Cryogenic Heat Exchanger Research Articles

Measured and predicted upward flow boiling heat transfer coefficients for hydrocarbon mixtures inside a cryogenic plate fin heat exchanger

Boiling of hydrocarbon mixtures inside plate fin heat exchangers (PFHEs) is most prevalent in many cryogenic processes of petrochemical plants. However, there is very little experimental data and a lack of prediction methods regarding boiling mixtures in PFHEs. This paper established an experimental setup that utilized a single-stage cryogenic cycle to measure the boiling heat transfer coefficients (HTCs) for hydrocarbon mixtures inside a perforated PFHE. The components of the mixtures include methane, ethylene, propane and isobutene. Twenty-seven sets of experiments were conducted under various operating conditions and 261 data points of the boiling HTC were obtained. The experimental conditions cover the mass fluxes of 3.69–19.38 kg m−2 s−1, pressures of 1.35–5.22 bar and vapor qualities of 0.05–0.77, which are representative of a wide range of actual industrial conditions. The corresponding heat fluxes only range from 55.7 to 3837.3 W m−2. The experimental boiling HTC varies from 21.4 to 1055.7 W m−2 K−1, and it is a strong function of the heat flux. Moreover, twelve existing correlations were assessed by using the present experimental data. The results show that the HTCs predicted by Chen type correlations were not consistent with the experimental results, and a new correlation developed from mini-channel correlations was recommended.

Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes

A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency. A detailed model of a plate-fin heat exchanger is presented that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.

Experimental study on the thermal hydraulic performance of plate-fin heat exchangers for cryogenic applications

Efficient and compact plate-fin heat exchangers are critical for large-scale helium liquefaction/refrigeration systems as they constitute major part in the cold box. This study experimentally explores the heat transfer and pressure drop behaviors of helium gas at low temperature in four types of plate-fin channels, namely offset-strip and perforated fins, with different geometrical parameters. A series of cryogenic experiments at approximately liquid nitrogen temperature are carried out to measure the Colburn j factors and Fanning friction f factors with a wide range of Reynolds number. Besides, to reveal the performance variations under different operating temperatures, comparative experiments respectively conducted at room temperature and liquid nitrogen temperature are implemented. The results show that in comparison with the performance data at room temperature, most of j factors are relatively smaller perhaps because the lower aluminum thermal conductivity and higher Prandtl Number at low temperature. Meanwhile, the f factors corresponding to cryogenic conditions exhibit slightly larger even though the core pressure drops show considerable reductions. In contrast to the calculated results from the frequently-used performance curves (Chen and Shen, 1993), the Root Mean Squared Errors of j and f values are correlated within 8.38% and 6.97% for one perforated fin core, 41.29% and 34.97% for three OSF cores, respectively. For OSFs, further comparisons with the previous empirical correlations from literatures are conducted to verify the accuracy of each correlation. Generally, most of the calculated results predict acceptably within the deviations of ±25% for the j factors, while the predicted results express relatively large deviations for the f factors. Therefore, it may be revealed that most of the existing correlations were not able to accurately predict the experimental data in consideration of the performance differences under realistic cryogenic operating conditions, which could have significant influences during the design process of cryogenic heat exchangers.

액체질소 냉각용 극저온 열교환기의 최적설계를 위한 열전달 및 압력강하 특성 분석

액체질소 냉각용 극저온 열교환기의 최적설계를 위한 열전달 및 압력강하 특성 분석

Rapid Simulation of Solid Deposition in Cryogenic Heat Exchangers To Improve Risk Management in Liquefied Natural Gas Production

The carryover of heavy hydrocarbons into cryogenic heat exchangers, even at low concentrations, represents a significant risk to reliable liquefied natural gas (LNG) production given their propensity to freeze, deposit, and block flow. Calculating blockage risk requires not only freeze-out temperature predictions but also estimates of where solids will form and the associated operational symptoms. We present here a new simulation tool capable of such predictions in industrial cryogenic heat exchangers, which has been tuned to solid formation data available in the literature. Simulation of the LNG plant upset detailed by Ismail and Al Thani (LNG-18, 2016) confirms the location of the initial deposition that occurred upon restart and provides insights into possible remediation strategies that could have helped avoid the subsequent shutdown. The analysis also reveals that predicting water crystallization in cryogenic LNG systems requires a substantial extrapolation of current thermodynamic models, highlighti...

