【Drying】 The sheet, as it passes from the press ……

Lesson Eighteen Drying

The sheet, as it passes from the press section, should be as dry as possible so that it can be made mechanically. From this point it is necessary to apply heat in order to reach the desired state of dryness. The most usual method of drying uses steam-heated dryers.

The standard paper dryer is a 4 to 5 feet 1.2 to 1.5m diameter cast iron shell, machined to a fine and smooth finish on the outer surface. It should also be board out on the inner surface to give a uniform wall thickness allowing for an even transfer or heat. This meticulous machining also provides an important balance factor.

On some paper machines the number of dryers is well over one hundred. The dryers are arranged so as to facilitate the passing of the sheet from one dryer to another. They are usually in two horizontal rows. This arrangement has been found to give the best operating results. There are several other dryer arrangement in one case the dryer is held in high “stack” so that the paper runs vertically up one “stack” and down an adjacent one. The stack system is used mainly to conserve space, and as can be imagined is rather hard to service.

The most usual arrangement of the dryer is in sections. The number of dryers in a section is governed by the shrinkage of the drying sheet. There is no specific rule for the number dryers in one of these groups or sections. Each section is driven at a slightly different speed to compensate for the shrinkage occurring in the sheet during the drying operation.

Papermaking machines, with the exception of paperboard machines, have a dryer felt to hold the sheet in contact with the dryer. Heat from the steam introduced into the dryers is conducted through the dryer shell to the sheet, evaporating the moisture. The vapor is then carried away by a ventilating system which exhausts outside of the machine room. The moisture-laden air from the area of the dryers is usually removed by means of a huge hood completely covering the dryer section and the utilization of powerful circulation and exhaust devices. Not all of the dryers in the dryer section of the machine are paper dryers. Some are used to dry the felt.

On slow-speed machines, when a dryer is started up, it is possible to thread the sheet through the dryer section by hand. However, as machine speeds increased, a device known as the “rope carrier” was developed. The rope carrier consists of two ropes which run in grooves machined or bolted to the dryer heads on the tending side. A tail is cut from the sheet and passed into the nips of the rope. The two ropes run together, threading the sheet through the dryer section, then separate and return on pulleys to the wet end of the dryer section. After the sheet has been threaded through the dryer section it falls away from the rope nip as the ropes separate for the return run. The rope carrier is a very efficient system as well as an excellent safety device.

Steam to heat the dryer is introduced through the hollow back journal. The exhausted steam and condensate are removed through the same journal. As can be imagined the transfer of steam and condensate is a complex operation and its operation is important to the correct function of the dryer. This transfer of steam and condensate is accomplished by a device called a steamfit or a steam joint. The steamfit allows for the passage of steam into dryer and the evacuation of condensate from the interior of the dryer.