Overall optimization and performance analysis of an air conditioning system by adopting series –series couter flow to central water chillers

공학박사 학위논문

중앙집중 물냉각기에 직렬-직렬 대향류
방식을 채택한 공기조화시스템의 성능
해석과 최적화
Overall Optimization and Performance Analysis of
an Air Conditioning System by adopting Series-Series
Counter-flow to Central Water Chillers

울산대학교 대학원
기계공학과
Nguyen Minh Phu


Doctor of Philosophy

Overall Optimization and Performance Analysis of
an Air Conditioning System by adopting Series-Series
Counter-flow to Central Water Chillers

The Graduate School
of the University of Ulsan
Department of Mechanical Engineering
Nguyen Minh Phu


Overall Optimization and Performance
Analysis of an Air Conditioning System by
adopting Series-Series Counter-flow to
Central Water Chillers
지 도 교 수 이근식

이 논문을 공학박사 학위 논문으로 제출함
2012 년 5 월

울산대학교 대학원
기계공학과
Nguyen Minh Phu


Overall Optimization and Performance Analysis of
an Air Conditioning System by adopting Series-Series
Counter-flow to Central Water Chillers
Supervisor: Prof. Geun Sik Lee

A Dissertation

Submitted to
the Graduate School of the University of Ulsan
In partial Fulfillment of the Requirements
for the Degree of

Doctor of Philosophy

by

Nguyen Minh Phu

Department of Mechanical Engineering
University of Ulsan, Republic of Korea
May, 2012

Nguyen Minh Phu 의 공학박사 학위 논문을 인준함

심 사 위원장

김무현

인

심 사 위 원

이근식

인

심 사 위 원

원성필

인

심 사 위 원

이목인

인

심 사 위 원

김무근

인

울산대학교 대학원
2012 년 5 월


Overall Optimization and Performance Analysis of
an Air Conditioning System by adopting Series-Series
Counter-flow to Central Water Chillers
This certifies that the dissertation
of Nguyen Minh Phu is approved.

Committee Chairman Prof. MOO HYUN KIM

Committee Member

Prof. GEUN SIK LEE

Committee Member

Prof. SUNG PIL WON

Committee Member

Prof. MOK IN LEE

Committee Member

Prof. MOO GEUN KIM

Department of Mechanical Engineering
University of Ulsan, Republic of Korea
May, 2012


ACKNOWLEDGEMENTS
I would like to express my heartfelt gratitude to my advisor, Prof. Dr.-Ing Geun Sik Lee,
who not only gives me priceless guidance in scientific work but also supports me the financial
help for three and half years studying in the University of Ulsan.
I would like to thank professors in the committee for their suggestions and comments
throughout the research. Sincerely thank to University of Ulsan that grants me the scholarship
and opportunity for studying in Korea. I also express my gratitude to professors and staff
members of the Department of Mechanical Engineering for their kind help. Special thank to
Korea and Korean people for their civilization and friendship.
I would like to thank my family members, who encourage and suffer a separation during
the period I am away from home. Specially thank to all members of the Entire Energy
Harmony laboratory who support me a lot in my research work.
I’d like to thank the Union of Vietnamese Students at University of Ulsan for spiritual help
and consulting abroad living experience, and close friends in Vietnam who always look
forward to my returning our motherland.

Ulsan, Republic of Korea. May, 2012
NGUYEN MINH PHU


CONTENTS
Abstract ................................................................................................................................... iii
List of figures .......................................................................................................................... vi

List of tables ............................................................................................................................ ix
Nomenclature ........................................................................................................................... x
Abbreviations ......................................................................................................................... xii
Chapter 1. Introduction ........................................................................................................... 1
1.1 Introduction ................................................................................................................ 1
1.2 Problem statement, necessity, objectives and thesis layout ....................................... 4
Chapter 2. Analysis and design of a water chiller using commercial enhanced tubes ..... 8
2.1 Introduction ................................................................................................................ 8
2.2 Model formulation ..................................................................................................... 9
2.2.1 Heat transfer model ..................................................................................................10
2.2.2 Water pressure drop model ......................................................................................14

