U.P.B. Sci. Bull., Series C, Vol. 74, Iss. 2, 2012 ISSN 1454-234x OPERATION PRINCIPLE OF CYLINDRICAL GEOMETRY OZONE GENERATORS Ionel COLT1 Se prezintă un principiu de funcţionare al generatoarelor de ozon cu geometrie cilindrică, alimentate de la surse statice de înaltă tensiune alternativă, în regim de rezonanţă în tensiune, având curentul şi tensiunea cvasisinusoidale. În lucrare sunt tratate: analiza funcţionării ozonizorului pe trei intervale de timp dintr- o semiperioadă, cu determinarea expresiilor şi a formelor de undă ale mărimilor electrice specifice fiecărui interval; relaţiile de calcul ale mărimilor electrice specifice ozonizorului; graficele unui exemplu de calcul. The paper presents an operating principle of the cylindrical geometry ozone generators, supplied by alternating high voltage static sources, in voltage resonance regime, with cvasisinusoidal current and voltage. This paper covers: the ozonizer’s operating analysis on three time intervals from a semi-period, determinating the wave forms and expressions of the electric quantities specific to each interval; calculation of the electric quantities specific of the ozonizer ; graphs for an example of calculation. Keywords: ozonizer, operation principle, mathematical model 1. Introduction The ozone (O3) is obtained industrially by creating “still” corona type dischar- ges, using prepared air, dried up to dew point −500 C or oxygen as discharge environment. The corona discharge is carried out in plane or cylindrical geometry ozone generators, the last being the most used. 1.1. Cylindrical geometry ozone generators. Fig.1 schematically represents the longitudinal sections of the industrial variants of cylindrical geometry discharge tubes used by different producers. Variant a): used on a large scale, it has two concentric tubes, the external one made from stainless steel connected to the earth terminal and the internal one made from glass (ceramics), centered with spacers, and connected to the high voltage terminal of the source through the metallic layer on the internal side of the tube. The electrical gas (air or O2) discharge that produces the ozone takes place in the circulary space between the tubes, the radius between the tubes is of 1-3 mm. The stainless steel tube is cooled with water. 1 Eng., PhD, S.C. ICPE SAERP S.A. of Bucharest, Romania, e-mail: [email protected] 342 Ionel Colt Variant b): the discharge tube is made of a glass (ceramic) cylinder over which is coated with a metallic (copper/silver) layer – the high voltage electrode and an insulating layer (enamel/polymer) which is centered inside a metallic recipient tube connected to the earth terminal. Inside the discharge tube the second metallic earth link electrode is centered, so that the corona discharge is produced on both sides – double discharge. Both tubes are earth linked and cooled with water. The ozone generator is also called an ozonizer. a) b) Fig. 1. Schematic representation of industrial discharge tubes variants used in cylindric geometry ozone generators 1.2. Recent status of the subject. There are a number of works worldwide concerning the simulation and the improvement of ozone generating systems. Paper [1] highlights the liniar (RC) and non-linear (RCVz) models of industrial ozone generators. This paper also presents theoretical results of the variation simulation of the converter’s voltage frequency on the voltage from the ozonizer, obtaining a maximum at voltage resonance (f=f0). Paper [2] presents the functioning of the ozone generator using the inverter’s switching at zero voltage with period modulation (PWM) adjusting the ozone production through the variation of the inverter’s frequency by maintaining the voltage on the ozonizer constant. The controlled and still corona discharge can be obtained only by using a glass (ceramic) dielectric barrier that prevents the discharge degeneration in a distructive electric arc. These aspects of the microscopic corona discharge and ozone forming reactions are presented in paper [3]. 