The Fermilab Central Helium Liquefier (CHL) facility is operated 24 hours-a-day to supply LHe at 4.6"K for the Fermilab Tevatron superconducting proton-antiproton collider Ring and to recover warm return gases. The centerpieces of the CHL are two independent cold boxes rated at 4000 liters/hour and 5400 liters/hour with LN2 precool. These coldboxes are Claude cycle and have identical heat exchangers trains, but different turbo-expanders. The Tevatron cryogenics demand for higher helium supply from CHL was the driving force to investigate an installation of an expansion engine in place of the Joule-Thompson valve. A mathematical mode1 was developed to describe the thermo-and gas-dynamic processes for the equipment included in the helium coldbox. The mode1 is based on a finite element approach, opposite to a global variables approach, thus providing for higher accuracy and conversion stability. Though the coefficients used in thermo-and gas-dynamic equations are unique for a given coldbox, the genera1 approach, the equations, the methods of computations, and most of the subroutines written in FORTRAN can be readily applied to different coldboxes. The simulation results are compared against actual operating data to demonstrate applicability of the model.