What Is an Ozone Generator?
Ozone is produced by high-voltage ionisation (or chemical or photochemical reaction), which decomposes and recombines part of the oxygen in the air into ozone — an allotropic transformation of oxygen; it can also be obtained by the electrolysis of water. Ozone's instability makes bottled storage very difficult, so it is generally produced on site by an ozone generator and used as it is generated. Classification of ozone generators. By the way ozone is generated, ozone generators fall mainly into three types: high-voltage discharge, ultraviolet irradiation, and electrolytic.
The circuit of an ozone generator consists of transistors VT1 and VT2, inductors L1–L13, pulse transformer T, current-limiting resistor R1, charging capacitor C3 and bidirectional trigger diode VD5, which together form a push-pull oscillator circuit; filter inductor L0, rectifier diode VD1 and filter capacitors C1 and C2 form a half-wave rectifier and filter circuit. When the power is switched on, the 220 V AC voltage is filtered by L0 and rectified by VD1, producing a voltage of about +280 V across C1 that supplies the push-pull oscillator circuit. At the instant of power-on, VT1 conducts. Owing to the charging action of C3, the bidirectional trigger diode VD5 is cut off. When the charging voltage across C3 rises to 32 V, VD5 is triggered and conducts, turning VT2 on. While VT2 conducts, C3 gradually discharges, which in turn cuts VT2 off. After VT1 conducts, positive feedback voltages are generated across L1 and L2 by the action of pulse transformer T; these voltages are applied to the bases of VT1 and VT2 respectively, making VT1 and VT2 conduct and cut off alternately (that is, when VT1 conducts VT2 is cut off, and when VT2 conducts VT1 is cut off), so that the push-pull oscillator circuit oscillates. Once the push-pull oscillator circuit is operating, a pulsed high voltage is generated in the secondary winding L6 of pulse transformer T, driving the ozone-generating plate VG to produce ozone.
The core technology and equipment of an ozone system is the discharge tube inside the generator, which directly affects the operating efficiency and reliability of the equipment. The ozone generator uses a micro-gap dielectric-barrier discharge design, which not only greatly improves operating efficiency but also increases the safety and reliability of continuous system operation. The technical parameters of the equipment have reached the internationally advanced level.
Because micro-gap discharge technology is used, the system operating voltage is reduced to 6–8 kV, far below the withstand voltage of the glass-tube insulating dielectric, which effectively prevents dielectric breakdown and short-circuit faults and improves operational reliability.
The modular design adopted for the ozone generator's discharge unit makes installation, servicing and maintenance of the equipment much easier. Provided the quality of the feed gas is guaranteed, the maintenance-free period for continuous operation of the discharge unit can be as long as 5 years.
Unlike conventional medium-frequency (<1 kHz) ozone power supplies, high-frequency high-voltage ozone systems use 3–6 kHz high-frequency power supply technology; combined with a micro-discharge gap design, this can effectively raise ozone generation efficiency and reduce the size and footprint of the generator, thereby cutting civil-engineering design and investment costs. The inverter power supply system adopts mature high-frequency power supply technology, and long-term on-site operation has proven that it ensures the stability of long-term system operation. The high-frequency output passes through a step-up system to produce a sinusoidal high voltage, which is connected to the generator by cable; under the action of the high frequency and high voltage, the discharge gap produces a cold plasma discharge that generates ozone.
Although the efficiency of modern ozone generators has improved markedly compared with conventional products, about 90% of the electrical energy is not used to generate ozone but is converted into heat. If this heat is not effectively dissipated, the temperature of the ozone generator's discharge gap will keep rising and may even exceed the design operating temperature. High temperature is unfavourable to ozone generation but favourable to ozone decomposition, which reduces ozone output and concentration. We therefore designed a single-pass cooling-water unit; when the cooling-water temperature exceeds the system design temperature or the water flow is insufficient, the system automatically issues an alarm signal.
