The design of semiconductor vacuum chambers often requires that overall chamber conductance be optimized. In high-density plasma (HDP) chemical vapor deposition (CVD) systems for dielectric deposition in STI applications, a high chamber conductance will promote lower chamber pressures over the wafer. These lower pressures can enhance the ability to perform high aspect ratio dielectric gapfill on the wafer. Flow modeling is routinely used in the design of such CVD systems, employing either Monte Carlo methods or Navier-Stokes solvers (CFD) using slip boundary conditions. In this work, a thermal radiation analogy to free molecular flow is used in place of a collisionless Monte Carlo computation for two different HDP-CVD chamber configurations. Modeling results show that with either the thermal radiation analogy approach, or the CFD approach, chambers with centrally-mounted pedestal chamber designs perform significantly better from a conductance/pressure perspective than do cantilever- mounted pedestal designs. Also, the thermal radiation analogy model approach allows complex 3D chambers to be modeled quickly with general-purpose, commercially available CFD codes. This CFD approach is preferable in industrial environments as opposed to using Monte Carlo methods, which require the use of a separate software modeling approach that is not available commercially, and is difficult to use for complex 3D geometries.