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International Journal on Research Innovations in Engineering Science and Technology(IJRIEST)

Website:www.ijriest.com Email:[email protected]

Volume2, Issue 5 ,May-2017

311

A Study on Performance of Air Cooled Steam Con-denser

Govind Reghunath

1 and Abin Mathew

2

Department of Automobile Engineering, Amal Jyothi College of Engineering kanjirapally, 686518

[email protected], [email protected]

Abstract - Air cooled steam condenser are introduced in the industry in the late 90s. Due to the growing demand for water ,Both in-

dustrially and domestically as well as considering environment safety, Air cooled steam condenser have gained considerable attention

in past 10 -15 years. This review paper is brief study about the performance of air-cooled steam condenser under different atmospher-

ic condition, and focuses mainly on performance of ACSC in windy condition. The performances of air-cooled condenser are affected

atmospheric temperature as well as outside wind, Unlike the water cooled steam condenser the performance is greatly affected by the

ambient condition.

Performance of ACSC are drastically affected by external wind. Ambient wind leads to a phenomenon of hot air recirculation

in condenser. External wind affects the fan performance thus leads to less volumetric effectiveness of fan, which in turn affects the

efficiency of condenser. Now a day’s windscreen are used to counter act the effect of wind. Thus, it is useful to understand the me-

chanism in order to improve the performance of condenser.

Keywords - cooled steam condensers (ACSC), Hot air recirculation (HAR), Water-cooled steam condenser (WCSC), Logarithmic

mean temperature difference (LMTD), ACC (Air cooled condenser).

I. INTRODUCTION

Condensers are heat exchangers that are used to condense a fluid from its gaseous state to liquid state.Condensers are of two

types air cooled condensers and water cooled condensers .

The former uses ambient temperature as sink where as the latter uses water.Air cooled steam condensers(ACSC) are heat ex-

changers that condenses high pressure steam to water ,Here heat transfer take place between the condenser coil and ambient

air.The condenser also lowers the pressure of steam which in turn help to increase power plant efficiency.

Water cooled steam condenser (WCSC)are heat exchangers that condense steam to water.The heat transfer takes place between

the condenser coil and water.The heated water is taken out and cooled at a spray pond ,which is then recirculated back to Con-

denser.

WCSC have high efficiency and is small in structure.Despite of many advantages WCSC cannot be used in areas scarce with

water. ACSC gained attention due to immense scarcity of water in industrial areas .Thus company are forced to install ACSC

.The less efficiency and high capital cost can be counter effect by longer life ,less operating cost ,less maintenance of ACSC.

II. AIR COOLED STEAM CONDENSER(ACSS)

ACSC are heat exchangers that are used to convert high pressure steam into water .The heat transfer takes place between con-

denser coil and atmospheric air. Air-cooled technology is the main cooling method in a region where is rich of coal and lack of

water.An air- cooled steam condenser (ACSC) in a power plant consists of an array of condenser cells. For each condenser cell,

finned tube bundles are arranged in the form of an A-frame fitted with an axial flow fan below .The steam flows the through

finned tube exchangers in the ACSC, and condenses while the cold ambient air is agitated by the fans to flow through the finned

tubes. The use of air as the cooling medium in an ACSC means that the heat transfer rate is directly influenced by ambient con-

ditions, such as wind, temperature, and atmospheric instabilities[1].The heat transfer performance of the ACSCs often deteri-




312

orates under strong natural wind which can generate flow distortions at the inlet of the axial fans to reduce the cooling air mass

flow rate[1].the ambient air flow causes many negative impact on the performance of ACSC also the wind causes excessive

vibration and stress on the fan blade.This review paper focuses on the impact of wind on the ACSC.