Ozone is a gas mixture, and its concentration is usually expressed as a mass ratio or a volume ratio. The mass ratio is the mass of ozone contained in a unit volume of the gas mixture, commonly expressed in mg/L, mg/m³ or g/m³. The volume ratio is the volumetric or percentage content of ozone in a unit volume, expressed as a percentage such as 2%, 5% or 12%. In the sanitation industry, ppm is commonly used to express ozone concentration: one part per million by volume of ozone in one cubic metre of ozone gas mixture is 1 ppm. Ozone concentration is an important indicator of the technical content and performance of an ozone generator. Under the same operating conditions, the higher the ozone output concentration, the higher the quality.
In addition, this technology also has wide application and practical experience in related fields.
Ozone is a highly oxidising and unstable gas, and its output concentration is affected by many factors, of which chamber temperature is one of the most important; at around 30 °C ozone decays by half within one minute. At 40–50 °C the decay reaches 80%. Above 60 °C ozone decomposes immediately.
In 2011 the state issued its latest national standard, regulating for the first time the concentration of ozone for different uses.
Beyond this, many other factors must also be considered in practical engineering applications.
Ozone-treated tap water is a high-quality drinking water. Passing O3 through each litre of water for just 2 minutes removes residual chlorine, sterilises and disinfects, removes odours and heavy metals, prevents the formation of the carcinogen chloroform, increases the dissolved oxygen content and produces ideal pure drinking water.
Placing washed tableware in water and passing O3 through it for 20 minutes removes detergent residues and kills bacteria and viruses, replacing an electronic sterilising cabinet and preventing tableware from transmitting disease. Clothes, towels, rags and socks can also be disinfected and deodorised in a water medium.
Hanging the ozone exhaust pipe at a height of more than 1.7 m and discharging O3 for 20–30 minutes effectively removes indoor smoke or the odour of decorative materials, reduces dust, sterilises, increases the oxygen content of the air and freshens it, letting you enjoy air as fresh as a forest after rain at home (suitable for smoke removal, dust removal, disinfection and deodorisation in homes, offices, meeting rooms and entertainment venues).
At the same time, the related auxiliary processes and equipment are also being continuously optimised and upgraded.
When purchasing an ozone generator, the first step is to determine its intended use — whether for air sterilisation and deodorisation or for water treatment. For air treatment, a low-concentration, economical open-type ozone generator can be chosen; open-type units come with or without an air source, and models with an air source are preferable. Such generators are simple in structure and inexpensive, but temperature and humidity affect the amount of ozone generated during operation. The open-type generator described above is the simplest ozone device; for demanding locations, a high-concentration ozone generator should also be selected for air treatment. For air treatment, dose at 20–50 mg/m³, choosing the higher value for the food and pharmaceutical industries. The total ozone demand (that is, the generator output) can then be calculated from the size of the space. For water treatment, a high-concentration ozone generator must be selected (ozone concentration greater than 12 mg/L); treating water with low-concentration ozone is ineffective. A high-concentration ozone generator is a standard configuration including an air source, air-source treatment device and ozone generation device. Small units can be designed as integrated models with outputs of 5–200 g/h, while medium and large ozone generators basically exist as skid-mounted units.
A high-quality ozone generator is designed, equipped and manufactured from materials according to its standards, and its cost is far higher than that of low-grade and low-specification generators. However, a high-quality ozone generator performs very stably: ozone concentration and output are not affected by environmental factors. A low-specification ozone generator, by contrast, is greatly affected by the environment during operation, and increases in temperature and humidity can sharply reduce ozone output and concentration, impairing treatment results. When purchasing, price and performance should be evaluated together.
Find out whether the ozone generator includes an air source — the cost difference between generators with and without an air source is very large. If you buy a generator without an air source because of its price advantage, you will need to supply your own air-source device and may ultimately spend more. 2. Understand the generator's structural form, whether it can run continuously, its ozone output concentration and other specifications. For example, if a generator is needed for water purification and an open-type ozone generator is chosen by mistake, it simply cannot be used. 3. Confirm the rated output marked on the ozone generator — whether it is rated with an air source or an oxygen source, because the ozone output of a generator using an oxygen source is twice that using an air source, and the cost of the two differs by nearly a factor of two. When purchasing an ozone generator, buyer and supplier should communicate comprehensively to avoid misunderstandings, and price should never be the main reference in judging an ozone generator.
