WO2005007207A1

WO2005007207A1 - Method and apparatus for the sterilisation of air that is intended to ventilate spaces requiring air with a low micro-organism content

Info

Publication number: WO2005007207A1
Application number: PCT/FR2003/001834
Authority: WO
Inventors: Georges Gau
Original assignee: Georges Gau
Priority date: 2003-06-17
Filing date: 2003-06-17
Publication date: 2005-01-27
Also published as: EP1633406A1; AU2003267484A1

Abstract

The invention relates to a method for the thermal sterilisation of air used to ventilate spaces requiring air with a low micro-organism content. The inventive method is characterised in that the air to be sterilised is circulated by forced flow in a cyclic manner in opposing alternate directions, for example using an air circulator (1) and solenoid feed valves (3), through a thermal sterilisation chamber (12) housing an electrical resistor (7) which is positioned between two stacks of wire grilles (8, 9), said air flowing perpendicularly to the stacks of grilles.

Description

Method and apparatus for sterilizing air intended for the ventilation of premises requiring air with a low content of microorganisms. The present invention relates to a method and an apparatus for sterilizing air intended for the ventilation of premises requiring air with a low content of microorganisms. This sterilization is obtained by a thermal process, that is to say by raising the temperature of the air using an electrical resistance and recovery of the enthalpy of the hot gas by transfer to the cold air introduced into the device and taken inside or outside the premises. The sterile air thus produced can be used for the pressurization of a room - hospital room, dental office, etc. - but also to create, inside a room, a sterile area near a table. surgery or a food or drug processing unit. Biological contamination of local air is mainly due to bacteria, some of which may exhibit, in the form of spores, increased resistance to relatively high temperatures. PRIOR STATE Mention is made, simply for the record, of chemical sterilization using certain products (ozone, formaldehyde, ethylene oxide) or sterilization by means of ultraviolet radiation. These methods are more generally used to sterilize instruments rather than to treat large airflows such as those necessary for partial or total ventilation of the premises. The technique generally used is fiitration on porous support; the filter retains both inert solid particles and, in some cases, bacteria whose size is generally less than 2 microns (micrometers). Filters called "absolute", or very high efficiency, or HEPA, have a fiitration power which stops bacteria of size equal to 0.3 microns. Their cost (purchase and maintenance) is high and their effectiveness is variable over time as a result of clogging. This cost is very sensitive to the size of the particles to be eliminated. Absolute fiitration must always be preceded by a very efficient classic fiitration. The device according to the invention has an efficiency independent of the size, and it does not undergo clogging, even after long periods of use. It can even, in some cases, simultaneously neutralize bacteria, viruses, epidermal scales, molds, fungi and solid or liquid aerosols, without the need to use a very advanced prior fiitration, which can be very useful when you want to recirculate the air in a room. In addition, absolute filter integrity measurements and aerocontamination measurements are difficult to automate and cannot be used for systematic and routine checks. The automatic regulation of the sterilization temperature offers a better guarantee in the case of the sterilizer according to the invention. It is certain that absolute fiitration will always retain its importance for modern operating theaters in which complex operations are carried out with deep wounds or bone prostheses. In general surgery or endoscopic surgery, the standards of sterility are less strict. There is therefore a very wide field of applications among which sterilization by heat treatment will find outlets, which will make it possible, thanks to an acceptable cost, to generalize decontamination by ventilation. The apparatus according to the invention is partially based on the fundamental principle of regenerative, cyclic exchangers with flow reversal. This principle was illustrated in the book: WM Kayes and AL London

