energy and exergy analysis for a steam cycle of north

Journal of Mechanical Engineering Research and Developments

ISSN: 1024-1752

CODEN: JERDFO

Vol. 44, No. 8, pp. 439-456

Published Year 2021

439

Energy and Exergy Analysis for a Steam Cycle of North

Refineries Company (NRC)/Baiji, Iraq

Hameed J. Khalaf †, Waad A. Salih

ffi, *, Aadel A. Alkumait

†

† Mechanical Engineering department, Tikrit University, Iraq.

ffi North Refineries Company (NRC)/ Baiji, Iraq.

* Corresponding author email: [email protected]

ABSTRACT

This study presents an energy and exergy analysis for steam cycle in a petrochemical refinery in North

Refineries Company (NRC)/Baiji, IRAQ. The study starts with setting up a mathematical model based on mass,

energy, and exergy balance equations for each component of the cycle separately, recording the operational data

(pressure, temperature, and mass flowrates) for the cycle at real and rated loading. Calculations were done by

using ‎(MATLAB) platform. The results of energy analysis showed that the total energy losses were 98.89 MW

and 119.85 MW for real and rated loading, respectively. The largest energy dissipation was found in the

condensers which represents 32.01% and 29.79% from the total energy loss at the two rating loads, respectively.

The 1st-law efficiencies for the cycle at real and rated loading were found to be 36.27% and 38.63%,

respectively. The exergy analysis based on the 2nd-law of thermodynamics showed different and interested

results, it explained that the total exergy destruction at real and rated loads are 61.58 MW and 75.13 MW,

respectively, and the largest destruction in exergy occurs in boilers and found to be 45.99% and 45.25% of the

total exergy destruction at real and rated loads, respectively. Turbines, condensers and heat exchangers are

following the boilers in the values of exergy destruction and found to be 15.54%, 13.64% and 5.98%,

respectively, at real load, and 14.53%, 12.7%, and 6.45%, respectively, at rated load. The 2nd-law efficiency for

the two loading rates were 23.65% and 22.6%, respectively.

KEYWORDS

t m n r t on pl nt, P trol um r n ry, En r y, Ex r y, 1st-l w o t rmo yn m s, 2n -l w o

t rmo yn m s, n r y and exergy efficiency.

INTRODUCTION

Steam energy is one of the most important resources in the oil and gas sectors. Steam generation plant which

provides this energy is considered the main source of the energy for refineries. Despite of development of

energy sources around the world such as, renewable energy and nuclear energy, the energy provided by these

plants is still the safest and simplest source of energy. For the above reasons, scientists focus on developing this

technology and doing more researches and updating studies related to these technologies. Based on the

directions of the ministry of oil in Iraq concerning the reducing and recovering the energy losses in the refineries

by reporting a periodic statistics and engineering solutions to the energy sector problems, the current research is

the first steps in solving the energy problems in this refinery since it presents numerical information about the

sites that need more attentions for development, and it expected to give data for best technical, economical, and

environmental improvements in the future.

This powerful tool which used in analyzing the performance of steam thermal plants is the technique of energy

and exergy analysis, gives a significant figure for energy loss and efficiency for each equipment in addition to

the whole cycle. The first-law deals with quantity of energy and asserts that energy cannot be created or

destroyed. This law serves as a necessary tool for adjust values of energy during processes and offers no

Energy and Exergy Analysis for a Steam Cycle of North Refineries Company (NRC)/Baiji, Iraq

440

challenges to engineers. The second-law, however, deals with the quality of energy. More specifically, it is

conserved with the degradation of energy during the process, the entropy generation, and the lost opportunities

to do work, and it offers a plenty range for improvement[1]. Also, the second-law analysis has a wide range of

applications such as prediction of process direction, establishing the equilibrium conditions of processes, and to

indicate the best thermal performance for the components and the cycle[2].

M. A. Rosen [3] utilized the energy and exergy approach to made a comparison between coal-fired and nuclear

power plants, the results showed that for the coal-fired plant, the energy and exergy overall efficiency were 37%

and 36% respectively, and for the nuclear process were 30% and 30% respectively. Many researchers[4]–[8] had

used the energy and exergy analysis to investigate the energy losses and exergy destruction of the steam power

plants, in spite of the major differences (such as combustion fuels, rating loads, environmental conditions) in the

design of these plants associated to theirs studies. But the results showed the same common conclusions related

to the sequence of the energy losses and exergy destruction, which were the condensers followed by the boilers,

are the major of energy losses. Boilers followed by turbines and condensers are the major components of exergy

destructions in these plants.

