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CASE STUDY
Energy Conservation in Air Cooled Condenser: A Case Study
D. S. Mallick • S. Paul
Received: 30 May 2012 / Accepted: 16 January 2014
� The Institution of Engineers (India) 2014
Abstract Air cooled condensers were first introduced in the
US power industry in the early 1970s, but only during the last
few decades has the number of installations greatly increased,
largely to mitigate the problem of available water supply. Air
may be used as a cooling medium in condensers where, pri-
marily, there is scarcity of water, or where the ambient remains
significantly cold for major parts of the year. Air cooled con-
densers are designed considering the design ambient condi-
tions of summer. During winter months, if the air flow rate over
the heat transfer surfaces is kept constant, it leads to improved
condenser vacuum, and consequently, improved heat rate.
Alternatively, the fans may be run at lower speeds, by using
variable frequency drives (VFD), so as to keep the condenser
vacuum constant, resulting uniform heat rate. This paper
compares the economics between the power saved by the use
of VFD in the condenser fans, keeping constant heat rate
throughout the year, vis-a-vis, the saving in fuel, effected when
the fans are operated at constant speed throughout the year and
thus achieving improved heat rate during colder ambient.
Keywords Air cooled condenser � VFD � Cogeneration �Affinity law
List of Symbols
A Area of finned tubes exposed for heat transfer (m2)
Cf Cost of fuel (USD/kL)
Cpa Specific heat capacity of air (kJ/kg �C)
DBT Dry bulb temperature (�C)
GCV Gross calorific value of fuel (kcal/kg)
hbd Enthalpy of blowdown water from steam drum
(kJ/kg)
hi Enthalpy of water at boiler inlet (kJ/kg)
ho Enthalpy of steam coming out from the boiler (kJ/kg)
LMTD Log mean temperature difference (�C)
Ls Latent heat of condensation of steam (kJ/kg)
M Mass of steam produced in the boiler (T/h)
Msav Mass of fuel saved per year (T/year)
ma Mass flow rate of air over the finned surfaces of
the condenser (T/h)
mbd Mass of water taken as blowdown from the steam
drum (T/h)
mo Fuel consumption (T/h)
ms Mass flow rate of steam in the condenser (T/h)
NPV Net present value (USD)
P Power consumed by the condenser fans (KW)
S Annual monetary savings obtained due to saving
in fuel consumption (USD/year)
Tf Final temperature of exit air from the condenser (�C)
Ti Initial temperature of air (ambient air
temperature) (�C)
Ts Temperature of condensing steam (�C)
U Overall heat transfer co-efficient from steam to
air (kcal/h m2 �C)
Va Volume flow rate of air in the condenser (m3/h)
gb Efficiency of boiler (%)
qa Density of air (kg/m3)
qf Density of fuel (kg/m3)
Introduction
With gradual scarcity of water, use of air as a cooling
medium has greatly increased in the last decade. This has
D. S. Mallick (&) � S. Paul
Development Consultants P Ltd, Kolkata, WB, India
e-mail: [email protected]
123
J. Inst. Eng. India Ser. C
DOI 10.1007/s40032-014-0095-3
resulted in increase in the number of installations of air
cooled condensers in power plants. Air can be conveniently
used as a cooling medium in places where there is scarcity
of water, i.e., where water cooled condensers cannot be
arranged for. It can also be used in places where the
ambient is significantly cold for major parts of the year.
Air cooled condensers utilize large fans to drive ambient
air over the heat transfer surfaces. The heat transfer surfaces
are finned tubes which carry steam. The finned tubes are
arranged in bundles on an ‘A’ frame. The cooler ambient air
extracts heat from the steam and condenses it. The vacuum
produced in the condenser depends upon the air temperature
and air flow rate over the finned tubes. If the fans are oper-
ated at fixed speeds, i.e., the air flow rate is kept constant,
then, during winter months, the vacuum in the condenser
improves. This, in turn, enhances the power generation. In
cases, where there is no provision to export the excess power
to the grid, this results in reduction of steam flow thru the
turbine to match the fixed electrical load.
A similar case has been studied in this paper where the
generator is not synchronized with the grid. In such a case,
power generation, at different ambient temperatures, is
made to remain constant, either by varying the air flow rate
or by varying the steaming rate.
