Coaxial heat transfer tube suitable for a fluidized bed boiler and a method for manufacturing same

A method for manufacturing a coaxial heat transfer tube, the method comprising the steps of:

• arranging an inner heat transfer tube and an outer heat transfer tube,
• inserting at least a part of the inner heat transfer tube into the outer tube such that the inner heat transfer tube extends at least through the outer heat transfer tube to form a straight coaxial heat transfer tube,
• arranging thermally insulating material in between the inner heat transfer tube and the outer heat transfer tube,
• hardening the thermally insulating material at a temperature of 100° C. to 400° C.,
• after hardening the thermally insulating material, bending the straight coaxial heat transfer tube to form a first primary curved part to the coaxial heat transfer tube, and
• before said bending, forming first protrusions on at least one of an inner surface of the outer heat transfer tube or an outer surface of the inner heat transfer tube,
• wherein, after said hardening, a thermal conductivity of the thermally insulating material is from 1 W/mK to 10 W/mK at a temperature of 20° C.