Study On The Performance Of Vaporizer In Fuel Gas Supply System For Ships At Low Pressure

In dual-fuel ships,liquid nature gas needs to be converted into gaseous state through the fuel gas supply system before it can be sent to the main engine for combustion.As the most important heat exchange equipment in the fuel gas supply system,the study on performance of LNG vaporizer is the key to ensure the effective operation of the fuel gas supply system of ships.The plate and shell heat exchangers and the spiral wound heat exchangers have superior heat exchange efficiency and compact structure.It makes them become one of the best choices for vaporizer in fuel gas supply system of ships.Therefore,by comparing the plate and shell heat exchangers and the spiral wound heat exchangers,it is meaningful to study the performance of both heat exchangers as a vaporizer in fuel gas supply system in ships at low pressure.For the study of the performance of LNG vaporizer,the whole process of the fuel gas supply system at low pressure in a specific ship type is analyzed firstly.Then calculate the process parameters of the vaporizer at steady state.Taking the marine low-pressure fuel gas supply system as the application object,the qualitative analysis of the type of vaporizer is carried out,and the comparison found that the spiral wound heat exchanger and the plate and shell heat exchanger are the preferred items of the LNG ships which at low pressure.According to the characteristics of LNG ingredients,the composition of LNG gas samples is determined.According to the definition of bubble point temperature and dew point temperature,the bubble point temperature and dew point temperature of LNG are calculated.In order to overcome the problem that the commercial software Refprop can not call the physical properties such as viscosity and specific heat in the transition zone,appropriate models for calculating the physical properties of the mixture is selected.The calculation results are compared with the results obtained by directly calling Refprop after programming in Matlab.It was found that the deviations of density,Cp,thermal conductivity,and dynamic viscosity are 7.25%,3.3%,2.25%,and 4.91%,respectively.Then,the boiling heat transfer model for LNG in tube and the winding tube and the boiling heat transfer model for LNG in microchannel are summarized,and the correlations of the flow heat transfer performance commonly used at home and abroad are listed.Matlab was used to design and calculate the structural parameters at qualitative temperature for two types of LNG vaporizer--the spiral wound vaporizer and the plate and shell vaporizer which commonly used in LNG.By comparing,it is found that under the condition of same design parameters,the heat transfer coefficient of the plate and shell heat exchanger is much larger than that of the spiral wound heat exchanger,which is 1.734 times,and the required heat exchange area is 0.707 times that of the spiral wound heat exchanger.The pressure drop of water-glycol side of the plate and shell heat exchanger is 1.833 times that of the spiral wound heat exchanger,while the pressure drop of LNG side is almost the same as that of the spiral wound heat exchanger.In general,in the limited space of the ship,the plate and shell heat exchanger has more advantages.Using the distributed parameter model,a mathematical model of the heat exchanger that can comprehensively measure the variable physical properties and different flow heat transfer models is established.The performance of the flow heat transfer of the spiral wound heat exchanger and the plate and shell heat exchanger in the actual low temperature environment was evaluated by programming calculations of Matlab.And the structure optimization analysis of the LNG vaporizer under the low pressure was carried out,based on the area control method.The analysis results show that the spiral wound heat exchanger and the plate and shell heat exchanger can obtain good operating performance under design conditions,and the flow and heat exchange effects are better than the design values calculated by the qualitative temperature.At the same time,for the problem that the design margin is too dependent on empirical selection,the suggestion that the heat transfer area on the LNG side of the spiral wound heat exchanger and the plate and shell heat exchanger can be reduced to 10.34 m~2 and 7.68 m~2,respectively.