DEHUMIDIFIER

Abstract: A rotary, parallel plate, regenerative dehumidifier comprises a rotor, itself comprising a multi-spoked frame supporting a spirally wound, plastics film ribbon (12) coated on both surfaces with a desiccant. Gas may flow axially through the rotor between the turns of ribbon so as to contact the desiccant thereon. The rotor is encircled by a casing and circumferential and diametral seals define two gas flow passages extending in the axial direction through the rotor. In use, gas to be dried flows through one said passage and a desiccant regenerating gas flows in the opposite direction through the other. The desiccant on the ribbon comprises silica gel covering that half (20) of the ribbon first met by the gas to be dried and sodium type X zeolite covering the other half (19) of the ribbon. The dehumidifier functions more efficiently than a like dehumidifier in which the ribbon is wholly coated with either desiccant alone.

Description: DEHUMIDIFIER TECHNICAL FIELD This invention relates to dehumidifiers for drying gases of the kind which operate by passing a stream of the gas to be dried over or about a desiccant. In those instances in which the dehumidifier is intended for more or less continuous or long term operation it is usual for it to include regenerating means whereby spent or used desiccant is regenerated for further use. Frequently those regenerating means direct a stream of warm, dry, regenerating gas over or about the desiccant, which is exposed alternately to the gas to be dried and to the regenerating gas in a continuous cycle. Typically such dehumidifiers are used in air conditioning systems wherein the gas to be dried is outside air entering the system and the regenerating gas is heated exhaust air from the conditioned space. BACKGROUND ART The invention was developed utilising a so-called parallel plate dehumidifier, wherein the desiccant is a surface coating on a matrix of closely spaced substrate layers with their width dimensions parallel to the direction of flow of the gas. Thus it is described hereinafter primarily with reference to that type of dehumidifier, but it will be appreciated that it is applicable quite generally to other types of regenerative dehumidifiers. The terms "upstream" and "downstream" are used herein with reference to the direction of flow of the outside air or other gas to be dried. For descriptive convenience, the terms "sector", "circumferential", "diametral" and others related to a circle, are used herein, where necessary, to embrace analogous concepts related to approximate circles which are in fact polygons. Likewise, the terms "spiral" and "spirally", which usually refer to a smooth curve, are used herein to refer also to near spirals comprising a series of angularly related straight lines. Indeed, all those terms are intended to be construed as relating not only to true circles and spirals but also, at the limit, to substantially triangular arrangements. Parallel plate, regenerative dehumidifiers are known wherein a stream of gas to be dried flows through one sector of a disk shaped matrix and a stream of regenerating gas flows through another sector of the same matrix, and wherein the matrix revolves about its own axis so that each part of it is periodically exposed to each gas stream in turn. In such dehumidifiers, usually referred to as rotary dehumidifiers, sliding seals are provided between the rotating matrix and its surrounding stationary structure to define and separate the two gas streams. Those seals comprise a circumferential seal and two radial seals. Usually the radial seals are aligned so as to form a single diametral seal, so that each said sector is substantially a semi-circle. It is also known, in other contexts, to dehumidify a quantity of material by exposing it to a first desiccant, perhaps a cheap desiccant provided in large amount, to extract the bulk of the moisture, and then to a more aggressive desiccant to remove the final vestiges of moisture; but hitherto that technique has been restricted to manual laboratory operations and the like. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a regenerative dehumidifier which is more operationally effective than prior known dehumidifiers. The invention achieves that object primarily by providing a dehumidifier of the kind comprising means to direct a stream of moist gas to be dried into contact with a desiccant, characterised in that the desiccant comprises at least two components which are segregated in the direction of gas flow so as to be exposed to the moist gas stream sequentially. In preferred embodiments there are means to regenerate the desiccant by directing a stream of dry regenerating gas into contact with the desiccant. In those instances the positions of the desiccant components are preferably chosen for optimum response to the operating conditions in both of the gas streams at their respective positions of exposure. This requires the regenerating gas stream to contact the segregated desiccant components in the reverse order to