2.3 Results and discussion ............................................................................................. 15
2.3.1 Condenser ................................................................................................................16
2.3.2 Evaporator ................................................................................................................22

2.4 Comparison to commercial software ....................................................................... 24
2.5 Summary .................................................................................................................. 29
Chapter 3. Effect of water temperature and water flow rate on wet cooling tower ....... 30
3.1 Introduction .............................................................................................................. 30
3.2 Merkel’s theory and the empirical correlations ....................................................... 31
3.3 Formulation of psychrometric properties of moist air ............................................. 39
3.4 Solution procedure and validation ........................................................................... 41

-i-


3.5 Results and discussion ............................................................................................. 45
3.6 Summary ................................................................................................................. 51
Chapter 4. Experimental and modeling studies of a dehumidifying cooling coil ............ 52

4.1 Introduction .............................................................................................................. 52
4.2 Enthalpy potential method ....................................................................................... 53
4.3 Experimental set-up ................................................................................................. 63
4.4 Uncertainty analysis ................................................................................................. 65
4.5 Results and discussion ............................................................................................. 66
4.6 Summary ................................................................................................................. 76
Chapter 5. Overall optimization and exergy analysis of the air-conditioning system with
series-series counter-flow chillers ........................................................................................ 77
5.1 Exergy model ........................................................................................................... 77
5.2 Description of the central air conditioning system and solution procedure ............ 79
5.3 Results and discussion ............................................................................................ 87
5.3.1 Energy analysis ........................................................................................................87
5.3.2 Exergy analysis ........................................................................................................98

5.4 Summary ................................................................................................................ 100
Chapter 6. Discussion, conclusions, and future work ...................................................... 102
6.1 Discussion and conclusions ................................................................................... 102
6.2 Future work ............................................................................................................ 104
References ............................................................................................................................. 105
List of publications ................................................................................................................ 110
Appendix ............................................................................................................................... 112
I.

Solution procedures .............................................................................................. 112

II. Matlab codes ......................................................................................................... 114

-ii-

Overall Optimization and Performance Analysis of an Air
Conditioning System by adopting Series-Series Counter-flow to
Central Water Chillers
By
Nguyen Minh Phu
Department of Mechanical Engineering
Graduate School, University of Ulsan
ABSTRACT
When central water chillers in air conditioning are arranged in series-series counter-flow
(SSCF), compressor lift of each chiller decreases in comparison with that of water chillers in
parallel. That means that the compressor power of the chillers in SSCF is lower than that of
chillers in parallel. However, the water pressure drop of the SSCF chillers increases, i.e.
increase in power of water pumps, because the water flow rates of SSCF chillers are the flow
rates of the whole system. This disadvantage will be solved by increasing the temperature
difference of water flow through an evaporator and condenser, but the water flow rates will
decrease. Under the conditions of high temperature differences and low water flow rates,
cooling coil and cooling tower may be affected. Hence, in this thesis, we consider overall air
conditioning system including SSCF water chillers, cooling coil and cooling tower. Those
components are modeled and simulated in viewpoints of the first and second laws of
thermodynamics. Model formulations of transfers and pressure drops are performed in the
thesis, namely as follows.
The water chiller model is based on the logarithmic mean temperature difference (LMTD)
method. Refrigerant R134a condenses and evaporates on the tube bundles of condenser and
evaporator of the chiller; whereas, chilled water and condenser water pass inside tubes. The
tubes are commercial enhanced tubes so that heat transfer areas can be diminished and the
problem can be reached reality. Refrigerant pressure drop is neglected. But water pressure

