Canister Heater With PTC Wafer

FIG. 1 is a view, partly in top plan and partly in section, of the improved heater assembly of the invention, particularly illustrating a disk-like insulating support structure and eight retainer lugs carried thereon, by which the support structure can be releasably snapped into an annular recess at the bottom of a charcoal-containing fuel-vapor recovery canister.

FIG. 2 is a vertical section taken on the line 2--2 of FIG. 1.

FIG. 3 is a fragmentary view taken on the line 3--3 of FIG. 2.

FIG. 4 is a fragmentary section taken on the line 4--4 of FIG. 1.

FIG. 5 is a fragmentary section taken on the line 5--5 of FIG. 1.

FIG. 6 is a view like that of FIG. 2, except that it is fragmentary and enlarged.

FIG. 7 is an edge view of the slab-like heating element or wafer employed in the assembly of FIGS. 1-6, the arrows illustrating the path along which air flows while it is being heated, and prior to its being drawn into the interior of the canister, during purging thereof, and

FIG. 8 is a fragmentary view of a modified heater construction wherein both the air intake disk and the baffle plate are constituted of metal-clad circuit boards, this construction constituting another embodiment of the invention.

FIG. 2 illustrates in fragmentary vertical section a generally cylindrical fuel vapor storage canister 10 for use in an automotive pollution control system, the canister having a side wall 12 and an inner bottom flange 14. The canister is filled with a quantity of activated charcoal or carbon 15 which adsorbs vapors that are collected through a vent line (not shown) extending to the vehicle's fuel tank, and which releases the vapors to the vehicle's carburetor (not shown) after the engine is started. This operation is set forth in the specification of the co-pending application above identified.

In accordance with the present invention there is provided, as a retro-fit for an existing fuel vapor recovery canister, a novel and improved heater construction 16 for raising the temperature of the charcoal 15 in the canister 10 so as to facilitate purging of the stored fuel vapor/condensate, and to enable the purged condensate to be burned in the vehicle's engine. The heater construction 16 comprises a solid, imperforate wafer 18 preferably constituted of positive temperature coefficient (PTC) ceramic material that is mounted in a molded plastic heater housing 20 constituting an enclosure means. The housing 20 has a series of upstanding resilient integrally formed lugs or spring fingers 22, shown in FIG. 1 as being eight in number, by which the housing 20 can be snapped into the circular opening formed by the bottom flange 14 of the canister 10. The heater housing 20 comprises a base portion 21 provided with an upstanding annular flange 24 and a bottom wall 26 which together enclose portions of the wafer 18 and provide support thereto. The housing bottom wall 26 has an air inlet opening 28 as shown.

Disposed against the opposite faces of the wafer are ring terminals 30, having integral spade lugs or blades 32 that are connected to electrical leads 34, 36, respectively, shown in FIG. 1. FIG. 4 also shows these terminals. Grooves 38 that provide clearance for the leads 34, 36, as shown in FIGS. 1 and 5, are provided in a bottom plate described below. The leads extend to suitable control circuitry (not shown) for effecting electrical energization of the wafer 18.

Disposed against the bottom wall 26 of the heater housing 20 is an air intake regulating disk or wall 40, preferably constituted of aluminum, copper, or copper-clad circuit board (printed circuit board) having the conductive or metal-clad side facing the wafer 18. The wall 40 has an opening or aperture 41. A pair of wave washers 42 is provided, one being disposed between the air intake disk 40 and one ring terminal 30, and the other being disposed between the other ring terminal 30 and an air exhaust disk or baffle plate 44, the latter having a series of discharge apertures 46, as shown in FIG. 1.

Fitted over the upstanding flange 24 is a molded plastic retainer cap 48. The arrangement is such that the wafer 18, ring terminals 30, wave washers 42, air intake disk 40 and baffle plate 44 are held in the relative positions shown in FIG. 2, with the wave washers 42 providing a spring bias to the wafer 18 through the ring terminals and causing them to press against the opposite faces of the wafer 18 so as to establish good electrical and thermal contact therewith.

