Effect of various twisted-tape designs on the thermal and environmental performance of line-heaters in city gate stations

A city gate station is a point in natural gas transmission pipeline at which the pressure of gas flow should be reduced significantly. For preventing gas hydrate forming in the gas flow, the gas is heated before the throttling process. This is accomplished via a line-heater. The major problem of the heater is its low energy efficiency (below 50%), leading to wasting a big portion of the combustion released energy through the chimney. As such heaters burn natural gas, this low efficiency causes them to release a large amount of greenhouse gas. This study investigates the effects of using various twisted-tape designs with different pitch lengths and the number of twists on the thermal and environmental performance of line-heater. The proposal is numerically analyzed for a line-heater in a real city gate station. The results show that a larger number of twists and shorter pitches better enhance the efficiency so that twisted-tapes with the total numbers of 17 and 68 twists improve the average Nusselt number by 12.5% and 36.5%, respectively. These heat transfer enhancement figures are associated with preventing a total annual of 24 and 50 mil. tonnes of equivalent carbon dioxide being released by the case study heater.