-iii-

drop is considered. Results of the water chiller model are verified by the commercial software
HTRI Xchanger suite. Simulation results show that with larger number of tubes than a
specified one, the length of the tube can be short if a tube of small diameter is used. It avoids
a design of large number of tube and large tube diameter. This is because inner thermal
resistance can be lessened with the tube of small diameter. This conclusion reduces in capital
cost of heat exchanger. Tube diameter must be as low as 10.08 mm when number of tubes is
more than 630 if the condenser capacity is 3412.4 kW with the common design conditions of
cooling water and refrigerant R134a. However, tube of small diameter results in high water
pressure drop. Therefore, an optimization can be carried out by trade-off of capital and
operating costs.
A cooling tower model is derived by using Merkel’s theory in company with the empirical
correlations of transfer and air pressure drop from previous studies. The model optimizes
power of the cooling tower fan. The optimization problem is compared with the previous
study that used the optimization package DICOPT (DIscrete and Continuous OPTimizer). A
prominent result can be deduced. A condition of high temperature of water entering the
cooling tower and low water flow does not bring an effective energy consumption of the fan.
With that condition, the air flow rate can be reduced but the length of cooling tower fill has to
be longer in order to increase the contact time between water of high temperature and air. This
trade-off leads to maximum power consumption of the fan at a certain value of the entering
water temperature. That means the water temperature should be higher or lower than this
value. From this work, the water temperature is 36oC and doesn’t depend on the thermal duty
of the tower. Only at a rather high water temperature, potential of heat and mass transfer can
be strengthened and therefore fan power can be lowered.
The cooling coil model is based on the enthalpy potential method and sustainable
correlations of McQuiston about continuous plate fin under wet and dry environments. An
experimental apparatus is set up to validate the model and carry out further studies. From
analysis, it can be deduced that air pressure drop under fully wet and fully dry conditions are
well predicted by the correlations of McQuiston. However, the correlations have not covered
the prediction in partially wet condition. Hence, a linear correlation between fully wet and

-iv-


fully dry zones is proposed to predict the air pressure drop in partially wet condition and
proposition is confirmed by the experiment.
A combination of the above models to simulate overall air conditioning system is then
conducted in this study. The water temperature entering cooling coil, the water temperature
leaving cooling tower, condenser water flow rate and number of SSCF chillers are assigned as
key parameters. Results can be seen as follows. The COP (Coefficient of performance) of the
entire system can obtain maximum at a certain number of SSCF chillers when SSCF chillers
system is operated with the same operating parameters of conventional parallel chillers
system and when number of SSCF chillers is more than 4, the air conditioning system with
SSCF chillers is ineffective. Trade-offs of power consumptions were pointed out as functions
of the water temperature entering cooling coil, the water temperature leaving cooling tower,
and the condenser water flow rate in order to see that an overall optimization is necessary.
The COP after optimization can be greater up to 26% than that of conventional parallel
chillers and the COP reduces as the number of chiller in SSCF increases. Optimal parameters
corresponding to number of SSCF chillers were shown so that they are can set in reality.
Exergy analysis was performed with the parameters that make the maximum COP at each
number of SSCF chillers. Result shows that the exergy efficiency of SSCF chillers system
(83%) is higher than that of parallel chillers system (77%). Irreversibilities of the components
in SSCF chillers are lower than those of components in parallel chillers. But irreversibilities
of cooling coil and cooling tower in SSCF chillers are slightly higher and lower than those in
parallel chillers, respectively.
Key words: Air conditioning, Overall optimization, Heat exchanger, Series-series counterflow.