The case discussed in this paper is a 2 9 15.5 MW co-
generation plant in Dammam, Saudi Arabia. Due to scar-
city of water in Dammam, the choice of air cooled con-
densers was automatic. From the monthly design dry bulb
temperature profile of Dhahran, a place close to Dammam
and for which meteorological data is available in ASHRAE
Handbook—Fundamentals [1], it is noted that the design
dry bulb temperature in the month of July is as high as
44.2 �C, whereas, the design dry bulb temperature in the
month of January falls to 23 �C.
Apart from generating power, the plant also produces
140 T/h of process steam.
Since, the plant is not synchronized with the grid, the
power generation cannot exceed 31 MW throughout the
year. One way of realizing it is by controlling the con-
denser vacuum, by varying the air flow rate. In other words,
this can be done by varying the speed of the fans, i.e., by
use of variable frequency drive (VFD) in the fans. Alter-
natively, it can be achieved by keeping fixed air flow rate,
but, by varying the steaming rate, which in turn results in
saving in fuel consumption.
In the present context, a comparative study between the
above two cases has been presented in this paper.
Plant Configuration
The 2 9 15.5 MW Co-generation Power Plant, considered
for the analysis, generates process steam and power for a
paper plant. It has three (two working ? one standby) light
crude oil/natural gas fired boilers, each having the capacity
to generate 118 T/h steam at 83 kg/cm2 (absolute) and
510 �C, two extraction-cum-condensing turbines, each
with a gross output of 15.5 MW, coupled with air-cooled
condensers.
Steam from the boilers will be connected to a high
pressure header. Two exhaust lines from this HP header
delivers steam to each of the 15.5 MW TG set. Steam
parameters at turbine are 83 kg/cm2 (absolute) and
510 �C, ignoring line losses. 75.5 T/h of steam, including
deaerator heating steam, is extracted at 11.5 kg/cm2
(absolute) from each of the two turbines. The extraction
steam from each of the turbine is collected in the medium
pressure process header. From this header, process steam
of 140 T PH at 11.5 kg/cm2 (absolute), 200 �C is trans-
ported to paper machine complex of the associated pro-
cess plant.
The exhaust steam from each turbine is fed to an air
cooled condenser. The condensate is collected in a con-
densate storage tank and is pumped to the deaerator by
2 9 100 % condensate extraction pumps. Three boiler feed
pumps (two working ? one standby) pump the water back
to the boilers. The corresponding heat and mass balance
diagram, for an ambient temperature of 43.5 �C, is shown
in Fig. 1.
Calculation Procedure
The solution of the problem involves analysis of the fol-
lowing two scenarios:
Case 1 Calculation of the monetary savings obtained by
saving power consumption in the condenser fans, by use of
VFD to provide the same vacuum, thus providing gross
power generation and the process heat load constant
throughout the year.
Case 2 Calculation of the monetary savings obtained
by saving fuel consumption by varying the steaming rate
so as to maintain the same gross power generation and
same process heat load throughout the year. In this case,
the fans are assumed to operate at a fixed speed all the
year round.
Basis of Calculation
The calculation has been done based on the following
considerations:
(i) The plant runs throughout the year.
(ii) The monthly average dry bulb temperature of Dam-
mam is considered same as that of Dhahran, which is
J. Inst. Eng. India Ser. C
123
close to Dammam, for which meteorological data is
available.
(iii) Light crude oil was considered as the fuel.
GCV of light crude oil was considered as
10,000 kcal/kg.
(iv) While developing the heat and mass balance dia-
grams, to work out various steaming rates corre-
sponding to various time of the year for Case 2, the
following assumptions were made:
a) Turbine efficiency is 82 %.
b) Condenser is designed for 47 �C ambient and 52
T/h flow total.
c) Required power at the Generator terminal is
31 MW.
d) Required process steam is 140 T/h at
11.5 kg/cm2 (absolute), 200 �C.
e) Pressure drop in piping is ignored.
f) Boiler efficiency is 82 %, based on GCV of fuel.
(v) Cost of light arabian crude is $40/kl.
(vi) Cost of electric power is $0.04 per KWh.
