US3807495A - Heat exchange apparatus - Google Patents

Abstract

This invention provides a heat exchange device for gas, which includes a cross-flow blower and a plurality of electrically heated fins within the centre core of the blower. Preferably, the fins are parallel and normal to the axis of the blower, and are shaped so as to avoid the location of the vortical centre within the blower.

Description

United States Patent 1191 Skarecky 1451 Apr. 30, 1974 1 HEAT EXCHANGE APPARATUS [75] Inventor: Rudolf E. Skarecky, Waterloo,

Ontario, Canada [73] Assignee: Furnofan Limited, Kitchener,

Ontario, Canada 22 Filed: June 12, 1972 21 Appl. No.: 261,785

52 us. c1 165/122, 219/374, 415/54, 34/97 [51 Int. Cl. F24h 3/04 [58] Field of Search.....'.. 165/122; 415/54; 219/371, 219/372-376, 407

[56] References Cited UNITED STATES PATENTS 3,275,223 9/1966 Fowell 415/54 1,868,905 7/1932 Kelley 219/371 3,481,530 12/ l 969 Korovkim 684,459 10/1901 Porter 219/372 Primary Examiner-Charles J. Myhre Assistant Examiner-Theophil W. Streule, Jr. Attorney, Agent, or FirmSim & McBurney ['57] ABSTRACT This invention provides a heat exchange device for.

gas, which includes a cross-flow blower and a plurality of electrically heated fins within the centre core of the blower. Preferably, the fins are parallel and normal to the axis of the blower, and are shaped so as to avoid the location of the vortical centre within the blower.

9 Claims, 7 Drawing Figures HEAT EXCHANGE APPARATUS This invention relates generally to heat-exchange apparatus intended to effect a change in the temperature of a gas, preferably air, while that gas is drawn into and expelled from the apparatus. More particularly, this invention has to do with an electrically-powered air heater especially adapted for maintaining the temperature of an enclosed living space, such as a house or a cottage.

GENERAL BACKGROUND OF THE INVENTION:

This invention involves the use of a cross-flow blower consisting of a rotating, hollow squirrel cage rotor, with an electrically-powered, gas-heating apparatus situated In a cross-flow blower design, the flow of air or other gas through the rotatingsquirrel-cage rotor is generally in planes normal to the axis of rotation, and usually a greater or lesser change of direction of the gas takes place as it passes through the rotor. It is understood that the rotation of the cross-flow rotor sets up within the hollow core of the rotor an eccentric vortex whose direction of rotation is-the same as that of the rotor, and whose centre is generally located between the axis of rotation and the peripheral blades of the rotor on the downstream side of the rotor.

An example of the use of a cross-flow blower in an air-heating situation is disclosed in U.S. Pat. No. 3,397,463, issued Aug. 20, 1968 to N. Laing, and entitled Hair Dryers. Laing generally discloses the use a cross-flow blower with electrical heating elements eitherwithin the hollow core of the rotor or in the path of the air outside of and downstream of the rotor.

Another example of the use of a cross-flow blower for heating air is disclosed in U.S. Pat. No. 3,494,415, issued Feb. 10, 1970 to J.C. McNabney, and entitled Room Heating and Cooling Apparatus Including a Cross-Flow Blower. McNabney discloses the use of a cross-flow blower, within the hollow core of which is situated a plurality of disk-like fins oriented transversely to the rotational axis, the fins being adapted to heat or cool air passing through the blower. A central pipe through which a heating or cooling medium may pass is adapted to heat or cool'the fins by conduction.

DISADVANTAGES OF THE PRIOR ART: l The primary disadvantages of both of the designs 'disgas, or may necessitate operating temperatures so high that either the life of the heating means is shortened or undesirable radiation losses to the housing are encountered. The McNabney device suffers from the first of these two problems, the Laingdevice from the second. The central pipe axially located within the crossflow rotor of McNabney for carrying the heating or cooling medium essentially destroys or greatly hampers the eccentric vortex upon which the efficient operation of a cross-flow blower depends, while the single wire heating element proposed by Laing within the hollow core of the cross-flow blower simply has too small a surface area to heat the air appreciably without requiring temperatures so high as to shorten the life of the heating elements to an undesirable degree.