Factors affecting the performance of ACSC are:

a:Ambient environment condition

b:Number of condenser cells in ACSC.

c:Type of fins used in ACSC

d:Type of convection -Natural or forced

e.Local interference

III. EFFECT OF AMBIENT ENVIRONMENT

CONDITION ON ACSC

Ambient environment condition has huge impact on the performance of ACSC .These factors mainly include atmospheric tem-

perature and ambient windy.When the ambient temperature rises or during windy periods when the ACSC effectiveness is re-

duced there will be a rise in steam temperature and a corresponding turbine back pressure rise in order to reject the same amount

of heat.The effect of ambient environmental condition can be easily analysed .

IV. EFFECT OF TEMPERATURE

Consider a simple condenser

Q = UATMN , Where

U = Overall heat transfer coefficient between two fluids .

A = Effective heat transfer area.

TMN = Logarithmic mean temperature difference

from the above equation it is evident that the heat transfer depends upon the ambient temperature condition (Note : here the cold

fluid refers to the ambient air) , thus as the atmospheric temperature increases, LMTD decreases and heat transfer rate decreas-

es hence the efficiency of power plant decreases.




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V. EFFECT OF WIND

Wind has negative impact on the performance of ACSC, according to the study conducted by Kroger and Duvenhage [1],The

ambient wind causes two effects on condenser that are,

• Hot plume air recirculation occurs when a fraction of the hot buoyant outlet air plume is drawn back into the ACSC inlet. This

results in an increase in the effective temperature of the cooling air with a corresponding reduction in heat rejection rate;

• Fan performance is reduced (decrease in air mass flow rate through the system )due to distorted inlet air flow conditions. It

also affects the static inlet pressure of the fan. It is important to note that these two effects are not coupled ,either any one

of the effect is predominant ,it depends upon the geometry of ACSC ,wind or both.wind also causes excessive vibration to the

fan blades.

A. Fan Performance

As discussed earlier fan performance are affected by wind.numerous study have been conducted on fan unit model of ACSC

and these results are integrated to predict the actual performance of ACSC ,Assumption taken for these studies are:

1. an A-frame plenum chamber is simplified as a rectangular box.study conducted by Owen (2010) shows that difference be-

tween these two models is only .72%,there exist a minimum difference in volumetric effectiveness between these two mod-

els ,Therefore simplified model are adopted for studies.Actual Fan unit model of ACSC are shown in the figure (a), but stu-

dies conducted in simplified design as shown in figure(b).

2. Obstacles or appurtenances such as supports, beams, inlet screens, electrical fan drives, and ducting are not individually

modelled in the simplified model of a condenser cell.

3. The mechanical energy losses (or corresponding pressure drop) resulting from the obstacles, heat exchanger, plenum cham-

ber, and outlet losses are taken into account in the heat exchanger model .Numerical modelling are done with FLUENT

software .

fig 1(1)

Fan performance are usually described by stating the volumetric effectiveness

The volumetric effectiveness of a fan is defined as the ratio of the actual air volume flow rate through the fan Vf

divided by the

ideal volume flow through the fan Vf id; i.e., that is the case where air flow inlet conditions to the fan are undisturbed




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volumetric effectiveness = actual volume flow rate

ideal volume flow rate

effect of wind on volumetric effectiveness of fan differ from each other based upon the arrangement of fan .

Based on study conducted by J A van Rooyen and D G Kroger[1] on ACSC(Numerical model as shown in (fig 2(1)).It

represents the arrangement of 30 fans in ACSC ,during windy condition different fan has different volumetric effectiveness .the

result of study conducted are shown in( fig 3).

fig 2 (1)

fig 3(1)




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by analysing the graph it can be concluded that every fan does not have a negative impact of wind in terms of reduced volume-

tric efficiency.The wind may actually improve the volumetric effectiveness of some fan The volume flow rate through fan (4,3)

, fan (5,3), and fan(6,5) also exceeds the ideal. This trend may be ascribed to the relatively high momentum and low distortion

of the air entering these fans.

