Shaft seals keep the fan airstream inside and help isolate it from the atmosphere. They are not required but are generally employed on larger fans. Ceramic felt and Teflon shaft closures are the least expensive options. There are several version of labyrinth seals which are available, including mechanical seals which feature a single or double carbon rings. The double carbon ring seal may be arranged for a purge between rings to ensure there is no migration between the atmosphere and airstream.