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This paper describes the development of a Back Pressure Regulating Valve (BPRV) model for thermal-hydraulic performance analysis of the Space Station Freedom (SSF) Central Thermal Bus (CTB). The CTB is a two-phase ammonia system which transports waste energy generated by station electrical loads to final rejection to space. The BPRV controls the operating temperature of the CTB by controlling the system pressure. The BPRV operates to maintain the CTB system pressure at a prescribed setpoint using a spring-biased mechanism which adjusts the valve area based on the system pressure. The setpoint is changed by varying the load on a control spring. The BPRV model accounts for the mechanical response of the BPRV in transient thermal-hydraulic analysis of the CTB. The model uses force balances to simulate the actions of the valve components in order to calculate the flow area. Transient responses are provided.