PACKING OR BODY FOR MOISTURE EXCHANGER
United States Patent 3782081
A packing element for heat and moisture exchange assembled of thin corrugated sheets of asbestos fiber, the corrugations serving to form cells and separate the cells one from the next, whereby air can be passed from one side of the packing to the other for modification of either its heat or moisture content, the asbestos fibers of the packing being first treated with an organic binder for temporary stability in assembling the thin corrugated sheets into a green packing element, further impregnated with inorganic binder and moisture absorptive substance to provide structural stability and moisture absorbency after the organic binder has been burned out, so that the packing assembly is composed entirely of inorganic fiber and binder substance which may be a hygroscopic salt to enhance the moisture absorbency.
US Patent References:
/1972500.html
Toohey et al. - September 1934 - 1972500
Heat exchanger
Kramig - May 1961 - 2986379
Aluminum phosphate bonded asbestos insulating material
Spooner - February 1955 - 2702068
Method of purifying waste water
Quinn et al. - February 1966 - 3232865
Method of heat treating an inorganic fibrous body for moisture transfer and/or heat
Munters - January 1966 - 3231409
/1972500.html
Toohey et al. - September 1934 - 1972500
Heat exchanger
Kramig - May 1961 - 2986379
Aluminum phosphate bonded asbestos insulating material
Spooner - February 1955 - 2702068
Method of purifying waste water
Quinn et al. - February 1966 - 3232865
Method of heat treating an inorganic fibrous body for moisture transfer and/or heat
Munters - January 1966 - 3231409
Application Number:
05/010610
Publication Date:
01/01/1974
Filing Date:
02/11/1970
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
96/392, 261/112.200, 55/521, 55/527, 55/524
International Classes:
B01J19/32; F24F3/14; F24F3/147; F24F3/12
Field of Search:
161/135,137,205 117/126AF 165/60,10 261/28,112,151,153 55/278,388,521,524,527
US Patent References:
| 3377225 | Method for the manufacture of gas conditioning packing | April 1968 | Munters |
Primary Examiner:
Lesmes, George F.
Assistant Examiner:
Dixon Jr., William R.
Attorney, Agent or Firm:
Wiczer, Sol B.
Parent Case Data:
This application is a continuation-in-part of my co-pending application, Ser. No. 505,501, filed Oct. 28, 1965, now abandoned, in turn a division of my copending application Ser. No. 164,880, filed Jan. 8, 1962, now issued as U.S. Pat. No. 3,231,409. The method of producing the improved coating is fully set forth in said patent.
Claims:
What is claimed is
1. A packing for a moisture exchanger for air conditioning systems comprising a series of thin partition walls formed of corrugated sheet members spaced by intermediate spacing sheets, each of said sheets being composed of natural asbestos fiber, said packing being free of organic binder, said corrugated members and spacing sheets defining a plurality of channels for passing air to be dried from side to side, the asbestos fiber of said sheets being stabilized and stiffened by being impregnated with an insoluble silicate precipitated in situ thereon by reaction of water glass with an insolubilizing agent.
2. The packing according to claim 1 in which the packing is stabilized and stiffened by being impregnated with both a hygroscopic substance and an inorganic stiffening material comprising an insoluble silicate precipitated in situ thereon from water glass.
3. The packing as defined in claim 2 wherein the hygroscopic substance is a member of the group consisting of lithium chloride and calcium chloride.
4. A packing according to claim 1, said body having a hygroscopic substance added to its sheets after it was heat treated to remove organic binder substances so that the moisture-absorbing capacity of the body was increased.
5. A packing for a moisture exchanger for air conditioning systems comprising a series of thin partition walls formed of a series of corrugated sheet members spaced by intermediate spacing sheets, each of said sheets being composed of natural asbestos fiber, said packing being free of organic binder, said corrugated members and spacing sheets defining a plurality of channels for passing air to be dried from side to side, the asbestos fiber of said sheets being stabilized and stiffened by being impregnated with an insoluble calcium silicate formed by being precipitated in situ upon the packing fibers from water glass and a soluble calcium salt.
1. A packing for a moisture exchanger for air conditioning systems comprising a series of thin partition walls formed of corrugated sheet members spaced by intermediate spacing sheets, each of said sheets being composed of natural asbestos fiber, said packing being free of organic binder, said corrugated members and spacing sheets defining a plurality of channels for passing air to be dried from side to side, the asbestos fiber of said sheets being stabilized and stiffened by being impregnated with an insoluble silicate precipitated in situ thereon by reaction of water glass with an insolubilizing agent.