-v-


LIST OF FIGURES

Fig. 1.1 Chillers arrangements. a) Parallel, b) Series-series counter-flow. ................................ 2
Fig. 1.2 Trends of water temperatures, condensation temperature and evaporation temperature.
a) Parallel chillers, b) SSCF chillers. ..................................................................................... 3
Fig. 1.3 Conceptualization of reduced lift (Groenke and Schwedler, 2002). ............................ 4
Fig. 1.4 Structure of the present thesis with a central air conditioning system. ........................ 7
Fig. 2.1 Pressure-enthalpy diagram of the considered water chiller. ...................................... 10
Fig. 2.2 Notations used in the finned-tube. ............................................................................. 11
Fig. 2.3 Enhanced boiling surface of turbo-B tube (Browne and Bansal, 1999). .................... 12
Fig. 2.4 Boiling heat transfer coefficient on Turbo-B tube (Wolverine, Inc.) ........................ 14
Fig. 2.5 Tube length vs. the number of tubes and tube passes. ................................................ 18
Fig. 2.6 Reynolds number vs. the number of tubes and tube passes. ....................................... 18
Fig. 2.7 Water pressure drop vs. the number of tubes and tube passes.................................... 19
Fig. 2.8 Allowable tube side ( Pt ) and shell side ( Ps ) pressure drops (Muralikrishna and
Shenoy, 2000). ..................................................................................................................... 19
Fig. 2.9 Tube length vs. the number of tubes and tube inner diameter. ................................... 21
Fig. 2.10 Thermal resistances for four cases. ........................................................................... 21
Fig. 2.11 Reynolds number vs. the number of tubes and tube inner diameter. ........................ 22
Fig. 2.12 Water pressure drop vs. the number of tubes and tube inner diameter..................... 22
Fig. 2.13 Tube length vs. the number of tubes and tube passes. .............................................. 23
Fig. 2.14 Reynolds number vs. the number of tubes and tube passes. ..................................... 24
Fig. 2.15 Water pressure drop vs. the number of tubes and tube passes.................................. 24
Fig. 2.16 An interface of HTRI software. ............................................................................... 26
Fig. 2.17 Graphical results from HTRI software. a) HTRI condenser, b) HTRI evaporator. . 28
Fig. 3.1 A sketch of cooling tower and its notations................................................................ 32
Fig. 3.2 A differential control volume of a counter-flow cooling tower. ................................ 33

-vi-


Fig. 3.3 Equilibrium and operating lines on enthalpy-temperature diagram (Serna-González et

al., 2010). ............................................................................................................................. 35
Fig. 3.4 Algorithm for searching the optimal fan power ......................................................... 42
Fig. 3.5 Comparison about a) TAC, b) Me and c) Kfi to examples in previous study (SernaGonzález et al., 2010). ......................................................................................................... 44
Fig. 3.6 Merkel number against Tw,in for all the considered Q ................................................. 46
Fig. 3.7 Dry air flow rate against Tw,in and Q ........................................................................... 46
Fig. 3.8 Cross sectional area of fill against Tw,in and Q ............................................................ 47
Fig. 3.9 Fill height against Tw,in for all the considered Q ......................................................... 48
Fig. 3.10 Loss coefficient against Tw,in for all the considered Q .............................................. 49
Fig. 3.11 Total pressure drop for all the considered Q ............................................................ 49
Fig. 3.12 Fan power against Tw,in and Q ................................................................................... 50
Fig. 4.1 Circular fin in fully dry (a), fully wet (b), and partially wet (c) surface conditions
(Pirompugd et al., 2009). ..................................................................................................... 53
Fig. 4.2 Tube and wet fin with notations. ............................................................................... 54
Fig. 4.3 Photo and the detailed test coil of the present experimental setup. a) Photo of
experimental setup, b) Geometry of test coil. ...................................................................... 64
Fig. 4.4 Some devices in the apparatus. a) Differential pressure transducers, b) Magnetic flow
meter, c) Stirrer motor and d) Data logger with propeller, and temperature and relative
humidity sensor. .................................................................................................................. 65
Fig. 4.5 Theoretical and experimental water pressure drop ..................................................... 67
Fig. 4.6 Cooling load components with the inlet water temperature. ...................................... 68
Fig. 4.7 The effect of entering water temperature and air face velocity to total heat transfer
rate. ...................................................................................................................................... 68
Fig. 4.8 The effect of entering water temperature and water flow rate on total heat transfer
rate. ...................................................................................................................................... 69
Fig. 4.9 The effect of entering water temperature and air face velocity on air pressure drop. 70
Fig. 4.10 The wet fin with notations. ...................................................................................... 71
Fig. 4.11 Air pressure drop under three conditions of the coil surface. ................................... 74
Fig. 4.12 Coil surface at various inlet water temperatures with va = 4 m/s. ............................ 75