"Compact Heat Exchangers", 2 ^nd Edition, Me Graw-Hill, New York, 1964

(see p 27) as well as in the first edition published in 1952. The cyclic flow reversal device has rarely been used and, to our knowledge, has not found any application in the treatment of air for ventilation Locals. It was preferred to cross plate heat exchangers although they have lower efficiencies. Two patents describing the implementation of the cyclic device have been granted to the author of the present application. European Patent 1.029.203 describes the use of an exchanger in the recovery of enthalpy and humidity from the air extracted from the premises to transfer them to the incoming air; this device therefore corresponds exactly to what is designated as a double flow ventilation device with energy recovery. It operates with a clean air circuit and a stale air circuit with two tanks and two fans. European patent 0.607.379 is intended for the catalytic combustion of hydrocarbons, that is to say the elimination of chemical pollutants. It describes an apparatus comprising two stacks intended for the storage of heat, which are not placed in two separate tanks but in the same tank which also contains the catalyst. The total pressure drop (stacks + catalyst) is relatively high, but we are not looking for a purification efficiency or a very high heat transfer efficiency. A cyclic device which can be considered as conventional is therefore sufficient in spite of its drawbacks. The design of a cyclic device, intended to produce sterile air close to the user, in the same room, with a very high sterilization efficiency, close to 99.9%, requires the resolution of serious problems. The invention aims to provide an apparatus making it possible to solve these problems and to obtain, simultaneously, a significant reduction in the volume of the apparatus, in its energy consumption and in its investment cost and consequently in obtain a system that can be installed not only in medical or dental establishments but also in homes, schools, workshops, shops, or other premises. In its application to the equipment of medical care establishments, the non-noisy operation of the device makes it possible to envisage its installation near the sick. SUMMARY DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for sterilizing the air in premises; it is essentially composed of a heat exchanger, cyclic, with flow reversal, making it possible to recover at least a fraction of the enthalpy necessary to raise the temperature of the ambient air to the sterilization temperature and transmit it to the air to be treated; the heat exchanger comprises two zones included in a single enclosure which can be installed horizontally or vertically, and, in which are arranged two stacks of metal grids intended to accumulate the heat (enthalpy) of the air passing through them; these grids are arranged perpendicular to the gas flow, that is to say perpendicular to what we will designate "main axis" or "axis of symmetry"; between these two zones, and, more precisely, between the two stacks of grids metallic, an electrical resistance is provided providing the necessary electrical energy; the grids consist of continuous metallic fabrics woven with threads whose diameter is between 0.1 mm and 1 mm; these metal grids have a volume porosity of between 75% and 95% and, consequently, the stacks of grids have a slightly higher porosity; each stack of grids has a higher thermal conductivity in the direction perpendicular to the main axis and to the gas flow than in the direction of this flow, direction parallel to the main axis; a centrifugal fan is used to circulate the air in the direction of the main axis, in one direction or, after inversion, in the opposite direction, perpendicular to the metal grids; a cyclic programming system makes it possible to open or close the solenoid valves of a set of solenoid valves, and this with a cycle time of less than one minute. At the intake of the centrifugal fan, a low efficiency and easily removable solid particle filter is placed. According to the method of the invention, the fan blows the ambient air sucked through one of the main solenoid valves, during a half-cycle, the air entering through this solenoid valve passes through a distribution chamber (plenum), then a stack of grids, then an electrical resistance, then the other stack of grids, then a second main solenoid valve, to then be either injected into the room, or recycled to the suction of the fan, through a third solenoid valve (purge solenoid valve) . During the next half cycle, the flow is reversed. The two main valves are therefore kept either open or closed for half a cycle. The two purge valves are also actuated by the programmer, but their opening time is relatively short and depends on the amount of air that one wishes to recycle. To improve the distribution of the speed profile at the entrance to a stack of grids, the distribution chamber (plenum) which precedes it must have a relatively large volume, greater than or equal to the volume of the stack of grids. We can therefore consider that the air contained in this chamber as well as that contained in the corresponding stack of grids is not sterile and that, in the absence of recycling, at the start of the inversion, the air would be polluted. The average efficiency of the device would be reduced. The use of the two purge solenoid valves is therefore one of the important characteristics of the apparatus according to the invention. Another important characteristic of the invention is the simultaneous implementation of a packing of high volume porosity and very low pressure drop, of a relatively short length, associated with a high inversion frequency, the metal constituting the grids being a very good conductor of heat (aluminum or copper). The effect of the conduction would be very weak in permanent regime: it is very important in cyclic regime because the heat accumulated during a half-cycle can redistribute throughout a grid and thus compensate for the relative ineffectiveness due to a distribution imperfect air in the device. Consequently, thanks to the combination of its different characteristics, the device presented makes it possible to obtain the desired objective: efficient and economical sterilization of a large air flow, with a compact, autonomous and low noise device. DESCRIPTION OF THE DRAWING Figure 1 shows the block diagram of the sterilizer or thermal sterilization device of the invention shown during a half-cycle during which the air circulates from left to right, perpendicular to the stacks of grids. Figure 2 is a schematic view similar to Figure 1 and showing the flow of air flow in the opposite direction during the next half cycle. Figure 3 is a schematic view similar to the previous ones and showing one of the purge solenoid valves in the open position, so as to allow the recycling of untreated air at the end of each half-cycle. According to the method of the invention, the air to be sterilized is forced into circulation, in a cyclic manner and according to flows of alternately opposite directions, for example by means of a fan and of distribution solenoid valves, through an enclosure. thermal sterilization containing an electrical resistance positioned between two stacks of metal grids, perpendicular to said stacks of grids. Advantageously, energy is dissipated from the gas flow by passing it through this empty zone or plenum reserved for the inlet and the outlet of the thermal sterilization enclosure. On the other hand, preferably, the frequency of the inversion of the direction of the air flow is greater than one inversion per minute and each cycle consists of two half-cycles, of equal duration, and the untreated air. at the end of each half cycle is recycled to the fan suction. DETAILED DESCRIPTION OF THE INVENTION The apparatus comprises a centrifugal fan 1, provided with a filter 2 intended to remove dust and suspended particles, and which is positioned to blow air from the street or from the ventilation ducts from the building to four solenoid valves, two of which designated by reference 3 can be considered as the main solenoid valves and the other two designated by reference 4 as the purge and recycling solenoid valves. These solenoid valves are actuated by electromagnets or by small electric motors or the like, activated by a cyclic programmer (not shown) known per se. The main valves 3 make it possible to introduce the air to be treated to one or other of the two inlets of the enclosure in which the sterilization elements 7-8-9 described below are assembled. In the position indicated in Figure 1 of the drawing, corresponding to the first half of the cycle, the air enters at 5 and exits at 6. In the second half of the cycle (Figure 2), after reversing the state of the solenoid valves, the air enters at 6 and exits at 5. References 5 and 6 designate empty zones or chambers, or "plenums". These empty zones or "plenums" 5 and 6 have volumes at least equal to or greater than the volumes of the sterilization zones 8 and 9. The sterilization zone comprised between the inputs-outputs 5, 6, consists of three parts included in a single enclosure 12:

- An element 7 consisting of an electrical resistance well distributed in a section perpendicular to the flow and of the type mentioned below.

- two zones 8 and 9 filled with stacks of metal grids arranged perpendicular to the gas flow or to the main axis of symmetry and which act as heat accumulators. The characteristics of these grids are also specified in the following description. Thanks to these devices, a cyclic heat exchanger with reverse flow operating by accumulation of heat (enthalpy) of the air in the metal of the metal grids is obtained. This exchanger operates in a cyclic regime, that is to say in a dynamic regime which is designated by the name of pseudo-stationary regime. Its operation is as follows (FIG. 1): after a period of heating of the grids, the air which enters at 5 to 25 ° C. is preheated by the stack of grids 8 to a neighboring temperature (190 ° C.) of the maximum desired temperature (200 ° C) then reheated by 10 ° C by the electrical resistance 7 before being cooled by the stack of grids 9 to about 33 ° C. After reversing the state of the two main solenoid valves 3, the stack of grids 9 serves as a preheater and the stack of grids 8 as a cooler (FIG. 2). The heat input obtained by the resistor 7 could be very low if it were possible to cool the air to a temperature equal to its inlet temperature. To achieve this, it would be necessary for the two stacks of grids to have a very long length, that is to say a large number of grids and a large exchange surface which obviously is not economical (large volume and pressure drop high). The outlet temperature in 6 is necessarily higher (by 7 ° C with the data indicated above) than the inlet temperature in 5. We then say that the actual efficiency of the exchanger is: (200 - 33 ) / (200 - 25) = 95.4% The enthalpy, corresponding to 10 ° C, provided by the electrical resistance is therefore used to compensate for the loss of 7 ° C (inefficiency of the exchanger 4.6%) and a loss equivalent to 3 ° C to compensate for the losses through the thermal insulation 10. This simplified, but more pictorial presentation of the enthalpy balance, would not be modified if one took into account the evolution of the exit temperatures during the duration of half a cycle. The electrical resistance 7 must have a relatively high exchange surface and be arranged across the square section or rectangular of the sterilization zone, so as to distribute the energy as uniformly as possible over the entire section. The simplest method is to use a bare nickel-chromium resistor which is in the form of a long coil (helical spring) and which crosses this section many times, at regular intervals. The start-up and stabilization period is as follows: the button on the switch of the appliance's electrical installation is pressed, which simultaneously activates fan 1 and the electronic programmer (not shown) ) and the electrical resistance 7. The maximum temperature of the lining (measured at a point located in the middle of the appliance) gradually increases to reach, after approximately sixty inversion cycles, that is to say approximately twenty minutes, the temperature chosen when designing the device. A temperature regulator is not essential. The pseudo-stationary regime is therefore reached and the performance of the device remains constant over time. An essential property of the device is to minimize the supply of energy to the resistor and to the fan, without increasing the investment cost. For this, we have found that the storage heat exchange system must meet the following characteristics:

1) Use of metal grids (woven fabrics or expanded metal) with a large specific surface (square meters of surface per cubic meter of stacking). The wire diameter (or equivalent diameter in the case of expanded metal) should be small to increase the heat transfer between the air and the metal. Furthermore, this large area must correspond to a relatively large stacking volume, obtained with grids of high volume porosity, or (which is generally equivalent) a high percentage of vacuum, an essential condition for reducing the pressure drop required. and supplied by the fan. 2) Use of metal grids having a volume porosity of between 75% and 95%, and wires of diameter between 0.1 mm and 1 mm, constituting the best solution to minimize energy consumption (resistance and fan). 3) The frequency of the inversion cycle must be high, greater than 1 cycle per minute. For example, the duration of a cycle can be 20 seconds, so that the direction of air flow through the sterilization chamber is reversed every 10 seconds. 4) A fourth characteristic is the obtaining of the flattest possible speed profiles at the entrance and inside the two stacks. A non-flat profile results in the equivalent of a bypass (foxing) and therefore in a reduction in the efficiency of the heat exchanger, which leads to increasing the number of grids and increasing the pressure drop. A relatively satisfactory speed profile is obtained by the combination of the following arrangements:

- Use of air distribution flaps extending over the entire depth of the appliance, oriented obliquely (around 45 °); preferably, the section of the sterilization enclosure is square or rectangular and the component flaps of the valves have the same length as the largest side of the section; it is observed that the flaps of the distribution valves 3 also act as deflectors or distributors making it possible to distribute the air over the entire inlet section of the sterilization zones.

- Dissipation of kinetic energy from the gas flow in empty zones or "plenums" 5 and 6 whose volume is equal to or greater than the volumes of the zones

8 and 9 of the stacks (which has not been shown in FIG. 1, for the sake of simplification).

- Use, in the same enclosure, of the preheating stack and the recovery stack, which helps to level the imperfections of the speed front at the entrance because the “useful” pressure drop is double and, d on the other hand, the flat profile obtained at the exit of a stack is transmitted to the stack which follows it. We can speak of a “useful” pressure drop because it is well known in aeraulics that a high pressure drop reduces the bypass effect. - Each stack of grids 8, 9 is preceded by a perforated plate 11 (distributor), pierced with numerous holes of different diameters, the number and size of which are defined, in each particular case, after a study of the distribution of speeds measured with suitable instruments (hot wire anemometer for example); in other words, a perforated plate 11 is placed between each empty zone 5 or 6 and the adjacent sterilization zone 8 or 9, this distributing grid alternately materializing the inlet face and the outlet face of each stack of grids or sterilization zone. 5) Another essential characteristic is the use of metal grids made with a metal which is a very good conductor of heat. Indeed, even when all the conditions mentioned above are met, the speed distribution will never be perfectly flat. But the aim is not, as such, to obtain this flat profile but a temperature front as flat as possible so that all the solid particles (bacteria or spores) are brought to the maximum temperature (and what whatever their point of entry into the stack of metal grids). The thermal conduction, along the wires of a grid contributes well to flatten the temperature front; but calculation and experience show that, in steady state, this effect is not very significant, because the air crosses each stack in less than 1/10 th of a second, a time too short for the balancing of temperatures of the same grid can be effective. But it is not the same in pseudo-stationary cyclical regime. For cycles of 20 sec. (half cycle: 10 sec), conduction allows temperature rebalancing for a much longer time (100 times more), provided of course that the metal is a good conductor as shown by the gain in thermal efficiency obtained in l Example 2 below. It is moreover very obvious that the conduction of a grid towards the two neighboring grids has a zero effect both in steady state and in pseudosteady state, because this conduction can only be done on a few contact points (including the equivalent area is zero). Another phenomenon, probably secondary and difficult to assess, is the migration of solid particles perpendicular to the gas flow due to their multiple impacts with the wires of the grids; this phenomenon, purely aeraulic, is not amplified in cyclic regime but it certainly contributes to reduce the effect of bypass. Advantageously, the metal grids forming the stacks of grids housed in the sterilization zones are made of aluminum, or copper, or galvanized iron, or stainless steel. These grids are by example executed in the form of metallic fabrics or in expanded metal having similar aeraulic characteristics. 6) One of the drawbacks of the cyclic regime is that it makes it difficult to obtain a heat exchange efficiency greater than 97%. In fact, at the end of the half-cycle corresponding to Figure 1, zones 5 and 6 are cold and contain air that is not sterilized or heated. The sweep from zones 6 and 9 in the following half-cycle rejects this air which will then be mixed with the sterile air. For a complete cycle of 20 seconds, this volume of air, for the device described in Example 1, is 0.025 m3 compared to the 1.1 m3 treated during this cycle. There is therefore, from the point of view of sterilization a loss of efficiency of 2 to 3%, which has no notable consequence for energy consumption but can have a significant influence on the quality of the air obtained . So far, an apparatus has been described which operates only with the two main solenoid valves 3. The incorporation of the two additional solenoid valves 4 which will be called "purge solenoid valves" eliminates this drawback. The electronic timer of the cyclic programming system makes it possible to open one or the other of these solenoid valves for a determined time (1 second for example) and to recycle the untreated air towards the suction of the fan (figure 3) . In this case, sterile air is therefore produced for only 9 seconds, per half-cycle. This scanning also makes it possible to recycle microorganisms which, possibly, would have remained “hooked” on the grids. The recycling rate can be increased at will, which by multiple passes increases the efficiency of sterilization. It is observed that the efficiency (conversion rate) of sterilization, apart from the purging phenomenon, can be much higher than the thermal efficiency of the exchanger, because the destruction of a microorganism depends on both temperature and the time spent at that temperature but the function of temperature is an exponential while the function of time is linear. The role of the exponential does not exist in the heat exchange which, on the other hand, is favored by the improvement due to the regime cyclic, which, in turn, results in a leveling of temperatures and, consequently, an improvement in the efficiency of sterilization. From kinetic studies published concerning sterilization phenomena, it can be estimated that a difference of 15 ° C between two portions of the same heat exchanger grid would result in an increase (or decrease) by a factor of 30 the local sterilization speed. PROTOTYPE AND TEST PROTOCOL The tests described in the following three examples were all carried out in a single device, as shown in Figure 1. To facilitate the implementation of the tests, the dust filter (2) was removed and the apparatus was placed directly in a laminar flow laboratory hood equipped with a HEPA filter, the air flow of which is greater than the air flow of the prototype. The air flow in the sterilization zone was equal to 200m3 / h, which was obtained by varying the speed of the motor via an auto-transformer. The duration of a complete cycle was chosen equal to 20 seconds, each purge lasting 1 second. The main solenoid valves 3 are therefore inverted every 10 seconds, and the purge solenoid valves 4 inverted for 1 second at the start of a half-cycle. The air flow actually produced is therefore 180 m3 / h. The sterilization and heat exchange zone consists of a metallic box of thin stainless steel (0.2mm), in order to reduce the longitudinal thermal conduction through the walls. Its section is square, 30 cm side and its length 25 cm. ; it is insulated on all four sides with 2.5 cm of rock wool. Each stack 8 and 9 is, for example, constituted by 120 contiguous (but not compressed) grids for a total length of 10 cm. These grids are 30 cm x 30 cm squares cut out of metal fabrics sold by the Gantois Company. The wires of these grids pass right through the square (or other) section without breaking the thermal conduction along the wires. With a flow rate of 200 m3 / h the pressure drop corresponding to the 240 grids is of the order of 2 mbar (i.e.: 2 hecto pascal). The pressure drop in the diffusers 11 is equal to 0.1 mbar. Plenums 5 and 6 are 10 cm long. Resistor 7 is connected to an auto-transformer, which allows the maximum sterilization temperature to be selected. The thermal efficiency of the exchanger is calculated from the measurements of several thermocouples installed in the device. The effectiveness of sterilization is measured by injecting an aerosol of a spore very resistant to dry heat at the inlet of the ventilator, Bacillus Subtilis (Niger variety), generally used to check the sterility of materials after heat treatment. in a closed enclosure. These bacilli are not pathogenic. Several suspensions are prepared at different spore contents; the liquid is introduced using a variable speed peristaltic pump into a 20 kHz atomizer-sonicator (of Sonics and Materials brand), the droplets obtained having an average of 90 μm in diameter. These droplets are instantly vaporized inside the fan and a solid aerosol is obtained and dispersed homogeneously in the flow of 200m3 / h. A SAS Super 100 biocollector is used to take samples before and after sterilization by impacting microorganisms on agar after suction of a certain volume of air. This hygiene controller is equipped with a sampling head for 90 mm diameter petri dishes. The bacillus being relatively inert, it is advisable to use as culture medium, trypticase - soy agar (TSA) and an incubation temperature of 56 ° C. The samples were entrusted to a private laboratory of medical biology analyzes both for the incubation and the counting of the colonies and for certain problems related to the sterilization of the materials.

