Shell boiler designed as horizontal 3-pass flame-tube smoke-tube boiler often also named “three-pass boiler” for this reason.

The structure of the three-pass boiler is simple. The boiler pressure vessel is a horizontal cylindrical tube (diameter 1 to 5 metres depending on the overall size) closed with bottoms on both sides and insulated all around. The inside of the pressure vessel is equipped with a flame tube (1st pass) fired by a burner and there is an internal / external reversing chamber that reverses the flue gases and transports them back through the 3-pass boiler in the 2nd flue tubing. The front side of the three-pass boiler is equipped with an external reversing chamber that diverts the flue gases and transports them to the boiler end in the 3rd flue tubing. The lower ¾ of the 3-pass boiler are filled with water during operation; the upper quarter is the steam space. All heated parts (heating surfaces) of the three-pass boiler are surrounded by a water jacket during operation.

The design principle of the three-pass boiler contributes to the reduction of pollutant emission. As three-pass boilers achieve a high efficiency, fuel consumption and emissions are reduced. Thanks to the generous dimensioning of the flame tube and the separate internal reversing chamber of the 3-pass boiler flame cooling is good and reduces the formation of nitrogen oxide (NOx).

The outstanding success of the three-pass boiler which has been unmatched to date is based on the LOOS three-pass patent from 1952. Shell boilers are available as single or double-flue boilers for steam generation up to 55,000 kg/h and hot water generation up to 38,000 kW. The different shell boilers are manufactured in all pressure stages and for all fuel types and can be equipped with heat recovery systems for increasing the efficiency of all heating processes.


 

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