Effect of Uneven Cooling on Performance of Air-Cooled Condenser


Wissenschaftlicher Aufsatz, 2017
21 Seiten

Leseprobe

Contents

Abstract

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

Glossary

Indices

Bibliography

Abstract

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.

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.

illustration not visible in this excerpt

Fig. 1: General ACC Arrangement

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.

To avoid negative consequences it must be ensured that all primary modules have at least some amount of remaining vapor at tube outlet because otherwise the inert gas carried along with the steam flow will be accumulated at the locus of minimum pressure (i.e. tube end). Locally, this results in a sharp temperature decline on the steam side and, over time, a so-called “cold spot” may develop. As a result, the capacity of the condensing system is reduced. At ambient frost conditions even freezing may occur which endangers the structural stability of the exchangers. Furthermore, cold spots promote tube side corrosion. The thermohydraulic details have been extensively discussed in the literature. All this applies to multi-row ACC’s as well as single row type.

Air side maldistribution effects can only be counteracted by an appropriately sized dephlegmator which evacuates the primary section under all circumstances. However, as the cooling effectiveness of the secondary section is worse than that of the primary sections ACC suppliers tend to design the primary section as large as possible. It is therefore extremely important to understand if the dephlegmator has been properly sized. Dephlegmator sizing recommendations have been published covering the so-called “tube row effect” [1, 2]. The following report outlines a simple theoretical procedure to assess the potential of cold spot development caused by airside maldistribution.

2 ACC Model

For parameter definitions and abbreviations see the attachment.

Fig. 1 shows the general situation with a side view of an ACC street consisting of a multitude of adjacent primary and secondary modules. Steam enters the primary module tubes from the top and is condensed by cross-flowing air. Depen­ding on local cooling airflow the steam exit quality from the primary section varies over the street length. All residual steam is conveyed by the condensate collection line (“CC line”) to the dephleg­mator(s) where the terminal condensation takes place in condensation reflux mode and inert gases are extracted from the top end. The condensate is accumulated in the CC line and connected to the condensate tank for permanent drain.

The current investigation is focused on the primary condenser section. Generally, the distribution of cooling airflow over the street is arbitrary leading to different condensation rates of parallel tubes or exchangers. The larger the cooling airflow duty the higher the condensation rate will be which results in enlarged steam flow into the tubes. As a first simplification we assume that cooling capacity dominates the local steam flow so that steam flow momentum changes in manifold and CC line have practically no effect. This assumption allows a simplified ACC layout where all primary modules are arranged in a consecutive line (fig. 2) – leaving out internal dephlegmator sections.

illustration not visible in this excerpt

Fig. 2: Re-arranged ACC Primary Modules

With neglect of momentum effects in manifold and CC line the pressure difference top to bottom remains the same over the whole street length. If all tubes are cooled at the same equal rate the condensation capacity is constant. With varying cooling air speed each tube performs individually as local condensation duty depends on local cooling airflow. It must be made sure that in no condition the condensation end is reached within any primary module tube so that a cold spot evolves.

Given the simplifications above the air velocity profile may be re-arranged without affecting the total conden­sation duty. The profile starts at the lowest air speed and rises gradually along coordinate ‘ ’ to the maximum (fig. 3). This modification allows to identify a location =𝛽 which divides the range of thermally fully active tubes from partly thermally active ones – the latter tubes characterized by no exit steam flow and cold spot development.

Steam enters each tube with initial speed and leaves it with terminal speed . At cold spot formation the terminal condensation point is located within the tube at distance . Local cooling air velocity is taken as an average over the tube length. The thermally passive region is located between end of active tube length and tube exit.

3 Boundary Conditions

The base ACC calcu­lation model has been described elsewhere (e.g. §5 of the VGB acceptance test stan­dard [3]). We assume that an ACC has been designed for ideal conditions – i.e. an even airflow profile. In the following the ambient air temperature does not change so that airside process and transport properties will not change. To restrict the solution to the maldistribution effect the total airside flowrate will always be kept constant with all flow profiles. These air flow profiles will be considered as imposed boundary condition.

The overall heat transfer coefficient is dominated by cooling air velocity [1, 2]. All adja­cent tubes are modeled as a combined straight channel of constant width along co­ordinate ‘ ’. Momentum effects in manifold and CC line will be neglected so that initial temperature difference (“ITD”), manifold and CC line pressure as well as total steam side pressure drop are constant along co­ordinate ‘ ’.

Generally, we consider only the dry fraction of the approaching steam. Total manifold flow as well as residual steam flow to the dephlegmator and condensation heat are taken constant. Thus, the heat duty of the primary condenser does not change. Cooling capacity is deteriorated as soon as the air velocity profile becomes uneven. This would lead to local increase of residual steam flow which the secondary condenser cannot handle. Therefore, to support condensation capacity the manifold pressure must rise until the nominal condensation duty – which we consider constant for the purpose of this investigation – is reached. Variation of steam pressure andITDwill therefore be variable in the general procedure. Note that this investigation is restricted to the performance of primary condensers alone – possible variations of secondary condensation capacity will be excluded for sake of simplicity.

4 Saturation Line

Manifold pressure and condensation temperature definingITDare related via the steam saturation line. Assuming small relative variations of steam pressure this relation may be linearized to

illustration not visible in this excerpt

In a similar wayITDratio and manifold pressure are related as

illustration not visible in this excerpt

5 Heat Balance of Primary Section

Heat is exchanged along the active tube length. The small loss of saturation temperature caused by pressure changes within the tube will be similar in all adjacent tubes. Therefore, if the air flow profile is constant the exchanged heat duty anywhere in the thermally active region ‘I’ is the same. Assuming all tubes fully active at design conditions (i.e. no passive region) the condensed steam duty may be expressed by the difference of inlet and outlet steam velocity.

Per unit length of the primary section the heat duty is

The relative heat balance of the primary section may be written as

With the definition of a combined airside boundary function

illustration not visible in this excerpt

In this equation the groups are defined as

illustration not visible in this excerpt

For relative exchanger effectiveness the simplification of [3] may be used with

illustration not visible in this excerpt

6 Steam Side Pressure Drop

For pressure drop we use the simplified correlation of [1] – however, modified for mass fluxes instead of velocities. Assuming pre-dominant manifold steam pressure the pressure drop is

where

illustration not visible in this excerpt

The pressure drop ratio at constant CC line pressure is

illustration not visible in this excerpt

7 Solution Regions

Equations (3) and (4) enable to calculate inlet and outlet steam mass fluxes. Two solution regions must be considered - active as well as partly active zone (cf. fig. 3).

illustration not visible in this excerpt

At the intersection point where the following equation holds:

[...]

Ende der Leseprobe aus 21 Seiten

Details

Titel
Effect of Uneven Cooling on Performance of Air-Cooled Condenser
Veranstaltung
Heat transfer engineering
Autor
Jahr
2017
Seiten
21
Katalognummer
V418540
ISBN (eBook)
9783668676169
ISBN (Buch)
9783668676176
Dateigröße
2360 KB
Sprache
Englisch
Schlagworte
Air-cooled condenser, ACC, cold spot, fan speed control, dephlegmator sizing
Arbeit zitieren
Dipl.-Ing. Hans Georg Schrey (Autor), 2017, Effect of Uneven Cooling on Performance of Air-Cooled Condenser, München, GRIN Verlag, https://www.grin.com/document/418540

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