Heat Transfer Structure Design Of Submerged Combustion Vaporizer (SCV)

Submerged combustion vaporizer (SCV) is a common kind of vaporizer in liquefied natural gas (LNG) terminal. Recently, it has been widely used in the developing natural gas industry. In China, SCV is usually used as peak shaving type vaporizer to help the major vaporizer (open rack vaporizer or intermediate fluid vaporizer) to fulfill the needed capacity when the users'demand is increasing or the weather is bad. SCV belongs to combustion type vaporizer, compared to other types of vaporizer, it has higher heat transfer efficiency and more compact structure. Design and manufacture of SCV have been monopolized by Linde and Sumitomo. Analysis of heat transfer in SCV is the fundamental for structure design. In order to design the heat transfer structure in SCV, following tasks have been conducted in this paper.The theoretical calculation has been done for the three typical heat transfer process in SCV:the submerged combustion heat transfer, the trans-critical heat transfer and the two-phase flow sweeping across tube bundle heat transfer. For the submerged combustion heat transfer, it should be controlled in the water temperature rising section. The heating efficiency is influenced by the water temperature, submergence and the excess air coefficient in the fuel gas combustion. For the trans-critical heat transfer, three kinds (nine in total) of models have been used for the prediction of heat transfer coefficient. It is found that the Petukhov model can not only predict the heat transfer coefficient precisely but also can show the influence of the variation of LNG properties on the heat transfer coefficient. For the two-phase flow sweeping across tube bundle heat transfer, the characteristics of two-phase flow, the specific structure of SCV and the mechanical energy conservation law have been combined together, it is concluded that using average sweeping velocity in Zhukauskas equation can predict the heat transfer coefficient outside the tube bundle. A SCV in service has been designed as a case study, the designed value of heat transfer tube bundle length is within 1% deviation from its real value, thus the feasibility of these heat transfer structure design method provided in this paper is confirmed.The computational fluid dynamics numerical simulation for the characteristics of two-phase flow outside the tube bundle and the analysis of operating factors have been conducted. It can be concluded from the results that the pressure distribution is consistent with the liquid static pressure distribution except the little fluctuation in the tube bundle zone; the temperature of two-phase flow is relative stable in the tube bundle zone; the phase state in tube bundle zone is uniformly; although the instantaneous velocity in tube bundle zone is disorderly, the average velocity is relative stable. What is more, reducing or increasing of flue gas flux and reducing of initial water height could weaken the heat transfer outside the tube bundle.A cold model experiment setup has been designed to investigate the heat transfer characteristics both inside and outside the tube bundle. It can be concluded that under certain initial water height the heat transfer coefficient outside the tube bundle increases with the hot air flux. There is a optimize initial water height. Both increasing and decreasing of initial water height would lead to heat transfer deterioration. A semi theoretical and semi empirical correlation has been obtained to predict the heat transfer coefficient of the half-submerged immiscible two-phase sweeping heat transfer with different media, tube pitch, initial water height and hot air flux.Using changing diameter tube to optimize the heat transfer structure has been proposed when analyzing the weakness of heat transfer in SCV, the heat transfer area reduced 7.9% and 10.3% using the changing diameter of 15 mm and 13 mm respectively. Based on this paper, a visual programming interface has been made by using Visual Basic 6.0, it can be used to design the heat transfer tube bundle, sparge pipes and weirs in SCV.