Air bar arrangement for drying tissue on a belt
Apparatus and methods particularly suited for drying tissue grade paper. In certain embodiments, a wet web, such as a web exiting a tissue manufacturing machine, is transferred to an air floatation dryer with specially arranged air bars and impingement nozzles described herein where the wet web is dried while still at least partly supported by an air-permeable belt. In certain embodiments, the specially arranged air floatation dryer comprises one or more air floatation dryer units through which the wet web is conveyed by way of a supporting endless loop or belt of air-permeable construction. The air bars and opposing impingement nozzles are so arranged as to force the web into contact with the belt and then subsequently lift the web from the belt surface in a repetitive and alternating fashion as the web and belt travel through the dryer.
1. A web dryer comprising an endless woven fabric loop having an upper side supporting and conveying a wet tissue web, and a plurality of spaced apart air bars that discharge air toward said wet tissue web, comprising a first set and a second set, each air bar being elongate and arranged such that its longitudinal axis is transverse to the longitudinal axis of the dryer, wherein the first set of air bars arranged above the web supported on said endless loop in the dryer comprises adjacent Coanda air bars that emit air at a velocity of 4000 to 10,000 feet per minute, and the second set of air bars arranged below the web and endless loop in the dryer comprise jet impingement air bars that emit air at a velocity of 2000 to 8000 feet per minute, and wherein each jet impingement air bar is positioned opposite two Coanda air bars of said first set and is centered between them.
2. The web dryer of claim 1 , wherein the space between said jet impingement air bars comprises a removable blank.
3. The web dryer of claim 1 , wherein said endless loop comprises an air-permeable belt.
4. The web dryer of claim 1 , wherein the velocity of air issuing from said jet impingement air bars is 20 to 50% of the velocity of air issuing from said Coanda air bars.
5. The web dryer of claim 1 , wherein all of the second set of air bars arranged below the web are jet impingement air bars.
6. The web dryer of claim 1 , wherein said second set of air bars arranged below the web comprise first and second jet impingement air bars spaced a distance from each other sufficient to accommodate a third jet impingement air bar.
7. A web dryer comprising an endless woven fabric belt for supporting and conveying a wet tissue web, and an array of air bars that discharge air toward said wet tissue web, comprising a first set of adjacent Coanda air bars that are positioned above the surface of said web and endless belt that emit air at a velocity of 4000 to 10,000 feet per minute, and a second set of opposing impingement nozzles positioned below said belt that emit air at a velocity of 2000 to 8000 feet per minute, wherein each impingement nozzle is positioned opposite two Coanda air bars of said first set and is centered between them, and the air pressure in said first set of adjacent Coanda bars is balanced in relation to the air pressure in said second set of impingement nozzles to force said web to the surface of said endless belt, said second set of opposing impingement nozzles positioned to subsequently lift said web from said belt by forcing air through said belt.
8. The web dryer of claim 7 , wherein the velocity of air issuing from said jet impingement air bars is 20 to 50% of the velocity of air issuing from said Coanda air bars.
9. The web dryer of claim 7 , wherein all of the second set of air bars arranged below the web are jet impingement air bars.
10. The web dryer of claim 7 , wherein said second set of air bars arranged below the web comprise first and second jet impingement air bars spaced a distance from each other sufficient to accommodate a third jet impingement air bar.