CFD modelling of a novel liquid Nitrogen/Air engine and cryogenic heat exchanger for small scale applications

Liquid nitrogen/air is considered as a promising energy storage vector where a number of mathematical and thermodynamic studies have shown its feasibility to provide cooling and power for both domestic and commercial applications. The current work presents a CFD modeling of storage system that uses liquid Nitrogen/Air generated using surplus electricity at off peak times or renewable energy sources, to provide cooling and power for domestic applications. Ansys Fluent 17.2 software was used in this study to model the main two components of the proposed system (cryogenic heat exchanger which provides cooling effect and expander (cryo-engine) that generates electrical power). The CFD modelling of boiling process of LN2 in the cryogenic heat exchanger was compared with an experimental data that available in the literature and showed a good agreement. Also, the model showed encouraging results where the engine power output reaches 2.2 kW at inlet pressure of 20 bar. Taking to account the generated cooling energy the total energy output reaches 14.6 kW which is enough to run domestic hoses at the peak times.

Performance evaluation of cryogenic counter-flow heat exchangers with longitudinal conduction, heat in-leak and property variations

Counter-flow plate-fin heat exchangers are commonly utilized in cryogenic applications due to their high effectiveness and compact size. For cryogenic heat exchangers in helium liquefaction/refrigeration systems, conventional design theory is no longer applicable and they are usually sensitive to longitudinal heat conduction, heat in-leak from surroundings and variable fluid properties. Governing equations based on distributed parameter method are developed to evaluate performance deterioration caused by these effects. The numerical model could also be applied in many other recuperators with different structures and, hence, available experimental data are used to validate it. For a specific case of the multi-stream heat exchanger in the EAST helium refrigerator, quantitative effects of these heat losses are further discussed, in comparison with design results obtained by the common commercial software. The numerical model could be useful to evaluate and rate the heat exchanger performance under the actual cryogenic environment.

Constraint Formulations for Optimisation of Dual Mixed Refrigerant LNG Processes

Optimisation of a dual mixed refrigerant LNG process is challenging due to its rather complex structure. Therefore, appropriate constraint formulations are essential to solve the optimisation problem. In this paper, the AP-DMR process was optimised to minimize the power consumption with different constraints in order to find proper constraint formulations. The case studies indicate that refrigerant superheating does not always increase the power consumption. Instead, the best-known solutions required specific values of refrigerant superheating. Moreover, maximum heat exchanger area constraints for cryogenic heat exchangers delivered higher energy efficiency, compared to the constraints of minimum temperature difference, which is common practice for LNG processes.

Экспериментальное исследование процесса охлаждения газа в криогенном теплообменнике

Экспериментальное исследование процесса охлаждения газа в криогенном теплообменнике

Superstructure based techno-economic optimization of the organic rankine cycle using LNG cryogenic energy

A process design of the organic Rankine cycle utilizing LNG cryogenic exergy is proposed using superstructure optimization. The superstructure imbeds about 1024 possible process alternatives, and the most profitable process configuration and the operating condition are decided simultaneously using a stochastic optimization solver and Aspen Plus-MATLAB interface. The optimum process configuration includes a multi stream cryogenic heat exchanger, a five-stage turbine with reheaters, three stage vapor re-condensation processes and direct contact heaters. In addition, the exergy transfer from the LNG to the working fluid is maximized by using a multi component mixture as working fluid. The 1st law efficiency of the proposed process reaches about 26.2% with 85 °C of waste heat source and it is about 42% higher than that of the conventional ORC. The annual profit of the optimum process is about 39 M$ and it can be interpreted as 24$ of profit per kg LNG evaporation. Sensitivity analysis is also presented to show the reliability of the stochastic solution found in this study.

Flow mal-distribution study in cryogenic counter-flow plate fin heat exchangers

Plate Fin Heat Exchangers are one of the main components in modern large capacity helium liquefiers/refrigerators. These heat exchangers should have more than 95% effectiveness in order to liquefy helium gas. Design of these heat exchangers are complex one and govern by many parameters such as flow mal-distribution, variable fluid properties, heat-in-leak from ambient and longitudinal heat conduction through wall. Among these design parameters, flow mal-distribution within heat exchanger layers is one of the important design parameters for achieving higher effectiveness. Flow mal-distribution reduces the effectiveness significantly and may bring down it to a much lower value than design value. In the present study, a numerical model has been developed which is capable to evaluate layer by layer temperature distribution in presence of flow mal-distribution. Two major mal-distribution conditions, linear weighted and center weighted, might occurs when flow leads from nozzle to different layers in plate fin heat exchangers. Present work studied the effect of these two conditions on performance of plate fin heat exchanger in the temperature range of 300-90 K as a case study.

