The Loeffler boiler is a high-pressure, forced-circulation boiler in which water never flows through the furnace tubes. Instead, superheated steam is blown into an evaporating drum, where it mixes with the feed water and boils it; a steam circulating pump then sends saturated steam from the drum through the superheaters. About one-third of the superheated steam goes to the turbine and about two-thirds returns to the drum to evaporate more water. Textbooks give its output as about 100 tonnes of steam per hour at around 140 bar and 500 °C.
It sits in the forced-circulation group of water-tube boilers in the classification of boilers, alongside the La Mont, Benson and Velox designs.
The core idea: keep water out of the hot tubes
In the La Mont boiler, water boils inside tubes that sit in the furnace. At high heat flux, water evaporates right at the tube wall, and every kilogram that evaporates leaves its dissolved salts behind on the metal. The scale insulates the tube, the metal runs hotter, and eventually the tube fails.
The Loeffler boiler removes the problem at its source. The furnace tubes carry only steam, which leaves no deposits. The water is boiled in a drum outside the furnace by direct contact with superheated steam, so any salt settles in the drum, where it can be blown down, instead of on a heated tube wall.
The boiler is named after the German engineer Stephan Loeffler (Löffler), who worked out the steam-circulation method in the early 1920s. Some textbooks give his first name differently, and dates quoted range from about 1923 to 1925.
Parts of the Loeffler boiler, in flow order
| No. | Part | What it does |
|---|---|---|
| 1 | Feed pump | Raises feed water to boiler pressure |
| 2 | Economiser | Preheats the feed water with outgoing flue gas |
| 3 | Evaporating drum (with mixing nozzles) | Holds the water; superheated steam is injected through nozzles below the water surface and evaporates it. It is placed outside the furnace and is not fired |
| 4 | Steam circulating pump | Draws saturated steam from the top of the drum and pushes it through the superheaters |
| 5 | Radiant superheater | Tubes lining the furnace, carrying steam only, heated by radiation |
| 6 | Convective superheater | Tubes in the flue-gas path that raise the steam to its final temperature |
| 7 | Steam outlet and return line | About one-third of the superheated steam goes to the turbine; about two-thirds returns to the drum nozzles |
Loeffler boiler working, step by step
- Feed. The feed pump sends water through the economiser, where it is heated but not boiled, into the evaporating drum.
- Evaporation by mixing. Superheated steam returning from the superheater is blown into the water in the drum through nozzles. It gives up its superheat to the water, which boils. The drum now contains saturated water and saturated steam at boiler pressure.
- Steam circulation. The steam circulating pump draws saturated steam from the top of the drum.
- Radiant superheating. The pump drives this steam through the radiant superheater tubes that line the furnace walls.
- Convective superheating. The steam then passes through the convective superheater in the gas path and reaches about 500 °C.
- Split. At the superheater outlet, about one-third of the steam goes to the turbine. The other two-thirds return to the evaporating drum to boil more feed water, and the cycle repeats.
The flue gas passes over the radiant superheater, then the convective superheater, then the economiser, and out to the chimney.
Worked example: why about two-thirds of the steam goes back to the drum
The 1/3 : 2/3 split is not arbitrary; it follows from an energy balance on the evaporating drum.
Assumptions: drum pressure 140 bar; superheated steam enters the drum at 140 bar and 500 °C; feed water enters at 140 bar and 250 °C from the economiser; the drum is insulated; the steam leaving the drum is dry saturated; pump work and pressure losses are neglected. Enthalpies are from the IAPWS-IF97 steam tables.
| State | Condition | Enthalpy, kJ/kg |
|---|---|---|
| A: superheated steam into drum | 140 bar, 500 °C | 3324.1 |
| B: feed water into drum | 140 bar, 250 °C (compressed liquid) | 1086.0 |
| C: dry saturated steam leaving drum | 140 bar, 336.7 °C | 2638.1 |
Let m kg of superheated steam be needed to evaporate 1 kg of feed water. Both leave the drum as saturated steam at state C:
Heat given up by the steam = heat taken by the water
m × (3324.1 − 2638.1) = 1 × (2638.1 − 1086.0)
m × 686.0 = 1552.1
m = 2.26 kg of superheated steam per kg of feed water
So for every 1 kg of steam sent to the turbine (which must be replaced by 1 kg of feed water), about 2.26 kg is recirculated. The steam passing through the superheater is 1 + 2.26 = 3.26 kg, and the fraction going to the turbine is 1 / 3.26 = 0.31, or about one-third. The textbook 1/3 : 2/3 split checks out.
