ABSTRACT
This chapter considers the principles of fluid flow and heat transfer in vacuum systems. The flow regime for vacuum systems may be determined from the value of the ratio of the gas molecule mean free path to the characteristic dimension of the container. The flow process in free molecular flow is more a diffusion than a convection process. The flow regime at the highest pressure and in the largest flow passage is free molecular flow; therefore, the flow will also be free molecular throughout the remainder of the system. Free molecular conduction differs from continuum-type conduction, which usually occurs in gases at ambient pressure. A high-energy molecule travels across the space between the containing surfaces, strikes the lower-temperature surface and transfers energy directly to the surface. The free molecular gaseous conduction may be reduced to very low values by reducing the gas pressure to a sufficiently low value.
