In this paper, a comprehensive model of indirect water heater for a pressure reduction station of natural gas has been presented. Effiiency and fuel consumption of heater have been estimated by applying energy and mass conservation equations. Three terms of energy including heat absorbed by natural gas, heat transferred to environment from stack and heat loses to environment from heater body have been evaluated. Thermal properties of natural gas has been calcualated using AGA8 state equation. Therefore, the model is a function of climate, heater geometry and natural gas characteristics including temperature, pressure and its composition. The results are compared with data recorded from real station in Iran (Arak, Kheriabad and Malekabad stations) and very good agreement has been observed. Reducing temperature and increasing pressure of inlet gas, thickening glass wool insulation and rising fire tube radius led to an increase in heater’s thermal efficiency from 20 to 60 percentage and consequently fuel consumption of heater was reduced. Finally, choosing the sutible geometery yeilded a higher heater efficincy with a raise from 44% to 70%.
