The development of a sub-atmospheric two-phase thermosyphon natural gas preheater using a lumped capacitance model

Natural gas (NG) for domestic and industrial use is distributed around the country at high pressure. To counteract the Joule-Thompson effect, pre-heating of natural gas is required prior to pressure reduction. Most preheaters used on national gas networks are in the form of water bath heaters, where a closed tank houses a burner which delivers energy to the fluid in situ, which in turn heats an exchanger through which the NG flows. These systems usually have a low efficiency, and as a result of thermal inertia have a long time lag to accommodate changes in NG mass flow rates. A new preheating system has been designed utilising sub-atmospheric two-phase type thermosyphon theory. The main advantage of using a sub-atmospheric thermosyphon to preheat NG is the improved response time, due to the reduction in working fluid volume and the thermosyphon operating at lower temperatures resulting from partial vacuum. A numerical model of the thermosyphon has been developed using a lumped capacitance method. Simulations of the thermosyphon preheater system over a 24 hour load cycle suggest that the thermosyphon system could reach an efficiency of approximately 84% over the period.