energy saving in cooling towers by using variable frequency drives

Proceedings of the 2nd

International Conference on Current Trends in Engineering and Management ICCTEM -2014

17 – 19, July 2014, Mysore, Karnataka, India

148

ENERGY SAVING IN COOLING TOWERS BY USING VARIABLE

FREQUENCY DRIVES

Sowmya G1, S.Nagendra prasad

2, N.Kumar

3

1(E & E Department, VVIET, Mysore, Karnataka, India) 2,

3(E & E Department, NIE, Mysore, Karnataka, India)

ABSTRACT

This paper deals with energy savings in cooling towers by using variable frequency drives.

An economic evaluation has been performed to determine the potential annual savings of 25% is

achieved by using adjustable-speed AC drives on cooling tower fans to track water temperature

throughout the year. PWM was found to be the optimum solution for efficiency, harmonic distortion

feedback, and power-factor considerations. The digital drive also offered the control options

necessary to successfully operate with two fans in parallel on one drive. These options included the

capability for bypass starters, automatic transfer, and selectable fault condition responses. In addition

to the energy savings offered by the digital drives, being able to adjust the speed of the fans would

result in smoother plant operations due to constant water temperatures. The adjustable-speed drives

also enable the fans to be soft started.

Closed loop PWM controlled Inverter fed 3 phase Induction Motor model is developed

followed by simulation results using Matlab/Simulink. The control block diagram of the proposed

system is given in this paper.

Keywords: Cooling Tower, Model Simulation, Variable operating conditions, Variable Frequency

Drive.

I. INTRODUCTION

The manufacturing industry can be classified as the process industry and discrete

manufacturing. The process industry transforms the inputs, through various conversion methods, into

a new product with significantly different physical and chemical properties from the unprocessed

substance. It can be further categorized into various market segments including food and beverages,

chemicals, petrochemicals, paper, pesticides, fertilizers, dyes and pigments and drugs and

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &

TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 5, Issue 8, August (2014), pp. 148-160

© IAEME: www.iaeme.com/IJEET.asp Journal Impact Factor (2014): 6.8310 (Calculated by GISI) www.jifactor.com

IJEET

© I A E M E




149

pharmaceuticals. The end products usually act as inputs for the discrete manufacturing industry,

which includes electronic, power equipment, automobiles and a range of consumer goods.

To sustain in the increasingly competitive environment, the process industry needs to reduce

its operational costs without impacting profitability and product quality. This multiproduct industry

is energy intensive and provides a major opportunity to implement energy efficiency solutions.

Through strategic investments in energy efficiency initiatives, the process industry can reap the

benefits of reduced energy consumption- lower energy and operation and maintenance costs. In

addition, these initiatives have a positive impact in terms of reduced dependence on fossil fuels and

lower carbon emissions.

Solutions for achieving energy efficiency

Every plant has a unique configuration for process equipment, input supply pattern, a power

generation and distribution system and grid power supply pattern. Therefore, the energy saving

solution needs to be customized for each facility.

The equipment and systems used in process industries are usually old and inefficient. Since

energy consumption is barely monitored in these industries, the management, in most cases, is not

aware of the energy inefficiency in the systems. Thus, the first step towards adopting an energy

efficiency solution is measuring both electrical and thermal energy consumption through regular

audits. Following this, the organization needs to prepare a road map highlighting future energy

efficiency projects, the requisite investments in these projects, and the payback period.

Process industries can also adopt specific and generic energy efficiency improvement

solutions. The specific solutions are related to process improvement, process integration and control,

combined heat and power methods, load distribution and optimization and process automation.

Generic energy efficiency improvement solutions are related to the replacement, modification

and installation of heaters, furnaces, motors, pipes, lights, turbines, boilers, cooling towers and other

components used in the process industry. These solutions typically do not entail significant

investments. For instance, the installation of variable frequency drives (VFDs) in motors offers

significant energy savings. The operating speed of a motor connected to a VFD is changed by

varying the frequency of motor supply voltage, which allows continuous process speed control.

Another generic low investment measure includes the replacement of metallic blades with

fibre- reinforced plastic (FRP) blades in the cooling towers. Metallic blades are heavy and consume

more power. Replacing them with the lightweight FRP blades helps in reducing power consumption

by 20-40 percent. Similarly, the replacement of shell and tube heat exchangers increases the rate of

heat transfer, leading to thermal energy savings. Further, the replacement of exhaust fans with

natural air draft turbo ventilators reduces the power requirement. The ventilators run on wind while

the exhaust fans use electrical energy to operate the motor.

