A drying system may comprise of any one, two or all the above three methods.
In most cases air dryers are used alone or in combination with cylinder dryers. In certain cases air dryers are preceded with IR dryers, which prevent binder migration. The major part of the drying is usually made by the air dryer. The cylinder does mainly serve as a conveyor for the web (which becomes specially important if the basis weight is low) and for maintaining the web tension, which is needed to give a smooth finish to the paper.
DESCRIPTION OF AIR DRYERS
Depending on the amount of water to be evaporated, there may be one or a number of air dryers in series.
In Air dryers the water removal is effected by means of blowing hot air against the paper web through nozzles. The evaporated water vapor along with the return air is sucked out of the dryer through return air passages provided in the dryer. The major portion of the return air is re-circulated back into the dryer via a set of heat exchangers and a centrifugal fan. The remaining small portion of the return air is vented out through an exhaust air fan. An equivalent amount of fresh air (make-up air) is introduced into the system through air filters. Normally, the amount of air to be exhausted out is small and as such incorporating heat recovery system for recovery of exhaust heat is not worthwhile, particularly in tropical countries.
Depending on the requirement of air impingement temperature, the heating medium for heating the re-circulated air can be:
(d) Steam (for air temperature up to 150 to 160°C)
(e) Thermic fluid (for air temperature up to 200°C)
(f) Gas or Oil (for air temperature more than 200°C)
Circulation air equipment (ducting, circulation air fan and burner or heating coil) is usually installed on a platform or on floor on the drive side of the paper machine.
In certain cases, especially if the machine is narrow and the amount of circulated air small, the air circulation equipment can be made in-built inside the dryer boxes.
When a web break occurs, the dryer box is lifted up automatically or manually to allow cleaning and ease tail threading.
The Sonar- DNB designed coating dryers are tailor-made to offer: