Abstract:
A cleaning method for removing solid deposits of the oxides of nitrogen, especially dinitrogen pentoxide, from ozone generator tubes and dielectrics is described. The method circulates warm dry gas in the tube section of the generator, warm water in the shell section or both to clean the ozone generator. The oxides are evaporated and evacuated from the system. The method substantially reduces or eliminates the formation of nitric acid on the tubes and dielectrics when the generator is exposed to humidity upon being opened to the atmosphere.

Description:
FIELD OF THE INVENTION 
     The present invention is related to a method for cleaning electrical discharge ozone generators. More particularly, the present invention relates to circulating a warm fluid within the ozone generator to evaporate solid deposits of nitrogen oxides, including dinitrogen pentoxide, from the tubes and dielectrics of the generator. 
     BACKGROUND TO THE INVENTION 
     Ozone (O 3 ) is a strong oxidizing agent (2.07V) that is used as a disinfectant in various applications, such as wastewater treatment, cooling towers, air treatment, swimming pool cleaning, food processing, hydroponics, and meat processing. Ozone is particularly effective in aqueous environments. Ozone is, however, very reactive and cannot be stored for any significant period of time. As a result, ozone must be generated at the site where it is to be used. Two common means by which ozone is generated are by subjecting oxygen gas (O 2 ) to ultraviolet radiation or to an electrical discharge. 
     One type of electrical discharge generator is an electrical barrier discharge ozone generator, commonly known as a silent discharge generator. One such generator is a corona discharge generator. Corona discharge generators are commonly used to generate ozone on a large scale. The basic principle of electrical discharge ozone generators is that a feed gas is fed through a high voltage electrical discharge field between two electrodes. The oxygen is then ionized as it passes through the electrical field which will cause at least some oxygen to be converted to ozone. For corona discharge generators, the feed gas, usually dry air or oxygen, is subjected to coronal discharges created by high voltages between two electrodes, one of which is contained within a dielectric material. In a tubular ozone generator, a dielectric is supported within a tube and a central cathode within the dielectric is subjected to a high voltage relative to an outer anode. The anode is often grounded. High voltage phenomena occur inside the dielectric envelope and induce a corona discharge field between the outside of the dielectric envelope (hereinafter referred to as “dielectric”) and the outer anode material. An oxygen-containing feed gas is fed into this space and through this field, and the oxygen (O 2 ) molecules are split to form atomic oxygen, which then reacts to form ozone. 
     
       
         3O 2 +Energy→2O 3   (Eq. 1) 
       
     
     The quantity of ozone generated depends on several factors, such as for example the voltage, the frequency of AC current, the gap between the dielectric and the cathode and the concentration of O 2  and other gases in the feed gas. The feed gas may be dry, clean air; dry, clean oxygen; or dry, clean oxygen containing small amounts of other relatively inert gases such as nitrogen (N 2 ) or argon (Ar). It is important that dry feed gas be used, as water interferes with the reaction and also reacts with gases in the ozone gas to create contaminants, most notably nitric acid (HNO 3 ). 
     Much of the energy required for the reaction is lost as heat; therefore ozone generators should be cooled to operate more efficiently. One of the ways that cooling the generator increases the efficiency of the generator is by causing fewer O 3  molecules to be lost due to decomposition or collision. A good description of ozone generating equipment can be found in U.S. Pat. No. 4,954,321 by Jensen issued Sep. 4, 1990, and also in “Ozone Technology and Equipment Design”, Ozonia North America, USA 1996, the information in, and contents of, both documents hereby being incorporated herein by reference. 
     Large amounts of ozone are not easily generated. For example, an ozone generation system employing an electrical discharge and which uses liquid oxygen at ≧99.5% purity, that has been vaporized and has had between 2 and 3% N 2  by weight and a certain amount of argon added, will typically produce 10-13% ozone by weight. Because of the relatively low rate of ozone generation, large plants may require several ozone generators to meet the demand for ozone. In turn, each generator may contain many anode/cathode/dielectric units. 
     As noted above, the amount of ozone generated depends on several factors, one of which is the amount of N 2  in the feed gas. When oxygen separated from air is used as a feed gas, nitrogen may be present. This is because of the method used to separate the oxygen from the air, e.g. vacuum or pressure swing adsorption or cryogenic separation. Nitrogen may also be present in the feed gas because it has been introduced to act as a catalyst. Nitrogen allows production of a higher ozone concentration or the reduction of the power consumed in generating the ozone. For example, large commercial ozone generators using pure oxygen generally create between 6-10% ozone by weight, instead of the 10-13% available when a small amount of N 2  is added. It is therefore not desirable to remove all of the N 2 . 
     Unfortunately, it has been discovered that the presence of nitrogen in the feed gas results in a solid residue, mainly composed of dinitrogen pentoxide (N 2 O 5 ), with some of it being deposited within the generator system, including on the tubes and the dielectrics. The residue may also contain other solid oxides of nitrogen (NO x ). The oxide deposits on the support tubes and dielectrics and may eventually clog the passageway between the dielectric and the support tube. 
     Regular maintenance of ozone generators typically involves an inspection and repair of the electrical connections. However, because of the problems inherent in cleaning the generators described in greater detail below, opening the ozone generator to the atmosphere is avoided whenever possible. From time to time, however, ozone generators may require special or preventative maintenance. Such maintenance may be occasioned by failure of more than approximately 10% of the dielectrics or by deposits that clog the passages between the dielectrics and their support tubes in some systems. 
     Current methods of cleaning ozone generators consist of turning off the power supply and cooling water and purging the generator by circulating dry oxygen gas at room temperature through the system. The purging continues until the residual ozone has been removed from the inside of the generator for the safety of the workers. Thereafter the system is opened up to the atmosphere. 
     When the ozone generator is opened for regular maintenance, if it is opened for long enough, the water in the ambient air reacts with any residual solid nitrogen oxides to form nitric acid (HNO 3 ). The reaction with N 2 O 5  for example, proceeds as follows: 
     
       
         H 2 O+N 2 O 5 →2HNO 3   (Eq. 2) 
       
     
     Nitric acid is an oily, yellow residue, and any nitric acid in the generator needs to be removed. 
     The cleaning typically requires that all dielectrics and the tubes holding them be cleaned with a proper solvent. Generally, the dielectrics and tubes are removed from the system, cleaned with water and then with an industrial organic solvent such as acetone or a chlorinated organic solvent such as perchloroethylene, or methanol. This cleaning work is time consuming, and may require more than 14 days for an industrial scale ozone generator. In addition, removal and cleaning of the dielectrics will result in some breakage (perhaps 10%), thereby requiring their replacement. Finally, the chlorine containing solvents and the disposal of the contaminated cleaning solvents represent additional cost and safety issues that must be considered. 
     It is therefore desirable to have a less onerous cleaning method that would decrease the time and expense required for special maintenance of electrical discharge ozone generators, particularly large-scale corona discharge ozone generators. 
     SUMMARY OF THE INVENTION 
     The current invention relates to a method of removing solid deposits of the oxides of nitrogen, including in particular dinitrogen pentoxide, in an ozone generator thereby avoiding the need to open the generator to atmosphere. If it is necessary to open the generator, to replace dielectrics for example, the inventive method will significantly reduce or eliminate the creation of nitric acid residue within the system. The inventive method can significantly reduce the maintenance time required from perhaps two to three weeks for each generator to perhaps as little as three days. In addition, damage to the dielectrics is minimized or eliminated, as is the need for solvents to remove the nitric acid. The method can provide significant costs savings in personnel time and materials. 
     A preferred embodiment of the inventive method uses warm gas circulation, preferably at 47-65° C., within the generator dielectric support tubes and warm water circulation in the shell section of the generator, preferably at 47-65° C. If the physical components of the generator can withstand temperatures above 65° C. then the temperature of the gas can be increased well above 65° C., although this is not necessary to remove dinitrogen pentoxide, and heating the gas to a higher temperature may make the cleaning process more expensive. The fluid circulation within the system raises the temperature within the generator, and various solid oxides of nitrogen, which have boiling points less than the temperature of the circulated gas, including dinitrogen pentoxide which has a boiling point of about 47° C., are evaporated and thereafter evacuated from the system by the gas stream. The temperature within the generator is sufficient to ensure that the deposits do not re-form within the ozone generator. 
     The progress of the cleaning can be monitored by bubbling a portion of the evacuated gas through a water trap and measuring the change in pH caused by the HNO 3  formed by interaction of the NO x  and the water. Fluids are circulated within the generator until the pH of the water used as a reference is not appreciably lowered by the gas exiting the tubes of the generator. 
     If the need for maintenance was caused only by a build-up of NO x , including in particular N 2 O 5 , the system is ready to return to production without requiring the generator to be opened to the atmosphere. Cleaning time and potential contamination are reduced. If the maintenance was required because of damaged dielectrics, when the system is opened to ambient air after being sufficiently cleaned, no nitric acid is formed. Only those dielectrics requiring replacement need be removed and replaced. Again, there are significant benefits in terms of both time and material savings. 
     In one aspect of the invention there is provided a method of cleaning an electrical discharge ozone generator comprising passing a warm cleaning gas between an inlet of the generator and an outlet of the generator to evaporate at least some of the NO x  deposited in the ozone generator. 
     In another aspect of the invention there is provided a method for removing solid deposits of NO x  from an ozone generator comprising a first and second electrode, the electrodes being spaced from each other and having a passageway therebetween. The solid deposits of NO x  are located within the passageway. The method comprises the step (i) of passing a warm cleaning gas through the passageway to evaporate the solid deposits of NO x  with boiling points equal to or less than 65° C. which are deposited therein. The warm cleaning gas exiting the ozone generator is at a temperature sufficient to maintain the NO x  in a gaseous state until the NO x  exits the ozone generator. 
     In another aspect of the invention there is provided a method for removing solid deposits of NO x  from an ozone generator comprising a housing enclosing an interior having an inlet and an outlet and a pair of spaced electrodes mounted within the interior. The electrodes are spaced apart from each other. The solid deposits of NO x  are located within the interior. The method comprises the step of passing a warm cleaning gas through the interior from the inlet to the outlet to evaporate at least some of the NO x  deposited therein. The warm cleaning gas exits the ozone generator at a temperature sufficient to maintain the NO x  in a gaseous state until the NO x  exits the ozone generator. 
     In a further aspect of the invention there is provided a method for removing solid deposits of NO x  from an ozone generator comprising a housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each of the support tubes has an inner wall. A passageway is formed between the inner wall of the support tubes and the dielectrics. The passageway has solid deposits of NO x  therein. A support tube inlet is in flow communication with a support tube outlet through the passageway. The method comprises the step (i) of passing a warm cleaning gas through the passageway to evaporate at least some of the solid deposits of NO x  which are deposited therein and carry at least some of the evaporated NO x  from the ozone generator. 
     In another aspect of the invention there is provided a method for removing solid deposits of NO x  from an ozone generator comprising an outer housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each of the support tubes has an inner wall and a passageway between the inner wall and the dielectrics. The passageway communicates between a support tube inlet and a support tube outlet. The housing has a shell that defines an interior surrounding the support tubes, the interior communicates between a shell inlet and a shell outlet. The method comprises step (i) of circulating a warm fluid within the shell and the concurrent step (ii) of evacuating the support tubes to remove the evaporated NO x  with boiling points less than 65° C. that had been deposited therein. 
     In a further aspect of the invention there is provided a method for removing solid deposits of NO x  from an ozone generator comprising an outer housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each of the support tubes has an inner wall and a passageway between the inner wall and the one or more dielectrics. The passageway communicates between a support tube inlet and a support tube outlet. The housing has a shell that defines an interior surrounding the support tubes. The interior communicates between a shell inlet and a shell outlet. The method comprises step (i) of circulating a cleaning gas within the support tubes and concurrent step (ii) of circulating a warm fluid within the shell to heat the cleaning gas, thereby removing the NO x  with boiling points less than 65° C. deposited therein. The temperature of the warm fluid is sufficient to ensure that the temperature of the cleaning gas exiting the ozone generator is sufficient to maintain the NO x  in a gaseous state until the NO x  exits said ozone generator. 
     In yet another aspect of the invention there is provided a method for removing dinitrogen pentoxide deposits from an ozone generator comprising an outer housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each support tube has an inner wall and a passageway between the inner wall and the dielectrics. The passageway communicates between a support tube inlet and a support tube outlet. A shell surrounds the support tubes, the shell defining an interior surrounding the support tubes. The interior communicates between a shell inlet and a shell outlet. The method comprises circulating a clean, dry mixture of oxygen, nitrogen and argon at 55° C.-60° C. between the shell inlet and shell outlet; supplying the shell with warm water at 55° C.-60° C.; diverting a portion of the gas exiting the support tubes to a liquid ring compressor; adding a neutralizing agent to the water in the compressor to maintain the pH in the liquid ring compressor at an approximately constant pH using an in-line process pH controller; and continuing the cleaning until the addition of neutralizing agent terminates as it is no longer required to maintain the constant pH. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific, preferred embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a ozone generating system having an ozone geneator which can be cleaned using a method in accordance with an embodiment of the invention. 
     FIG. 2 is a cross-sectional view of part of an ozone generator, that can be cleaned using an embodiment of the inventive method. 
     FIG. 3 is a cross-sectional view at  3 — 3  of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1,  2  and  3 , an ozone generator system  10  is illustrated which includes an ozone generator  14 . It will be appreciated that FIGS. 2 and 3 show only part of an ozone generator. Typically such electrical discharge generators employ numerous dielectric support tubes through which oxygen is passed during the ozone generation process. 
     In the Figures, generator  14  has a housing formed from a shell  14 A. In this case, it is shown in FIGS. 1 and 2 with a jacket  14 B surrounding shell  14 A, although a jacket is not typically employed in commercial generators. Between shell  14 A and jacket  14 B is a jacket passageway  54 . The jacket  14 B has an inlet (not shown) which is in flow communication with an outlet (not shown) through passageway  54 . Shell  14 A houses a plurality of dielectric support tubes  28  which are mounted on supports within the interior  34  of shell  14 A. Between the support tubes in interior  34  is an interior space  35  which surrounds the support tubes. Each support tube  28  houses one or more dielectrics  27 . Each tube  28  has an inlet  25  which is in flow communication with an outlet  26  through a passageway  33  which is provided between the inner wall  50  of the support tube  28  and the outer wall  52  of the dielectrics  27 . In operation to make ozone, ozone is created in the corona electrical discharge immediately outside or around dielectric  27 . During the production of ozone, deposits  29  of nitrogen oxides, especially N 2 O 5 , may build up on the dielectric  27  and the inner wall support tube  28 . 
     The system  10  also provides for cool water to flow through the interior space  35  of generator shell  14 A around the tubes  28 . During operation of system  10  to produce ozone, the flowing water will cool generator  14 , thereby increasing the efficiency of ozone production. The flow of water  17 A enters inlet (shown schematically as  18 A) of shell  14 A via conduit  36  and exits at outlet (shown schematically  18 B). The water fills interior space  35 , which as described above generally comprises the portion of interior  34  of the housing not filled by support tubes  28 . However, when used in the inventive method of cleaning, the water flowing through interior space  35  is warm and may be provided by water source  15  and heated at heater  16  or water may be provided by a source of warm water  32 . 
     Also, during operation of system  10  to produce ozone, gas enters the ozone generator  14  at inlet  60  through conduit  31  (FIG.  1 ), and then the gas flow divides within the generator  14  so that a portion of the total gas flow will flow through each of the passageways between the inlets and outlets of each support tube. At the outlets  26  of each support tube  28 , the separate flows re-unite and then exit the generator at a common outlet  40 . Providing a positive flow of gas is also preferred in practicing the inventive method. Thus during the cleaning process, a flow of gas  13 A enters support tubes  28  at each inlet  25  and exits at each outlet  26 . The cleaning gas may be provided by gas source  11  and heated at heater  12  or gas may be provided by gas source  30 , which is already heated. After travelling through support tubes  28 , the cleaning gas exits shell  14 A via outlet  40  and may enter a water trap  21  via conduit  19 . A portion of the cleaning gas that exits outlet  40  is diverted to outlet  20 . Water trap  21  is provided with a source of reference water  23 , and a pH monitor  22 . Water exits water trap  21  through exit  24 . 
     When ozone production is stopped at installation  10  to undertake special maintenance, occasioned, for example, by the support tubes  28  being plugged, such as by solid deposits of N 2 O 5  and perhaps other solid oxides of nitrogen in passageway  33  or by too many of the dielectrics  27  being damaged, the system is first purged of all O 3 . This can be done, for example, by using the feed gas generally used to create ozone or by using industrial grade oxygen. The gas is fed through support tubes  28  while no electric discharge is present. 