The other cost effective measures include the replacement of sodium vapour lamps with

compact fluorescent lamps, switching off equipment when not in use, installation of a centralized air

conditioner (AC) in place of separate ACs, and the use of fiberglass sheets on rooftops for better

lighting on plants premises during daytime. In addition, process industries can deploy renewable

energy-based power systems to reduce their dependence on the grid supply. For instance, solar water

heaters can be installed to meet hot water requirements.

II. BACKGROUND INFORMATION

The purpose of a cooling tower is to provide process cooling water within a specified

temperature range, usually between 28-29ºC. Since the cooling tower will operate most of the time at

less than design capability, an economic evaluation was performed to determine the potential annual

savings achieved by using adjustable speed AC drives on the cooling tower fans. The outcome of this

evaluation had to not only justify the increased capital expenditure for the adjustable speed drives,




150

but also had to maintain a very high level of reliability due to the critical nature of the cooling tower.

This paper reviews the key points of the economic evaluation, and then discusses the application of

adjustable speed drives to cooling tower fans.

The cooling Water System

The cooling tower [1],[2] is used to provide process cooling water to heat exchangers for five

separate production plants. Loss of this system would create a process upset in these plants. The

cooling tower had to be designed with the highest degree of reliability possible to insure the integrity

of the cooling water system.

COOLING TOWER PAHARPUR MODEL NO --23204 T--0

SERIAL NO---004--20--0025

GEAR BOX MODLE---22.2

NOZZLE SET----1.

DRIVE SET--------1

DRIVE SOFT SET --1.

6 Q BUSH---8 NOS

HOT WATER BASIN SET---1.

MOTOR DETAILS FRAME NO—200L, H P---20,

KW-15, RPM—1765, DUTY---S1.

PUMP BEARING 6307 --2z- &- 6308--2z.

COOLING TOWER PUMP IMPELLER - KDS 2050

PUMP DETAILS TYPE K D S 2050

SIZE 100X80, IMP DIA 197. 0

R P M 2915

HEAD RANGE--M----30--46.0

CAPACITY RANGE lps ---29--16.

KW/HP----15/20.

COOLING TOWER WATER TREATMENT CHEMICAL

SISIL--CT--104 8 LTRS PER DAY.

SISIL--CT--101 3 LTRS PER DAY

QUENCH OIL HEAT EXCHANGER PLATES TYPE M6 - M D F

GEAR BOX BEARING 34306, 34500 (CUP)

Electrical System

Incoming power at the 66 kV level is provided by means of overhead distribution from two

separate busses in a powerhouse. Each of the two feeders is capable of carrying the entire load

individually. 5 MVA transformer steps down this voltage to a level of 11kV and 5 distribution

transformers step down 11kV to 415 volt level. A secondary three breaker transfer scheme with a

normally open tie breaker is used to switch between the feeders in the event of an outage on one of

them. This system will provide high electrical reliability for the tower to reduce the possibility of a

power interruption due to the loss of one feeder.




151

The Control System

The cooling tower controlled by means of a star/delta starter and the cooling tower fan

operates at the same speed throughout the day irrespective of the weather condition. Cooling tower

fans gain a special advantage from the Variable Speed AC Drive. The cooling tower is outdoors

exposed to sun, wind, rain, sleet, and all kinds of other things. It is expected to dispose of a variable

amount of heat in a variable environment. The amount of forced air cooling required can be easily

controlled, it is not necessary to run the fan at 100% when the load is much lower due to a cool rainy

day. If the installation is in a northern climate, or in the mountains, the Variable Speed AC Drive

provides an easy way to reverse the fan, and dispose of any build up of ice.

III. VARIABLE FREQUENCY DRIVE

variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed

drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-

mechanical drive systems to control AC motor speed and torque by varying motor input frequency

and voltage.

VFDs are used in applications ranging from small appliances to the largest of mine mill

drives and compressors. However, about a third of the world's electrical energy is consumed by

electric motors in fixed-speed centrifugal pump, fan and compressor applications and VFDs' global

market penetration for all applications is still relatively small. This highlights especially significant

energy efficiency improvement opportunities for retrofitted and new VFD installations.

Over the last four decades, power electronics technology has reduced VFD cost and size and

improved performance through advances in semiconductor switching devices, drive topologies,

simulation and control techniques, and control hardware and software.

Although space vector pulse-width modulation (SVPWM) is becoming increasingly popular,]

sinusoidal PWM (SPWM) is the most straightforward method used to vary drives' motor voltage (or

current) and frequency. With SPWM control quasi-sinusoidal, variable-pulse-width output is

constructed from intersections of a saw-toothed carrier frequency signal with a modulating sinusoidal

signal which is variable in operating frequency as well as in voltage (or current).

