Indirect water bath heaters are key components in natural gas pressure reduction stations, responsible for preheating the gas stream before it enters the pressure reduction process. Despite their essential role in preventing hydrate formation an‎d gas flow blockage, their high fuel consumption an‎d low thermal efficiency remain significant challenges in Iran’s gas industry. This study aims to identify the mechanisms of heat transfer an‎d the resistances along its path in order to propose solutions for improving energy efficiency an‎d reducing fuel consumption in gas heaters. To this end, the heater at the Zagheh pressure reduction station in Shahreza, with a capacity of 30,000 stan‎dard cubic meters per hour, was selec‎ted an‎d its performance eva‎luated through field measurements an‎d numerical simulations. Results showed that in the heat transfer process from the water bath to the gas stream, approximately 59% of the thermal resistance is attributed to the outer surface of the coil, 6% to the coil wall, an‎d the remainder to the inside of the gas flow path—resulting in a temperature difference of about 13°C between the gas an‎d the bath. Furthermore, investigations revealed the occurrence of subcooled boiling on the surface of the firetube; at a heat flux of 3350 W/m², the average tube surface temperature was estimated at around 103°C. In this regime, vapor bubbles collapse rapidly an‎d transfer a substantial amount of energy to the bath. Based on the energy balance analysis, the heater’s thermal efficiency under operational conditions was calculated to be 41%, which is significantly lower than the 71% efficiency reported by the manufacturer. Additionally, energy losses through the heater body an‎d the stack were estimated at approximately 1% an‎d 4.4% of the total combustion energy, respectively. Among the four coil configuration scenarios eva‎luated in this study, the best performance was observed when the primary coil was split into two parallel coils with doubled flow rate. In this scenario, a 5°C increase in bath temperature led to a twofold increase in heat transfer to the gas stream. In addition to these technical findings, the study highlights the effectiveness of strategies such as removing ethylene glycol to promote subcooled boiling an‎d using a pump to enhance natural convection an‎d reduce thermal resistance along the coil surface. These measures are economically viable an‎d can be implemented within short timeframes.

محسن نصراصفهاني

نسرين اعتصامي , احمد محب