that of the moist gas stream. This is achievable either by changing the positions of the desiccant components or arranging for the moist gas and the regenerating gas to flow in opposite directions relative to the desiccant. The latter is preferred. BRIEF DESCRIPTION OF THE DRAWINGS By way of example, an embodiment of the above described invention is described in more detail hereinafter with reference to the accompanying drawings. Figure 1 is a diagrammatic perspective view of a rotary type parallel plate dehumidifier. Figure 2 is a plan view of a short length of the peripheral surface of the rotor of the dehumidifier of figure 1 as it would be seen if viewed in the direction of the arrow 2 of that figure, showing the desiccant arranged as segregated components in accordance with the invention. Figure 3 is portion of a vertical diametral section of the dehumidifier of figure 1 taken on line 3-3 of that figure. Figure 4 is a graphical representation of the results of comparative tests on dehumidifiers according to the invention and dehumidifiers according to the prior art. BEST MODE OF CARRYING OUT THE INVENTION The illustrated dehumidifier is conventional in all respects except for the division of its desiccant into two segregated components of differing characteristics. Briefly stated its conventional mechanical structure comprises a rotor 4, itself comprising a frame and a flow-through matrix 5, comprising a plurality of spaced apart, axially extensive, substrate layers (the "plates" of this parallel plate humidifier), supported by said frame, drive means 6 to rotate the rotor about its axis, a casing 7 encircling the rotor 4, circumferential and radial sealing means to define two, stationary, gas flow paths extending respectively through upper and lower, substantially semi-circular sectors of the matrix 5 and a surface coating of desiccant adhered to and substantially covering said substrate layers. In this instance the rotor frame comprises two axially spaced apart spiders, each comprising a plurality of spokes 8 extending from a central hub 9, a plurality of spacers 10 extending from respective positions on the spokes 8 of one spider to corresponding positions on the spokes 8 of the other and an outer cover sheet 11. The matrix 5 comprises a tensioned, thin plastics ribbon 12, for example, of Mylar film, wound spirally upon the spacers 10. Each flight of that ribbon 12 extending from spacer to spacer constitutes a substrate "plate" of the parallel plate dehumidifier. The casing 7 comprises an outer drum 13 and cross-beams 14 furnished with bearings supporting the hub 9. The drive means 6 comprise a motor and worm gear unit 15 mounted on the drum 13 and driving a friction drive roller 16 making contact with the cover sheet 11. The circumferential sealing means of the illustrated dehumidifier comprise a resilient plastics or like band 17 underlying the cover sheet and projecting axially beyond it into sliding contact with circumferential edge flanges on the drum 13. The diametral sealing means comprise resilient plastics or like fins 18 extending along each of the spokes 8 adapted to make sealing contact with the inner faces of the cross¬ beams 14. The arrangement is such that the fins 18 of one pair of radially aligned spokes come into contact with a cross-beam 14 just prior to the fins of the pair ahead breaking contact therewith, to maintain a substantially continuous seal. In other embodiments there may be some intended leakage at the diametral seal but this is conventional and not germane to the present invention. Thus it will be apparent that the sealing means define two flow paths extending axially through the matrix 4, each of substantially semi-circular cross-section and disposed one above and one below the cross-beams 14 as drawn in figure 1. It will also be apparent that as the rotor turns each part of the matrix is alternately exposed to the gases flowing in the respective paths, namely the gas to be dried and the regenerating gas. In use in an air conditioning system, the dehumidifier would be mounted within a divided circular duct whereby incoming fresh air and exhaust air from the conditioned space, being the gas to be dried and the regenerating gas respectively, are passed through the dehumidifier in opposite directions and are kept separate from each other. As indicated in figure 2 the desiccant coating both sides of the ribbon 12 is segregated into two components applied respectively to an upstream zone 19 and a downstream zone 20 of each side of the ribbon. The characteristics of the two desiccant components differ from each other. In the present instance the desiccant component covering the upstream zone 19 of each side of the ribbon is silica gel and that covering the downstream zone is sodium type X zeolite (NaX), an alumino-silicate with molecular porosity. A significant parameter of a desiccant medium is its reduced isotherm slope, which is defined as the rate of increase in the moisture content of the medium with relative humidity of the ambient gas divided by