Cylindrical dryers are used in the paper making process. Webs of paper are passed over heated cylindrical drums to remove moisture from the web. The large cylindrical dryers, often referred to as “Yankee Dryers,” must be continuously heated to maintain an elevated temperature during the paper making process.
One type of Yankee dryer has an inner and outer shell. The space created between the inner and outer shell is fed with a heating medium, such as steam under pressure, to heat the outer surface of the dryer. The dryers are commonly made out of cast iron. A double shelled cast iron dryer is difficult to cast, costly and extremely heavy. Double shelled dryers were very rare and the idea was abandoned early.
Another type of Yankee dryer has a closed cylinder with pressurized steam fed into the cylinder. The pressurized steam raises the possibility of catastrophic explosion when the cylinder fails under the pressure. One possible solution to explosion risks in a pressurized cylinder type Yankee dryer is to fill the volume within the cylinder with spheres. Spheres occupy space within the cylinder and reduces the amount of pressurized steam. This reduced amount of pressurized steam lowers the risk of explosions. Problems with this approach include the need to use a non-compressible material for the spheres, increasing the weight of the dryer. Also, with spheres of equal size the total volume cannot be reduced more than approximately two thirds. This reduction is not enough for the purposes of reducing the amount of steam.
A dryer uses conduits to carry a heating medium, such as steam, to heat the outer surface of the dryer. The volume of steam is successfully reduced to non-explosive levels and the shell need not be designed to prevent an explosion. Conduits may be formed through the shell itself or grooves may be formed on the inner surface of the shell, with the conduits retained within the grooves. Also, the conduits can be placed against the inside surface of the dryer and a material, such as zinc, can be filled in about the conduits. The material serves to both retain the conduits in place and thermally couple the conduits to the dryer to assure efficient heat transfer between the conduits and dryer. These modifications relieve the dryer from the Unfired Pressure Vessel classification to the classification of a piping assembly under ASA code regulations. This results in savings in operation safety, installation cost and operating costs due to the absence of costly inspections.
a and 4b shows end and side views of an alternative arrangement for providing steam to conduits;
An alternative arrangement shown in
a shows an alternative arrangement, providing each conduit with an inlet and outlet for steam. This arrangement can be used when using iron pipes and offers easier assembly and maintenance. A circular header 28 providing steam has a series of conduits 30 attached thereto. As seen in
The use of conduits on the inner surface of a dryer shell allows higher pressure steam to be used. Existing dryers can be retrofit with grooves and conduits at little cost. The system has a longer life span and less down time than prior yankee dryers leading to great savings for the manufacturing plants.
While the invention has been described with reference to preferred embodiments, variations and modifications would be apparent to one of ordinary skill in the art. The invention encompasses such variations and modifications.
This application claims benefit of provisional application 60/793,657, filed Apr. 21, 2006.
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