Stock material or miscellaneous articles – Composite – Of carbohydrate
Reexamination Certificate
1998-03-16
2001-05-08
Loney, Donald J. (Department: 1772)
Stock material or miscellaneous articles
Composite
Of carbohydrate
C428S182000, C428S507000, C427S337000, C427S372200, C427S396000, C096S125000, C096S153000, C096S154000, C055S524000
Reexamination Certificate
active
06228506
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a new sensible and latent heat exchange device, hereinafter termed an enthalpy exchanger, and to a method for its manufacture. This enthalpy exchanger consists of a cellulose-based substrate impregnated with a polymeric desiccant. This device has possible applications in the field of air treatment, such as dehumidification, in systems for the transfer of moisture and heat between two air streams, in HVAC systems and in other applications involving water vapour control and recovery.
BACKGROUND
The solid desiccants used in air treatment systems or other applications are primarily inorganic (silica gel, molecular sieves, etc). They take the form of fine powders which must be bonded to a rigid substrate. There are a number of techniques for depositing these desiccants, some of which have been patented. Examples include patents filed in the United States under U.S. Pat. No. 3,338,034; 4,769,053; 5,052,188; 5,120,694; and 5,496,397. U.S. Pat. No. 5,542,968 describes a method which involves mixing the desiccant powder with fibres in a solution containing a binder and fire retardants, among other ingredients. A manufacturing process borrowed from the paper industry is then used to produce sheets of this compound.
Another technique disclosed in Canadian Patent No. 1,285,931 involves coating a metallic substrate with a mixture consisting primarily of an inorganic desiccant and a heat-curable binder or adhesive in a solvent. The powder is then bonded to the substrate by heating the article. U.S. Pat. No. 4,172,164 describes the use of a solvent to dissolve the surface of the thermoplastic substrate, leaving the polymer particles imbedded in it following evaporation of the solvent. These techniques have the disadvantage of inhibiting to some extent the absorption of water by the desiccant powder, which may deliquesce and become detached under conditions of actual use.
SUMMARY OF THE INVENTION
The technique presented here involves the production of an enthalpy exchanger consisting of a cellulose substrate coated with a polymerized desiccant. Polymer-based desiccants have the advantage of being readily modified to obtain the desired absorptive properties, as well as other properties of interest for certain applications. The polymers themselves have relatively good thermal transfer properties thereby enabling sensible heat transfer. They can also be obtained in a number of geometric forms. Some have a certain affinity for structural products used as substrates, which can facilitate bonding.
The product developed consists, preferably, of a potassium salt of acrylic acid polymerized on a corrugated cardboard substrate. The technique used to bond the desiccant to the substrate does not alter the properties of the desiccant, even when the proportion of polymer by mass is relatively low. In addition, the article has good fire resistance and acceptable mechanical strength.
In this application, applicants are also seeking a high rate of absorption and desorption, so that the article reacts rapidly to a sudden variation in the concentration of water vapour or other gases in the flow of air in contact with the desiccant. Deposition of the polymer in a thin layer on the walls of the substrate makes it possible to obtain very rapid sorption kinetics.
In general, the technique consists of preparing a solution with a base of acrylic, methacrylic or itaconic acid or a mixture thereof. The monomer is mixed with a sufficient quantity of a homolytic reaction initiator such as a peroxide, azabisisobutyronitrile or other initiator, in water, acetone and/or other solvents. A cross-polymerization agent such as trimethylolpropane triacrylate, divinylbenzene or other cross-polymerization agent is added to the solution in a quantity corresponding to the desired density of cross-linkages to be obtained in the polymer. The solution is well mixed, then applied evenly to the cellulose-based exchanger. Cellulose has been selected because of its chemical affinity for polymer. The article, impregnated with the solution, is placed in a chamber with low oxygen and heated to a temperature of between 50° C. and 80° C. The polymerization reaction thus initiated should be completed within a few minutes, depending on the rate of thermal exchange in the chamber. Once the polymerized solution is bonded to the substrate, the polymer is placed in contact with a hydroxide solution of sodium, potassium, lithium or other monovalent or bivalent cations. The polymer is transformed into a salt. The substrate and desiccant are then dried to form a rigid article.
When the desiccant absorbs water vapour or other products, its volume increases. In certain applications, the swelling may be excessive and must be controlled or minimized. This can be done by simply adjusting the proportions of the products used in the composition of the polymer, such as the cross-polymerization agent. The swelling of the polymer is, in fact, related to the degree of cross-linkage.
The concentration of the base monomer in the solution can be adjusted on the basis of the desired proportion of desiccant by mass to be obtained. Generally, the amount of polymer present in the article or substrate is from about 5 to 65% by mass, and preferably from about 15 to 30% by mass.
The enthalpy exchanger contains passageways which allow the air to circulate and to come into contact with the substrate walls and the desiccant. The enthalpy exchanger permits exchanges of sensible heat and moisture with an air stream. It may be a rotary or static type with parallel or cross streams. The first case involves wall-to-air exchanges of sensible and latent heat while, in the second, heat and moisture pass through the wall of the exchanger for transfer from one air stream to another.
The enthalpy exchanger of the present invention can be manufactured by either of two techniques. The first consists of constructing the exchanger in its final form and impregnating it with the monomer solution, which is then polymerized. The exchanger can be constructed by rolling the substrate to form a wheel, or by stacking the substrate to produce a static exchanger with cross or parallel streams. The substrate may consist of corrugated cardboard, corrugated paper or any other cellulose-fibre-based product. The second technique involves impregnating a continuous flat sheet of cellulose-based substrate with the monomer solution, which is then polymerized. The composite sheet thus obtained is corrugated before drying to obtain the desired passageway geometry, then rolled to form a wheel, for example.
The technique used to bond the desiccant to the substrate does not affect the desiccant's absorptive properties. This process does not alter the desiccant's absorptive properties. In addition, it gives the substrate a number of other interesting properties, including mechanical rigidity and fire resistance. Due to the thermal properties of the polymer, the desiccant article is also capable of transferring heat from a warm air (gas) stream to a cool air (gas) stream within a recovery system. The bond obtained between the desiccant and the substrate is very strong and permits it to withstand a large number of absorption and desorption cycles without any deterioration in absorptive properties of physical characteristics. A further advantage of this technique is the fact that these properties can be controlled by adjusting the composition and quantity of polymer. Most supports made from natural or synthetic cellulose fibres are permeable to air, which can pose a contamination problem in certain application such as air exchangers. Treatment of this type of support with the polymer makes it much more air-tight and also more rigid, even using amounts of the polymer on the order of only 10%. For an application in the field of total heat exchangers made from paper or cardboard, a quantity of polymer on the order of 15% to 20% of total mass has been found to result in a product with very good sorption capacity and kinetics, good mechanical resistance as well as ve
Amazouz Mouloud
Cote Roland
Hosatte Sophie
Jones, Tullar & Cooper
Loney Donald J.
Natural Resources - Canada
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