EXAMPLE 1 The prototype described above is equipped with aluminum cloths with a nominal opening 1.4 mm and whose volume porosity is equal to 0.875. The wire diameter is 0.265 mm. After a period of 20 minutes, corresponding to 60 cycles of 20 seconds, the pseudo-stationary regime is reached. The entry temperature into a stack is 25 ° C (2 ° heating in the fan), 190 ^C C at the exit of the stack and 200 ° C after crossing the section occupied by the electrical resistance. The average outlet temperature is 33 ° C, which therefore corresponds to a thermal efficiency of 95.4%. The electrical energy consumed at the resistance is 0.660 kwh per hour, to which should be added to 0.100 kwh per hour consumed by the fan, or a total of 0.760 kwh for 180 m3 of sterile air. At the price of € 0.07 per kwh, the expenditure is € 0.04 per hour. In a typical test, the air to be sterilized contained 1,200 bacteria / m3, or more exactly 1,200 cfu (colony-forming unit) per m3. The treated air contained 6 cfu / m3, that is to say that the sterilization efficiency was 99.5%. This efficiency is much higher than thermal efficiency (thermal ineffectiveness is 4.6%; sterilization ineffectiveness equal to 6/1200, is 0.5%, therefore approximately 10 times lower. sterilization efficiency should increase rapidly with the maximum operating temperature, but the inaccuracy of colony counting does not allow this increase to be evaluated. However, thermal efficiency is practically independent of this temperature (energy consumption increases slightly as the temperature increases as a result of losses through the thermal insulation) EXAMPLE 2 The transverse thermal conduction obtained thanks to the metallic wires is illustrated by this example. The origin of this phenomenon, already mentioned, is obviously the leveling of the temperatures. the same grid by increasing the transfer time (neighboring but not equal to 10 seconds) in pseudo-stationary cyclical regime, much higher at the very short time (1/10 second) of crossing the stack. Three metallic fabrics (square mesh fabrics) from the Gantois Company were compared using the prototype. The three fabrics (Copper, Galvanized steel, 316 L stainless steel) have a nominal opening of 1.5 mm. and 0.5 mm wires. diameter, that is to say identical dimensions and the same volume porosity equal to 81%. An aluminum fabric having the same characteristics does not exist commercially and these fabrics do not exist at the nominal opening of 1.4 mm. from example 1. The following table allows you to compare the properties of the four metals (in relative values). In the three tests (Copper, Galvanized Iron, Stainless Steel) the number of grids was identical (60 in each of the stacks). All the other operating conditions were identical to those of Example 1, as well as the efficiency measurement protocol.