Other researchers [9]–[11], conducted an exergy analysis to indicate the component with the largest share of

exergy destruction, their results showed that the boilers are the major exergy destruction component in the cycle

followed by the turbines and then by condensers. For instance, M.N.Eke etl [12], studied the performance of

thermal power plant 220 MW through the concepts of energy and exergy analysis by using the design and

operational data of the plant. The results showed that the boiler is contributed 87 % of the total exergy

destruction followed by the three turbines which are forming 9%, and then by the condenser 2% from the total

exergy destructions in the cycle. They also investigate the effect of raising up the inlet pressure to the high-

pressure turbine and conclude that this will cause increasing in the exergy efficiency of this turbine and the

whole cycle. Also, the effect of changing the environment temperature was studied and its results showed no

valuable changes on the exergy efficiency of the steam generator.

Recently, the energy and exergy analysis has been used in wide range of studies, some researchers used it in

estimating the economic analysis of power plants [13], others used it to show the four effects (4E); energy,

exergy, exergoeconomic and environmental on the steam power plants[14]. Since this study is the only study

that analyze the petrochemical refinery steam cycle by the energy and exergy analysis in NCR, the objective

beyond this study is to verify if this type of complicated steam cycle (Refinery steam cycle) is satisfying the

concept of energy losses and exergy destructions, and if it matches the other steam cycles for electricity power

plant in its results. Also, it aims to determine the components that need more attentions if the improving of cycle

efficiency is desired, and study the effect of changing the environment temperature on the exergy destructions

and exergy efficiency for the components and the whole cycle.

DESCRIPTION OF THE STEAM CYCLE

North Refineries Company (NRC) /Baiji was constructed in 1978, it is one of the biggest refinery companies in

Iraq. It is located in Baiji city, in Salahuddin province at the north region of Iraq. The combined design capacity

is 310,000 BPD in total. The final products from these refineries are; Gasoline, Kerosene, Gas oil (Diesel), Fuel

oil, and LPG. It consists of four refineries (Salahuddin I, Salahuddin II, North, and lubrication oil refinery). The

steam generation plant included in this study is one for Salahuddin II refinery and it provides super-heated steam

for steam turbines and for petrochemical processes in this refinery. The schematic diagram, Figure 1, represents

the steam cycle included in this study. The plant contains three water tube boilers, each has an operation

capacity of 67 ton/h of super-heated steam. The boilers fuel is Liquid Petroleum Gas (LPG). The steam pressure

is classified according to its pressure to three types; high-pressure steam, medium-pressure steam and low-

pressure steam. Each type has its applications through the cycle.

The return condensate collected in headers with the make-up demineralization water to substitute the losses of

water mass in the cycle, then it returns to deaerator (Dea). The petrochemical processes (refinery) contain two

waste heat recovery boilers; one is (WHRB1) in hydro-treating distillation unit which used for superheating the

low-pressure steam before entering the atmospheric distillation tower (ADT) and the other is (WHRB2) in


441

reformate unit for generating super-heated steam from boiler feed water. These two waste heat recovery boilers

are designed ‎for petrochemical refining process such as raising up the temperature of ‎hydrocarbons mixture for

treating, and for steam generation.

For the last two purposes, the furnace in each one is designed ‎in a way that ensure the temperature of the furnace

not to exceed the ‎desired temperature (the separation temperature of hydrocarbons mixture), through controlling

it by the steam generation production, i.e., the steam generation process reduces the ambient temperature of

furnace. Mathematically, it means that the heat supplied to these types of furnaces is ‎reduced by a reduction

factors which denoted by (R.F), and their values are ‎inserted in the operational data Table 1 and included in the

calculations of energy and exergy ‎analysis for these equipment‎. Also, one of the uses of the steam is in some

heat exchangers, such as heat exchanger (H.E 5) in Liquid petroleum gas (LPG) unit which represents Amine

regeneration re-boiler (Kettle re-boiler type), it uses the heat rejected from low pressure steam to vaporize the

Amine and separate it by forming a liquid which dropped-out from bottom of the re-boiler and gas. Also, there

is heat exchanger (H.E 4) in the same unit, this heat exchanger is using steam for heating up the Naphtha

(hydrocarbons mixture) by medium pressure steam before entering a heating furnace.

