An “insulating sleeve” is a flanged tube that extends through the bore of a paper dryer journal.
The flanged end of the tube is usually bolted to the inside surface of the dryer head and sealed at the opposite end of the journal (the outboard end) by o-rings or packing in a journal adaptor flange. This sleeve creates an insulating air gap in the annular space between the tube and the dryer journal bore. This air gap greatly reduces the amount of heat that is transferred to the dryer journal from the steam that is passing through the inside of the tube. This reduction in heat transfer reduces the journal temperature and greatly reduces the risk of dryer bearing inner race cracking. It also reduces the amount of heat that goes into the bearing lubrication oil, helping it to maintain its viscosity thereby extending bearing life. Well-sealed sleeves are very effective, but even the slightest steam leak into this air gap will make the sleeve ineffective.