The three tests made it possible to measure the following thermal efficiencies: Copper 0.96 10 Galvanized Iron 0.93 Stainless Steel 0.92 The only factor explaining these differences is the thermal conductivity of the metal, the other two factors (specific heat and specific gravity) being practically identical. Note that the ineffectiveness increases from 8% (Stainless Steel) to 4% (Copper). Aluminum would have a performance close to that of copper. Analysis of the temperature map at the exit from the preheating zone has shown that temperature differences of the order of 15 ° C. can be observed at different points of the cross section, this in the case of the grids. stainless steel but the gaps are much smaller with the copper grids. There is therefore a leveling of temperatures, favored in cyclic regime by a high thermal conductivity. The results concerning the sterilization efficiency are as follows: Copper 99.5 25 Galvanized Steel 97 Stainless Steel 95 Inefficiency goes from 5% (Stainless Steel) to 0.5% (Copper), a factor of 10 more important than that observed (equal to 2) for thermal inefficiency. Assuming that the temperature difference observed (15 ° C) on a section corresponds to about 20% of the passage section and that this drop in temperature is due to a too high local flow of 20% in this part of the device , it can be calculated, by assessment, that the ineffectiveness due to the bypass is multiplied by 8. This for a factor of reduction of the sterilization speed of 30 for a drop of 15 ° C; this factor is of the same order as that observed and equal to 10. This calculation has only a qualitative illustration value and is very approximate. EXAMPLE 3 The purge, equal to 10% of the air treated in the previous examples, obviously plays an important role in the quality of the air obtained. The performance of the test of Example 1 is compared with that obtained by eliminating the purge. The purge solenoid valves 4 are permanently maintained in the closed position, that is to say in the positions shown in FIG. 1. The efficiency of the heat exchanger is not modified by the elimination of the two purges. At the end of a preheating cycle, the air volume in zones 5 and 8 (or 6 and 9) is equal to 0.0125 m3, i.e. 0.025 m3 for a full 20-second cycle and an air flow per cycle of 1.11 m3.

A purge time of 0.125 seconds on each of the solenoid valves 4 would, in principle, be sufficient to remove the untreated air, i.e. 2.25%. The overall ineffectiveness of the sterilizer of Example 1 then becomes equal to: 0.9775 x 0.005 + 0.0225 x 1 = 0.0274 That is to say 5.5 times higher than in the presence of the purge. It is not essential to use a purge rate of 10%, but in all cases a rate greater than 5% is used to compensate for problems of mixing in the purge circuit and to eliminate certain solid particles which could adhere to the cold grills. The loss of efficiency due to this “external” bypass cannot be modified by varying the uniformity of speeds and leveling of temperatures in an air passage section, any more than in increasing the number of racks or increasing the sterilization temperature. Purging is therefore essential, unless preference is given to external recycling in the room to be decontaminated. The influence of the "internal" bypass, as described in Example 2, on the contrary, can be reduced by standardizing the speeds and temperatures, by increasing the number of grids and the sterilization temperature.