Some petroleum liquids and final products from refineries such as crude oil, reduced-crud and asphalt ‎need to

maintain at a temperature upper the environmental temperature to keep it in a liquid state for ‎handling it by

pumps between the units, so there are tanks with steam coils used for that purpose, ‎these tanks represented by

(FPT) in the diagram. Turbines from (A) to (E) are used for driving forced draft fans, pumps of boiler feed

water, ‎demineralization water, return condensate and cooling water, respectively. High pressure steam

is ‎entering these turbines and leaving as condensate.‎ Turbine (F) is used for driving the hydrogen gas

compressor in reformate unit, it has two exhaust ‎stages for high power producing. The other collection of

turbines from (G) to (N) are used for driving ‎air compressors and pumps for handling Naphtha, crude oil,

Kerosene cut, heavy gas oil cut, light gas ‎oil cut and reduced-crude oil and booster, respectively.

The exhaust steam from these turbines is medium ‎pressure steam. The steam cycle also contains feed water heat

exchangers (H.E1, H.E2, H.E3) for raising up the ‎temperature of the boiler feed water and two condensers, one

is the main condenser (COND1) and ‎the other is the condenser (COND2) for the exhaust condensate from

hydrogen compressor steam ‎turbine. In addition, there are expansion vessels (E.V1, E.V2, E.V3, E.V4), this

equipment works as heat rejection ‎vessels to reject heat from condensate and convert it to liquid before entering

pumps to ensure ‎pr v nt n t bl s rom v t t on m s. T s v s r us n lo t ons t t on’t

have large quantities of steam flowrates and they are far away from the steam generation plant, so they are

necessary to use pumps to return the condensate steam to the main header.

Nomenclature

Terms Description Units

M ss flowr t ton/h

h ‎ Specific enthalpy kJ/kg‎

S p fi ntropy kJ/kg.K‎

ψ Exergy rate MW

Fuel exergy ‎ MW

E loss Energy losses rate MW

Exergy destruction rate MW

‎1st and 2

nd law efficiency ‎ ‎/‎

Power (Turbine/Pump)‎ MW

BPD Barrel Per Day

L.H.V ‎ Lower heating value of fuel ‎ KJ/Kg

R.F Thermal heat reduction factor for waste heat recovery boilers 1 ,

2 ‎ ‎/‎


442

Table 1. Operational Data

Operational conditions Value Unit

Number of boilers in service. 3 Boilers

Number of burners in each boiler. 2 Burners

Number of main condenser (COND1) ‎fans in service. ‎ ‎10 Fans

Number of condenser ( COND2) fans ‎in service ‎1‎ Fan

L.H.V 50050 KJ/kg

Reduction Factor of furnace ‎ (R.F) WHRB1 / WHRB2 0.22 /0.41 /

Fuel mass flowrate Boilers / WHRB1/ WHRB2

1.6/0.08544/2.6 Ton/h

884.7

177 Kg/s

4.186

6.6 MW

/ 25 °C / 1.0132 Bar /

Figure 1. Steam Cycle scheme

Heat rejected from Condenser 1 ‎ MW

Heat rejected from Condenser 2 ‎ MW

Heat transferred from steam to Naphtha by heat exchanger 4 ‎ MW

Heat transferred from steam to Amine by heat exchanger 5 ‎ MW

Subscripts

In, out ‎ Inlet, outlet ‎

f Fuel ‎

‎°‎ reference state conditions ‎

K source conditions ‎


443

METHODOLOGY

Mass, energy, and exergy destruction balance equations for steady state control volume systems with negligible

kinetic and potential change energies can be expressed as below [1]:

∑ ∑ (1)

(2)

∑ ∑ (3)

Where:

Also, the efficiency of 1st law of thermodynamics ( ) of a system and / or system component is ‎defined as the

ratio of the energy output to the energy input to systems / component:

(4)

The exergy analysis for steady state control volume systems based on equation (5) as explained below:

∑(

) ∑ ∑ (5)

Where:

(

)

Also, the 2nd law efficiency is expressed by:

(6)

Table 1, summarizes the energy losses, exergy destructions, 1st –law efficiency, and 2nd- law efficiency

equations for each component in the cycle separately. For the same component groups such as turbines,

condensers and heat exchangers, the energy losses had gathered in one term for each group ( ,