OBJECTS OF THIS INVENTION: I

In view of the shortcomings of the priorart outlined above, it is an object of this invention to provide apparatus for moving gas and at the same time changing the temperature of the gas, which incorporates a cross-flow blower design which includes heat-exchange elements designed for maximum heat-transfer surface area and for minimum interference with the eccentric vortex upon which theproper working of a cross-flow blower depends. I

It is a further object of this invention to provide heating elements for use within the hollow core of a crossflow rotor, which elements are adapted to be heated electrically.

GENERAL DESCRIPTION OF THE INVENTION:

Accordingly, this invention provides a heat-exchange device for a gas, comprising gas impelling means adapted for rotation about a central axis and including a plurality of axially extending elongated blades spaced equidistantly from said axis, a plurality of fins spaced closed in the above-mentioned prior patents are directly related to the nature of the heating means located within the hollow core of the cross-flow rotor. Because the gas flow in a cross-flow blower passes transversely across the core of the rotor, it will be understood that the positioning of any apparatus within the core of the rotor will reduce the efficiency of the rotor in terms of the work done per. unit time in moving the gas through it. However, too small a heating means within the hollow core of a cross-flow blower may be I insufficient to transfer a requiredamount of heat to the from one another and located radially inwardly of said elongated blades, means for electrically heating said tins, and means for rotating said gas impeller means about said central axis. Preferably, each fin has a recess in its periphery in order not to interfere with the highspeed central portion of the eccentric vortexset up within the rotor. A further preferred feature is that each fin is partly severed by a plurality of slits whereby to define a longer electrical current path with increased resistance.

GENERAL DESCRIPTION OF THE DRAWINGS:

One embodiment of this invention is shown in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which:

FIG. 1 is a partly broken away perspective view of a heater and/or blower embodying this invention;

FIG. 2 is a vertical section taken along the line 2--2 in FIG. 1;

FIG. 3 is .a plan view of an electrically heated fin for use with this invention;

FIG. 4 (a through e) shows five basic design configurations for an electrically heated element; and

FIG. 5 (a through d) illustrate the theory of partially severing or slitting a plate-type conductor to increase current path and thus increases resistance.

FIGS. 61: and 6b show alternate means of stiffening the heating element.

Attention is first directed to FIG. 1, in which an airblowing and air-heating apparatus is shown in partly broken-away perspective view. The apparatus10 includes a housing 12 consisting of two side walls 14 and 16 and a cylindrical bridging wall 18. The'side walls 14 and 16 are arranged in spaced, parallel relation, and have the general outline of a parabola, although this shape is not considered limiting. The two side walls' 14 and 16 are substantially identical in contour. The bridging wall 18 is cylindrical in the broadest meaning of that term, i.e., the curved surface generated by moving a curvilinear plane figure rectilinearly in a direction perpendicular to its plane. The term cylindrical as applied to the bridging wall 18 is not intended to imply a circular transverse section. I

In the embodiment shown, each side wall 14 and 16 has a rectilinear edge 20, and the edges 20 define, with the ends of the bridging wall 18, a rectangular opening 22 which is divided into an intake opening 23 at the bottom and an outlet opening 24 at the top by a transverse panel 26 extending between the rectilinear edges 20 at about their mid-portions.

Attention is now directed to both FIGS. 1 and 2, from which it will be seen that a squirrel-cage rotor 28 is mounted between the side walls 14 and 16 for free rotation about an axis of rotation 30 transverse to the side walls 14 and 16. The squirrel-cage rotor 28 is mounted between two bearings 32 (only one visible in FIG. 1), and includes a plurality of longitudinally extending curved blades 33 spaced equidistantly from the axis 30, and maintained in the desired relationshipbetween a pair of end plates 35 (only one visible in FIG. 1) to which they are secured.

A shaft extending from the centre of the closer end plate 35 in FIG. 1 projects outwardly of its respective bearing 32 and has a pulley wheel 37 fixed to the end thereof. An electric motor 38 mounted onthe bridging wall 18 of the housing 12 turns another pulley wheel 40 aligned with the first-mentioned pulley wheel 37. An endless belt 41 extends between the two pulley wheels 37 and 40 so that rotation, of the spindle of the electric motor 38 causes'the squirrel-cage rotor 28 to rotate in the direction of the arrow in FIG. 2.

