IMPROVING THE HEAT TRANSFER EFFICIENCY OF A TUBULAR HEAT EXCHANGER

ABSTRACT

Efficient heat removal is a key requirement in condensate stabilization processes to ensure stable operation and reduce vapor losses. This study presents an improved tubular heat exchanger design based on a combined internal water cooling and external air cooling approach. The proposed system uses a coaxial double-pipe configuration in which cooling water flows through the inner tube, stabilized condensate flows through the annular space, and forced air cooling is applied to the outer tube using an air cooling unit. The improved design enhances heat transfer efficiency and operational reliability without significant additional energy consumption. Analytical evaluation and comparative analysis confirm the effectiveness of the proposed solution.

АННОТАЦИЯ

Эффективный отвод тепла является важным условием стабильной работы процессов стабилизации конденсата и снижения потерь легких углеводородов. В данной работе предложена усовершенствованная конструкция трубчатого теплообменника, основанная на комбинированном внутреннем водяном и внешнем воздушном охлаждении. Конструкция выполнена в виде коаксиальной двухтрубной системы, где по внутренней трубе протекает охлаждающая вода, а стабилизированный конденсат движется в кольцевом зазоре, при этом внешняя поверхность охлаждается потоком воздуха. Результаты анализа подтверждают повышение эффективности теплообмена и надежности работы установки.

Keywords: condensate stabilization, tubular heat exchanger, double-pipe heat exchanger, air cooling, heat transfer efficiency.

Ключевые слова: стабилизация конденсата, трубчатый теплообменник, двухтрубная система, воздушное охлаждение, интенсификация теплообмена.

1. Introduction

Condensate stabilization is a critical technological process in oil and gas processing facilities, aimed at removing light hydrocarbon fractions to ensure safe storage, transportation, and further processing of condensate products. During stabilization, controlling the temperature of the condensate is of great importance, as excessive temperatures lead to increased vapor formation, product losses, and operational instability. The purpose of this study is to develop and evaluate an improved tubular heat exchanger design with combined internal water cooling and external air cooling to enhance heat transfer efficiency and operational reliability in condensate processing.

Tubular heat exchangers are commonly employed in condensate stabilization units due to their simplicity, reliability, and ease of maintenance. However, traditional single-pipe or shell-and-tube heat exchangers often exhibit insufficient heat transfer efficiency, particularly in regions with high ambient temperatures and limited cooling water availability.

Modern research on improving the efficiency of heat transfer in tubular heat exchangers is based on updated fundamental work on heat transfer and heat exchanger design. Recent monographs and manuals have examined the patterns of convective heat transfer in pipes, methods for calculating heat transfer coefficients and hydraulic resistances, as well as approaches to choosing coolant flow patterns and the layout of tubular apparatuses [1-3]. These studies form the theoretical basis for analyzing the relationship between thermal efficiency and pressure losses, as well as for setting optimization problems [1, 2].

A significant part of modern publications is devoted to the intensification of heat exchange due to modification of the inner surface of pipes and the use of developed surfaces. The review and guide for heat generator designers additionally analyzes the effects of oregony, real estate, local angles, and the use of (corrugated, ribbed tubes with recesses) is an integral part of the process [2-6]. It is shown that the formation of secondary flows and the destruction of the boundary layer can significantly increase the Nusselt number, but is accompanied by an increase in pressure losses, which requires a comprehensive assessment of efficiency according to thermohydraulic criteria [3, 6].

One of the key directions remains the use of internal turbulator inserts: screw tapes, wire coils, ribbon‑spiral and combined devices. A modern review summarizes the results of numerous experimental and numerical studies on pipes with screw bands, spiral inserts and their combinations, showing that an increase in the heat transfer coefficient can reach 1.5–3 times compared with smooth pipes [4]. At the same time, it is emphasized that it is necessary to evaluate not only the growth of the Nusselt number, but also the increase in the coefficient of friction and energy consumption for pumping [4].