Referring again to FIGS. 1, 2 and 6, there are provided in the inner surface of the upstanding flange 24 four connecting passages or channels 50 which provide communication between the area beneath the wafer 18 and that above the wafer. The passages 50 are shown slightly enlarged or exaggerated in FIG. 2, for purposes of clarity. Also, the heater housing 20 has a portion that closes wide grooves 52 in a separate disk-like bottom plate 53, defining air intake channels which communicate with the air inlet opening 28 in the bottom wall 26 of the heater housing 20. The air flow is as indicated by the arrows labelled with the capital letter A in FIG. 6. That is, during purging of the canister, air is drawn from the engine compartment in through the intake channels 52, through the opening 28 in the bottom wall 26 and the opening 41 in the air intake disk 40 and into the chamber formed by the disk 40 and the wafer 18. In FIG. 6, this lower chamber is designated 54. Air impinging on the central portion of the underside of the wafer 18 is directed radially outwardly, absorbing heat from both the wafer 18, the lower wave washer 42 which is in thermal contact with the wafer, and the air intake disk 40 which is receiving heat from the wafer 18 by both conduction and radiation. When the air reaches the periphery of the chamber 54, it is drawn upwardly through the four connecting passages 50 and arrives at the upper chamber formed by the upper surface of the wafer 18 and the baffle plate 44. In FIG. 6, this upper chamber is designated 56. Thereafter the air is directed radially inwardly to an extent, again absorbing heat from both the wafer 18, the upper wave washer 42 and the baffle plate 44, and thereafter exits through the apertures 46 in the latter and into the interior of the canister 10. Due to the spacing of the apertures 46, the air entering the interior of the canister is characterized by a divergent flow pattern which has been found to be especially effective in achieving a more even heat distribution throughout the canister. Experimentally it has been determined that the arrangement of two series each of six holes 46, each series being on a different diameter, and where the series disposed in the larger circle is interposed circumferentially between the holes lying on the smaller circle, provides highly satisfactory results. Such results have been found to be considerably better than if a single relatively larger central passage were to be provided in the baffle plate 44.

As can be readily seen in FIGS. 2 and 5, three walls of each of the channels 52 is formed by the bottom plate 53, and the other remaining wall of each channel is formed by the heater housing 20 at its base portion 21. The bottom plate 53 is preferably sonically welded to the base portion 21 of the heater housing 20 after the parts are assembled. Also, the retainer cap 48 is preferably sonically welded to the heater housing 20.

Where the heater construction is to be employed to fit an existing canister, by the invention there is provided an additional member at the bottom of the heater construction, in the form of a mounting ring 60 having an inwardly facing flange 62. The configuration and dimensions of this flange 62 are similar to those of the canister flange 14, and accordingly where this ring 60 is incorporated into the heater, the same mounting bracket (not shown) on the vehicle can be employed. The bracket is adapted to cooperate with the flange 62, whereas previously, where no heater was associated with the canister 10, the bracket was intended to engage the flange 14. The mounting ring 60 is preferably sonically welded to the bottom plate 53 at its periphery. Typically canisters of the type shown in FIG. 2 incorporate a bottom grid (not shown) above the location of the flange 14, in order to hold captive the charcoal. This grid has been omitted from FIG. 2 in the interest of clarity, since per se, it forms no part of the present invention.