the specific heat of the dry medium. In this definition, in the present context, the medium is the desiccant proper and its supporting substrate film. The reduced isotherm slope varies from one desiccant to another and for the one desiccant differs at different relative humidities. An effective desiccant for a given situation is one with a high reduced isotherm slope for the operating states at the situation concerned. The invention recognises this in that in dehumidifiers according to the invention each component of the desiccant is positioned where its reduced isotherm slope is the higher or highest for the local mean operating state. For typical air conditioning applications two desiccant zones suffice and the zones may be of approximately equal width. However, in other more sophisticated embodiments of the invention, three or more different desiccant components may be used segregated into a corresponding number of zones. In either instance the zones may be of different widths. In the illustrated preferred embodiment the spirally wound ribbon matrix is formed from a single ribbon, with all desiccant components, usually only two, adhered to discrete zones of it. Thus, if the ribbon is say 100 mm. wide, each zone may be a stripe of the ribbon's surface substantially 50 mm. wide extending for the length of the ribbon. One ribbon is preferred because it is easier to wind one ribbon than it is to wind two or more parallel ribbons, bearing in mind that the spacers 10 have to be individually attached to the spider spokes 8 as winding proceeds. Nevertheless in other embodiments each layer of the matrix may comprise two or more parallel ribbons disposed side by side. Those ribbons would be wound on simultaneously, and each would preferably be fully coated with a respective one of the desiccant components. This invention reduces the mass and cost of the materials to produce a dehumidifier and improves its performance by allowing it to dry air to a lower humidity with less regeneration heat. In this regard attention is directed to figure 4 of the drawings which shows two sets of graphs illustrating the performance of the above described dehumidifier of the invention utilising segregated silica gel and NaX, compared with the performances of identical dehumidifiers, except that in one instance only silica gel was used and in the other instance only NaX was used. For all the graphs the relevant characteristics were reduced to dimensionless parameters before plotting. The top set of graphs are plots of the "Coefficient of Performance" against a dimensionless length. That coefficient is a measure of the thermal efficiency of the dehumidifier taking into account the heat needed for regeneration of the desiccant. The bottom set of graphs are plots of the "Cooling Effect" of the dehumidifier as a component in a building air conditioning installation, that is to say the specific latent heat difference, between the regenerating air entering the humidifier and the dehumidified air leaving the dehumidifier, brought about by the dehumidifier, . It will be seen that in both instances the NaX only units performed better than the silica gel only units, but that the dehumidifier according to the invention performed better than either.

Claims: CLAIMS 1. A dehumidifier of the kind comprising means to direct a stream of moist gas to be dried into contact with a desiccant, characterised in that the desiccant comprises at least two components which are segregated in the direction of gas flow so as to be exposed to the moist gas stream sequentially. 2. A dehumidifier according to claim 1 further characterised in that an upstream one of said desiccant components is silica gel and a downstream one of said components is sodium type X zeolite. 3. A dehumidifier according to either claim 1 or claim 2 further comprising means to regenerate said desiccant. 4. A dehumidifier according to claim 3 wherein said means to regenerate the desiccant comprise means to direct a stream of regenerating gas through the dehumidifier in a direction opposite to the flow direction of the moist gas. 5. A rotary regenerative dehumidifier of the kind comprising a rotor, itself comprising a frame and a flow-through matrix comprising a plurality of spaced apart, axially extensive, substrate layers, supported by said frame, drive means to rotate the rotor about its axis, a casing encircling said rotor with circumferential and radial sealing means to define two, stationary, gas flow paths extending through respective sectors of the matrix and a surface coating of desiccant adhered to and substantially covering said substrate layers, characterised in that each side of each said layer has at least two axially discrete surface zones and in that the desiccant component on each zone of each side has different properties from that on the other, or each other, zone on that side.

Inventors: MACLAINE-CROSS IAN LACHLAN, MACLAINE-CROSS, IAN, LACHLAN
Applicants: UNISEARCH LTD, UNISEARCH LIMITED
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, C01B39