(vii) Bank discount rate is 10 %.
Formulation
Case 1
This case involves the calculation of annual energy con-
sumption by the condenser fans and its monetary equiva-
lent as follows:
(i) Vacuum created in the condenser being constant
throughout the year, Ts and Ls were known. U and A
were obtained from the condenser manufacturer’s data.
Using these values in Eq. (1), LMTD was calculated:
ms � 103 � Ls ¼ 4:18 � U � A � LMTD ð1Þ
The variation in the value of U, with respect to different
ambient conditions, was not available from the manu-
facturer, and hence, constant U has been considered for
the calculations. However, the variation of U being
5–10 % at the most, its effect is not significant and may
not change the overall result.
(ii) Assuming values of Tf, LMTD was also found out
using Eq. (2):
Fig. 1 Heat and mass balance diagram
J. Inst. Eng. India Ser. C
123
LMTD¼ Ts� Tið Þ� Ts� Tfð Þ
lnTs� Tið Þ
Ts� Tfð Þ
� � ð2Þ
(iii) The two LMTDs, thus obtained, were checked to
examine whether they match each other. Trial and
error was conducted, by assuming different values of
Tf, to see that LMTD obtained by (1) and (2) are
close to each other by 0.002 �C.
(iv) With the final value of Tf, obtained after achieving
desired level of convergence, ma was calculated
using Eq. (3).
ma x Cpa x Tf� Tið Þ ¼ ms x Ls ð3Þ
(v) Volume flow rate of air flowing into the condenser
was found out using Eq. (4).
Va ¼ ð1; 000 � maÞ =qa ð4Þ
(vi) Power consumed by the fans at different air flow rate
was then calculated using the affinity law:
P a Vað Þ3 ð5Þ
(vii) The energy consumed by the condenser fans at
different times of the year was calculated.
(viii) The energy consumed by the fans, at rated condition,
was found out from the fan manufacturer’s data.
(ix) The difference between the results of steps (viii) and
(vii) yielded the savings in energy consumed by the
fans at different times of the year, which, when
summed, indicates the annual energy saving.
(x) With the knowledge of the present cost of energy, finance
charge and the expected life of the plant, the present
value (PV) of this saved annual energy was calculated.
(xi) The NPV was calculated by subtracting the capital
expenditure from the result of step (x).
Case 2
The annual savings in the fuel consumption and its mon-
etary equivalent was calculated as follows:
(i) Assuming a value of Ls, Tf was calculated using
Eq. (3).
(ii) The saturation temperature corresponding to the
assumed Ls was taken as the condensing temperature
of the steam, Ts.
(iii) LMTD was then found out using Eq. (2).
(iv) LMTD was also found out using Eq. (1).
The variation in the value of U, with respect to different
steaming rate, was not available from the manufacturer,
and hence, constant U has been considered for the calcu-
lations. However, the variation of U being 5–10 % at the
most, its effect is not significant and may not change the
overall result.
(v) The two LMTDs, thus obtained, were checked to
examine whether they match each other. Trial and
error was conducted, by assuming different values of
Tf, to see that LMTD obtained by (1) and (2) are close
to each other by 0.002 �C.
(vi) The saturation pressure corresponding to Ts was taken
as the condenser pressure.
(vii) The above steps were repeated to calculate the
condenser vacuum for every month.
(viii) For simplicity, the twelve months were clubbed into
four groups and Heat and mass balances were,
subsequently, conducted for every group, taking the
average values of condenser pressures for that group.
The heat and mass balances were conducted consid-
ering 2.5 % blowdown and neglecting any pressure
drop in the pipings.
(ix) Using these heat and mass balances, the consumption
of fuel was calculated using Eq. (6).
mo ¼mbd hbd� hið Þf g þ M ho� hið Þf g
gb=100ð Þ � GCVð6Þ
(x) The savings in the consumption of fuel with respect to
rated ambient condition was calculated over the year.
The price of the fuel saved was found out using Eq. (7).
S ¼ ðMsav=1; 000 � qfÞ � Cf ð7Þ
(xi) With the knowledge of the present cost of fuel,
finance charge and expected life of the plant, the NPV
of this saved fuel was calculated.