2. The packing according to claim 1 in which the packing is stabilized and stiffened by being impregnated with both a hygroscopic substance and an inorganic stiffening material comprising an insoluble silicate precipitated in situ thereon from water glass.
3. The packing as defined in claim 2 wherein the hygroscopic substance is a member of the group consisting of lithium chloride and calcium chloride.
4. A packing according to claim 1, said body having a hygroscopic substance added to its sheets after it was heat treated to remove organic binder substances so that the moisture-absorbing capacity of the body was increased.
5. A packing for a moisture exchanger for air conditioning systems comprising a series of thin partition walls formed of a series of corrugated sheet members spaced by intermediate spacing sheets, each of said sheets being composed of natural asbestos fiber, said packing being free of organic binder, said corrugated members and spacing sheets defining a plurality of channels for passing air to be dried from side to side, the asbestos fiber of said sheets being stabilized and stiffened by being impregnated with an insoluble calcium silicate formed by being precipitated in situ upon the packing fibers from water glass and a soluble calcium salt.
Description:
This invention relates to a packing or body for the transfer of moisture and heat.
More particularly the invention relates to improvements in a packing or body for the transfer of heat and/or moisture which packing is composed of sheets or layers made of asbestos or other equivalent, non-metallic, inorganic fibrous material and directly or possibly indirectly bearing against one another at mutually spaced points and therebetween forming open channels or passageways extending throughout the packing or body. Usually, at least every second sheet or layer is undulated or corrugated, the spaced points of contact of adjacent sheets being constituted by the ridges of the undulations or corrugations.
An important field of application of the invention is the transfer of moisture and heat between two air flows. The transfer packing or body may in this connection be formed as a rotor moving in a closed path between two passages traversed by one and the other of the air flows, respectively.
The sheets or layers of asbestos shall, in order to be capable of fulfilling their moisture and heat transferring function in the most effective manner, have a thickness not exceeding about one-tenth or some few tenths of a millimeter. Such thin sheets cannot be given the required mechanical strength unless a bonding agent of organic nature such as starch or cellulose fibers contained in the stock is used to cement the asbestos fibers together.
It has been observed that the constituents of material of organic nature in the asbestos sheets in spite of their small proportion relative to the total sheet material are capable of burning as a fuel for a fire in the packing and releasing enough heat to destroy the asbestos. Asbestos has a relatively high fusing point, but the combustion may be so intensive as to convert the asbestos fibers into a powder causing the sheets to disintegrate into dust. A fire may even be started on a moderate heating of the sheets, for example when the packing or body employed for the transfer of moisture is regenerated by means of a heated gaseous medium such as air. The combustion may be started some time after the regenerating flow has been stopped and while the mass of sheets in the packing is still hot. A packing composed of thin asbestos sheets with small channels is excellent heat insulation and consequently constitutes an effective bar to emission of heat remaining in the interior of the packing. The temperature rises to the ignition point when the cooling effect produced by the circulation of air or gas through the packing is terminated. In this way an oxidation is produced in the hot body of sheets and due to the fact that the air within the channels of the body is almost stationary, the temperature rises gradually until the critical point is reached.
The asbestos fibers contain water of crystallization which escapes when the fibers are heated to a sufficiently high temperature and which causes the fibers to decompose into a powder. The temperature at which the crystal water is expelled is approximately about 500°C. in the case of relatively long heating periods, but may be higher if the time of combustion is reduced to seconds or still less. As soon as the critical temperature has been reached, the curve indicating the reduction of weight takes a steep descent which implies that the water of crystallization escapes rapidly.
The principal object of the present invention is to produce a packing or body for the transfer of moisture and heat which eliminates the inconveniences which may result from an unintentional ignition of the packing or body. Another object of the present invention is to provide a packing or body for the purpose above mentioned, and which shall be composed of the material herein described and which insures that organic constituents in the material of which the sheets are made, are rendered innocuous with respect to ignition or combustion or neutralized prior to the employment of the packing or body for its purpose.