Even though the volumetric effectiveness of some fans are increased but the net volumetric effectiveness of the entire ACSC

decreases measurably with increasing wind speed.its well documented in the study[1]. fig 4 shows graph between wind speed

and volumetric effectiveness of ACSC.It is evident from the graph that as the wind speed increases the effectiveness decreases.

fig 4(1)

• Fan inlet temperature

Under windy condition fan inlet temperature also increases this is due to the large pressure difference between areas under and

below ACSC (5).

• Other factors Affecting Fan Performance

1.Platform Height

It was found that lowering the platform height resulted in a reduction in flow rate through the fans ,as the flow rate through

the fans is reduced it leads to reduce the effectiveness.(2).

based upon the various studies conducted on effect platform height the following equation is obtained

V/Vid = 0.985 — exp ^( -x)

where;

V/Vid = volumetric effectiveness

X is a dimensionless platform height and defined by

X= (1+45/ nFR) HFP




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(6.35 dF)

dF= Diameter of the fan

HFP =Platform height

nFR = Number of fan rows

based on this equation graph has been plotted between system effectiveness and dimensional platform height (X)

fig 4b(1)

on analysing the graph we can conclude that by increasing the platform height system volumetric effectiveness is increased,This

is due to the reduction in cross section

2.Type fan used also affects the flow rate[2].

Based type of fan used fan flow rate will change, hub ratio ,blade angles of fan also affects the flow rate and hence sys-

tem effectiveness.

3.Fan inlet section also affect the flow rate [2]

Based upon study conducted by Meyer ,it was found that by removing the fan inlet section of the edge fan reduces inlet flow

losses and consequently increased the flow rate through this particular fan .

B .Hot air recirculation

The main factor which influences the heat transfer performance of air cooled condenser is the back-flow or recirculation of hot

air caused by the environmental strong wind. [3]

Hot air Recirculation is defined as the entrainment of a portion of the hot air leaving the ACSC into the inlet air stream drawn

by the fans from the surrounding atmosphere. This results in an average inlet air temperature to the ACSC that is higher than the




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ambient temperature. The rise of temperature of ACSC will lead to increase in turbine back pressure ,simultaneously leads to

reduce power production.

• Mechanism of HAR

The ambient air circumventing the ACSC and hot air rising from the condenser form a complex field around the ACSC, re-

ferring to the interaction and disturbance between the hot air and separated vortex.Under the windless condition, the hot air ex-

hausted from the ACC through the finned tubes rises and diffuses into the upper atmosphere (shown in Fig. 5a). However,

When the intensive wind is blowing along the upwind of the ACSC ,will lead to the formation of two intake vortices as( shown

in fig 5b) as similar to whirlwinds in the downside of platform, also pumping action of the axial flow fans, leading to the ob-

vious pressure difference between the upper and lower surfaces of the ACSC platform. Therefore, the atmospheric pressure at

the bottom of the ACSC platform becomes lower .Under the inertia-force action of local air streams, the rising hot air exhausted

from the ACC returns to fan-inlet of the ACC platform again, which will lead to the increase of fan-inlet temperature near the

edge of the ACSC platform. This phenomenon is known as the hot air recirculation (HAR), which has a significant impact on

the normal operation of the ACSC

(Fig5(a) and 5(b)) :: Schematic of a representative section of a large ACC system depicting the interaction between ambient

cooling air and hot exhaust air.

• Concept of HAR

The unfavourable ambient wind field will lead to the occurrence of the HAR. The HAR results in the reduction of fan perfor-

mance and cooling efficiency of the ACSC. Therefore, the steam-turbine increases so rapidly that it seriously causes turbine

trips. However, the HAR caused by different ambient winds has different impacts on cooling efficiency of the ACSC.The con-

cept of HAR according to Peiqing Liu, Huishen Duan, Wanli Zhao is given by

where, THAR

is the average air temperature of fan-inlet at the bottom of the ACSC platform after the HAR arises; Ta

is the

ambient air temperature ;Tout

is the temperature of the ACC outlet .