Capacity enhancement of indigenous expansion engine based helium liquefier

Development of technology and understanding for large capacity helium refrigeration and liquefaction at helium temperature is indispensable for coming-up projects. A new version of helium liquefier designed and built to provide approximately 35 liters of liquid helium per hour. The refrigeration capacity of this reciprocating type expansion engine machine has been increased from its predecessor version with continuous improvement and deficiency debugging. The helium liquefier has been built using components by local industries including cryogenic Aluminum plate fin heat exchangers. Two compressors with nearly identical capacity have been deployed for the operation of system. Together they consume about 110 kW of electric power. The system employs liquid Nitrogen precooling to enhance liquid Helium yield. This paper describes details of the cryogenic expander design improvements, reconfiguration of heat exchangers, performance simulation and their experimental validation.

Dynamic heat transfer analysis of liquefied natural gas ambient air vaporizer under frost conditions

Liquefied natural gas (LNG) ambient air vaporizer (AAV) is a cryogenic heat exchanger, which utilizes ambient air to evaporate LNG in gas terminal stations. The moist air around the AAV fin tube may freeze during the long-time operation, leading to a worse heat transfer performance. Study on the dynamic heat transfer performance of AAV is required for its optimal design and operation. This paper presents a coupled dynamic heat transfer model, which incorporates the cryogenic frost formation and flow boiling heat transfer of LNG. The model was demonstrated in the heat transfer analysis of a typical star-fin type AAV. The model results were validated by the test data of a commercial HAI848F type AAV. The dynamic heat transfer performance of AAV under frost conditions was also studied.

Optimization of modified single mixed refrigerant process of natural gas liquefaction using multivariate Coggin’s algorithm combined with process knowledge

The optimization of a mixed refrigerant liquefaction process is a challenge because of its non-linear characteristics with stringent multiple process constraints. This study proposes a novel hybrid approach for the optimization of a newly developed, modified single mixed refrigerant process of natural gas liquefaction targeted for offshore applications. This contribution focuses on interpreting the geometric pattern of a plot of the temperature difference between the hot and cold composite curves in a cryogenic heat exchanger to understand the profound effects of the flow rates of the individual refrigerant components and the operating pressure on the liquefaction efficiency. From this, an effective method to generate a proper initial approach temperature profile was developed to ensure robust convergence of the main optimization step. An enhanced coordinate descent methodology was implemented in the main optimization procedure to accelerate the optimization of the modified single mixed refrigerant liquefaction process. The proposed knowledge-inspired hybrid optimization approach showed a robust convergence on determining the optimal design condition. The total energy requirement for natural gas liquefaction cycle was reduced by 21.9% compared to the base case. The proposed methodology can be extended directly to solve optimization problems for other mixed refrigerant based natural gas liquefaction processes.

Numerical modeling of recuperative cryogenic matrix heat exchangers and the experimental validation

The recuperative matrix heat exchanger (MHE) has advantages of large specific surface area, compactness, high effectiveness, and low flow resistance, which make it promising for applications in Joule-Thomson cryocooler and reverse Brayton cryocooler. In this study, a numerical model is developed to predict the thermal performance of MHEs. The effects of axial conduction of both inner and outer walls as well as radial heat conduction through mesh-screens and the heat leakage are considered. The temperature distribution over the fluids and mesh screens in MHEs can be determined using this model. The model is validated by analytical solution of effectiveness-NTU method and the results of the experimental tests on two MHEs with different structures at cryogenic temperature. Using the proper correlations for heat transfer coefficients, the relative deviations of the ineffectiveness predictions from experimental results are below 13.2% in the mass flow range of 0.4 g/s – 2.1 g/s. The numerical results show that the axial conduction of inner and outer walls has similar effects on the thermal performance of MHEs. The increase of axial conduction can substantially lower the effectiveness of the MHEs at mass flow rates below 1 g/s, while the influence of the number of transfer unit (NTU) on the MHE effectiveness becomes dominant at mass flow rates above 1 g/s.