Special attention has recently been paid to combined intensification methods, when a geometrically modified tube is combined with internal inserts. Pipes with local depressions (ripple stoppers) equipped with various versions of screw tapes were experimentally studied; correlations for the Nusselt number and the coefficient of friction were obtained, and integral efficiency indicators were calculated [5]. It is shown that a properly selected combination of a profiled surface and a turbulator provides higher values of the complex criterion "heat transfer–resistance" than using each method separately [5].

Research conducted on the computing platform (CFD) makes a significant contribution to the development of computerization methods. A numerical analysis of heat transfer in round pipes with various internal surface configurations, including ribs and recesses, was carried out [6]. Modeling makes it possible to identify in detail the flow structure, the distribution of local heat transfer coefficients and zones of increased hydraulic resistance, which makes it possible to perform parametric optimization of geometry at the design stage 6. The manuals on the calculation and design of heat exchangers also emphasize the role of the Central Federal District in reducing the volume of field experiments and improving the accuracy of predicting the characteristics of devices [3].

A separate area of modern research is related to the multipurpose optimization of tubular and shell-and-tube heat exchangers. The use of multicriteria optimization methods (including evolutionary algorithms) to simultaneously account for thermal efficiency, pressure drop, and economic performance was examined [7]. The results demonstrate that the optimal design option in terms of thermohydraulic parameters is not always optimal in terms of cost and energy consumption, which requires an integrated approach to selecting the parameters of the device [7].

In modern sources on heat and mass transfer, special emphasis is placed on energy efficiency and environmental aspects of the operation of heat exchangers. It is shown that an increase in heat transfer efficiency in tubular heat exchangers is directly related to a reduction in fuel consumption and greenhouse gas emissions in the energy sector and industry, as well as a reduction in the size and material consumption of equipment [1, 8]. Methods for increasing the compactness of devices, including through the use of intensified surfaces and optimization of operating modes 1, 8, are considered.

The use of nanofluids as a heat carrier in tubular and minichannel heat exchangers was considered [9]. It is noted that the addition of nanoparticles to traditional heat carriers can lead to an increase in effective thermal conductivity and heat transfer coefficient, but is accompanied by an increase in viscosity, possible problems with suspension stability and erosive effects on the walls of pipes [9]. In this regard, nanofluids are considered as a promising direction, but requiring further research, to increase the efficiency of heat transfer, especially in compact and highly loaded devices.

Modern reviews on intensification methods emphasize that to assess the effectiveness of a solution, it is not enough to analyze only an increase in the heat transfer coefficient of [4, 10]. Complex dimensionless indicators such as performance criteria (evaluation criteria, PEC) are increasingly used, taking into account both a relative increase in the Nusselt number and a relative increase in pressure losses of [4, 5, 10]. This approach allows you to correctly compare different designs of intensified pipes and inserts, as well as choose options that ensure the maximum thermal effect at an acceptable energy cost.

Analysis of the literature shows that increasing the efficiency of heat exchange in tubular heat exchangers is realized through a combination of measures: geometric intensification (fins, profiled and dimpled pipes), the use of internal turbulators, the use of CFD modeling and multi‑purpose optimization, as well as the promising use of nanofluids [1-7, 9, 10]. At the same time, the key scientific and practical task remains the development of constructive and operational solutions that ensure a significant increase in thermal efficiency with a limited increase in hydraulic losses and compliance with energy efficiency and operational reliability requirements [3-8, 10].

Thus, the purpose of the work is to develop and justify constructive and operational solutions to increase the efficiency of heat exchange in a tubular heat exchanger through the use of intensification methods (geometric modification of the flow part, the use of turbulators, etc.) while ensuring an acceptable level of hydraulic losses and taking into account real operational limitations.

2. Methods

Therefore, the development of improved heat exchanger designs that enhance heat transfer efficiency while maintaining compactness and operational reliability remains an actual scientific and engineering task. This study focuses on improving the heat transfer performance of a tubular heat exchanger used in condensate stabilization by introducing a combined water–air cooling approach.

The proposed method is based on constructive modernization of a tubular heat exchanger by applying a coaxial double-pipe configuration [11-13]. Cooling water flows through the inner tube, while stabilized condensate moves through the annular space between the inner and outer tubes. Additionally, forced air cooling is applied to the outer surface of the exchanger using an air cooling unit.

