Air-cooled condensers (“ACC”) operating in vacuum are widely used at the cold end of contemporary thermal power plants. Proper functioning of the condenser is paramount for power plant efficiency.
To adapt to changing process conditions because of changed ambient air temperature or power station load ACC streets are either taken out of or into service or, module fan speed settings are selected appropriately. Even if all fans are running at same speed local variations of cooling air flow may arise as a consequence of e.g. fan location or local wind impact. The designer must ensure that no negative effect (i.e. no cold spot) may evolve at the steam side of the ACC caused by uneven air side cooling or fan control.
The following note describes a theoretical method to assess the effect of air flow variations on ACC performance and provides simple rules to avoid potentially risky situations. An important role for safe operation plays the size of the secondary condenser. The procedure may be used as a guideline for proper sizing of the secondary condenser with respect to airflow maldistribution.
Table of Contents
1 Introduction
2 ACC Model
3 Boundary Conditions
4 Saturation Line
5 Heat Balance of Primary Section
6 Steam Side Pressure Drop
7 Solution Regions
8 Steam Flow
9 Cold Spot
10 Steam Side
11 General Airside Velocity Profile
12 Fan Speed Control Primary Condenser
13 Fan Speed Effect on Steam Pressure
14 Cold Spot Counteraction
15 Conclusion
Objectives & Research Topics
The work aims to establish a theoretical method for assessing the impact of air-side flow maldistribution on the performance of air-cooled condensers (ACC), with a specific focus on the prevention and management of "cold spot" formation. The research investigates the relationship between cooling air velocity profiles, dephlegmator sizing, and steam-side pressure, providing guidelines for safe operation and optimal design.
- Theoretical modeling of ACC modules under uneven cooling air velocity.
- Identification of parameters leading to "cold spot" development in primary condenser sections.
- Analysis of the influence of fan speed control strategies on condensation efficiency.
- Development of guidelines for proper sizing of secondary condensers to mitigate air-side maldistribution risks.
Excerpt from the Book
1 Introduction
A typical ACC consists of one or more parallel streets with a multitude of heat exchangers arranged in an A-frame geometry at approximately 60° base angle. The condensation is done in two steps with primary and secondary condenser (fig. 1). Heat Exchanger modules are similar in geometry. The set of heat exchangers forming a module is normally served by one fan. Steam supplied via the turbine exhaust duct enters the primary condenser section at top manifold of the exchangers and flows down the tubes to the condensate collection line (“CC line”). A considerable fraction of steam is condensed along the flow path in the primary condenser. Condensate and residual steam are collected in the CC line which conveys the remaining steam to the secondary condenser section (“dephlegmator”) and condensate via the condensate line to the condensate tank. At dephlegmator inlet the remaining steam enters from below and is condensed during up-flow in the exchanger tubes. The secondary condensate is drained back to the CC line by gravity. The dephlegmator outlet connects to the inert evacuation system which keeps the vacuum permanent.
Load adapation of the ACC is normally done by fan speed control. Variable frequency drives are typical for small ACC’s. However, in large ACC units, it is sufficient to use one or two-speed fan motors. In this case, fan speed adjustment leads to stepwise cooling airflow duties resulting, in turn to discrepancies of individual module condensation capacity and different steam exit quality in primary condenser modules. Wind may have a similar effect if ACC modules are not evenly affected. Depending on arrangement individual modules may encounter poor fresh air supply. Therefore, the ACC always faces some degree of airside maldistribution - at least on a low scale.
Summary of Chapters
1 Introduction: Provides an overview of typical ACC arrangements and introduces the challenge of airside maldistribution and its impact on condensation performance.
2 ACC Model: Outlines the theoretical simplification of the ACC layout, focusing on primary condenser modules and assuming cooling capacity dominance over steam momentum.
3 Boundary Conditions: Defines the governing assumptions for the model, including constant total airside flow rate and heat transfer coefficients as a function of air velocity.