Directly inwardly of the transverse panel 26 is an airscoop structure 42 defined by an elongated panel 43 of S-shaped cross-section having one edge 44 attached to the upper margin of the transverse panel 26, andhaving the other edge 46 supported in the position shown in FIG. 2 by a support panel 48 extending from the lower margin of the transverse panel 26.

Co-operating with the air-scoop structure 42 is an elongated air foil 50 having its head end 51 spaced from the dished portionvof the elongated panel 43 and centred with respect thereto, and having its tail 53 extending away from the elongated panel 43 to a location closely adjacent the curved blades 33 of the squirrelcage rotor 28.

As shown in FIG. 2, the air-scoop structure 42 in combination with the air foil 50 produces a small amount of recirculating flow indicated by the curved arrow, and this recirculated air is injected back into the squirrel-cage rotor 28 where it helps to maintain a stable vortex. While the gas-flow dynamics of a cross-flow blower are not totally understood, it has been found empirically that the stability of the vortex depends on gas being supplied at a point where the flow changes direction from outward to inward. In this connection,

' result is that air passes inwardly through the intake opening 23 and outwardly through the outlet opening 24, as shown by the arrows.

Mounted within the hollow core of the squirrel-cage rotor 28 is a plurality of fins 56 which are spaced from one another in the longitudinal direction of the rotor 28, and which are preferably arranged in parallel relationship transversely to the axis of rotation 30.

All of the fins are preferably identical in outline, and

5 FIG. 2 shows the fins of the illustrated embodiment to be substantially crescent-shaped. In the embodiment shown, the tin configuration is one in whichthe contours of a fin approximate two arcs of a circle, the arcs having the same radii ofcurvature and being both convex in the same direction, the centre of curvature of one arc lying betweenthe two arcs.

As seen in FIG. 2, the fins are arranged such that the missing portion is located in the area of the centre 55 of the'vortex. Another way of expressing the configuration is to say that the fins are circular but have a portion removed where the centre 55 of the vortex is expected to be located. The primary reason for this configuration is to interfere aslittle as possible with the centre 55 of the vortex, where the highest rotational air speeds are encountered.v If the fins were to pass through the centre of the vortex, the stability and speed of the vortex wouldbe reduced and the performance of the crossflow blower would suffer. Naturally, even the presence of the fins in the configuration disclosed has some effect upon the performance of the cross-flow blower, but this effect is minimized by arranging the fins 56 in such a way that they do not interfere directly with the centre 55 of the vortex.

While the foregoing is a satisfactory configuration, it will be understood that a variety of alternative configurations could also be employed without adverse effect. It is clear that there is a gradual increase in the effect of a tin upon performance as the fin area increases, and

particularly as the fin encroaches upon the vortical centre 55. However, there must be some passage of the gas past the fins in order to achieve heat-transfer. Thus, a large number of different fin designs could be developed which would have varying heating characteristics and would interfere with the vortex to different degrees. The fin configuration shown in the drawings has the advantage of relatively large surface area without interfering with the vortical centre. However, it is conceivable that other shapes,-- circular, semi-circular, elongated, etc.- would also perform satisfactorily. It is to be understood that the fin configuration is limited only by particular recitations in the appended claims and is not deemed to be restricted by the specific nature of the drawings.

This invention contemplates that the plurality of fins 56 be electrically heated in order that the gas being urged past the fins 56 by the cross-flow blower will rise in temperature.

It is preferred that the fins 56 be used without insulation, i.e., that there be direct contact between the gas where, L length This invention overcomes certain problems inherent in the use of electrically-heated fins within the hollow core of a cross-flow rotor, and a brief digression into theory is required to illustrate the nature of these problems. Essentially, the primary problem relates to finding a compromise between (a) the mechanical strength and self-supporting ability of the individual fins 56, and

. (b) the requisite electrical resistance which the fin must exhibit in order that normal line voltage will produce a given surface temperature on a large area fin.

Let us assume that the power for the electrical heating of the fins is supplied at 230-240 volts. Using the dimensional and operational parameters of the example given later in' this disclosure (using several fins in series), it is calculated that the necessary resistance per fin is between 1.2 and 1.5 ohms. Assuming that the full cross-section of a fin'we re to be used for the passage of the current, and assuming that each fin had a radius of about 3% inches and therefore a surface area of about 22 square inches on each side, the thickness of each fin would have to be extremely'small. If one were using, for example, Kanthal (Reg. T.M.) as the resistance mate- The latter arrangement gives the greatest increase in resistance. However, structurally it is the least desirable arrangement since especially for thin plates, there remains very little rigidity in the plate.