     With reference to FIGS. 1-3, after purging the generator of ozone, in one embodiment of the invention, the inside of generator  14  where the dielectrics  27  and support tubes  28  are located is supplied through inlet  25  with a warm cleaning gas. The cleaning gas may be any dry, clean gas that is compatible with the ozone generator system, such as oxygen; nitrogen; a mixture of nitrogen and oxygen that may contain argon; or industrial grade helium, argon, air or possibly even carbon dioxide (CO 2 ), although the latter will have the effect of lowering the pH to 7.0-8.0. It may be most convenient to utilize oxygen since that is the gas used in the ozone generating process. The cleaning gas must be dry, sufficiently contaminant-free and compatible with a system used for generating ozone, i.e. it should not detrimentally affect the physical system or interfere with production of ozone when the system is returned to production. Any such gas should be dry or substantially dry. In this embodiment, the cleaning gas evaporates and entrains the NO x    29  deposited on the dielectrics  27  and support tubes  28 . 
     Herein, the term “cleaning gas”, when used in this specification and claims includes any of the cleaning gases described thus far in the specification, and any other suitable gases. 
     In one preferred embodiment, the cleaning gas  11  is circulated into inlet  25  via conduit  31  and inlet  60  after heating at heat source  12  which may be any conventional heat source, for example, a water bath, or an electrical or steam heater. In another preferred embodiment, a source of hot gas  30  may also be used. The gas enters the support tube  28  at inlet  25  at a temperature of preferably between 47-65° C. and most preferably 55-60° C. The cleaning gas exits support tube  28  at outlet  26 . Cleaning gas is circulated through the system  10  until the solid deposits of NO x  that have boiling points of about 65° C. or less, including in particular N 2 O 5 , have been substantially removed from the ozone generator. 
     The temperature of the warm gas  19 A that exits tube  28 , and later outlet  40 , is preferably between 47-65° C. to ensure that the evaporated NO x  does not re-deposit as solids within the ozone generator. While not strictly necessary, it is good practice to ensure that NO x  also does not re-deposit within conduit  19 . 
     In the same preferred embodiments, while the gas is circulating in tube  28 , water is circulated in the interior  34  of generator shell  14 A. In one such preferred embodiment, the circulating water  15  is heated before entering the interior space  35  of shell  14 A via conduit  36  and inlet  18 A using a conventional heat source  16 . As stated above, the heat source may, for example, be a water bath or an electrical or steam heater. In another embodiment, a source of warm water  32  is used. The water enters interior space  35  at inlet  18 A at a temperature of between 29-65° C., preferably between 47-65° C. and most preferably between 55-60° C. and exits at outlet  18 B at a temperature sufficient to ensure that, in combination with the cleaning gas temperature, the evaporated NO x  remains in a gaseous state until it exits outlet  40 . 
     While water circulation is preferred, it is not necessary. In most applications, gas circulation alone, should normally be sufficient to clean the system of the NO x  deposits as long as the temperature reached inside the support tubes is sufficient to evaporate the NO x    29 , as the gas passes over the NO x  solids  29  and maintain the NO x  in a gaseous state until the NO x  exits the generator  14 . It will also be appreciated that, at relatively high flow rates of cleaning gas, the NO x  solids may be evaporated at a temperature that is significantly below their boiling points due to the vapor pressure effects. 
     In yet a further embodiment, the gas  13 A or water  17 A may be heated after entering the generator  14  by causing the generator itself to be heated. Such heating may take many forms, such as for example by applying a heat source directly to the outside of shell  14 A or by circulating hot water or steam through a jacket  14 B mounted on the outside of shell  14 A, as long as the generator can withstand such heating. 
     Additionally, in a further embodiment of the invention there is no need to heat the cleaning gas directly if the temperature and the effect of the fluids circulating in interior space  35  or jacket  14 B has a sufficient effect on heating the cleaning gas, that the cleaning gas can evaporate substantially all of the deposited NO x    29  and maintain the NO x  in a gaseous state until it exits the generator  14  at outlet  40 . 
     Additionally, it may also possible to remove the deposits of NO x    29  using water circulation only in generator shell  14 A, as long as the temperature inside the support tubes  28  of the generator  14  is sufficient to evaporate the NO x  deposit  29 . However, if this embodiment of the invention is used, a means for creating a flow of gas out of support tube  28 , such as a vacuum pump will be required. In that case, the ozone generator used must be rated to withstand the physical stresses that may result. 
     If warm water is used, it exits generator shell  14 A at outlet  18 B. The water exiting generator shell  14 A has a temperature of preferably between 47-65° C. The water may be discharged in an environmentally safe manner or it may be re-circulated to inlet  32  if it is still warm although more likely it would be returned to inlet  15  and reheated prior to re-circulation through the generator. 