Influence of tube wall longitudinal heat conduction on temperature measurement of cryogenic gas with low mass flow rates

Longitudinal heat conduction is an important parameter in the cryogenic field, especially in cryogenic heat exchangers. In the present study, the parasitic effect of tube wall longitudinal heat conduction on temperature measurement within the tube has been studied for cryogenic gas with low mass flow rates by finite element method and experimental tests. The effects of various parameters such as tube outlet temperature, tube wall thermal conductivity, mass flow rate, and tube wall thickness have been investigated. Axial positioning errors of temperature sensor due to tube wall longitudinal heat conduction were higher for lower gas flow rates. The results showed that the tube wall thermal conductivity leads to axial heat conduction within the tube wall, but the higher tube wall thermal conductivity does not lead to bigger axial positioning error of temperature sensor at tube outlet. According to data obtained from simulations and experiments, sensor with distance of 5mm from tube outlet had 14.92% and 8.51% temperature measurement error (with respect to gas flow temperature at tube outlet) for tube wall thermal conductivities of 16 and 400Wm−1K−1, respectively.

Design and comprehensive optimization of C3MR liquefaction natural gas cycle by considering operational constraints

Liquefied natural gas (LNG) plays a key role in gas transits. Hence, attempts to improve the current liquefaction technology to increase its efficiency and economic advantages have earned a place in the master plan of many countries. Propane precooled mixed refrigerant process (C3MR) is one of the most successful cycle in the liquefaction industry. In this study, an enhanced process including a three stage propane precooling cycle and two cryogenic heat exchangers with mixed refrigerant (MR) has been modeled in ASPEN-HYSYS software by 1 MTPA production capacity. Saturated temperatures of Precooling stages, outlet temperatures of heat exchangers (HE) and aftercoolers will determine the quality and efficiency of the precooling stage. Minimization of energy consumption per 1 kg LNG production (specific energy consumption) has been defined as the main objective function. Optimization of the cycle variables has been performed, intended to minimize the specific energy consumption. Subsequently, comprehensive and integrated optimization has been developed. Mixed refrigerant composition has been recognized as the most effective parameter in the performance of the cycle. Therefore, mixed refrigerant composition has been optimized by two methods. HYSYS optimizer functions and a self-initiated method entitled as “Observation of governing trend in mixed refrigerant cooling curve behavior by an approach to maximization of possible fit in cryogenic heat exchangers composite curve”. Although these methods will cause a great amount of energy saving, their results are not practically achievable. Therefore the cycle has been optimized by considering operational constraints. The ultimate results of the optimization are not only theoretical, but also practical. The optimized point introduced here is obtainable in real performance, and cycle results are constant despite perturbation. Thus specific energy consumption decreases from 1028.94 kJ/kg at initial condition to 973.93 kJ/kg at sustainable optimized condition, by 5.35 percent of specific energy consumption saving.

A cryogenic heat exchanger with bypass and throttling and its thermodynamic analysis

A precooled Joule-Thomson (J-T) cooler refrigerates at liquid helium temperature. Its third stage heat exchanger works below 20 K. Hot fluid cannot be sufficiently cooled due to nonidealism of the heat exchanger and helium-4 properties. In a J-T cycle of low pressure ratio, the heat exchanger with bypass and throttling improves the refrigeration capacity. Bypass and throttling reduces the temperature difference and entropy generation within the heat exchanger.

Low-temperature thermal conductivity of highly porous copper

The development and characterization of new materials is of extreme importance in the design of cryogenic apparatus. Recently Versarien® PLC developed a technique capable of producing copper foam with controlled porosity and pore size. Such porous materials could be interesting for cryogenic heat exchangers as well as of special interest in some devices used in microgravit.y environments where a cryogenic liquid is confined by capillarity.In the present work, a system was developed to measure the thermal conductivity by the differential steady-state mode of four copper foam samples with porosity between 58% and 73%, within the temperatures range 20 - 260 K, using a 2 W @ 20 K cryocooler. Our measurements were validated using a copper control sample and by the estimation of the Lorenz number obtained from electrical resistivity measurements at room temperature. With these measurements, the Resistivity Residual Ratio and the tortuosity were obtained.

Method for defrosting heat exchangers using an air-particle jet

We report test results on a method for defrosting heat exchangers in which solid particles accelerated by an air jet impinge on heat exchange surfaces. This study was motivated by the development of a cryogenic heat exchanger for a hypersonic aircraft engine. We conducted experiments to remove frost from a single-row heat exchanger cooled with liquid nitrogen as the refrigerant. In the experiments, particle-jet defrosting was conducted for 1 s at intervals of 50–300 s over total experimental durations of 600 s. The experiments were performed using different intervals between defrost cycles, different particle diameters, and different numbers of particles. The effects of these parameters on the performance of the heat exchanger were evaluated by measuring pressure losses and heat transfer rates. The results show that the method prevents buildup of frost and is more effective than defrosting with only an air jet.