This weakness can be countered by providing several parallel paths for the current, bridging over some of the cuts in an alternate pattern as shown in FIGS. 50 and 5d.

Because in these instances several resistance paths are in parallel, the overall resistance of the element is reduced to the following:

with n being the number of strips and m the number of parallel paths.

Thus, a large number of parallel paths improves the rigidity, but reduces. the resistance. Some compromise is required that would give a satisfactory solution in both respects.

Essentially, this invention achieves such a compromise by using sufficient parallel paths to obtain acceptable rigidity in the individual fins, and compensating for the reduced resistance by placing more fins in series across the available constant line voltage, thereby lowrial, the thickness of the fin would be of the order of 0.00004 to 0.00005inches. Clearly, the fin would not be self-supporting, and would require some backing or support, which in turn would interfere with the flow of gas and lower the efficiency of the cross-flow blower.

This invention provides that the resistance of each fin be increased by a suitable distribution of perforations Attention is directed to FIG. 5, in connection with which the theory involving the provision of slits or cuts in a plate willbe discussed. In FIG. 5, the cuts indicated by the lines are to be understood to fully separate the material along the cut, so that .the'current has to take a meandering round-about way.

The resistance of a conductor can beexpressedas R=PL/A (in.)' g

A cross-sectional area (in?) t p resistivity (Q in.'/in.) For Kanthal and similar materials, an approximate value of resistivity p is between (50m 60) X 10" (I in."

/in. (i.e. 5.5 X 10* average) A rectangular plate with dimensions a X b X c as shown in FIG. 5a with a current passing through the full cross-section a X c in the direction of b offers a resistance The same plate, divided by cuts into n strips (FIG. 5b) and providing a single winding path for the current, has a resistance ering the voltage potential across each fin. Since power output is proportional to V /R, where V equals voltage and R equals resistance, the number of fins in series and the resistance of each fin can be juggled to achieve a desired output per group of fins in series.

One arrangement of cuts or slits in a fin'is shown in FIG. 3, to which attention is now directed.

- In FIG. 3, the fin 56 is assembled from a single piece of a material sold under the name Nikrothal (Registered Trade Mark). Only the right-hand side of the fin 56 shows the location of slits, it being understood that the left-hand side is perforated'in the same way. The slitsare arranged to provide three parallel paths, and there are fifteen slit-lines in each half, plus the centre slit. Contact point 58 and 59 provide both the electrical connections,.and two support locations for the fin. A

third support location 60 is provided on the midline of the fin.

The fins are supported on appropriate mounting posts. 1

FIG. 4.shows schematically five alternative perforation arrangements.

. FIG. 4a shows an arrangement similar to that detailed in FIG. 3; I FIG. 4b represents a similar arrangement to FIG. 4a,

with the perforations followingconcentric circles. It is believed that this could providehigher rigidity of the fin if it were corrugated along the cuts, the waves spreading in two dimensions over the plate rather than linearly.

FIG. 4c shows cuts arranged in the shape of concentric annular segments connected at alternating points.

FIG. 4d is a simple spiral representing a single path and therefore a desirable solution because it gives the widestpossible strip. However, due to its length, the strip has very little support and an externalinsulated cross-member of some width would have to be usedfor actually clamping the spiral across the diameter. With a properly designed air foil shape, such a supporting member need not adversely affect the performance of the cross-flow blower.

FIG. 4e is an alternative of FIG. 4d, with the spiral divided into two smaller ones and a continuous ring around the rim of the fin.

In all of the alternatives shown in FIG. 4, the contact points are indicated by round dots.

In the construction of the fins, it may be found desirable to provide extra rigidity in the plate material by corrugation or by flanging adjacent the slits. Prefera bly, such corrugation or flanging should be arranged in the primary direction of gas flow past the fin, so as to interfere as little as possible with the flow. With the fin 56 shown in FIG. 3, corrugation or flanging aligned with the slits would meet this criterion. FIG. 6a illustrates corrugation, while FIG. 6b illustrates flangmg.

EXAMPLE: AN AIR-HEATING DEVICE Cross-Flow Blower Rotor diameter 9 in.

Rotor length 12 in.