The sheets of the body or packing before their use, such as in the moisture or heat transferrer, are subjected to heat treatment by which the organic constituents contained in the sheet material are oxidized. The neutralization of the organic constituents may be effected by combustion inasmuch as adjusting of the conditions for this combustion renders it possible to control the oxidation to thus avoid reaching a dangerous temperature. One possibility of attaining this object is to blow air through the channels of the transferrer packing with such velocity as to prevent the heat developed in the packing during the combustion from surpassing a critical value.
The oxidation may even be conducted so as to heat the transferred packing slowly to a temperature such as between 400° and 550°C. Under this heating various products of distillation will be formed in succession to one another or escape from the packing so that any direct ignition will never occur. The oxidation can in this manner be advanced until the packing becomes non-combustible without any visible combustion in the specific meaning of this term which means combustion producing a flame or temper coloring of the packing. Experiments have revealed that a packing if directly exposed to temperature reaching as high as about 500°C. was immediately subjected to a combustion creating smoke and incandescence.
In some cases a pyrolysis can be carried out until a carbonized residue of organic constituents remains in the asbestos. This residue is of such kind as not to involve any danger of subsequent combustion.
In the production of the improved packing it is essential that the oxidation or combustion is controlled so as to insure that the water of crystallization present in the asbestos is not expelled. A transferrer packing or body of asbestos paper treated according to the invention has the remarkable and surprising effect that it allows the drying of air to a lower moisture content than has hitherto been attainable with transferrer packings of the general type under consideration.
It is known per se in the manufacture of transferrer bodies of ceramic material to apply a ceramic paste onto a strip-formed carrier of a combustible material. This carrier is wound in layers positioned one upon another and causes the transferrer body to retain its form during the firing of the paste to take its solidified state. In connection therewith the carrier is eliminated by combustion which may be conducted at a high temperature without any danger of the ceramic material becoming destroyed or damaged. In this case, the initial carrier is thus replaced by ceramic material which withstands a temperature far exceeding the temperature developed by forced combustion.
The method of producing the improved packing is fully set forth in my co-pending application, Ser. No. 164,880, filed Jan. 8, 1962, now issued as U.S. Pat. No. 3,231,409.
Further objects and advantages of the present invention will become apparent from the following description considered in connection with the accompanying drawing which forms part of this specification and which, in a diagrammatic manner illustrates an apparatus for carrying out the production of the improved packing, and in which:
FIG. 1 is a longitudinal vertical sectional view of an apparatus constructed for the production of a packing according to the present invention, and
FIG. 2 is a partial top view of a transferrer packing shown in a larger scale than that used in FIG. 1.
Referring to the drawing, 10 denotes a kiln having walls of a heat insulating material. Formed in one of said walls is an inlet opening 12 and an outlet opening 14, said openings being provided with a damper indicated at 16 and 18 respectively, or a similar throttling device. The kiln encloses an internal casing 20 made of sheet metal or similar material defining a chamber 22. Positioned within said chamber is a screen or lattice 24 intended to carry a transferrer packing 26 placed thereon for the purpose of being treated. The portion of the lattice 24 not covered by the packing can be screened off by means of shields 28 so as to force all air passing through the lattice to pass through the transferrer packing 26. A perforated plate 30 or similar member may be located below the lattice 24 and has for its purpose to bring about a uniform distribution of the air over the cross-section of the chamber 22. A gaseous medium, in most cases air, is caused to circulate upwardly through the chamber 22 and downwardly past the heater 32 by means of a fan 34 driven by a motor 36 through a shaft 38.
The transferrer packing 26 is composed of thin sheets of asbestos formed by undulation or in any other suitable way so as to bear against one another at mutually spaced points so as to form flow passages extending throughout the packing between the sheets. In the embodiment illustrated in FIG. 2 the packing is composed of alternately plane sheets 40 and corrugated sheets 42. The spacing between the plane sheets is suitably less than 3 millimeters such as between 1 and 2 millimeters, when a packing for a transferrer intended for dehydrating or influencing the temperature of one air flow by means of another flow of air or a gas is under consideration. The sheets at their points of mutual contact are rigidly secured to one another by means of a bonding agent, preferably of inorganic nature, preferably a soluble silicate such as water glass.
The asbestos sheets are made of asbestos fibers which usually are composed of filaments having an extremely small diameter. In order to impart to the sheets required mechanical strength, organic bonding agents such as starch, are added and in addition a minor quantity of cellulose fibers is usually admixed with the asbestos fiber stock, since cellulose fibers possess a higher felting capacity than the asbestos fibers.