-vii-

Fig. 4.13 Air pressure drop vs. face velocity under fully dry and fully wet conditions. ......... 76
Fig. 5.1 A schematic diagram of an air conditioning system. .................................................. 79
Fig. 5.2 Air conditioning process (a) and its psychrometric chart (b). .................................... 81
Fig. 5.3 Central air conditioning system with conventionally parallel chillers ....................... 82
Fig. 5.4 Central air conditioning system with proposed SSCF chillers. .................................. 83
Fig. 5.5 Solution procedure for the system with SSCF chillers. .............................................. 86
Fig. 5.6 Power consumption of components against number of chillers. a) Full y-axis, b)
Limited y-axis. .................................................................................................................... 88
Fig. 5.7 Variation of the total COP with number of chillers. .................................................. 89
Fig. 5.8 Trends of water temperatures, condensation temperature and evaporation temperature
with 3 chillers in arrangements. a) Parallel chillers (A typical chiller), b) SSCF chillers. . 89
Fig. 5.9 Cooling capacity-dependent configuration with mixed parallel and SSCF
arrangements. ..................................................................................................................... 90
Fig. 5.10 Power consumption of components against the water temperature entering coil. a)
Full y-axis, b) Limited y-axis. ............................................................................................ 91
Fig. 5.11 Variation of the total COP with the water temperature entering coil. ...................... 92
Fig. 5.12 Power consumption of components against the water temperature leaving cooling
tower. a) Full y-axis, b) Limited y-axis. .............................................................................. 93
Fig. 5.13 Variation of the total COP with the water temperature leaving cooling tower. ....... 94
Fig. 5.14 Power consumption of components against the condenser water flow rate. a) Full yaxis, b) Limited y-axis. ....................................................................................................... 95
Fig. 5.15 Variation of the total COP with the condenser water flow rate. .............................. 96
Fig. 5.16 Qualitative illustration of global maximum COP with 2 independent parameters. 97
Fig. 5.17 Various COP against number of chillers and parameters which lead to the optimum
COP. .................................................................................................................................... 98
Fig. 5.18 Exergy efficiency against number of chillers. .......................................................... 99
Fig. 5.19 Irreversibility of components in arrangements. ...................................................... 100
Fig. A1 Solution procedure for investigating cooling coil. ................................................... 112
Fig. A2 Solution procedure for the cooling coil subroutine. ................................................ 113

-viii-


LIST OF TABLES
Table 2.1 Low fin tubes for condenser. Smooth bore. Nfin=26 fpi,  = 0.33mm. .................. 17
Table 2.2 Comparison of this work and HTRI software. ........................................................ 27
Table 3.1 Coefficients in Eqs. (3.10) and (3.11). .................................................................... 37
Table 4.1 Experimental errors. ................................................................................................ 66
Table 5.1 Exergy loss equations for each system equipment. ................................................. 79
Table 5.2 Range and resolution of the independent parameters for searching optimum COP.97

-ix-


NOMENCLATURE
A
b
cp
D
f
g
Gc
h
i
I
ifg
j
k
L

Le
m
Me
nchillers
nt
ntp
p
Pr
Q
r1
Re
s
S
T
Tw1
Tw2
Tw3
Tw4
UA
v
Vw
W
Xl
Xt
yw

: Area (m2)
: Ratio of enthalpy to temperature (kJ/kg.K)
(Defined by Eqs. (4.2), (4.5), (4.16a) and (4.16b))
: Specific heat at constant pressure (kJ/kg.K)