Looking at the global ozone technology industry, sterilisation and purification for water treatment is the main market, and water purification ozone equipment consists of two parts: the ozone source and the gas–water mixing device. The ozone generator should provide ozone of sufficient concentration and output, and the mixing device should dissolve the ozone in water with high efficiency, that is, achieve a certain dissolved ozone level. A good ozone generator must therefore have an excellent gas–water mixing device, so that ozone can be mixed efficiently with water and the dissolved ozone level in the water is sufficient for complete sterilisation.
In addition, this technology also has wide application and practical experience in related fields.
Operating mode — co-current, counter-current, or multi-stage series alternating counter/co-current flow, with continuous or intermittent batch operation. Advantages: relatively low energy consumption. Disadvantages: when the nozzles clog, gas distribution becomes uneven and mixing is poor; large air compressors and expensive oxidation reaction towers are required.
Operating mode — forced gas–water mixing. Advantages: low investment, good mixing and short contact time; after passing through a jet mixer, the ozone concentration in the water can be several times that of the aeration method.
Operating mode — making full use of the turbulence of the water to create a large negative pressure that draws in ozone, generating vortices in the mixing chamber that rotate and collide with one another. Practical use has shown that the single-stage ozone absorption rate exceeds 90%, and with the same ozone generation system, the dissolved ozone level in the water is about twice that of a Venturi jet mixer. The outer body of the gas–liquid intensive mixing chamber is made of 304 stainless steel, while the inner chamber is made of 316 stainless steel and partially ozone-resistant ABS material; the inner chamber of the SSLW series is made of stainless steel and ozone-resistant PVDF. The product is sturdy and durable, free of mechanical faults and maintenance, and easy to install and operate — currently the least expensive and best-mixing equipment available.
Beyond this, many other factors must also be considered in practical engineering applications.
The suction inlet of a gas–liquid mixing pump can draw in gas by negative pressure, and the high-speed rotating impeller mixes and agitates the liquid and gas. Because of the pressurised mixing inside the pump, the gas dissolves fully in the liquid with high dissolution efficiency. A single gas–liquid mixing pump can draw in and mix the gas and liquid, dissolve them, and deliver the highly dissolved liquid directly to the point of use.
High-voltage discharge generator. This type of ozone generator uses a high-voltage current of a certain frequency to create a high-voltage corona field, causing the oxygen molecules in or around the field to undergo electrochemical reaction and thereby produce ozone. Such generators have the advantages of mature technology, stable operation, long service life and large ozone output (up to 1 kg/h per unit), and are therefore the most widely used ozone generators in related industries at home and abroad. High-voltage discharge ozone generators are further divided into the following types:
1. By the high-voltage electrical frequency of the generator, there are power frequency (50–60 Hz), medium frequency (400–1000 Hz) and high frequency (>1000 Hz). Power-frequency generators have basically left the market because of their large size and high power consumption. Medium- and high-frequency generators offer a small size, low power consumption and large ozone output, and are the most commonly used products today.
2. By the gas feedstock used, there are oxygen type and air type. The oxygen type is usually supplied with oxygen from an oxygen cylinder or an oxygen concentrator. The air type usually uses clean, dry compressed air as feedstock. Because ozone is produced from oxygen, and air contains only 21% oxygen, air-type generators produce a relatively low ozone concentration, whereas the purity of oxygen from cylinders or oxygen concentrators is above 90%, so oxygen-type generators produce a higher ozone concentration.
It is worth noting that the technology and standards in this field are also constantly developing and improving.
It is widely used in ozone generators, but its manufacturing cost is relatively high. By structure, ozone generators are divided into gap-discharge (DBD) and open types. The gap-discharge type is characterised by ozone being generated in the gap between the inner and outer electrodes, so the ozone can be collected in a concentrated way and delivered for use at a relatively high concentration — for example, for water treatment. In the open type, the electrodes are exposed to the air and the ozone produced diffuses directly into the air; because the ozone concentration is low, it is usually used only for air sterilisation in small spaces or for surface disinfection of certain small items. Gap-discharge generators can be used in place of open-type generators. However, the cost of a gap-discharge ozone generator is far higher than that of an open-type one.
