This work is concerned with the effects of fouling on different fin tubes and exchangers cooled by air.
During operation of heat exchangers layers of deposits or corrosive products may be formed and accumulated on heat exchanger surfaces over time. This leads to additional heat transfer resistance and constriction of fluid flow area. In consequence, the exchanged heat duty is badly affected. The loss of heat duty is extreme if local heat transfer coefficients are high at clean conditions. However, maintaining cooling effectiveness is paramount in most applications. As a remedy, surfaces must be regularly cleaned.
Fin tubes are core elements in air cooled exchangers or condensers to transfer heat. Fin tube exchangers are characterized by a multitude of circular, elliptical or channel type core tubes with air-side finning. Generally, the process medium flows on the tube internal side with air as coolant on the external fin side. The report deals with air cooled heat exchangers and condensers under forced or natural draft in dry cooling applications with the focus on the effect of fin side fouling. Water spray injection into the cooling air flow is excluded.
Consequently, the effect of fin side fouling layers will be assessed as well as the consequence for air flowrate and heat duty at different convection types. Special attention is given to the effect of fouling on the performance of dry air cooled condensers. Also, differences of forced, induced or natural draft dry cooling applications will be covered.
Table of Contents
1. Introduction
2. General Fin Tube Geometry
3. Heat Balance
4. Pumping Power
4.1 Bundle Pressure Drop
4.2 Natural Draft Pressure Gain
4.3 Air Inlet Pressure Drop
4.4 Air Outlet Pressure Drop
5. Determination of Air Flow
6. Results Discussion
Objectives and Core Topics
This report investigates the detrimental impact of fin-side fouling on the thermal performance and operational efficiency of dry air-cooled heat exchangers and condensers. It focuses on how fouling-induced flow area constriction and the subsequent reduction in cooling air mass flow affect overall heat duty, especially under varying forced, induced, and natural draft conditions.
- Mechanisms of fin-side fouling and flow area constriction.
- Impact of fouling on air flow rate and heat exchanger effectiveness.
- Performance degradation in air-cooled condensers (ACCs) and its economic implications.
- Comparison of different draft types (forced, induced, natural) regarding fouling sensitivity.
- Development of a calculation procedure for determining the net fouling effect on steady-state heat rejection.
Excerpt from the Book
1. Introduction
Fouling has a detrimental effect on heat exchanger performance. In case of dry air-cooled exchangers specifically two factors are relevant on the fin side:
(a) Increase of heat transfer resistance by formation of a fouling layer on surfaces,
(b) Constriction of air side flow area, loss of cooling air flow.
Generally, fin side heat transfer coefficients are in the range of only 35 to 50 W/m²K whereas tube side coefficients may be 20 to 100 times higher. Large surface extensions are needed to make up for this discrepancy. Compact finning with small inter-fin gaps (in the low mm range) are a typical consequence of surface enlargement.
As for heat transfer resistance - fin side fouling resistance of typically 1/2000 m²K/W or less has only marginal influence on total heat transfer (fig. 1). For typical design values of NTU and fouling it is easy to see that the effect of fouling layer heat transfer resistance alone accounts for much less than 1% over the typical range of fin side heat transfer coefficients and fin surface extension ratio, φ. This is in contrast to the tube side where typically much higher heat transfer coefficients will be severely affected by tube side fouling.
Summary of Chapters
1. Introduction: Outlines the negative impacts of fin-side fouling on dry air-cooled exchangers, specifically focusing on flow area constriction and heat transfer resistance.
2. General Fin Tube Geometry: Defines the common parameters of fin tube systems and establishes the importance of the minimum inter-fin gap as the primary bottleneck for airflow.
3. Heat Balance: Establishes the mathematical framework for the heat duty balance and defines the relationship between exchanger effectiveness, number of transfer units (NTU), and capacity flow ratios.
4. Pumping Power: Analyzes the relationship between fan volume flowrate, air pressure drop, and the impact of natural draft contributions on total system performance.
5. Determination of Air Flow: Derives the calculation methods for relative air flowrate under various convection conditions by implicitly solving the pumping power requirements.
6. Results Discussion: Evaluates the fouling impact on performance through practical examples, illustrating the sensitivity of various cooling systems to fouling layer thickness.
Keywords
Scaling, Fouling, Fin Tubes, Air Cooled Condenser, ACC, Air Cooler, Natural Draft, Vacuum Decay, Heat Duty, Mixed Convection
Frequently Asked Questions
What is the primary focus of this study?
The study examines the effect of fin-side fouling on the performance of dry air-cooled exchangers and condensers, particularly addressing how deposits reduce cooling air flow and overall heat duty.
Which cooling systems are specifically analyzed?
The report focuses on air-cooled heat exchangers and condensers under forced, induced, or natural draft configurations in dry cooling applications.
What is the main objective of the research?
The objective is to quantify the net fouling effect on steady-state heat rejection and to identify the resulting loss of performance, which can lead to profit loss in industrial power plants.
What methodology is used to assess fouling?
The author uses a theoretical derivation and modeling approach, establishing ratios for heat transfer property changes based on flow area restriction and fouling layer thickness.
How is the performance of a fouled system calculated?
Performance is determined by analyzing the pressure drop balance and fan power requirements, leading to an estimation of air flow reduction and its impact on the number of transfer units (NTU).
What are the critical performance indicators mentioned?
Key indicators include the air side mass flow ratio, heat exchanger effectiveness, face area ratio, and the sensitivity of the system to fouling based on its design NTU.
Why are air-cooled condensers (ACCs) particularly sensitive to fouling?
ACCs are highly sensitive because even a small loss in cooling capacity significantly increases vacuum pressure, which directly reduces generated power and overall plant profitability.
How do different draft types differ in their response to fouling?
The study finds that sensitivity to fouling increases with the degree of natural convection; thus, systems with higher natural draft contributions require more frequent cleaning than forced draft systems.
Does the choice of fan arrangement significantly impact fouling resistance?
The analysis demonstrates that the difference between forced and induced fan arrangements regarding fouling impact is negligible.
- Citar trabajo
- Dipl.-Ing. Hans Georg Schrey (Autor), 2018, Effect of Fouling on Performance of Exchangers Cooled by Air. Ramifications for Exchanged Heat and Cooling Effectiveness, Múnich, GRIN Verlag, https://www.grin.com/document/453957