: Diameter (m)
: Friction factor (-)
: Gravity acceleration (m/s2)
: Mass velocity based on the minimum free flow area (kg/m2.s)
: Heat transfer coefficient (kW/m2K)
: Enthalpy (kJ/kg)
: Irreversibility (kW)
: Latent heat of vaporization (kJ/kg)
: The Colburn factor (-)
: Thermal conductivity (W/m.K)
: Length (m)
: Lewis factor (-)
: Mass flow rate (kg/s)
: Merkel number (-)
: Number of chillers (-)
: Total number of tubes (-)
: Number of tube pass (-)
: Pressure (Pa)
: Prandtl number (-)
: Heat transfer rate (kW)
: Outside radius of tube (m)
: Reynolds number (-)
: Entropy (kJ/kg.K)
: Fin spacing (m)
: Temperature (oC or K)
: The water temperature entering cooling coil (oC)
: The water temperature leaving cooling coil (oC)
: The water temperature leaving cooling tower (oC)
: The water temperature entering cooling tower (oC)
: Overall conductance (kW/K)

: Velocity (m/s)
: Volumetric flow rate of water (Lit/min)
: Power (kW)
: Longitudinal tube spacing (m)
: Transverse tube spacing (m)
: Thickness of water film (m)

-x-


Greek symbols

: Difference (-)

: Efficiency (-)










: Dynamic viscosity (Pa.s)
: Density (kg/m3)
: Ratio of minimum free flow to face area (-)
: Fin thickness (m)
: Relative humidity (%)

: Specific exergy (kJ/kg)
: Humidity ratio (kgw/kgda)

Subscripts
00
: Reference state
a
: Air/Airside
av
: Moist air
cf
: Cooling coil fan
Comp. : Compressor
Cond. : Condenser
cwp : Condenser water pump
da
: Dry air
Evap. : Evaporator
ewp : Chilled water pump
f
: Saturated liquid
fi
: Fill
ft
: Fin tip
g
: Saturated vapor
in
: Inlet/Inner
m

: Medium
out
: Outlet/Outer
p
: Pump
R
: Refrigerant
r
: Root
sat
: Saturation
sur
: Surface
t
: Tube
tf
: Cooling tower fan
v
: Vapor
w
: Water/Wet
wb
: Wet bulb
wf
: Water film

-xi-


ABBREVIATIONS

AHU : Air handling unit
COP

: Coefficient of performance

EES

: Engineering equation solver

LMTD : Logarithmic mean temperature difference
SHF

: Sensible heat factor

SSCF : Series-series counter-flow

-xii-


Chapter 1

Introduction

1.1 Introduction
Air conditioning, or more specifically, heating, ventilating, air ventilating, air conditioning,
and refrigeration (HVAC&R), was first systematically developed by Dr. Willis H. Carrier in
the early 1900s. Because it is closely connected with the comfort and health of the people, air
conditioning has become one of the most significant factors in energy consumption. Most
commercial buildings in the United States are air conditioned after World War II (Wang,
2001). In recent years, more attentions are paid on energy savings throughout the world. Due

