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The weakest part of a conventional fire tube boiler is found in the combustion gas turnaround chamber, located just after the boiler furnace and comprised of the refractory frame and the back lid, the latter of which is made of refractory and insulating materials.

Refractory materials are notoriously fragile and can crack easily, especially during the transportation, loading, and unloading of the boiler. Moreover, because they contain a considerable amount of aluminum, refractory materials expand and contract with temperature changes.  Because of this, refractory material has a defined, normally short, useful life.  The problem intensifies as the boiler capacity increases since the dimensions of the turnaround chamber and the back lid increase as well.

When the turnaround chamber gets damaged, changing the refractory materials is a labor intensive, costly, and time-consuming task.  The boiler must be turned off and left to cool down slowly, and the back lid, given its weight, must be dismantled using a crane.  The damaged refractory frame must then be removed, a new turnaround chamber poured and left to set, then the lid must be put back, and the boiler sealed and reheated slowly.  This process leaves the boiler out of commission for at least three to four days.

Global leaders in boiler technology and manufacturing, including the US, Germany, the Netherlands, France, and Japan, eliminated the use of refractory and insulating materials in the turnaround chamber from their boiler designs over 80 years ago and also developed a simpler back lid.

Their designs include a turnaround chamber made of steel, the same material used in the boiler furnace, in which the chamber is submerged in the water that contains the boiler.  In this way, not only are refractory and insulating materials eliminated, but additionally, the area now adds to the heat transfer, providing a perfect solution to what was a weak point of fire tube boilers  and offering great operating advantages. This design increases both the reliability of the boiler operation and its efficiency (by eliminating heat leakage in the back lid). It also leads to a considerable reduction in annual maintenance costs (now that repairing/replacing the turnaround chamber is no longer necessary) and eliminates the costs of maintaining an out-of-service boiler which result from a partial or total reduction in production.

This type of turnaround chamber is called a “Wet-Back” in contrast to the “Dry Back” that refers to traditional boilers with turnaround chambers made of fragile refractory and insulating materials.

The Wet-Back design provides the following advantages over a traditional (Dry Back) boiler:

  • Greater boiler efficiency (less fuel consumption)
  • Greater operational reliability (no time-consuming refractory material repairs)
  • Lower operating and maintenance costs


For the past 30 years Calderas Powermaster has been proud to offer our clients the Wet-Back boiler design.  We make it our business to seek out and implement the most cutting edge technology in boiler design in order to maintain our position at the forefront of boiler manufacturers worldwide and also to provide our clients with the world class products they deserve.