Heat transfer analysis was performed using classical heat transfer relations, considering convective and conductive resistances. Comparative evaluation was carried out between the conventional single-pipe design and the proposed combined cooling configuration.

The proposed heat exchanger is designed as a coaxial double-pipe system consisting of an inner tube and an outer tube. The flow arrangement is organized as follows:

• Inner tube (diameter 20 mm): cooling water flows through the inner tube and removes heat from the condensate through the tube wall.
• Annular space: stabilized condensate flows in the annular gap between the inner and outer tubes.
• Outer tube (diameter 32 mm): the outer surface of the tube is cooled by forced airflow generated by an air cooling unit (HVO fan).

This configuration enables simultaneous internal and external cooling, significantly increasing the total heat transfer rate.

Figure 1. Schematic diagram of the improved tubular heat exchanger

1 – Cooling water inlet; 2 – Inner tube (Ø20 mm); 3 – Condensate flow in annular space; 4 – Outer tube (Ø32 mm); 5 – Forced air flow (HVO fan); 6 – Air outlet

Heat transfer in the proposed system is based on classical heat transfer theory and includes convective and conductive mechanisms. The total heat transfer rate is calculated using the following equation:

,

where:  – heat transfer rate, ;

 – overall heat transfer coefficient,

 – effective heat transfer area, ;

– logarithmic mean temperature difference, .

The introduction of forced air cooling significantly increases the external heat transfer coefficient, thereby improving the overall thermal performance of the system.

Logarithmic mean temperature difference

The logarithmic mean temperature difference (LMTD) is determined as:

This parameter allows accurate estimation of the temperature driving force along the heat exchanger length.

Operating Parameters and Initial Data are presented in table 1.

Table 1.

Main operating parameters of the proposed heat exchanger

3. Results and Discussion

A comparative thermal performance analysis was conducted between a conventional single-pipe tubular heat exchanger and the proposed coaxial double-pipe heat exchanger equipped with external forced air cooling. The analysis demonstrates a substantial improvement in heat transfer characteristics for the proposed design, primarily due to the combined effect of internal water cooling and intensified external convection.

Figure 2. Comparison of overall heat transfer coefficients

The results indicate that the improved heat exchanger achieves an increase of approximately 18–25% in the overall heat transfer coefficient compared to the conventional design. This enhancement is attributed to the expanded effective heat transfer surface and the increased external heat transfer coefficient resulting from forced air cooling.

Figure 3. Temperature profile along the heat exchanger length

In the proposed design, the temperature reduction is more uniform and pronounced, indicating a higher cooling intensity and improved thermal control. The smoother temperature gradient also reduces localized thermal stresses, contributing to improved operational reliability.

The combined cooling mechanism enables more effective heat removal while simultaneously reducing the thermal load on the cooling water system. As a result, the proposed configuration ensures stable operation under varying process and environmental conditions.

The main advantages of the proposed tubular heat exchanger design can be summarized as follows:

• Enhanced heat transfer efficiency achieved through the integration of internal water cooling and external forced air cooling;
• Reduction in cooling water consumption due to partial replacement of water cooling by air cooling;
• Improved performance and thermal stability under high ambient temperature conditions;
• Compact and modular construction, facilitating installation and maintenance;
• High adaptability for integration into existing condensate stabilization units without the need for major structural modifications.

Scientific Novelty

The scientific novelty of this work is characterized by the following aspects:

• Implementation of a combined internal water cooling and external air cooling concept within a single tubular heat exchanger for condensate stabilization applications;
• Optimization of the coaxial double-pipe geometry, including the selection of inner and outer pipe diameters, to enhance heat transfer under typical stabilization operating conditions;
• Demonstration of improved thermal efficiency and operational effectiveness with minimal additional energy input, highlighting the feasibility of the proposed solution for industrial implementation.

5. Conclusion

The proposed improved tubular heat exchanger design provides an effective solution for enhancing heat transfer efficiency in condensate stabilization processes. The combination of inner water cooling and outer air cooling using an HVO fan system significantly improves thermal performance and operational reliability. The results of the study confirm the feasibility and practical applicability of the proposed design in industrial oil and gas facilities.

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