to development trend of the world and the human demand, air conditioning systems are being
installed popularly. Air conditioning systems consume considerable electric power in
commercial and residential buildings, especially in tropical countries, more than 50% of total
building energy (Lu et al., 2005a). Hence, there are many proposed solutions to save energy
and reduce power demand of the air conditioning systems such as cool thermal storage (Fang
et al., 2009; Erek and Dincer, 2008; Zhao et al., 2008; Habeebullah, 2007; Cliche and Lacroix,
2006; Ismail et al., 2003), absorption refrigeration/heat pump system and refrigeration/heat
pump system driven by gas engine or gas turbine with a combined cold, heat and power
system (Xu and Yang, 2009; Sun, 2008), solar powered air conditioning (Pongtornkulpanich
et al., 2008) or reduction in water consumption of cooling tower (Al-Bassam and Maheshwari,
2011), and so on.
In general, central water chillers in an air conditioning system are arranged in parallel as
shown in Fig. 1.1a. Lifts of chillers, i.e. the difference between the condensing temperature
and the evaporating temperature, are equal as seen in Fig. 1.2a. Chillers can be also built in
series as described in literatures (Wang, 2001 and Petchers, 2003). Fig. 1.1b shows another
series arrangement where chilled water flows through chiller 1 and chiller 2 in turn;

-1-


meanwhile, condenser water flows through chiller 2 and chiller 1 in turn. Hence, it can be
called series-series counter-flow (SSCF) arrangement. In the SSCF arrangement, the lift of
each chiller may be reduced in comparison with parallel chillers because the water
temperature leaving evaporator of the chiller 1 is higher and the water temperature leaving
condenser of the chiller 2 is lower as shown in Fig. 1.2b. Reduction in the lift leads the
reduction of power consumption of compressors of chillers. However, pumping power of
chilled water pumps and condenser water pumps may be raised in SSCF chillers system
because the water flow rate of each condenser and evaporator is the water flow rate of the
whole system and thus water pressure drop increased sharply that the pressure is almost
proportional to the square of flow rate and the power required is almost proportional to the

cube of flow rate. Consequently, optimizations and modifications can be imposed to adapt the
SSCF chillers to the whole system.

Chiller 1
Condenser
Condenser
water
Chilled
water

Evaporator
Chiller 2
Condenser
Evaporator
a)

Chilled
water

Chiller 1
Condenser

Chiller 2
Condenser

Evaporator

Evaporator

Condenser

water

b)

Fig. 1.1 Chillers arrangements. a) Parallel, b) Series-series counter-flow.

-2-


Chiller 1

Condensation temperature
Chiller 1

Condenser water
temperature

Lift

Lift

Lift

Chilled water
temperature

Evaporation temperature
a)

Chiller 2

b)

Chiller 2

Fig. 1.2 Trends of water temperatures, condensation temperature and evaporation temperature.
a) Parallel chillers, b) SSCF chillers.
The optimum performance of an air conditioning system with parallel chillers through set
points can be found in previous studies (Lu et al., 2005a; Lu et al., 2005b; Lu et al., 2004; and
Ma et al., 2008). For SSCF chillers, Groenke and Schwedler (2002) present results of a
system with 2 SSCF chillers where each chiller has two refrigerant circuits. Therefore, there
are four reductions of the lift as shown in Fig. 1.3. The water temperatures entering and
leaving SSCF chillers system were fixed in their study, and two condenser water flow rates
were considered. They prove that a power reduction by 14% is found by using the SSCF
arrangement. Directional analyses of effects of water flow rate and water temperature on air
conditioning system and need of SSCF chillers were reported in study of Eppelheimer (2003).
It was pointed out that high energy efficiency can also be obtained from other the
improvement of components of the system such as cooling tower fan and pumps.

-3-


Downstream chiller

Upstream chiller

o

Water temperature

37.2 C

Lift:
34.4oC

35.1oC
32.9oC
31.2oC

Lift: Lift:
30.2oC 29.9oC

Lift: Lift: Lift: Lift:
28.4oC 27.9oC 27.8oC 27.2oC
10.0oC
o

2.8oC
Chiller
(1 Circuit)

Upstream
circuit

7.3 C

Downstream
circuit

Chiller module

(2 Circuits)

o

5C

Downstream
Upstream circuit
Downstream circuit
circuit

Upstream
circuit

Chiller module
(4 Circuits)

