, and salts deposit where water evaporates at the wall. Both reduce heat transfer and can lead to tube overheating.