Another embodiment of the invention is shown in FIG. 8 wherein like reference numerals have been assigned to components similar to those of the embodiment of FIGS. 1-7. There is illustrated an assemblage comprising the wafer 18, ring terminals 30, and wave washers 42. By the invention, the air intake disk or wall can be constituted as a metal-clad circuit board 40a having its foil side or metal-clad side facing the wafer 18. Similarly, the baffle plate can be in the form of a metal-clad board 44a having a series of openings 46a similar to those of the baffle plate 44. Again, the metal or foil side of the board faces the wafer 18. It has been found that with the relatively thin metal layer provided on such boards, usually having a thickness on the order of five or six thousandths of an inch, a rapid heating of the metal occurs upon initial energization of the wafer. That is, due to the relatively small thermal masses represented by the foils on the boards, they absorb radiation and reach a relatively high temperature quickly. As they heat, they in turn re-radiate and re-conduct heat to the air passing through the chambers formed thereby. The insulating effect of the fiberglass or phenolic material of which the board is normally constituted significantly reduces radiation or conduction by the foil in an outward direction; thus most of the heat received by the foil is eventually transferred to the air stream flowing past it.

The engagement of the foil by the wave washers enhances the conduction of heat to the foil portions, as in the previous embodiment.

The disclosed heater constructions have the following important advantages as compared to previously devised arrangements. Most notable is that the various constructions shown eliminate the need for multiple PTC elements, or alternately a PTC element having a special configuration, such as a honeycomb. The units that have been disclosed employ a single thickness slab, which is preferably circular, but which could have other geometric outlines, if desired. Only one PTC element is required, as opposed to multiple units that were spaced from one another and which required multiple electrical connections.

By virtue of the circuitous or serpentine route taken by the air stream, as shown by the arrows labelled with the capital letter A in FIGS. 6 and 7, there is achieved a maximum transfer of heat from the wafer. The wafer is in thermal contact with both wave washers 42, and with the air intake disk 40, and the baffle plate 44. Thus there are formed the two circular chambers 54 and 56, one below the wafer and a second above the wafer. Air flow into the lower chamber 54 is in a radially outward direction, whereas in the upper chamber 56 it is generally in a radially inward direction and out through the apertures 46 in the baffle plate 44. Due to the fact that the intake disk 40 and baffle plate 44 are receiving heat from the wafer 18 continuously, by both conduction and radiation, heat is transferred to the air from these components as well as from the wafer. As presently understood, such an arrangement almost doubles the heat-conductive and heat-radiating surface area exposed to the air stream, and as a result, unexpectedly high efficiency is realized. In actual tests performed on operating models, it has been confirmed that there exists a pronounced increase in the temperature of the canister interior when thermally conductive components are employed for the air intake disk 40 and baffle plate 44, as opposed to the use of thermally insulating materials for these parts.

The provision of the wave washers 42 constitutes an important feature of the present construction. First, they provide the desired spacing between the wafer 18 and the air intake disk 40 on the one hand, and the wafer 18 and baffle plate 44 on the other hand, forming the two separate chambers 54, 56 which permit air to flow in the manner explained above. Second, they provide the desired contact pressure between the ring terminals 30 and the opposite faces of the wafer 18. This type of mounting minimizes possible damage to the wafer from vibration or shock. Also, the resilience of the wave washers minimizes the chance of breakage or fracture of the wafer during assembly.

In addition, any portion of the air stream passing close to the wave washers is heated thereby, since the washers themselves are relatively hot. Finally, the wave washers provide a conductive path for heat flow between the wafer and both the air intake disk 40 and the baffle plate 44.

The provision of the resilient fingers 22 permits removal of the heater construction from the canister if the need arises, thus facilitating replacement or repair of the various components. Also, the disclosed heater construction can be applied to existing canister designs virtually without alteration to or modification of the existing structure of the canisters.

The closed-bottom type of construction minimizes the possibility of water entering the canister from road splashes. Inadvertent build-up of moisture therein, which would be detrimental to proper operation, is thus avoided.

The disclosed constructions are thus seen to represent distinct advances and improvements in the field of fuel vapor adsorption canisters.

Variations and modifications are possible without departing from the spirit of the invention.

Each and every one of the appended claims defines an aspect of the invention which is separate and distinct from all others, and accordingly it is intended that each claim be treated as such when considered in the light of the prior art devices in any determination of novelty or validity.