Rotor speed 575 rpm Static pressure 0.25 in. WG

Flow rate 600 cfm Heater Electric input 6/l0/l5 kW Heating output Number of heating elements Outer diameter of heating elements Loading of heating elements Max temperature of heating 20,000/34,000/ l,000 BTU/hr 7 in. 0.3 to 1.5 kW each elements at full load 700F Air temperature at full load (75F inlet) 154F' each fin being shaped to avoid the location of the centre of said stable vortex, means for electrically heating said fins, and means for rotating said rotor.

2. The invention claimed in claim 7, in which each fin occupies most of the transverse area within the rotor, with the exception of the portion where the centre of said vortex is established by rotation of said rotor.

3. The invention claimed in claim 2, in which the surface of each fin is partly severed by a plurality of slits whereby to define a longer-electrical current path between two contact points than would be the case in the absence of slits.

4. The invention claimed in claim 3, in which at least some of the fins are stiffened by one of (a) corrugation, and (b) flanging.

5. A heat-exchange device for a gas, comprising:

gas impelling means adapted for rotationabout a central axis and including a plurality of axially extending elongated blades spaced equidistantly from said axis to define a hollow core radially inwardly of all blades, the gas impelling means when rotating inducing a rotational vortex in the gas having its vertical centre located eccentrically within said hollow core,

a plurality of stationary fins within said hollow core spaced from one another and disposed substantially perpendicular to said central axis, each fin being shaped so as to avoid the location of said vortical centre,

means for electrically heating said fins,

and means for rotating said gas impeller means about said central axis.

6. The invention claimed in claim 5, in which each fin is defined by two arcs of a circle, the arcs having the same radius of curvature and being both convex in the same direction, the centre of curvature of one are lying between the arcs.

7. The invention claimed in claim 5, in which said means for electrically heating said fins includes for each fin two contact points separated from each other, and electrical connections to said contact points.

8. The invention claimed in claim 5, in which the material of each fin is partly severed by a plurality of slits whereby to define a longer electrical current path than if the slits were absent.

9. The invention claimed in claim 8, in which each fin has a plurality of slits whereby to define at least one electrical current path, all electrical current paths being longer than the electrical path between said contact point through a fin without slits.

Claims (9)

1. A heat-exchange device for gas, comprising: a housing defining an intake opening and an outlet opening, a cross-flow rotor mounted for rotation within said enclosure, recirculation means within said enclosure for recirculating a portion of the gas back into the rotor to maintain therein a stable vortex of which the centre is located eccentrically with respect to the rotor, a plurality of spaced fins within said rotor oriented transversely to the axis of rotation of said rotor, each fin being shaped to avoid the location of the centre of said stable vortex, means for electrically heating said fins, and means for rotating said rotor.

2. The invention claimed in claim 7, in which each fin occupies most of the transverse area within the rotor, with the exception of the portion where the centre of said vortex is established by rotation of said rotor.

3. The invention claimed in claim 2, in which the surface of each fin is partly severed by a plurality of slits whereby to define a longer electrical current path between two contact points than would be the case in the absence of slits.

4. The invention claimed in claim 3, in which at least some of the fins are stiffened by one of (a) corrugation, and (b) flanging.

5. A heat-exchange device for a gas, comprising: gas impelling means adapted for Rotation about a central axis and including a plurality of axially extending elongated blades spaced equidistantly from said axis to define a hollow core radially inwardly of all blades, the gas impelling means when rotating inducing a rotational vortex in the gas having its vortical centre located eccentrically within said hollow core, a plurality of stationary fins within said hollow core spaced from one another and disposed substantially perpendicular to said central axis, each fin being shaped so as to avoid the location of said vortical centre, means for electrically heating said fins, and means for rotating said gas impeller means about said central axis.

6. The invention claimed in claim 5, in which each fin is defined by two arcs of a circle, the arcs having the same radius of curvature and being both convex in the same direction, the centre of curvature of one arc lying between the arcs.

7. The invention claimed in claim 5, in which said means for electrically heating said fins includes for each fin two contact points separated from each other, and electrical connections to said contact points.

8. The invention claimed in claim 5, in which the material of each fin is partly severed by a plurality of slits whereby to define a longer electrical current path than if the slits were absent.

9. The invention claimed in claim 8, in which each fin has a plurality of slits whereby to define at least one electrical current path, all electrical current paths being longer than the electrical path between said contact point through a fin without slits.

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