The following report attempts to make up for the deficiencies of theoretical Reynolds comparison by using an approach based on redefined parameter groups – specifically meant for cross-flow air coolers and/or air-cooled condensers. It will allow to directly evaluate different fin tube systems based on their performance characteristics alone.
Fin tubes are core elements of air cooled heat exchangers in industrial cooling applications. Thermohydraulic performance of the cooling system defines the overall size of the equipment. Fin tube characteristics vary over a large range resulting from geometry, material or manufacturing process. As a general rule - the better the performance the smaller and more competitive the design will be. Consequently, manufacturers strive for thermohydraulic optimization of their product line. A key factor is the ability to properly compare different cooling devices.
Comparison methods have been a topic in the academic field over a long period of time. Methods proposed so far have been based on classical dimensionless parameters, especially Reynolds number. Apart from transport properties Reynolds numbers include a geometric parameter pertinent to the individual heat exchanger system. If geometry is varied Reynolds changes as well. This is one of the reasons why Reynolds based comparison methods - although theoretically sound – lack practical applicability.
Table of Contents
1 Introduction
2 Classical Form of Fin Tube Characteristics
3 New Form of Fin Tube Characteristics
4 Base Evaluation
5 General Evaluation
6 Optimization Examples
7 Summary
Objectives and Topics
This report aims to overcome the limitations of traditional, Reynolds-based methods for comparing the thermohydraulic performance of fin tubes in industrial cooling applications. It introduces a redefined parameter approach specifically designed for cross-flow air coolers and air-cooled condensers, enabling direct performance evaluation independent of complex, varying geometric definitions.
- Development of a new "pseudo Reynolds" flow number based on face area velocity.
- Methodology for evaluating different fin tube systems under identical heat duty and power constraints.
- Impact analysis of pressure drop and flow regime (turbulent vs. laminar) on overall design performance.
- Assessment of boundary conditions, including parasitic pressure losses and noise restrictions.
- Optimization strategies for fin tube heat exchangers to enhance cooling effectiveness and cost-efficiency.
Excerpt from the Book
3 New Form of Fin Tube Characteristics
The proposed form of correlations uses a black box principle (fig. 2). For definition of parameters and variables see the attachment. Face area velocity which is independent from the type of fin tube system is used as defining air speed. The area concentration ratio combines face velocity directly to maximum air speed in the narrowest gap. By switching to face velocity the tube bundle internal geometry is made part of the correlation constant. Also, the hydraulic diameter may be defined with reference to face area and in this way is identical for all fin tube variations. Consequently, the hydraulic diameter can be made part of the correlation constant as well.
Thus, the new pseudo Reynolds flow number is defined as
Ry = (ρ ⋅ u_c ⋅ d_h / η) ⋅ (A / A_c)^-1 (1)
For convenience, the relation to the classical number is given. Heat transfer is also referred to face area. This gives a new heat transfer (pseudo Nusselt) number as
Ny = (h_A / λ) ⋅ (F / A) ⋅ Pr^1/3 = (Nu ⋅ d_h^-1 / Pr^1/3) ⋅ (F / A) (2)
Summary of Chapters
1 Introduction: Provides an overview of cross-flow atmospheric cooling systems and explains why conventional Reynolds-based correlations fail to account for the extreme variety in fin tube geometries.
2 Classical Form of Fin Tube Characteristics: Details standard correlations based on dimensionless numbers and highlights the critical issues regarding the definition of hydraulic diameter in existing methods.
3 New Form of Fin Tube Characteristics: Introduces a modified "black box" approach using face area velocity and new pseudo-numbers (pseudo Reynolds, pseudo Nusselt, and pseudo Euler) to simplify comparisons.
4 Base Evaluation: Explains a simplified comparison procedure for two systems by using direct correlation forms and exponent ratios when design conditions are similar.
5 General Evaluation: Derives an implicit equation for the flow number ratio to compare systems at identical design conditions, heat duty, and pumping power, including considerations for parasitic losses.
6 Optimization Examples: Applies the theoretical model to practical scenarios, comparing multi-row and single-row systems while discussing the impact of turbulence promoters, noise restrictions, and parasitic pressure drops.
7 Summary: Concludes that effective optimization requires considering pressure drop and flow regimes alongside heat transfer, emphasizing that cost-effectiveness is essential for practical engineering solutions.
Keywords
Fin tubes, heat exchangers, thermohydraulic performance, cross-flow, pseudo Reynolds number, pressure drop, heat transfer coefficient, optimization, cooling systems, air-cooled condensers, flow regime, turbulence, face area, pumping power, parasitic loss.
Frequently Asked Questions
What is the primary focus of this work?
The work focuses on developing a robust and practically applicable methodology for comparing the thermohydraulic performance of various fin tube systems used in industrial air-cooled heat exchangers.
What are the central thematic fields?
The central fields include fluid dynamics in cross-flow arrangements, thermohydraulic correlation development, exchanger design optimization, and the assessment of pressure drop versus heat transfer performance.
What is the main research objective?
The goal is to replace complex, geometry-dependent Reynolds-based comparison methods with a universal approach based on face area velocity, allowing for direct comparison of diverse fin tube geometries.
Which scientific methodology is utilized?
The author uses a "black box" modeling approach to redefine dimensionless parameters, resulting in pseudo Reynolds, Nusselt, and Euler numbers that simplify the evaluation of different tube bundles.
What is covered in the main section?
The main section covers the derivation of the new pseudo-numbers, the theoretical framework for evaluating systems under equal power and heat duty, and specific case studies comparing turbulent and laminar flow regimes.
Which keywords best characterize this publication?
Key terms include fin tubes, heat exchangers, thermohydraulic performance, pressure drop, pseudo Reynolds number, and optimization.
How does the proposed method account for "parasitic pressure loss"?
The method incorporates a parasitic pressure loss fraction into the calculation of total air side pressure drop, acknowledging that energy is consumed by air flow regardless of the internal bundle geometry.
Why is the "exponent ratio" significant in this study?
The exponent ratio (kappa) dictates the proportionality between heat transfer and pressure drop; it helps engineers understand how performance changes when air speed or tube geometry is modified.
What role does environmental noise play in the optimization process?
Noise restrictions force designers to reduce air speeds, which in turn requires higher number of transfer units (NTU); the study shows this makes the challenge of maintaining thermohydraulic performance significantly more difficult.
- Quote paper
- Hans Georg Schrey (Author), 2017, Thermohydraulic Comparison of Fin Tubes, Munich, GRIN Verlag, https://www.grin.com/document/414394