Claims

1. Method for thermally sterilizing the ventilation air of rooms requiring air with a low content of microorganisms, characterized in that the air to be sterilized is forced into circulation, cyclically and according to alternately opposite direction flows , for example by means of a fan (1) and distribution solenoid valves (3), through a thermal sterilization enclosure (12) containing an electrical resistance (7) positioned between two stacks of metal grids (8, 9 ), perpendicular to said stacks of grids.

2. Method according to claim 1, characterized in that the energy of the gas flow is dissipated by passing it through empty zones or plenums (5, 6) reserved for the entry and exit of the thermal sterilization chamber (8-7-9).

3. Method according to one of claims 1 or 2, characterized in that the frequency of the inversion of the direction of the air flow is greater than one inversion per minute.

4. Method according to any one of claims 1 to 3, according to which each cycle consists of two half-cycles, preferably of equal duration, characterized in that the untreated air at the end of each half-cycle is recycled to the fan intake (1).

5. Method according to any one of claims 2 to 4, characterized in that the fan (1) is installed so as to move the ambient air sucked, during the first half-cycle, through one of the solenoid valves of distribution (3), then of an empty zone (5) or plenum, then of one of the two stacks of metal grids (8), then of a heating zone (7), then of the second stack of metal grids (9), then a second distribution solenoid valve (3), then, finally, to the room to be ventilated or through a purge valve; the air follows the reverse path after the reversing of the distribution valves (3), for the entire duration of the second half-cycle.

6. Apparatus for thermally sterilizing the ventilation air of premises requiring air with a low microorganism content, comprising a thermal sterilization enclosure (12) and means making it possible to establish forced air circulation through said enclosure , these means comprising, for example, a centrifugal fan (1), and distribution solenoid valves (3) characterized in that said thermal sterilization enclosure (12) contains an electrical resistance (7) disposed between two stacks of metal grids ( 8, 9), and in that the means for establishing forced air circulation through said thermal sterilization enclosure include a cyclic programming system, solenoid valves (3) and an air circuit for directing the air flow produced, alternately, on one side and the other of said enclosure, perpendicular to the stacks of metal grids (8, 9).

7. Apparatus according to claim 6, characterized in that the air circuit comprises two empty areas or plenums (5, 6) formed in front of the inlet / outlet faces (11) of the thermal sterilization enclosure (8- 7-9).

8. Apparatus according to claim 7, characterized in that the volume of the empty areas (5, 6) formed in front of the inlet / outlet faces (11) of the thermal sterilization enclosure (12) is equal to or greater than the volume stacks of grids (8, 9).

9. Apparatus according to any one of claims 6 to 8, characterized in that the input face of each stack of grids (8, 9) is materialized by a perforated plate (11) pierced with many holes of different diameters.

10. Apparatus according to any one of claims 6 to 9, characterized in that the metal grids consist of continuous metallic fabrics, produced with a wire having a diameter between 0.1 mm and 1 mm.

11. Apparatus according to any one of claims 6 to 9, characterized in that the metal grids (8, 9) are made of expanded metal.

12. Apparatus according to any one of claims 6 to 11, characterized in that the metal grids (8, 9) have a volume porosity between 75% and 95%.

13. Apparatus according to any one of claims 6 to 12, characterized in that the metal grids constituting the stacks of grids (8, 9) are made of a metal having a very high thermal conductivity, for example aluminum, or in copper, or galvanized iron.

14. Apparatus according to any one of claims 6 to 13, characterized in that each stack of grids (8, 9) has a thermal conductivity which is very large in a section perpendicular to the main axis of said stack and practically zero in the direction of this axis.

15. Apparatus according to any one of claims 6 to 14, characterized in that the electrical resistance (7) is arranged in the middle part of the sterilization enclosure (12) and it is shaped to present an exchange surface significant through the square or rectangular section of said sterilization enclosure.

16. Apparatus according to any one of claims 6 to 15, characterized in that it comprises purge solenoid valves (4) making it possible to recycle the untreated air towards the suction of the fan at the end of each half-cycle .

17. Apparatus according to any one of claims 6 to 16, characterized in that the section of the sterilization enclosure (12) is square or rectangular, and in that the opercular members of the solenoid valves (3, 4) are formed by movable flaps having a length identical to that of the largest side of said section.