Question: Why is the internal liner system necessary?
Answer: The internal liner system is a layer of metal (typically 409SS) to protect the porous high temperature insulation from high exhaust velocities, which in turn protects the casing plate and keeps the external duct skin temperatures at a reasonable level.
Question: What is the common make-up of an Internal Exhaust Liner System?
Answer: The liner system consists of some type of internal support, liner pin, and liner sheets. The supports hold the liner system up to cover the insulation, and the liner pin allows the liner sheet to thermally move as the system heats up and cools down. The liner sheets typically have an oversized hole or slot to allow for thermal expansion.
Question: What is the best practice, a Scallop bar or direct pin system?
Answer: When designed properly, both systems can survive for decades. However, the Scallop bar system utilizes a 304SS stud welded directly to a 409SS scallop bar. This dissimilar metal has different thermal coefficients. Over time, these areas will crack, and the connection will fail. The Liner pin is also a dissimilar metal weld, but is located at the casing plate, therefore the temperature and the thermal coefficient is Minute.