The gaseous medium circulating within the kiln 10 is heated by means of the heater 32 to a temperature of between about 200 to 300 and about 500° to 600°C., in other words so high a temperature that the organic constituents in the transferrer packing 26 located on the lattice 24 can be oxidized under predetermined and controllable conditions. During this processing the gaseous medium contains oxygen in a quantity which can be adjusted by supply of combustion air into the closed circuit circulating within the kiln through the inlet opening 12, a corresponding quantity of medium being allowed to escape through the outlet opening 14. Further, the velocity of circulation within the channels or passageways of the packing is adjusted so as to keep the combustion at a temperature below the temperature at which the asbestos is attacked in a detrimental way. A combustion zone is produced which in a controllable manner migrates through the transferrer packing which is in this way relieved of its content of combustible substance.
If the treatment of oxidation or pyrolysis is effected by gradually heating the flow of gas while it passes through the channels or passageways in the packing, the neutralization or removal of the combustible constituents may be carried out without any visible combustion. More volatile products gasified by the heating will escape at a lower temperature than less volatile products until finally an oxidation of non-volatile residues such as carbon can take place at the highest temperature.
By the removal of the organic bonding agent from the packing, the mechanical strength of the sheets of which the packing is composed, is reduced in a corresponding degree. Due to the fact that the sheets are united with one another the entire structure possesses satisfactory strength. However, the strength reduced by removal of organic binder is desirably replaced by restrengthening the sheets after removal by impregnating the packing with a stabilizing inorganic substance. Such inorganic substance is per se a binder material such as water glass, but which preferably is hardened by precipitation of the water glass with an insolubilizing compound such as a soluble calcium or other alkali earth metal salt. Typically useful solubilizing salts such as calcium chloride, barium chloride, magnesium chloride or the like may be used. Such soluble salts react with the water glass in situ as impregnated in packing body to form an insoluble silicate of calcium, barium or magnesium which greatly strengthened the packing body as well as preserved its heat and moisture transfer effectiveness. Such impregnation may be effected before or after removal of the organic binder. The inorganic strengthened substance is preferably and usually applied after the packing body has been built up, and usually before the combustion treatment, but it may be added to the packing body after combustion.
It is particularly desirable where the packing is used in moisture exchange to enhance its moisture absorbency by adding an additional hygroscopic substance such as calcium chloride, lithium chloride or the like. Such hygroscopic substance as lithium chloride may be added together with the solution of water glass, but usually the hygroscopic substance is added after treatment of water glass and its percipitation to an insoluble inorganic binder. Thus, it is useful after silicate precipitation to then treat the packing in a final dip with a hygroscopic solution with one of these salts and finally drying in a warm air oven.
More particularly the invention relates to improvements in a packing or body for the transfer of heat and/or moisture which packing is composed of sheets or layers made of asbestos or other equivalent, non-metallic, inorganic fibrous material and directly or possibly indirectly bearing against one another at mutually spaced points and therebetween forming open channels or passageways extending throughout the packing or body. Usually, at least every second sheet or layer is undulated or corrugated, the spaced points of contact of adjacent sheets being constituted by the ridges of the undulations or corrugations.
An important field of application of the invention is the transfer of moisture and heat between two air flows. The transfer packing or body may in this connection be formed as a rotor moving in a closed path between two passages traversed by one and the other of the air flows, respectively.
The sheets or layers of asbestos shall, in order to be capable of fulfilling their moisture and heat transferring function in the most effective manner, have a thickness not exceeding about one-tenth or some few tenths of a millimeter. Such thin sheets cannot be given the required mechanical strength unless a bonding agent of organic nature such as starch or cellulose fibers contained in the stock is used to cement the asbestos fibers together.