, ). And so had been done for exergy destructions ( , ,

). The operational data for real load such as pressure, temperature and mass flowrates for each individual

equipment in the cycle had been recorded and listed in Table 3, enthalpies and entropies for each stream were

taken from property tables (A-4, 5,6) which available in reference book [1]. The exergy calculated according to

equation of exergy flow with negligible kinetic and potential energies [1]:

( ) ( ) (7)

And it represents the maximum value of ideal (theoretical) work acquirable from the overall thermal system

involving the system and the environment as the system comes through a process from its initial state to

equilibrium condition with the environment (the dead state)[15]. This means that the exergy is an environment-

combination property. Also, calculations were conducted by (MATLAB) platform, which considered a helpful

tool for running the huge calculations.


444

Table 1 : Energy and exergy equations for steam cycle

Co mpo nent Energy balance equation and 1st law

efficiency

Exergy balance equation and 2nd law

efficiency

Boilers ( )

( ) ( ) ( )

( ) ( ) ( )

( )

( )

WHRB1 ( )

( )

( )

(

) ( )

WHRB2 ( )

( )

( )

(

) ( )

Turbines ( )

( )

( )

( )

( )

( )

( )

( )

Pumps ( )

( )

( )

( )


445

Condensers

(COND)

( )

( )

( )

( )

( )

( )

( ) (

)

(

)

( )

( )

(

)

(

)

( )

Expansion

Vessels(E.V)

Heat

Exchangers

(H.E)

( ) (

)

( )

( )

( ) (

)

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( ) (

)

( )

( )

( ) (

)

( )

( )

( ) (

)

( )

( )

( ) (

)

(

)

( )

( ) (

)

(

)

( )

Line Headers

(L.H)


446

Deaerator

(Dea)

( ) ( )

Atmospheric

Distillation

Tower (ADT)

( )

( )

Final

Products

Tanks (FPT)

( )

( )

Miscellaneous

( )

( )

Cycle

.

.

Table 3. Operational Readings for cycle component

Stream

Pressure

Pressure

Temperature

Enthalpy

(

Entropy

(s)

Exergy

( )

Mass

flowrate

Exergy


447

7.

0

1019.0

9

6

211

7

293.0


448

-81

RESULTS AND DISCUSSIONS

The steam generation plant has two common different operating loads, one of them is the real load, which used

in normal operation state of refinery production. On the other hand, when the demand for some petrochemical

products is desired such as Kerosene, Gasoline and LPG through a period of time (days or weeks), the plant

operating mode is converted to the second operating load which is the rated load. The two loads are included in

this study in order to get a comprehensive view on the thermal performance of the steam cycle and to dedicate

t lo t ons o t b st loss s’ lo t ons n n r y loss s n x r y stru t ons. To investigate the effect

of reference temperature on the 2nd

-law cycle efficiency, and since the exergy of streams is related to reference

states as it appears in equation, the study conducted with selecting a range of reference temperatures which

match with Iraqi weather varies from 10 °C in winter to 50 °C in summer. The energy analysis results for real

and rated loads are represented in Table 4.

Table 4. Energy losses at real and rated load

Real Load Rated Load

Fuel mass flowrate

(ton/h) 4.3 5.13

Total energy Input (MW) 150 179.5

Work net (MW) 7 10.76

Co

mp

on

ent

En

erg

y

Lo

sses

(MW

)

Per

cen

t o

f

En

erg

y

Lo

sses

(%

)

1st

-La

w

effi

cien

cy (

%)

En

erg

y

Lo

sses

(MW

)

Per

cen

t o

f

En

erg

y

Lo

sses

(%

)

1st

-La

w

effi

cien

cy (

%)

Boilers 24.36 24.63 81.7 29.88 24.93 81.14

WHRB1 0.136 0.14 88.5 2.24 1.87 35.44

WHRB2 2.4 2.43 83.7 3.4 2.84 80

Turbines 2.05 2.07 70.5 2.07 1.73 73.56

Pumps 0.16 0.16 69 0.32 0.27 56.76

Condensers 31.65 32.01 63.22 35.7 29.79 64

Expansion vessels 4.73 4.78 17.66 5.72 4.77 24.35

Line Headers 3.42 3.46 98.59 4.04 3.37 95.58

Heat exchangers 13.82 13.98 68.25 17.17 14.33 79.48


449

Deaerator 4.38 4.43 77.5 5.04 4.21 75.4

Atmos. distillation tower 10.83 10.95 3.3 12.69 10.59 3.32

Final product tanks 0.15 0.15 95.98 0.18 0.15 95.9

Miscellaneous uses 0.8 0.81 3.6 1.395 1.16 3.66

Total Energy Loss ( Cycle) 98.89 100.00 119.85 100.00

1st -law efficiency (Cycle) 36.27% 38.63%

Figure 2; shows the percentage distribution of energy losses at the real operating load. It is clearly that the