4 Saturation Line: Presents linearized relationships between manifold pressure, condensation temperature, and ITD.
5 Heat Balance of Primary Section: Derives the heat balance equations for the primary condenser, accounting for steam velocity changes along the tube.
6 Steam Side Pressure Drop: Modifies existing correlations to calculate pressure drop based on mass fluxes, crucial for modeling cold spot tendencies.
7 Solution Regions: Distinguishes between fully active and partially passive zones to calculate inlet and outlet steam mass fluxes.
8 Steam Flow: Establishes equations for beta and ITD, identifying the conditions under which steam flow contributes to the dephlegmator.
9 Cold Spot: Details the calculation of the active length of passive regions and the relative size of cold spots.
10 Steam Side: Summarizes how to calculate missing variables once the intersection point for cold spot formation is identified.
11 General Airside Velocity Profile: Introduces mathematical representations of non-uniform air velocity profiles to model realistic operating conditions.
12 Fan Speed Control Primary Condenser: Applies the model to assess step-wise air velocity variations caused by different fan speed settings.
13 Fan Speed Effect on Steam Pressure: Investigates the impact of fan control strategies on steam pressure increases and vacuum loss.
14 Cold Spot Counteraction: Proposes operational and design strategies, such as fan switching or over-designing, to mitigate the risks of cold spot development.
15 Conclusion: Summarizes the findings, confirming that the developed model allows for the effective estimation of maldistribution effects during the design phase.
Keywords
Air-cooled Condenser, ACC, Cold Spot, Steam Condenser, Dephlegmator, Maldistribution, Heat Transfer, Cooling Airflow, Fan Speed Control, Vacuum Performance, Primary Condenser, Secondary Condenser, Thermal Power Plant, NTU, Pressure Drop.
Frequently Asked Questions
What is the primary objective of this work?
The work provides a theoretical method to assess the impact of uneven airside cooling flow on the performance of air-cooled condensers, specifically to prevent and manage the formation of cold spots.
What are the central themes discussed?
The core themes include ACC design, airside maldistribution, condensation thermodynamics, dephlegmator sizing, and fan control optimization.
What methodology is used in the study?
The author develops a simplified theoretical model for ACC primary modules that calculates steam mass flux, cold spot fractions, and pressure drops based on air velocity distribution profiles.
What is a "cold spot" in the context of an ACC?
A cold spot is a localized area in the primary condenser where steam condensation ends prematurely, leading to inert gas accumulation, temperature decline, and potential structural damage due to freezing or corrosion.
How do fan speed controls affect ACC performance?
Stepwise fan speed adjustment can create uneven cooling air duties, potentially causing discrepancies in condensation capacity and leading to inefficient operation or cold spot development.
What are the primary factors influencing cold spot development?
Key factors include the design NTU (Number of Transfer Units), the uniformity of the cooling air velocity profile, and the relative size of the dephlegmator.
Why is the dephlegmator size critical?
An appropriately sized dephlegmator is essential to evacuate the primary section and handle residual steam, effectively acting as a safeguard against cold spot formation.
How does this model help with practical design?
The model provides analytical expressions (such as the "minimum dephlegmator fraction") that allow designers to estimate the potential for maldistribution effects and refine equipment sizing at the design stage.
Does the model address wind impact?
The model addresses airside maldistribution caused by wind-induced velocity variations using standard velocity profile approximations, although it does not model hot air recirculation.
What operational advice is given to counteract cold spots?
The author suggests using frequency-controlled fan motors to maintain uniform speed or, if fixed-speed fans are used, implementing an interchanging fan switching sequence to prevent prolonged inert gas accumulation in specific modules.
- Quote paper
- Dipl.-Ing. Hans Georg Schrey (Author), 2017, Effect of Uneven Cooling on Performance of Air-Cooled Condenser, Munich, GRIN Verlag, https://www.grin.com/document/418540
