The Case Study of 5 KHz – 25 KHz Generate Ozone

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AU J.T. 11(1): 42-47 (Jul. 2007)

The Case Study of 5 kHz – 25 kHz High Frequency Adjustment in

Converter Circuit to Generate Ozone Gas

Siseerot Ketkaew

Faculty of Engineering, Ramkhamhaeng UniversityBangkok, Thailand

<[email protected]>

Abstract

This paper presents a case study on changing the switching frequency in a

converter circuit to generate ozone gas using a high-voltage, high-frequency, switching

power supply. This supply uses a flyback converter of 100 VA (Output 3 kV), the

operational frequencies are from 5 kHz to 25 kHz, and the control circuit uses a pulse-

width-modulation (PWM) technique. Under the one-hour test of the ozone generator at

wind velocity of 0.259 x 10-6

m3

/sec, the switching frequency of 5 – 25 kHz can generateozone gas of 99.6 – 120.5 mgO3 /hr.

Keywords: switching frequency, flyback converter, ozone gas, control circuit.

Introduction

Nowadays, the ozone gas is widely

brought to use for living needs such as using

ozone to clean vegetables instead of using

manganese, to kill diseases and to reduce the

quantity of chlorine in water. However, bringing ozone to clean the air has some

drawbacks. For example, if it is used at high

concentration, it can irritate the body. It is

health effective only if its quantity is properly

controlled for a given application. Therefore,

the control of the quantity of ozone gas should

match some required technical specifications.

This is the main reason to study the artificial

generation of ozone gas. This paper presents an

evaluation of the effect of changing the

switching frequency in a converter circuit togenerate a particular ozone quantity. The high-

voltage high-frequency circuit is constructed

by using the principle of switching the ac

power supply. A high-voltage high-frequency

signal is supplied to the load consisting of two-

layer electrodes connected in series to produce

ozone gas. The generation of ozone gas is

based on the principle of spreading molecules

of oxygen. One can produce ozone gas on the

basis of the equation O2

+ O = O3. The ozone

gas can withstand a high voltage level. Also,

the heat affects the quantity of the occurring

ozone gas. Therefore, both the voltage level

and the applied frequency have to be controlled

during the generation of ozone gas (Dalarat et

al. 2004).

The energy range being used to produce

ozone gas from chemical components is from

493 kJ/mol to 762.23 kJ/mol. The adapting unit

has the required energy within the range from

5.583 kWh/m3 to 8.631 kWh/ m3. Since there is

only 21% oxygen in the air, the required

energy should be within the range from

1.17243 kWh/m3 to 1.620 kWh/m3. It is

enough to generate ozone gas in the gap of

two-layer electrodes connected in series.

(Dalarat et al. 2004).

The Process of Generating Ozone Gas

The air comprises mainly of 79%

nitrogen (N2) and 21% oxygen (O2). The rest

are inert gases and steam. The ozone gas is a

gas consisting of 3 oxygen atoms under

unstable status with easy dispersion that

depends on both the environment and the

density of the produced quantity. The

production procedure relies on the process of

generation of a free oxygen atom from an

oxygen molecule in the air. After that the free

oxygen atom is combined together with an

oxygen molecule to obtain ozone gas (O3)

Technical Report 42



AU J.T. 11(1): 42-47 (Jul. 2007)

which is brought to use in both industrial and

health systems. The occurring process of ozone

gas generation forms from two sub-processes –

ionization process and dissociation process.

The ionization, spreading of gas, is based on

the increment of electron avalanche leading toan insulator breakdown where the electric

current flows through the border line of the

insulator. This follows to the occurrence of

heat due to the current flow of the insulator

breakdown which causes the ozone gas to

disintegrate due to its lower energy. Therefore,

the ozone gas production should not result in a

breakdown, i.e., the electron energy from the

electric field should be lower than the

ionization energy, but it should be high enough

to separate the oxygen atoms (Ketkaew 2005).

Co-core Cylinder:

Ozone Tube’s Structure

The Gauss's law for the electric field says

that the electric flux through any closed surface

is proportional to the amount of electric charge

contained within that surface, .

The electric field of a co-core cylinder oflength l and radius r , r

∫ →→

⋅=S

Ad DQ

1 ≤ r ≤ r 2, (see Fig. 1) is

given by (Dalarat et al. 2004):

r l

Qr E

1

2)(

πε

= . (1)

The voltage, V , across between both cylinders

is given by:

1

2ln 22

)(

1

2

1

r

r

l

Q

r

dr

l

Qdr r E V

er

r

r

r

πε πε

=== ∫∫ .