condensers ‎have the largest share of energy losses 31.65 MW, and it forms a percentage of 32% of the total

energy ‎losses in the cycle, because the huge amount of heat energy rejected in the condensers to the ‎air, and low

quality of steam. Boilers also contribute in energy losses from the cycle by an amount of ‎‎24.36 MW forming a

percentage of 24.63% of the total energy loss from the cycle. Also, heat exchangers ‎‎(H.E), atmospheric

distillation tower (ADT) and expansion vessels showed a valuable amounts of ‎energy losses but less than

condensers and boilers, which about 13.82 MW, 10.83 MW, and 4.73 MW, respectively, and ‎forming the

following percentages of 13.98 %, 10.95 %, and 4.78% of the total energy losses respectively. For the rated

operating load, the energy losses results are illustrated in Figure 3, showed the same ‎sequence of the energy

losses at real load. Condensers, boilers followed by the heat exchangers ‎‎(H.E), atmospheric distillation tower

(ADT) and expansion vessels (E.V), which were forming a ‎percentages of 29.79%, 24.93%, 14.33%, 10.59%,

and 4.77%. Respectively. Also, Grassman diagrams Figure 4 and Figure 5 they illustrate the distribution of

energy losses and its percentage at real and rated operation loads for each equipment included in the study.‎‎The

1st-law efficiency for the whole cycle at real load and rated load are 36.27% and 38.63%, ‎respectively. But

t s n s ( n r y n s) on’t v subst nt l m n n n v lu bl ‎results in comparison with

2nd-law efficiency (Exergy efficiency).

Figure 2. Energy losses distribution for real load


450

Figure 3. Energy losses distribution for rated load

Figure 4. Grassman diagram for energy losses distribution at real load


451

Figure 5. Grassman diagram for energy losses distribution at rated load

Exergy destruction results presented in Table 5, for the two operation loads, reveals that at real load and rated

load, the highest exergy destructions are in the boilers, since the boilers are dominant upon all other

irreversibilities in the cycle. They destroy 28.32 MW and 34 MW which represent about 45.99% and 45.25% of

the total exergy destruction at real and rated load, respectively. The fact that boilers have the major of exergy

destruction in the cycle at various loads is due to the nature of the entropy generation (lost chances for doing

work) in it, because of the combustion process in the boilers and the heat transfer between finite temperature

differences.

Table 5. Exergy destruction at real and rated load

Real Load Rated load

Fuel mass flowrate (ton/h) 4.3 5.13

Total exergy Input (MW) 80.7 96.57

Work net (MW) 7 10.76

Co

mp

on

ent

Ex

erg

y

des

tru

ctio

n

(MW

)

Per

cen

t o

f

exer

gy

des

tru

ctio

n

(%)

2n

d-L

aw

effi

cien

cy (

%)

Ex

erg

y

des

tru

ctio

n

(MW

)

Per

cen

t o

f

exer

gy

des

tru

ctio

n

(%)

2n

d-L

aw

effi

cien

cy (

%)

Boilers 28.32 45.99 60.67 34 45.25 60.22

WHRB1 0.14 0.23 76.7 1.23 1.64 30.72

WHRB2 3.13 5.08 61.11 3.84 5.11 58.34

Turbines 9.57 15.54 33.85 10.92 14.53 34.53

Pumps 0.4 0.65 24.55 0.6 0.80 18.74

Condensers 8.4 13.64 48.65 9.54 12.70 49.44

Expansion vessels 1.23 2.00 14.93 1.54 2.05 19.9

Line Headers 2.74 4.45 95.76 3.41 4.54 95.48

Heat exchangers 3.68 5.98 65.14 4.85 6.45 60.96

Deaerator 0.8 1.30 80.76 0.91 1.21 81.08


452

Atmos. distillation tower 2.91 4.73 / 3.88 5.16 /

Final product tanks 0.06 0.10 93.02 0.06 0.08 93

Miscellaneous uses 0.2 0.32 / 0.35 0.47 /

Total exergy destruction 61.58 100.00 75.13 100.00

2nd

-law efficiency (Cycle) 23.65% 22.60%

Barographs in Figure 6 and Figure 7 showed that after boilers and turbines have a considerable value for