Fig. 1.3 Conceptualization of reduced lift (Groenke and Schwedler, 2002).
To reduce the high water pressure drop of SSCF chillers, the water flow rates should be
reduced by increasing water temperature differences. The increase in water temperature
differences may increase the water temperature entering the cooling tower (Condenser water
side) and decrease the water temperature entering the cooling coil (Chilled water side). When
the water temperature of the cooling tower is higher and the condenser water flow rate is
lower than those in the parallel chillers system, the pumping power of the condenser water
loop may be reduced and the fan power of the cooling tower may be also reduced due to high
potential of heat and mass transfer between the cold air and hot water. For the chilled water
side, the pumping power of the chilled water loop may be reduced and also we need to make
clear the phenomena occurred in the air side of the cooling coil. These judgments should be
figured out clearly when chillers are piped in SSCF.

1.2 Problem statement, necessity, objectives and thesis layout
Energy consumption of air conditioning systems is a remarkable problem of all investors
and managers. Hence, improving strategies should be issued continuously and systematically.
In this study, we deal with water-cooled central air conditioning system by applying SSCF
arrangement for chillers. Model formulation and analysis of main components of the system

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are presented in this thesis. The main components are water chiller, wet cooling tower and
cooling coil. Air conditioning schematic, cooling load and outdoor conditions are given
previously to limit the scope of the thesis. Calculation programs are written by using Matlab
R2008a. All properties of water, moist air and refrigerant are taken from EES software (Klein,
2003). The set-points of air conditioning system such as water temperatures and water flow
rates are assigned as key parameters. Most of design parameters are taken from the ASHRAE
standard and ARI standard.
The objectives of this research are as follows:


Analyzing the effects of structure of shell-and-tube condenser and evaporator on water

pressure drop in order to limit the inherent obstacle of high water pressure drop in SSCF
chillers. Heat transfer tubes, which are commercial enhanced tubes, are used so that heat
transfer areas can be small and the heat exchangers considering a real situation can be
designed. A water chiller is designed as one in the parallel chillers and SSCF chillers in
order to serve the next chapter.


Quantitatively analyze the impact of the high water temperature and the low water

flow rate on the performance of a cooling tower.


Cooling coil is modeled in order to investigate transfer characteristics and air pressure

drop under different humid conditions with respect to water temperature and water flow
rate.


Analyze the effects of the set-points of an air conditioning system and the number of

chillers on the performance of the whole system when water chillers are piped in SSCF.
When the trade-offs of power consumptions have been predicted previously, it is necessary
to carry out an overall optimization.


Analyze exergy to point out the irreversibilities of components in SSCF chillers

system.


Perform the energy and exergy analyses of the air conditioning systems with both

parallel and SSCF chillers and compare the results.


To obtain reliability, model formulations are validated by one way or another.

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Judging from the objectives, the dissertation can be organized as follows and organization
of the thesis for next chapters can be seen in Fig. 1.4.


Chapter 2 presents a heat transfer model in conjunction with the water pressure drop

of condenser and evaporator using commercial enhanced tubes. Advantages and
disadvantages of the size of heat transfer surface are analyzed and discussed in this chapter.


Chapter 3 presents the Merkel theory and the empirical correlations from previous

studies to predict heat and mass transfer and air pressure drop of an induced draft cooling
tower. Impact of high water temperature and low water flow on performance of the cooling
tower is pointed out in this chapter.


Chapter 4 introduces the enthalpy potential method and sustainable correlations of

McQuiston for dehumidifying air coil. The coil is studied theoretically and experimentally
so that air pressure drop under different fin conditions (Fully wet, partially wet, and fully
dry) is emphasized in this chapter.


Chapter 5 presents energy and exergy analyses of SSCF chillers systems in

comparison with those of parallel chillers systems. An optimization of the overall air
conditioning system with SSCF chillers is presented in order to show that how much
energy can be saved when the system is operated at optimum set-points.



Chapter 6 reports results from this study.

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