An embedded microprocessor governs the overall operation of the VFD controller. Basic

programming of the microprocessor is provided as user inaccessible firmware. User programming of

display, variable and function block parameters is provided to control, protect and monitor the VFD,

motor and driven equipment.

Powerflex 400

PowerFlex 400 AC Drives are optimized for control of commercial and industrial fans and

pumps. Built-in features such as purge and damper input provide a cost-effective solution for speed

control in a broad range of variable torque fan and pump applications. An available packaged

PowerFlex 400 Fan and Pump drive provides additional control, power, and enclosure options in

standardized designs for a cost-effective solution for speed control in variable torque fan and pump

applications.

Operator interface The operator interface provides a means for an operator to start and stop the motor and adjust

the operating speed. Additional operator control functions might include reversing, and switching

between manual speed adjustment and automatic control from an external process control signal. The

operator interface often includes an alphanumeric display and/or indication lights and meters to

provide information about the operation of the drive. An operator interface keypad and display unit is

often provided on the front of the VFD controller. The keypad display can often be cable-connected

and mounted a short distance from the VFD controller. Most are also provided with input and output




152

(I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control

signals.

IV. ANALYSIS AND DISCUSSION

On studying the meteorological data of Mysore, it was observed that the variation of weather

conditions between day & night & also seasonal variation was quite substantial. A rough idea about

the same can be made in table below.

Table 1: Meteorological data of Mysore city & temperature distribution in Primary and

Secondary circuit of cooling system

Month Wet Bulb Temp

(ºC)

CTW- Cold water

return temp (ºC)

Secondary Coolant Cold

Water return temp (ºC)

Max Min Max Min Max Min Desired

Temp

Jan 21.5 16.0 22.6 17.4 25.8 17.8 31.0

Feb 20.0 18.5 23.2 19.2 27.0 22.8 31.0

Mar 21.0 17.0 26.8 20.5 27.3 24.0 31.0

Apr 25.5 21.0 30.6 22.2 28.0 24.4 31.0

May 24.5 19.5 24.6 21.8 28.4 25.2 31.0

Jun 24.5 21.5 26.5 22.2 29.7 25.3 31.0

Jul 24.0 21.0 29.3 22.3 29.4 24.3 31.0

Aug 23.5 21.0 21.8 21.2 27.0 23.4 31.0

Sep 22.5 19.5 21.8 21.8 27.6 24.7 31.0

Oct 22.5 20.5 25.5 21.8 28.2 24.3 31.0

Nov 21.0 16.5 24.0 23.0 26.6 25.0 31.0

Dec 21.5 16.0 23.6 19.3 25.8 18.6 31.0

The design cold water inlet temperature in the secondary circuit of the cooling system, to

various systems is approx. between 31-32 ºC and the maintenance of this design temperature was

very important from the point of view of process performance. However, due to the wide fluctuation

of weather conditions & also due to the partial loading of cooling tower, it had virtually become

impossible to maintain optimum cooling tower cold water return temperature i.e. 28-29 ºC and

operating cold water inlet temperature to various system i.e. 31-32 ºC. A rough idea about the extent

of variations that has been taking place can be made from the following table below.

From table 1 & the graph shown, it can be due to the partial loading of the cooling tower and

also due to seasonal fluctuation of weather conditions frequently, it is not able to maintain the




153

optimum cooling tower cold water return temperature. So, the main objective was to take some

measures in order to achieve optimum cooling tower cold water return temperature.

By using VFD, the fan speed & the quantity of air being supplied to the cooling tower can be

varied in the entire range from zero speed to maximum fan speed & from zero air supply to

maximum air supply as per the requirement of the process. By varying the frequency of the supply

power with the help of VDF, the speed of motor can be varied.

For achieving the optimum CTW return temperature, it was necessary to operate the cooling

tower at zero fan speed & at full fan speed in order to see the extent the temperature control possible

with the installation of a VFD. The result of the experiment is described in the Table below.

Table 2: Variation cooling system temp with respect full speed & zero speed of cooling tower

fan

Fan Speed CTW return

temperature(ºC)

Secondary Coolant Cold

Water return temp (ºC)

FULL 22.5 28

ZERO 32 37

So, from table.2, it is clear that the extent temperature control possible, by varying the speed

of the fan full speed to zero speed is approx, 9-10ºC & the same fulfills the requirements.