exergy ‎destructions, which reached a percentage of 15.54% at real load, and 14.53% at rated load, this is

because the ‎various types of irreversibilities in turbines such as (expansion, friction, and leakages). Also,

condensers formed a ‎percentage of 13.64% and 12.70% from total exergy destruction in the cycle at real and

rated load. Gr ssm n diagrams in Figure 8, Figure 9 also show the comparison in exergy destructions for

each ‎equipment in the cycle ‎for real and rated load, respectively.‎

Figure 6. Exergy destruction distribution for real load

Figure 7. Exergy destruction distribution for rated load


453

Figure 8. Grassman diagram for exergy destruction at real load.

Figure 9. Grassman diagram for exergy destruction at rated load.

The effect of changing environment temperature on the exergy destruction and 2nd

-law efficiency for the main

components at real load are shown in Figure 10 and Figure 11, and for rated load in Figure 12 and Figure 13.‎‎It

can be noticed from these figures that the effect of changing the environment temperature is effectively occurs

in boilers and condensers. As the exergy destructions are increased for boilers but decrease for condensers when

the environment temperature is increased. On the other hand, the rest components in the cycle showed a small

response towards environment temperature changing as illustrated in the last figures. These results are matched

previous studies[6], [7]. The results of the exergy analysis also showed the effect of changing the environment

temperature on the 2nd-law efficiency of the cycle as in Figure 14. It can infer two things, the first is that the

2nd

-law efficiency for the real load is greater than that for rated load. This is true as the loading rate is higher in

case of rated load, in other words the exergy in (exergy expanded- equation (6) is high, and as a result of the

rating load increasing, the entropy generation in each equipment in the cycle will increase, causing increasing in

the total exergy destruction, i.e. the total exergy destruction increases as the entering exergy increase at rated

load, and gives a ‎ratio of total exergy destroyed to the exergy expanded (equation (6)) greater than the ratio in

case of ‎real load.‎ The second is that the 2nd-law efficiency for the cycle at both rating ‎loads is decreasing as the

environment temperature increased. This can be clearly noticed and satisfied ‎particularly in the plant, especially

in hot weather (summer). The performance of plant equipment is ‎decreases as the environment temperature

increase and this mainly happens for the equipment that reject ‎heat (heat-output devices) to dead state ‎such as

condensers, as illustrated graphically in Figure 11, and Figure 13.


454

Figure 10.‎ Effect of environment temperature on exergy destruction for main components at real load‎

Figure 11: Effect of ‎ environment temperature on 2nd-law efficiency for main components at real load

Figure 12: Effect of environment temperature on exergy destruction for ‎main components at rated load


455

Figure 13. Effect of ‎ environment temperature on 2nd-law efficiency for ‎main components at rated load ‎

Figure 14. Effect of environment temperature on 2nd-law efficiency of the cycle

CONCLUSIONS

The conclusions from this study are can be divided into two groups, the first are conclusions that related to

answering if this type of refinery steam cycle is verifying the concept of the energy and exergy analysis, and to

explain the results of this study if matches with the previous studies that deals with the power plant steam

cycles. The steam cycle had showed the applicability of the concepts of energy and exergy analysis and also, it

showed that its results matched with the previous studies that related to other power plants, this means the

recommendations, improvements, and upgrading systems of these studies can be applied to the steam generation

plants for refineries.

The second group, are conclusions that related to the operational results such as:

the condensers are the major source of energy losses in the steam cycle, and the boilers are the major

source of exergy destructions at any operation load.

At the both rating loads, the boilers showed a little increase in exergy destruction, while condensers

showed an obviously fluctuating in the exergy destruction.

The 2nd

-law efficiency for most equipment showed a bit decrease, but for the condensers showed a sharp

lowering as the environment temperature is increased.

The 2nd

-law efficiency for the cycle also affected by increasing the environment temperature, and it

showed a decreasing as the environment temperature is increasing at both rating loads.

Finally, the important note can be concluded through this study, is that for the low 2nd

-law efficiency devices, it

shows that there is a chance for doing an improvement in performance for such devices and should be kept in

mind in rehabilitation of the plant.


456

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