2. The operation analysis of the ozonizer with alternating high voltage In this paper the author analyzes a non-linear model of the cylindrical geometry ozone generator, the voltage u1 (t) being alternative, rectangular (easy to Operation principle of cylindrical geometry ozone generators 343 obtain in practice) (fig.2), with adjustable amplitude U1 and auto-adjustable frequency f with zero current switching (ZCS) - voltage resonance regime (fig. 4). Fig. 2. Schematic representation of the high voltage source connected to the ozonizer 2.1. The physical-electrical parameters of the cylindrical geometry ozonizer; parallel n-tube ozonizer. This paper analyzes the functioning of the ozonizer tube with simple discharge. The glass tube from an electrical point of view is a cylindric capacitor of capacity cd [4]. Similarly, the air space between surfaces(2) and (3) is also a capacitor with air (void) as a dielectric εra=1, of capacity ca: 2πε ε l 2πε l 10 − 9 c = 0 rd , c = 0 ; ε = [ F / m ] (1) d ln(1 + d r ) a ln(1 + d r ) 0 36 π 1 1 2 2 cd and ca from a tube are series connected and form the equivalent capacity c: c = σ ⋅ cd /(1+ σ ) where:σ = ca / cd . (2) In practice, the increase in production of an ozone generator is done by mounting n parallel identical tubes so the total electric capacities of the ozonizer are: Cd=ncd and Ca=nca. (3) During the operation process with the alternating voltage u1(t)=±U1, on a voltage alternation, the author identified three functionally distinct time intervals: -interval 1 (1+ and 1-)−the maximum electric field on the air space is below the air ionization level: the air space is capacitive; -interval 2 (2+ and 2-)−the maximum electric field on the air space is over the air ionization level: the air space resistive; -interval 3 (3+ and 3-)−the corona electric discharge is carried out at the threshold voltage Up=constant: the air space is an ideal source of voltage Up. 2.2. The ozonizer operation analysis in the 1+ interval below the corona discharge threshold ua<Up; t=[0,τ1]. The electric field in the tube dielectric and air space are (fig.3 a, b): q (t) q (t)+q (t) e (x ,t) = 1 ;e (x ,t) = 1 2 (4) d 1 2π(r + x )lε ε a 2 2π(r + x )lε 1 1 0 rd 2 2 0 344 Ionel Colt formulas valid on all intervals 1, 2 and 3. In interval 1, the maximum electric field in the air ea(0,t) is below the ionization level Er: │ea(0,t) │<Er , so in the discharge space the conduction current is null, the equivalent diagram can be seen in fig.3c. a) b) c) Fig. 3. Representation of a sector of the ozone tube cross-section in the 1+interval: a)-the electric state at the start of interval 1+; b)-the electric state at the end of interval 1+ and startof interval 2+ ; c) the equivalent electric diagram of the ozonizer (n parallel tubes) 2.2.1. The voltage and current through the ozonizer. r1+d1 r2 +d2 q (t) q (t)+q (t) q (t) q (t) u (t) = e (x ,t)dx + e (x ,t)dx =u +u = 1 + 1 2 = 1 + 2 (5) 2 ∫ d 1 1 ∫ a 2 2 d a c c c c r1 r2 d a a i2(1) / n = dq1(1) / dt = c du2(1) dt ; (6) ˆ Note 1. Load q2 = +Q2 = cdU 2 − (ca + cd )U p =constant, (7) is the stationary space charge from the surface (2) of the dielectric; Differential equation of the diagram ( R 2 ≈0)(fig. 3c): 2 2 2 2 d u2(1) dt + ω1 u2(1) = ω1 U1 (8) where: ω1 = 1 L2 C - the oscillating circuit’s own angular frequency. With the initial requirement that voltage u2(1)(0)=−Û2 , these are obtained: ˆ ˆ ˆ u2(1) (t) = (U1 +U 2 )(1− cosω1t) −U 2 ; i2(1) (t) = I2(1) sinω1t with I 2(1) = Cω1 (U1 +U2 ) (9) 2.2.2.The threshold voltage Up at the corona discharge.The electric field in the discharge space is given by formula (4) where (q 1+q2)/ca is substituted from relation(5) u a(1) e a(1) (x 2 , t) = ; (10) (r2 + x 2 )ln(1+ d 2 r2 ) The gas discharge voltage is defined in the ionization electric field Er – threshold voltage Up, as a result of formula (10) at x2=0: Operation principle of cylindrical geometry ozone generators 345 U p = u a (0) = u a (τ 1 ) = g 0 (T0 T )( p p0 )r2 ln(1 + d 2 r2 ) (11) g0=3kV/mm –dry air; g0=2,8kV/mm –damp air; T- absolute temperature of the gas (T0=273,15K); p-gas pressure in [atm.](p0=1atm) (12) 2.2.3. The explicit expressions of the voltages on the dielectric, discharge space and of the current through the ozonizer. Relations (5) and (6) are obtained in a primary form, which substituted in (9) take a final, explicit form of the voltages and current through the ozonizer in interval 1. 2(σ +1)U 1− cosω t U + U = p ; u (t) = 2σU 1 −U ; (13) 1 2 1 − cosω τ d (1) p 1− cosω τ d 1 1 1 1 1− cosω t 1− cosω t u (t) = (2 1 −1)U ; u (t) = 2(1+ σ ) 1 U −Uˆ ; (14) a(1) 1− cosω τ p 2(1) 1− cosω τ p 2 1 1 1 1 i (t) = I sinω t with Iˆ = (2(1 + σ ) (1 − cosω τ ))U L ω ; (15) 2(1) 2(1) 1 2(1) 1 1 ( p 2 1 ) In relations (13), (14) and (15) the U voltage is a system parameter that p remains constant for certain geometry of the ozone tube and certain gas environment con- ditions.