     In a further embodiment of the invention, at least a portion of the cleaning gas exiting support tube  28  through outlet  26  enters a water trap  21  via conduit  19 . The pH of the water in water trap  21  is monitored continuously or manually by a pH meter  22 . The water trap  21  may be any one of several water containers such as a barrel, a tank or a liquid ring compressor, as long as it is sufficiently stable to withstand the expected gas flow into the water. The liquid ring compressor uses an elliptical liquid ring around an impeller to compress the ozone gas. As the ozone is compressed, it gives off heat, but this heat is absorbed by the ring of water. This water is continuously re-circulated through the compressor and through a heat exchanger to cool the water. 
     The majority of the gas exits outlet  20  in front of water trap  21  to an approved scrubbing or capture system. 
     If NO x    29 , including in particular N 2 O 5 , is present, it will react with reference water  23  flowing into the trap  21  and form nitric acid, thereby reducing the pH  24  of the water in the trap  21  below that of the incoming reference water  23 . 
     The reference water  23  is fed into the trap  21  continuously at a certain flow rate, which will depend on the particular system being cleaned, sufficient to record an appreciable pH change at the beginning of the cleaning cycle when the cleaning gas and warm water are at the proper temperature. The value of the appreciable pH change will depend on the pH monitoring system being used. The pH may also be monitored manually. The value of the pH drop can be approximately 3 pH units. The water trap  21  may be any size. The pH is allowed to vary and is monitored. When the monitored pH returns to the same pH as the incoming water, and stays constant, the cleaning will have been completed. 
     In the preferred embodiment, the existing installation liquid ring compressor  21  and in-line pH control system, which includes a pH meter, are used to monitor the pH  24  of the water containing gas from conduit  19 . When a compressor is used, it is not desirable to allow the pH to vary significantly as that might damage the compressor. Therefore, the method employed is that when the cleaning first begins, the pH of the reference water  23  in the liquid ring compressor  21  will start to drop. In response, the in-line pH control system starts to add a neutralizing agent, for example, trisodium phosphate (TSP), to maintain the pH at a substantially stable level. Thus the presence of nitric acid can be ascertained by whether or not the neutralizing agent is continuing to be added. When the addition of the neutralizing agent stops, (i.e. no neutralizing agent is needed because the incoming gas no longer contains significant amounts of NO x  and therefore no nitric acid is formed) the cleaning is complete. 
     The circulation of warm gas  13 A and/or warm water  17 A is maintained until substantially all NO x    29  with boiling points less than 65° C., especially N 2 O 5 , have been removed from support tube  28  and dielectric  27  of the generator. Typically, this is when there is no longer an appreciable difference in pH between the pH  24  of the water exiting the trap  21  and the pH of the reference water  23  entering the trap  21 , if no pH adjustment is applied. 
     It will be appreciated by those skilled in the art, however, that pH monitoring, or more generally monitoring for the presence of NO x  in the cleaning gas, is not necessary for the generator cleaning to be effectual. The cleaning may also be carried out for particular periods of time for which it is known that sufficient cleaning will have occurred, rather than monitoring. However, monitoring will clearly be a more accurate way of ensuring that the generator has been cleaned sufficiently. 
     It will be appreciated by those skilled in the art that while water has been used to describe the above embodiments, other fluids including gases may be used within interior  34  and jacket  14 B as long as they are compatible with the physical characteristics of the ozone generator being used. 
     The aforementioned temperature ranges are influenced by the physical characteristics, including partially the boiling points of the NO x    29 , and the physical limits of the particular generator used. The upper temperature limit of 65° C. may be increased in generators that are constructed to withstand elevated temperatures. The upper limit of the temperature range is then dictated by other concerns such as safety concerns. 
     In the preferred embodiment, the O 3  is purged from the system before the generator cleaning process is started. However, if the ozone generating plant has available means of disposing of water contaminated with O 3 , the cleaning procedure can be commenced without first performing the O 3  purge. 
     The foregoing description is necessarily described with reference to the preferred embodiments of the inventive method applied to a particular ozone generator system but of course, the method may also be applied to other ozone generating apparatus. For example, the ozone generator may consist of spaced apart electrodes that are electrode plates and that have a passageway therebetween. While a plurality of embodiments of this invention has been illustrated in the accompanying drawings and described above, it will also be evident to those skilled in the art that changes and modifications may be made therein without departing from the invention. All such modifications or variations are considered to be within the scope of the invention as defined by the claims appended hereto.