It is worth noting that the technology and standards in this field are also constantly developing and improving.
5. Because the ozone concentration produced is high, for the same amount of ozone dosed into the water this method achieves a higher residual ozone concentration in the water, so drinking water can be thoroughly sterilised and disinfected and high-concentration ozone water (disinfectant) can be produced.
Beyond this, many other factors must also be considered in practical engineering applications.
"Ozone Generators for Water Treatment" CJ/T 322-2010, an industry standard of the People's Republic of China for urban construction, replacing "Ozone Generators" CJ/T 3028.1-1994 and CJ/T 3028.2-1994 "Measurement of Ozone Concentration, Output and Power Consumption of Ozone Generators"
Beyond this, many other factors must also be considered in practical engineering applications.
(5) For air-cooled generators, the cooling air must be free of moisture, impurities, corrosives, aerosols, oil or conductive substances and visible dust. Under normal conditions, unless the unit is in an extremely dusty industrial atmosphere, the air mostly does not need filtering.
When ozone generators are used in water treatment engineering and similar applications, many factors can affect the generator's efficiency and reliability, so the following issues must be noted when designing an ozone generator application system:
It is well known that the three elements of an explosion are fuel, oxidant and ignition source, and two of these — oxidant and ignition source — already exist in the corona environment of an ozone generator. Hydrocarbon fuel substances must therefore be prevented from entering the feed gas; if hydrocarbons may be present, a hydrocarbon analyser must be installed so that the power is cut off when the hydrocarbon concentration approaches 25% of the lower explosive limit (LEL).
In addition, particles of around 5 μm should be filtered out of the feed gas to prevent small desiccant powder or other fine particles from entering the corona zone of the generator and impairing corona efficiency.
At the same time, the related auxiliary processes and equipment are also being continuously optimised and upgraded.
For water-cooled generators, the cooling-water quality is very important in order to minimise scaling on heat-transfer surfaces. Scaling reduces heat-transfer efficiency, thereby lowering ozone output and increasing maintenance costs. Technically, tap water is the preferred coolant, but for the water consumption required by large industrial generators, using tap water is economically unattractive — perhaps except where the generating system is used in a water treatment plant. In contrast to tap water quality, generally treated sewage used as cooling water does not perform well, because it tends to cause scaling. If high-quality water or another fluid is used in a sealed secondary cooling loop and the final heat exchanger is specially designed to minimise scaling and be easy to clean, the treated sewage effluent can also be used for final-stage heat dissipation. To achieve the best balance between water costs and equipment maintenance costs, most system designs use cooling-tower water or, for heat exchangers, high-quality potable water (no suspended solids, chloride <5 mg/L).
On this basis, industry experts have also carried out extensive research and improvement.
The ozone generator applies an advanced physical oxygen-generation principle: through the pressure-swing adsorption of an oxygen-generation tower, oxygen and nitrogen in the air are separated directly at ambient temperature and pressure to obtain high-purity oxygen; ozone is then obtained by the corona discharge method, in which oxygen-containing gas under an alternating high-voltage field produces a corona discharge at atmospheric pressure to generate ozone; after the water and ozone are mixed by a gas–liquid mixing system, ozone water of a certain concentration is obtained.
Public canteens in schools, military units, government offices and factories use it to sterilise and disinfect vegetables, fruit and meat and to reduce pesticide residues. Drinking water plants, food factories, pharmaceutical plants, cosmetics factories, aquaculture farms, wastewater treatment plants, swimming pools, warehouses and community water supplies use ozone water for sterilisation and disinfection.
It is worth noting that the technology and standards in this field are also constantly developing and improving.
The 120 kg/h large-scale ozone generator and its product series can be widely used in municipal drinking water treatment, wastewater treatment, industrial wastewater treatment, flue-gas denitrification, pulp bleaching and fine chemical oxidation. It is an important development direction for the state under the 12th Five-Year Plan and beyond in terms of "transforming the growth model and adjusting the structure" for drinking water safety, environmental protection and energy saving and emission reduction, and it is a major technical equipment for China's environmental-protection engineering applications that can replace imports.