On the basis of above study, to operate the cooling tower fans at variable speed SPWM drive

were installed. The VFD device has the facility to operate both in fixed drive (FD) mode. So,

depending upon the requirements, the VDF device can be operated at full speed in FD mode or at

variable speed in VD mode between 6Hz to 60Hz to fulfill the temperature requirement.

Fig 1: Graph Showing Monthly Distribution of Coolant Temperature

0

5

10

15

20

25

30

35

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature in ºC Max

Temp

Min Temp

Desired Temp

Months

Temp in

ºC

Monthly distribution of coolant Temperature




154

Table 3: Seasonal variations of fan speed over a period of 1-year and cooling tower return

temperature

Months Period

(Months)

Fan Speed (Hz) CTW

return

Temp.

Secondary ckt.

Inlet Temp.

Day Night Avg ºC

ºC

Mar Apr

May June

4 40-50 35-40 40 26-29 29-31

July Aug

Sep Oct

4 30-35 25-30 35 26-29 29-31

Nov Dec

Jan Feb

4 35-40 20-30 30 26-29 29-31

Considering fluctuation of weather conditions & also the partial loading of cooling

tower(depending on the Production), it is decided to run the Cooling tower fan at the following

frequencies in order to have the CTW return temperature approximately equal to the design

temperature.

So, from the above table, it is concluded that the regulation of cooling tower fan speed and &

air flow using a VFD -device helps in maintaining optimum cooling tower return temperature even in

the circumstances when cooling tower is partially loaded and also weather & climatic condition

fluctuation is substantial.

V. MATLAB/SIMULINK MODEL

SPWM Control of an induction motor

In Sinusoidal PWM three phase reference modulation signals are compared against a

common triangular carrier wave to generate the PWM signals for the three phases. Fig:3 represents

the closed loop speed control of 3 phase IM (represents the fan of the Cooling Tower). To have

closed loop control temperature sensor is used to sense the temperature of the inlet water to the

cooling tower and the feedback is given to VFD to accordingly set the speed (frequency settings) of

the IM to get the constant water outlet from the Cooling Tower.

To get the constant water outlet from the Cooling Tower, model consists of a dynamic lookup

table. The dynamic parameters of the lookup table are Temperature and the corresponding Frequency

at which the 3 phase IM has to run. From the detailed analysis and discussion made in the previous

chapter, lookup table has been derived. The inlet water temperature of the Cooling Tower varies

between 40 ºC to 55 ºC and the corresponding frequency at which the motor has to run to get the

constant water outlet temperature of 28 ºC to 29 ºC has been arrived by running motor at full speed

and zero speed. Table below gives the temperature and the corresponding frequency.

The machine's rotor is short-circuited, and the stator is fed by a PWM inverter, built with

Simulink blocks and interfaced to the Asynchronous Machine block through the Controlled Voltage

Source block.

The inverter uses sinusoidal pulse-width modulation. The base frequency of the sinusoidal

reference wave is set at 50 Hz and the triangular carrier wave's frequency is set at 1980 Hz.




155

The Simulink input of the block is the mechanical torque at the machine's shaft. When the

input is a positive Simulink signal, the asynchronous machine behaves as a motor. When the input is

a negative signal, the asynchronous machine behaves as a generator. Here the 20 HP machine is

connected to a constant load of nominal value (11.9 N.m).

Fig 2: Simulink model of a PWM Controlled Inverter fed 3Phase Induction motor

vab (V)

v+-

vab

-K-

rpm

-K-

pu2radpersec

Discrete,

Ts = 2e-006 s.

powergui

-K-

peak2rms

ir,is (A)

s -+

Vbc

s -+

Vab

sin

x

xdat

ydat

y

Temperature v/s

Frequency

Lookup Table

-C-

Temperature

Te (N.m)

RelayC

RelayB

RelayA

1460

Rated speed

RMS Vab voltage

Product

N (rpm)

rem

Math

Function Look-Up

Table

Fourier

Mag

Phase

Fourier

50

Feedback from

Temperature sensor

DivideDemux

Demux

-C-

Corresponding

Frequency

1/1980

2*pi/3*[ 0,-1,1 ]