Eg let Ta

= 30 °C ,Tout

= 61.7 °C ,if the temperature of fan-inlet increases 1 °C higher than the ambient temperature, the HRR

of the ACSC will be the following value.




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1. Effect of wind direction on HAR

A study on wind direction has been conducted by Peiqing Liu, Huishen Duan , Wanli Zhao .their study was based on three

wind speed against eight different angles of wind direction and obtained the following results.It is shown in the fig 6.

(fig 6 ) : effect of HAR with wind angle

Based on the study it is found out that under different wind speeds, the three curves of the HRR are similar .the most unfa-

vourable wind directions occurs within a = 180

°

± 90 .When the wind direction occur at range of a = 0–90

°

or 270–360

°

, the

HAR is very weak and is almost undetectable. In these cases, the HRR is quite low, and the wind effects on the HAR of the

ACSC system can be neglected. The effect of HAR is maximum when a=90.

2.Effect of wind velocity on HAR

A study on wind velocity has been conducted by Peiqing Liu, Huishen Duan , Wanli Zhao. They have conducted study on

ten different wind speed and plot has been made between HAR % and wind velocity. (fig 7 )shows the plot.




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(fig 7) : variation of HAR with increase in wind speed

It is clearly seen that when the wind speed is less than 25 m/s, the HRR maintains the growing trend with the increase of wind

speed and the peak value of the HRR arises at wind speed of 25m/s. With the increase of the wind speed, the interaction and

disturbance between hot exhausted air and weak flow of leeward side of the boiler house and turbine room are gradually en-

hanced, and in the addition of intensive suction of axial flow fans, a lot of hot exhausted air involves into the fan-inlet of the

edge of the ACSC platform. However, when the wind speed exceeds 25m/s, the increasing wind speed will result in the reduc-

tion of the HAR of the ACSC system. The main reason that causes this reduction trend is that under the strong wind condItions,

the forced convection action of oncoming wind and hot exhausted air are evidently reinforced, therefore most of the heat quanti-

ty are transferred and diffused to the downwind of the ACSC platform. It leads to the decrease of the temperature of fan-inlet of

the ACSC and reduction of the HRR of the ACC system.

C . Other effects of wind on ACSC

1. Steam Turbine Back pressure

Turbine back pressure is the pressure at the outlet of the turbine. Under the windy operating condition, the volume flow rate of

air through the ACSC decreases and the fan inlet average temperature increases. If the turbine exhaust flow remains unchanged,

the air temperature leaving the finned tube exchangers increases to maintain energy conservation, which will cause the steam

turbine back- pressure increase . Due to the increase in steam back pressure the efficiency of turbine decreases.

2. Heat Transfer Effectiveness

Based on the study Xuelei Zhang

and Haiping Chen the following result are coined .Since the wind effect decreases the fan vo-

lumetric effectiveness and increases the fan inlet air temperature, the heat transfer effectiveness in the exchangers will certain-

ly be affected. It is found that the heat transfer effectiveness under windy conditions is lower than the ideal value under windless

conditions

VI. METHODS FOR IMPROVING PERFORMANCE OF ACSC

A. Measures to Minimise HAR effect

1. Increment of the wind - wall height

Increasing the wind wall can reduce the HAR effect significantly.Results of various study shows that HAR effect reduces

linearly with wind wall height, this is because the entrainment of hot air reduces with increase in travel length, consecutively

,the temperature of the hot air entering the inlet reduces.this effect is depicted in the fig 8.




