Single tier dryer section for curl control

Claim 1

A drying apparatus for drying a web of paper, said apparatus comprising: a plurality of drying sections for drying the web; each of said drying sections including: a plurality of dryers disposed in a single tier configuration; a plurality of vacuum transfer rolls, each vacuum roll being disposed between adjacent dryers of said plurality of dryers; a dryer felt extending alternately around each dryer and each vacuum roll, the arrangement being such that each of said dryers is top felted so that broke removal is facilitated, each of said drying sections being arranged in succession; a further single drying section only disposed downstream relative to said plurality of drying sections such that the web extends between said plurality of drying sections and said further drying section; said further drying section including: an upper tier of dryers; an upper plurality of rolls disposed between adjacent dryers of said upper tier; an upper felt extending alternately around each dryer of said upper tier and each roll of said upper plurality of rolls; a lower tier of dryers; a lower plurality of rolls disposed between adjacent dryers of said lower tier; a lower felt extending alternately around each dryer of said lower tier and each roll of said lower rolls, the arrangement being such that the web extends in open draw between each dryer of said upper and lower tiers so that any tendency of the web to curl is controlled during movement of the web through said further drying section; and said further single drying section further including: control means for controlling the stem pressure within each dryer of said upper and lower tier of dryers so that any tendency of the web to curl due to excessive drying from one of the sides of the web is compensated for by the application of differential steam pressure between succeeding dryers.

Claim 3

A method for drying a web of paper, the method comprising the steps of: restraining the web against cross-machine and machine directional shrinkage during passage of the web through a plurality of top felted, single tier drying sections, the arrangement being such that broke removal is facilitated during passage of the web through the plurality of drying sections; subsequently drying the web during movement of the web through a two tier, double felted further drying section disposed immediately downstream relative to the plurality of drying sections, the web extending in open draw between an upper and a lower tier of dryers of the further drying section so that any tendency of the web to curl is controlled during movement of the web through the further drying section; and transferring the web from the plurality of drying sections to the further drying section when the web has attained a dryness of at least M, as represented by the formula: M=101-0.246 (WRV) in which M equals dryness of the web (critical moisture content); and WRV equals water retention value of the web.

Claim 4

A method for drying a web of paper, the method comprising the steps of: restraining the web against cross-machine and machine directional shrinkage during passage of the web through a plurality of top felted, single tier drying sections, the arrangement being such that broke removal is facilitated during passage of the web through the plurality of drying sections; subsequently drying the web during movement of the web through a two tier, double felted further drying section disposed immediately downstream relative to the plurality of drying sections, the web extending in open draw between an upper and a lower tier of dryers of the further drying section so that any tendency of the web to curl is controlled during movement of the web through the further drying section; and transferring the web from the plurality of drying sections to the further drying section when the web has attained a dryness of at least M, as represented by the formula: M=101-0.246 (WRV) in which M equals dryness of the web (critical moisture content); and WRV equals water retention value of the web.