It has been observed that the constituents of material of organic nature in the asbestos sheets in spite of their small proportion relative to the total sheet material are capable of burning as a fuel for a fire in the packing and releasing enough heat to destroy the asbestos. Asbestos has a relatively high fusing point, but the combustion may be so intensive as to convert the asbestos fibers into a powder causing the sheets to disintegrate into dust. A fire may even be started on a moderate heating of the sheets, for example when the packing or body employed for the transfer of moisture is regenerated by means of a heated gaseous medium such as air. The combustion may be started some time after the regenerating flow has been stopped and while the mass of sheets in the packing is still hot. A packing composed of thin asbestos sheets with small channels is excellent heat insulation and consequently constitutes an effective bar to emission of heat remaining in the interior of the packing. The temperature rises to the ignition point when the cooling effect produced by the circulation of air or gas through the packing is terminated. In this way an oxidation is produced in the hot body of sheets and due to the fact that the air within the channels of the body is almost stationary, the temperature rises gradually until the critical point is reached.
The asbestos fibers contain water of crystallization which escapes when the fibers are heated to a sufficiently high temperature and which causes the fibers to decompose into a powder. The temperature at which the crystal water is expelled is approximately about 500°C. in the case of relatively long heating periods, but may be higher if the time of combustion is reduced to seconds or still less. As soon as the critical temperature has been reached, the curve indicating the reduction of weight takes a steep descent which implies that the water of crystallization escapes rapidly.
The principal object of the present invention is to produce a packing or body for the transfer of moisture and heat which eliminates the inconveniences which may result from an unintentional ignition of the packing or body. Another object of the present invention is to provide a packing or body for the purpose above mentioned, and which shall be composed of the material herein described and which insures that organic constituents in the material of which the sheets are made, are rendered innocuous with respect to ignition or combustion or neutralized prior to the employment of the packing or body for its purpose.
The sheets of the body or packing before their use, such as in the moisture or heat transferrer, are subjected to heat treatment by which the organic constituents contained in the sheet material are oxidized. The neutralization of the organic constituents may be effected by combustion inasmuch as adjusting of the conditions for this combustion renders it possible to control the oxidation to thus avoid reaching a dangerous temperature. One possibility of attaining this object is to blow air through the channels of the transferrer packing with such velocity as to prevent the heat developed in the packing during the combustion from surpassing a critical value.
The oxidation may even be conducted so as to heat the transferred packing slowly to a temperature such as between 400° and 550°C. Under this heating various products of distillation will be formed in succession to one another or escape from the packing so that any direct ignition will never occur. The oxidation can in this manner be advanced until the packing becomes non-combustible without any visible combustion in the specific meaning of this term which means combustion producing a flame or temper coloring of the packing. Experiments have revealed that a packing if directly exposed to temperature reaching as high as about 500°C. was immediately subjected to a combustion creating smoke and incandescence.
In some cases a pyrolysis can be carried out until a carbonized residue of organic constituents remains in the asbestos. This residue is of such kind as not to involve any danger of subsequent combustion.
In the production of the improved packing it is essential that the oxidation or combustion is controlled so as to insure that the water of crystallization present in the asbestos is not expelled. A transferrer packing or body of asbestos paper treated according to the invention has the remarkable and surprising effect that it allows the drying of air to a lower moisture content than has hitherto been attainable with transferrer packings of the general type under consideration.
It is known per se in the manufacture of transferrer bodies of ceramic material to apply a ceramic paste onto a strip-formed carrier of a combustible material. This carrier is wound in layers positioned one upon another and causes the transferrer body to retain its form during the firing of the paste to take its solidified state. In connection therewith the carrier is eliminated by combustion which may be conducted at a high temperature without any danger of the ceramic material becoming destroyed or damaged. In this case, the initial carrier is thus replaced by ceramic material which withstands a temperature far exceeding the temperature developed by forced combustion.
The method of producing the improved packing is fully set forth in my co-pending application, Ser. No. 164,880, filed Jan. 8, 1962, now issued as U.S. Pat. No. 3,231,409.
Further objects and advantages of the present invention will become apparent from the following description considered in connection with the accompanying drawing which forms part of this specification and which, in a diagrammatic manner illustrates an apparatus for carrying out the production of the improved packing, and in which:
FIG. 1 is a longitudinal vertical sectional view of an apparatus constructed for the production of a packing according to the present invention, and
FIG. 2 is a partial top view of a transferrer packing shown in a larger scale than that used in FIG. 1.
