An integrated SINDA85/FLUINT model has been developed for the central Active Thermal Control System (ATCS) of the Space Station Freedom. The model is based on the Phase I configuration of S.S. Freedom and a design similar to the LMSC thermal bus Ground Test Unit. The model simulates the ATCS which utilizes ammonia as the working fluid and analyzes both 70°F and 35°F thermal buses at various thermal loads. Both buses interact with nine heat acquisition devices representing the projected habitat and laboratory modules and attached payload heat exchangers. Performance of gas blocked and liquid blocked condensers, shear flow sub coolers, cold plate evaporators utilizing self-regulating capillary fluid management, liquid flow control orifices, pump, bellows accumulator, and liquid and vapor pressure control valves have been simulated and implemented into the integrated model. The model is capable of predicting flow rate, temperature, and pressure maps of the buses for steady state and transient cases under cyclic environmental heat loads and variable module heat rejection requirements. It also predicts the overall bus performance including sizing the orifices and setting valve stem positions and pump speed.