Table 1 Case 1 for same heat rate, same power generation, variable
fan speed (with VFD)
Month Monthly
DBT (�C)
Fan power
consumption
(KW)
Energy
consumption
in fan (MWh)
January 23 6.038 4.49
February 25 7.332 4.93
March 29.2 11.564 8.60
April 36.2 30.686 22.09
May 41.4 89.350 66.48
June 43.2 149.304 107.50
July 44.2 210.162 156.36
August 43.2 149.304 111.08
September 41.2 84.924 61.15
October 37.9 41.522 30.89
November 31.2 14.780 10.64
December 26.2 8.292 6.17
590.38
J. Inst. Eng. India Ser. C
123
(xii) Since, the capital expenditure in this case is zero,
the result of step (xii) yielded the NPV of annual savings.
Results
Case I has been tabulated in Table 1. The analysis of Case I
reveal that the annual consumption of energy by the con-
denser fans is 590.38 MWh, with the use of VFD, whereas,
that without VFD is 7,161.83 MWh (Table 3). The
monthly consumption of energy by the air cooled
condenser fans has been tabulated in Table 1. From the
table it is observed that the consumption of energy by the
fans has a large annual variation, ranging from 4.49 MWh,
in January, to 156.4 MWh, during July.
Case II has been tabulated in Table 2. From the analysis of
case II, it is observed that the annual saving in consumption
of fuel, in the two boilers together is about 1,140 tonnes.
Table 3 gives a comparison of the two cases. From
Table 3, it is observed that although the installation of VFD
requires an additional capital expenditure of about 47,254
Table 2 Case 2 for same power generation, same fan discharge rate, variable vacuum, variable steaming rate (i.e., variable fuel consumption)
Paired Months
MonthlyDBT(°C)
Temperatureof
condensing steam(°C)
Exhaust pressure
of turbine(kg/cm2)
Fuel consumption
rate inboilers(T/hr)
Saving infuel
consumptionrate inboilers(T/hr)
Saving in fuel
consumptionin boilers
(T)
December 26.2
24.73 32.58 0.0501 16.00 0.2980 643.68January 23
February 25
March 29.2
35.6 43.57 0.0908 16.18 0.1180 260.54April 36.2
May 41.4
June 43.2
43.53 51.57 0.1360 16.30 0.0000 0.00July 44.2
August 43.2
September 41.2
36.77 44.75 0.0965 16.19 0.1080 235.87October 37.9
November 31.2
1,140.10
Average Three-
MonthlyDBT(°C)
Table 3 Comparison of Case 1 and 2
Units Case 1 Case 2
Annual energy consumption in fans MWh 590.38 7,161.83
Annual savings in fan energy consumption MWh 6,571.44 BASE
Annual savings in fuel Tonnes BASE 1,140.10
Price of the saved energy in fans annually USD 2,66,143.41 BASE
PV of the saved powera USD 24,92,411.74 0.00
Price of the fuel saved annually USD BASE 53,778.11
NPV of the saved fuelb USD BASE 5,03,627.73
Capital investmentc USD 47,254.20 0.00
NPV of net savingsd USD 24,45,157.54 5,03,627.73
a the present value of power saved due to the use of VFD in condenser fansb the net present value of the savings incurred by saving on fuel consumption during power generationc the additional capital investment required for use of VFD in condenser fansd the net present value(s) of the overall monetary savings incurred by the use of VFD in condenser fans and the corresponding savings incurred by saving on fuel
consumption (by controlling steaming rate)
J. Inst. Eng. India Ser. C
123
USD, the NPV with the use of VFD is about 4.85 times the
NPV of annual monetary savings due to reduction in the
consumption of fuel.
Conclusion
The case study, represented here, reveals that although
VFD demands an additional capital investment, it is more
economical to use VFD to control the amount of power
generated for a power plant running in island mode with
constant power and heat demand. The plant highlighted in
this paper is based at Saudi Arabia, where the cost of fuel
oil and power is low. The result remains same for other
geographical areas also, though the extent of attractiveness
of VFD drive may not be the same.
Reference
1. ASHRAE Handbook—Fundamentals 2005
J. Inst. Eng. India Ser. C
123