(2)

Therefore:

1

2ln

)(

r

r r

V r E = , r 1 ≤ r ≤ r 2. (3)

The maximum electric field stress occurs on

the inside of cylinder’s surface and is given by:

1

21

1ln

)(

r

r r

V r E E

MAX

== . (4)

Fig. 1. Co-core cylinder: Ozone tube’sstructure.

Fig. 2. Block-diagram of ozone generator.

Block-Diagram of

Ozone Gas Generator

Fig. 2 shows the ozone gas generator,

which is constructed with the use of the high-voltage, high-frequency, switching power

supply. The ac input voltage is 180 V, 50 Hz,needed by the rectifier circuit to produce the dc

voltage of 255 V. The dc voltage signal isdelivered to the inverter controlled by Pulse-

Width Modulation (PWM) in order to obtainthe operational frequencies within the range 5

kHz – 25 kHz. The low voltage signal of theinverter of the primary circuit is amplified to

the 3 kV High-Frequency High-Voltage (HF-

HV) signal of the secondary circuit which isfed into the electrode tube for the generation of

the ozone gas.

Electrode Tube Design and Energy

Use

The principle of ozone tube design relieson an unsmooth electric field for the generation

of the ozone gas quantity. Therefore, a two-

layer electric insulator is chosen for theelectrode design due to the permittivity (ε )

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AU J.T. 11(1): 42-47 (Jul. 2007)

differences of the electric insulator. It is

suitable for the generation of a non-uniform

electric field to have variable but close to ε values of each layer under electric field stress.

As shown in Fig. 3, a two-layer co-core

cylinder for ozone tube design is chosen underthe following conditions (Ketkaew 2005):

- Silica is chosen for the 1st layer electric

insulator due to its effectiveness in generating

ozone gas, where ε 1 = 8, the diameter is 2.9 cm

and the length is 18 cm.- Air is chosen for the 2nd layer electric

insulator, where ε 2 = 1.- Cathode frilled aluminum (for rubbing

pots) in filament coil inside of the silica’selectric insulator is used. The reason is that

aluminum has a high conductivity.- The anode is a stainless steel cylinder,

where the diameter is 3.3 cm and the length is18 cm. Fig. 3. The structure of electrode tube.

Design of HF-HV Switching Power

SupplyCalculation of Electric Field ( E) and

Voltage (V) of Ozone Tube

The high-frequency, high-voltage, (HF-HV) switching power supply of high-ripple

voltage is controlled by IC LM555. Switchingdevices, power MOSFETs IRFP460, are usedin the flyback converter controlled by thePWM strategy from IC LM555 (National

Semiconductor 2006). The switchingfrequencies range from 5 kHz to 25 kHz. The

energy from the converter is transferred

through the HF-HV transformer to produce theHF-HV high-ripple voltage supplying theelectrode tube. The structure and the circuit of

this supply are shown in Figs. 4 and 5.

In Fig.3: r 1 = 1.35 cm, r 2 = 1.45 cm, r 3 =1.59 cm, l = 30 cm. For energies from 5.58

kWh/m3 to

7.73 kWh/m3, if the air is composedof 21% oxygen (O2), the chosen energy rangeis 1.172 – 1.620 kWh/m3 (Ketkaew 2005). As

Air volume = π (r 3 – r 2)2 x l

= π (1.59–1.45)2 x 30 = 1.846 cm

3, (5)

with maximum energy per volume (W max) of1.620 kWh /m3 and minimum energy per

volume (W min) of 1.172 kWh/m3, then:

W max = 1.620 x 103 x 1.846 x 10

-6 = 0.00299 Wh,

W min = 1.172 x 103

x 1.846 x 10-6

= 0.00216 Wh.

E min and E max are obtained from Eq. (6) below:

∫=Vol

dv E W 2

2

1ε , (6)

E min =262.210854.8

00265.02

212

min

××

×=

−Vol

W

ε

= 16.273 kV/cm,

E max =

262.210854.8

00366.02

212

max

××

×=

−

Vol

W

ε

Fig. 4. The structure of HF-HV convertercircuit. = 19.129 kV/cm.

Technical Report 44



AU J.T. 11(1): 42-47 (Jul. 2007)

Fig. 5. High-frequency, high-voltage, (HF-HV) converter circuit.

Experimental Results

Results of Measurements of Signals of the

IC LM555 and the Output Voltage of the

HF-HV Transformer

The switching frequency is 25 kHz. The

results of testing the breakdown voltage of the

electrode tube are shown in Table 1. Theaverage value of the breakdown voltage is

V Breakdown (avg.) = 35.38 / 10 = 3.538 kV.