11.9

Constant1

29.19

Constant

Clock

m

A

B

C

Tm

20 HP - 400 V

50 Hz - 1460 rpm<Rotor current ir_a (A)>

<Stator current is_a (A)>

<Rotor speed (wm)>

<Electromagnetic torque Te (N*m)>




156

Table 4: Temperature and corresponding frequency of dynamic lookup table

Temperature in

ºC

Frequency in Hz Speed in rpm Outlet water

Temperature in

ºC

40 20 584 28-29 ºC

41 22 642 28-29 ºC

42 24 700 28-29 ºC

43 26 759 28-29 ºC

44 28 817 28-29 ºC

45 30 876 28-29 ºC

46 32 934 28-29 ºC

47 34 992 28-29 ºC

48 36 1050 28-29 ºC

49 38 1110 28-29 ºC

50 40 1168 28-29 ºC

51 42 1225 28-29 ºC

52 44 1284 28-29 ºC

53 46 1342 28-29 ºC

54 48 1400 28-29 ºC

55 50 1459 28-29 ºC

IV. SIMULATION AND EXPERIMENTAL RESULTS

The modeling of a PWM Controlled Inverter fed 3Phase Induction motor is done. The results

of the simulation are as shown below.

Simulation results show better speed response of three phase induction motor (representing

cooling tower fan) with the change in temperature. The model also provides the better torque

response.The various currents, voltage response is obtained from the simulation results are as

follows.

Fig:4 shows the machine's speed going from 0 to 800 rpm at certain temperature and as the

temperature increases the speed increases automatically. Fig:5 shows the electromagnetic torque

developed by the machine for the same temperature change. Because the stator is fed by a PWM

inverter, a noisy torque is observed.




157

Fig 3: Output voltage of the SPWM Controlled Inverter

Fig 4: Speed Response of SPWM Controlled Inverter Fed I.M




158

Fig 5: Torque Response of SPWM Controlled Inverter Fed I.M

Fig 6: Rotor Current of SPWM Controlled Inverter fed I.M

However, this noise is not visible in the speed because it is filtered out by the machine's

inertia, but it can also be seen in the stator and rotor currents, which are observed next.




159

Fig 7: Stator Current of SPWM Controlled Inverter Fed I.M

Fig 6 and 7 shows the Rotor and Stator Line current response coming out from SPWM

controlled Inverter fed I.M for the different temperatures. We observe that initially variations occur

in the line currents and later it reaches the steady state within 0.1s. Initially Induction motor is at

rest, so it draws more current at the beginning.

VI. CONCLUSION

The Third Law of Affinity for fans states the ratio of the horse power for two operating

conditions is equal to the cube of the ratio of the flow rate at those conditions. Since flow rate is

proportional to speed and horse power is proportional to power (kVA), then the power used is

proportional to the cube of the speed. Therefore, reducing the fan speed by one-half requires only

one-eighth of the power.

Using any adjustable speed drive technology to track the wet bulb temperature throughout the

year will result in an annual energy savings of 25% when compared with running the fan at full-

speed all the time. This factor alone justified the additional capital expenditure for the drives.

However, due to the critical nature of the cooling tower other factors had to be weighed. In addition

to the tremendous annual energy savings offered by the drives, other operating criteria dictated the

necessity of using adjustable speed drives. Being able to adjust the speed of the fans would result in

smoother plant operations due to constant water temperatures.

Modeling and simulation of SPWM controlled Inverter fed 3 phase I.M drive has been done

by using MATLAB\SIMULINK. Simulation and experimental results presented are in agreement

with the theoretical analysis.

VII. REFERENCE

[1] The energy-saving benefit and economic evaluation analysis of cooling tower with flue gas

injection by Han, Q. ; Liu, D.Y. ; Chen, F.S. ; Yang, Z.

[2] DIII-D water-cooling system upgrades through modeling and power saving projects by Yip,

H.H. ; Mauzey, P.S. ; Anderson, P.M. ; Le, T. ; Hegstad, T. ; Thomas, A.; Leung, D.




160

[3] Energy Efficient Free Cooling System for Data Centers by Christy Sujatha, D. ;

Abimannan, S.

[4] Experiment Study on Tower Cooling Energy-Saving Technology by Ji Amin ; He Li ;

Yue Zhiqiang ; Jie Li ; Gang Yin ; Zhang Qinggang.

[5] Energy Efficiency Guide for Industry in Asia – www.energyefficiencyasia.org.

[6] Whiller A.A fresh look at the calculation of performance of cooling towers [G]. ASHRAE

Trans, 1976.

[7] Braun J E, Klein S A, Beckman W A, Methodologies for optimal control of chilled water

systems without storage[G].ASHRAE Trans, 1989.

[8] Jaber W. Design o f coo ling tower s by the effectiveness NTU method [C]. ASME Winter

Annual Meeting. Boston, 1989.

energy saving in cooling towers by using variable frequency drives

Technology

energy savings

process industry

energy efficiency initiatives

energy efficiency solutions

cooling tower fans

cooling towers

fans wouldresult

international conference