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fig (8) :The variation of HAR with wind wall height(4)

2. Accelerating the rotational speed of the edge fan

Increasing the rotational speed of edge fan can reduce the HAR effect.increasing the rotational speed of the edge fans leads to

the increment of fan flow rate, thus increasing the convective heat exchange between the cooling air and the finned tubes. The

temperature of the hot air exhausted from the ACSC has increased, but the kinetic energy along the vertical direction will in-

crease, which can weaken the entrainment phenomena of flow-field around the ACSC on the hot exhaust air. Finally, the hot air

involved into fan-inlet is to reduce and thus HAR effect. this is depicted in the figure (9).

fig (9) : Variation of HAR with fan speed

B. Using deflector plates

Heat transfer of ACSC can be improved by installing deflecting plates under the air cooled platform to guide the cooling air

flow installing of deflector plates increases the flow rate through the fan.the improved ACSC shows a large heat transfer en-

hancement while comparing with conventional ones under the strong winds.it shows better performance under windy condi-

tions. (fig 10) shows the arrangement of deflector plates




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fig(10):schematic digram of deflector arrangement

C. Increasing the platform height

As discussed earlier by increasing the platform height the flow rate through the fan can be increased it can also counter the ef-

fect during strong winds

D. By using walkway

Walkway are small extension of the platform height .By installing walkway the volumetric effectiveness of the fan is signifi-

cantly improved.This is mainly due reduced cross flow and reduction in the intel distortion occurring at the edge fans.

VII. CONCLUSION

A Study has been conducted on performance of air cooled steam condenser .study was based on ambient environment condi-

tion.In this review paper we explain about the effects of ambient wind on the performance of ACSC.The ambient wind condi-

tion drastically affects the performance of ACSC .Windy condition causes a phenomenon known as HAR which results in re-

duced system effectiveness, It also leads to reduced fan performance and causes excessive stress on fan blades.Another effect

caused by the wind is increased turbine back pressure which leads to lesser efficiency.

Based on the various studies, remedial measures to counter the effect of wind has been discussed.As the need of energy is in-

creasing day by day ,the study of the wind effects on the performance of ACSC is of benefit to the design and operation optimi-

sation of direct cooling systems.It will contribute to the recommendation of measures against the adverse impact of wind on

ACSC.

REFERENCES

[1] J.A. Van Rooyen, D.G. Kroger, Performance trends of an air-cooled steam condenser under windy conditions, Journal of Engineering for Gas Turbines and

Power 130 (2) (2008) 023006.

[2] J.R. Bredell, D.G. Kroger, G.D. Thiart, Numerical investigation of fan performance in a forced draft air-cooled steam condenser, Applied Thermal Engineer-ing 26 (2006) 846e852.

[3] Liu, P. Q., Duan, H. S., and Zhao, W. L. (2009). ―Numerical investigation of hot air recirculation of air-cooled condensers at a large power plant.‖ Applied.

Therm. Eng., 29(10), 1927-34

[4] Xuelei Zhang

and Haiping Chen ―performance Forecast of Air-Cooled Steam Condenser under Windy Conditions ―DOI: 10.1061/(ASCE)EY.1943-

7897.0000274. © 2015 American Society of Civil Engineers.

[5] Gao , X. F., Zhang, C. W., and Wei, J. J. (2010). ―Performance predication of an improved air-cooled steam condenser with deflector under strong wind.‖ Applied Therm. Eng., 30(17-18), 2663

[6] Owen, M. T. F., and Kröger, D. G. (2010). ―The effect of screens on air- cooled steam condenser performance under windy conditions.‖ Appl. Therm. Eng.,

30(16), 2610–2615.

[7] Owen, M. T. F., and Kröger, D. G. (2011). ―An investigation of air-cooled steam condenser performance under windy conditions using computa- tional fluid

dynamics.‖ J. Eng. Gas Turbines Power, 133(6), 604502.

[8] Yang, L. J., Du, X. Z., and Yang, Y. P. (2012a). ―Improvement of thermal performance for air-cooled condensers by using flow guiding device.‖ J. En-hanced Heat Transfer, 19(1), 63–74.




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[9] Yang, L. J., Du, X. Z., and Yang, Y. P. (2010). ―Measures against the adverse impact of natural wind on air-cooled condensers in power plant.‖ Sci. China

Tech. Sci., 53(5), 1320–1327.

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