One should use 3 kV because it is lower than

the breakdown voltage (see Fig. 6). Theexperimental set-up is shown in Figs. 7 and 8.

Table 1. Results of testing the breakdownvoltage of the electrode tube.

Order Breakdown voltage kV

1 3.51

2 3.53

3 3.51

4 3.52

5 3.58

6 3.53

7 3.52

8 3.59

9 3.53

10 3.56

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AU J.T. 11(1): 42-47 (Jul. 2007)

(a) (b)

Fig. 6. (a) VGS and VDS signals of the powerMOSFETs; (b) Output voltage of the HF-HVtransformer at 3 kV.

(a) (b)

Fig. 7. (a) The equipment for testing thebreakdown voltage; (b) The testing of thebreakdown voltage of the electrode tube(Ketkaew 2005).

Fig. 8. HF-HV transformer.

Results of the Used Switching Frequency

Adjustment and the Ozone Gas Quantity

The tests are conducted at 3 kV constantoutput. The experimental results are shown in

Table 2 and the meaning of each parameter isexplained below:

- f (kHz) is the frequency of the converter;

- V in(rms) (V) is the input voltage of the

converter;- I in(rms) (A) is the input current of the

converter;- Pin (W) is the input power of the

converter;

- PF is the power factor of the converter;- V out (kVdc) is the output voltage of the

converter;- Ozone quantity (mgO3/hr) is the ozone

gas generated by the ozone generator.

Conclusion

From the experiments for the study of theeffect of changing the switching frequency, onecan evaluate the process of ozone gas

production. The experimental results in Table 2

demonstrate the relationship between theswitching frequency and the quantity of

generated ozone gas. With the increase of theswitching frequency, increased quantities ofozone gas are generated because the shifting of

the frequency level in the converter circuit hasan effect on the production resonance at the

ozone tube. Therefore, the quantity ofgenerated ozone gas changes accordingly.

Table 2. Results of the used adjustment of the switching frequency and its effect on the ozone gasquantity of the ozone generator (the constant output voltage is set to 3 kV).

f

(kHz)Vin (rms)

(V)Iin (rms)

(A)Pin

(W) PFVout

(kVdc)Ozone quantity

(mgO3 /hr)

5 180 0.63 79.38 0.6 3 99.6

10 180 0.63 79.38 0.6 3 104.2

15 180 0.63 79.38 0.6 3 110.8

20 180 0.63 79.38 0.6 3 116.1

25 180 0.63 79.38 0.6 3 120.5

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AU J.T. 11(1): 42-47 (Jul. 2007)

Fig. 9. The chemical components andequipment being used during the tests.

The ozone gas quantity, which the ozonegas generator produces, can be used widely and

applied for solving the environmental problems.

Fig. 9 shows the quantity of ozone gas

occurring after using chemicals for testing. Thechemicals in this figure include also potassiumiodine (KI). The ozone gas is initially put into

KI. This affects the change of the solutioncolor from transparent to yellow. Starch liquid

pours into this solution that makes the solution

to change its color again. The color will bedark blue. Then, sodium thiosulphate is used in

tritratation in order to change the color from

dark blue to transparent in the end. Thismethod can detect and test the ozone gas

quantity.

Acknowledgements

The author wishes to thank the

Department of Environment, Faculty ofEngineering, Ramkhamhaeng University, forthe support in conducting the experiments. The

assistance of Ms. Munlica Kanjan is also

appreciated.

References

Chryssis, G. 1989. High frequency switching power supply. McGraw Hill, New York, NY, USA.

Dalarat, J.; Sreeuthaiporn, T.; Namkratok, Y.;and Rooptong, W. 2004. A study andconstruction of high voltage high frequency

full bridge inverter for ozonizer. MasterThesis in the Department of ElectricalEngineering, Faculty of Engineering,

Mahanakorn University of Technology,

Bangkok, Thailand.Ketkaew, S. 2002. Air cleaner by using high

voltage electrostatic. Proc. IEEE Int. Conf.

on Power System Technology. Kunming,

China, 13-17 October, pp.1611-1614.Ketkaew, S. 2005. The study of ozone gas

generating technique using high frequency,

high voltage dc switching power supply ofhigh ripple voltage (in Thai). Journal of

King Mongkut’s Institute of Technology LatKrabang 22 (2): 1-6.

Mohan, N.; Undeland, T.M.; and Robbins,W.P. 1989. Power electronics: Converters,

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Engineering, King Mongkut’s Institute of

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Technical Report 47

The Case Study of 5 KHz – 25 KHz Generate Ozone

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