Referring to the drawing, 10 denotes a kiln having walls of a heat insulating material. Formed in one of said walls is an inlet opening 12 and an outlet opening 14, said openings being provided with a damper indicated at 16 and 18 respectively, or a similar throttling device. The kiln encloses an internal casing 20 made of sheet metal or similar material defining a chamber 22. Positioned within said chamber is a screen or lattice 24 intended to carry a transferrer packing 26 placed thereon for the purpose of being treated. The portion of the lattice 24 not covered by the packing can be screened off by means of shields 28 so as to force all air passing through the lattice to pass through the transferrer packing 26. A perforated plate 30 or similar member may be located below the lattice 24 and has for its purpose to bring about a uniform distribution of the air over the cross-section of the chamber 22. A gaseous medium, in most cases air, is caused to circulate upwardly through the chamber 22 and downwardly past the heater 32 by means of a fan 34 driven by a motor 36 through a shaft 38.
The transferrer packing 26 is composed of thin sheets of asbestos formed by undulation or in any other suitable way so as to bear against one another at mutually spaced points so as to form flow passages extending throughout the packing between the sheets. In the embodiment illustrated in FIG. 2 the packing is composed of alternately plane sheets 40 and corrugated sheets 42. The spacing between the plane sheets is suitably less than 3 millimeters such as between 1 and 2 millimeters, when a packing for a transferrer intended for dehydrating or influencing the temperature of one air flow by means of another flow of air or a gas is under consideration. The sheets at their points of mutual contact are rigidly secured to one another by means of a bonding agent, preferably of inorganic nature, preferably a soluble silicate such as water glass.
The asbestos sheets are made of asbestos fibers which usually are composed of filaments having an extremely small diameter. In order to impart to the sheets required mechanical strength, organic bonding agents such as starch, are added and in addition a minor quantity of cellulose fibers is usually admixed with the asbestos fiber stock, since cellulose fibers possess a higher felting capacity than the asbestos fibers.
The gaseous medium circulating within the kiln 10 is heated by means of the heater 32 to a temperature of between about 200 to 300 and about 500° to 600°C., in other words so high a temperature that the organic constituents in the transferrer packing 26 located on the lattice 24 can be oxidized under predetermined and controllable conditions. During this processing the gaseous medium contains oxygen in a quantity which can be adjusted by supply of combustion air into the closed circuit circulating within the kiln through the inlet opening 12, a corresponding quantity of medium being allowed to escape through the outlet opening 14. Further, the velocity of circulation within the channels or passageways of the packing is adjusted so as to keep the combustion at a temperature below the temperature at which the asbestos is attacked in a detrimental way. A combustion zone is produced which in a controllable manner migrates through the transferrer packing which is in this way relieved of its content of combustible substance.
If the treatment of oxidation or pyrolysis is effected by gradually heating the flow of gas while it passes through the channels or passageways in the packing, the neutralization or removal of the combustible constituents may be carried out without any visible combustion. More volatile products gasified by the heating will escape at a lower temperature than less volatile products until finally an oxidation of non-volatile residues such as carbon can take place at the highest temperature.
By the removal of the organic bonding agent from the packing, the mechanical strength of the sheets of which the packing is composed, is reduced in a corresponding degree. Due to the fact that the sheets are united with one another the entire structure possesses satisfactory strength. However, the strength reduced by removal of organic binder is desirably replaced by restrengthening the sheets after removal by impregnating the packing with a stabilizing inorganic substance. Such inorganic substance is per se a binder material such as water glass, but which preferably is hardened by precipitation of the water glass with an insolubilizing compound such as a soluble calcium or other alkali earth metal salt. Typically useful solubilizing salts such as calcium chloride, barium chloride, magnesium chloride or the like may be used. Such soluble salts react with the water glass in situ as impregnated in packing body to form an insoluble silicate of calcium, barium or magnesium which greatly strengthened the packing body as well as preserved its heat and moisture transfer effectiveness. Such impregnation may be effected before or after removal of the organic binder. The inorganic strengthened substance is preferably and usually applied after the packing body has been built up, and usually before the combustion treatment, but it may be added to the packing body after combustion.
It is particularly desirable where the packing is used in moisture exchange to enhance its moisture absorbency by adding an additional hygroscopic substance such as calcium chloride, lithium chloride or the like. Such hygroscopic substance as lithium chloride may be added together with the solution of water glass, but usually the hygroscopic substance is added after treatment of water glass and its percipitation to an insoluble inorganic binder. Thus, it is useful after silicate precipitation to then treat the packing in a final dip with a hygroscopic solution with one of these salts and finally drying in a warm air oven.