  Many complex vacuum systems include edge-welded bellows. Their simulation in
the molecular flow regime with a Test Particle Monte Carlo (TPMC) code, such as
MolFlow+, can take considerable amounts of computing power and time. Therefore
we investigated the change of the transmission probability of a bellow compared
to a cylindrical tube in TPMC simulations. The results were used to develop an
empirical model to simplify the geometry of a bellow in a TPMC model by
replacing it with a cylindrical tube of extended length, consequently
compensating the reduced conductance of the bellow. The simulated transmission
probabilities of a variety of different bellow lengths have been used to fit
the parameters of the model. Each bellow geometry simulated with MolFlow+
comprised two cylindrical tubes, one at each end, and the central bellow
section. The geometry of each bellow is described by two quantities. The first
one is the total length of the geometry model normalized to the inner diameter
of the bellow. The second one is the fraction of the bellow section with regard
to the total length. Simulating a tube instead of a long bellow reduced the
simulation time by a factor of up to 1000, while the error introduced through
the replacement of the bellow with a cylindrical tube of modified length was in
most cases negligible.
