The Research On The Heat Transfer Computing Of LNG Ambient Air Vaporizers

LNG ambient air vaporizers (AAVs) have been widely used in the LNG gasification station and more recently, large scale applications of AAVs have been proposed for LNG import terminals for the advantages of lower operating costs and lower environmental impact. Because the air temperature is reduced during the operation of LNG ambient air vaporizers, frost is easily formed in the tube wall, and the fog will also appear when the cold air discharging from the AAVs arrays mixs with warm and humid air, and the running of the vaporizers is severely affected by the environmental conditions. All of the problems mentioned above will impact the heat transfer of the AAVs, resulting in the reduction of the NG temperature in the outlet and incompleteness of the gasification.Calculating the heat transfer of the single finned-tube of the LNG AAVs and analysing the heat transfer intensity and temperature distribution in the tube are the main works in the paper. Firstly, the research status of AAVs and fluid flow boiling in domestic and abroad are introduced briefly, and the theory of two phase flow and boiling heat transfer are recommended. Based on the instances of the gas imported by our contry, the article confirms the constitutions of LNG to be researched. Then according to the selected models and mixed rule, the physical parameters of LNG(NG) in different temperatures are computed, which are the basis for the heat transfer calculation.Based on the definitions of the bubble and dew point , the temperatures at these two points are calculated by the Matlab software, by which the LNG gasification process is divided into three areas: single-phase fluid, two phase and single-phase gas. The based on the gived thermodynamic properties and the theories of the heat transfer from air to the fluid in the finned tube , the paper establishes the mathematical models in the three areas and proposed the heat transfer calculation adopting the Matlab software. The calculation results include the length of each area,the distribution of heat transfer coefficients and the temperature of the LNG(NG) along the tube. When caculating the heat transfer process in the tube, the article adopts the forced convection functions in the single-phase fluid and single-phase gas areas, and adopts Klimenko relationship in the two phase area.Finally, based on the methods mentioned above, the paper studies the relationship between the heat transfer and the LNG components, air temperature. The results indicate that when the content of methane increases, the heat transfer intensity is strengthened, and the higher the air temperature, the stronger the heat transfer intensity, as the air temperature increases by 10K, the total length of the finned tube is shortened by 2m.The research method and the conclusions used in this paper can provide theoretical basis for the design and the operation regulation process of the LNG ambient air vaporizers.