The ratio depends on how hot the feed water is. With the same steam conditions, feed water at 300 °C gives m = 1.89 (about 35 percent to the turbine), while feed at 200 °C gives m = 2.60 (about 28 percent). A hotter economiser outlet means less steam needs to be recirculated.
How to draw the Loeffler boiler simple diagram
- Draw a tall rectangle for the furnace with a burner at the bottom. Line its walls with tubes and label them radiant superheater.
- Outside the furnace, draw a horizontal cylinder for the evaporating drum with a water level. Draw a perforated pipe (the nozzles) inside it, below the water level.
- From the top of the drum, draw a pipe to a circle marked steam circulating pump, then into the bottom of the radiant superheater.
- From the top of the radiant superheater, draw a pipe to a coil in the gas outlet marked convective superheater.
- At the convective superheater outlet, split the pipe: one branch with an arrow “1/3 to turbine”, the other with an arrow “2/3 back to drum”, ending at the nozzles.
- Draw the feed pump and economiser coil (in the gas path below the superheater) feeding into the drum, and an arrow “flue gas to chimney”.
The details examiners look for: the drum outside the furnace, steam (not water) in the furnace tubes, and the 1/3 : 2/3 split at the superheater outlet.
Advantages of the Loeffler boiler
- No salt or sludge deposits in the heated tubes, because only steam flows through them. This was the main aim of the design.
- Can use feed water of poorer quality than other high-pressure boilers, since deposits collect in the unfired drum where they can be blown down.
- High pressure and a large steam output from a compact unit.
- Steam in the tubes has high velocity, which gives good heat transfer and keeps the tube metal cool.
- Responds well to load changes.
Limitations of the Loeffler boiler
- The steam circulating pump handles a large volume of high-pressure steam, which is harder and costlier than pumping water.
- Steam is a poorer coolant than water, so the radiant superheater tubes run hot and need high steam velocity and good alloy steels.
- About two to three times the turbine steam flow must pass through the superheaters, so the superheater surface and pumping power are large.
- It is more complex than a natural-circulation boiler and has been largely superseded by modern drum and once-through designs, so it is mainly a textbook design today.
Loeffler vs La Mont in one line each
- La Mont: pumps water through furnace tubes at 8-10 times the steaming rate; water boils in the tubes.
- Loeffler: pumps steam through furnace tubes; water boils in an unfired drum by mixing with superheated steam.
The once-through, drumless option is the Benson boiler.
High-pressure boilers are part of thermal engineering in the AICTE model curriculum; NPTEL has lectures on steam generators.
FAQs
What is a Loeffler boiler?
It is a high-pressure, forced-circulation boiler in which feed water is evaporated in an unfired drum by mixing with superheated steam, and only steam flows through the furnace tubes. This prevents salt deposits in the heated tubes.
How does the Loeffler boiler work?
A steam circulating pump draws saturated steam from the evaporating drum and forces it through the radiant and convective superheaters. About one-third of the superheated steam goes to the turbine, and about two-thirds returns to the drum to boil more feed water.
Why is superheated steam used to evaporate water in a Loeffler boiler?
So that no water boils inside the heated tubes. When water evaporates at a hot tube wall it leaves salts behind as scale; mixing in an unfired drum keeps the deposits off the heating surfaces.
What proportion of steam goes to the turbine in a Loeffler boiler?
About one-third. An energy balance at 140 bar, with superheated steam at 500 °C and feed water at 250 °C, gives 2.26 kg of recirculated steam per kg of feed water, so about 31 percent goes to the turbine.
What is the function of the steam circulating pump?
It draws saturated steam from the top of the evaporating drum and drives it through the superheater tubes, providing the forced circulation that keeps the furnace tubes cooled by fast-moving steam.