Electric resistance heating devices – Heating devices – Immersion heater details
Reexamination Certificate
1998-11-06
2001-01-30
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Immersion heater details
C392S398000, C392S478000
Reexamination Certificate
active
06181874
ABSTRACT:
This invention relates to a heating element for heating fluids and to a heater incorporating such an element.
It is known to provide an electric heater in which current is passed through a mesh of interconnected wires. Such a heater, used for fusion welding of plastics, is described in U.S. Pat. No. 5,475,203. In this heater, the mesh is sandwiched between layers of plastics material in order to provide heat to weld same together. In the present invention a mesh is used to heat a flowing stream of fluid—gas or liquid—passing through it.
The invention is thus directed primarily at an electrically powered heating element of the type which is placed in a moving fluid stream so that the fluid is heated as it passes the element. Heaters made from such elements are widely used in many fields, commercial, industrial and domestic. It is anticipated that the heating element of this invention will find similar broad application.
In accordance with a first aspect of the invention, there is provided a heating element comprising a mesh made of intersecting strands of filamentary material arranged to define a plurality of apertures through which a fluid to be heated may pass, at least some of said strands being electrically conductive whereby current may be supplied to said strands to heat same, the element being characterised in that said apertures are sufficiently small that all or substantially all of the fluid passing through each said aperture is heated by conduction and/or convection.
In accordance with a second aspect of the invention, there is provided a heating element comprising a mesh made of intersecting strands of filamentary material arranged to define a plurality of apertures through which a fluid to be heated may pass, at least some of said strands being electrically conductive whereby current may be supplied to said strands to heat same, the element being characterised in that said apertures each have an effective diameter of less than 500 &mgr;m.
In a heater incorporating the heating element of the invention, means are provided for passing an electric current across the mesh, thus supplying the energy necessary for heating the fluid. The mesh, which will normally be generally planar, is mounted so as to at least partially intersect the fluid stream to be heated. In a particular embodiment, for example, the fluid to be heated may be passed, for example by pumping, along a conduit, and the mesh placed across the conduit so that all of the fluid is constrained to pass through one of the fine apertures in the mesh. In accordance with the invention the apertures should be fine enough to ensure that all or substantially all of the fluid passing through each aperture is heated by conduction and/or convection.
The mesh is attached to electrodes to which an electrical supply is connected to supply current to the mesh. To this end the mesh must be constructed so as to define an electrical path between the electrodes. Preferably the mesh is such as to give a substantially constant heating effect over its whole area; however, there may be circumstances in which the heating pattern could with advantage be tailored to suit particular specialist applications by providing, for example, relative cool areas of the mesh.
In order to maintain dimensional stability, it is preferred that the mesh is of woven construction; however, other techniques such as friction welding could be used to fabricate a non-woven mesh.
Whether woven or not, it is preferred that a simple construction of mesh is used, comprising two sets of filamentary strands crossing at right angles in the manner of the warp and weft of a conventional fabric. The strands of at least one of the sets should be of conductive material, and attached between the electrodes so that electrical current can be passed through them; not all of such strands in said one set need be of conductive material. It may be possible for non-conductive strands to be incorporated with the conductive strands, consistent with maintaining a reasonably constant overall heating effect, as aforesaid, or ensuring that a particular tailored heating effect is achieved.
The filamentary strands of the other set—those that extend laterally across those carrying the current—may also be conductive, or they may be non-conductive.
In a particular embodiment of the invention the mesh comprises a commercially-available woven wire cloth in which conductive wire is used in both warp and weft. The wire can be made from any suitable conductive material such as stainless steel, resistance wire, Nichrome wire, copper or aluminium wire or carbon fibre. In some applications, the wire may be made from a low melting point alloy (such as solder) to render the chance of overheating or combustion impossible. A material with a high positive temperature coefficient of resistance, for example barium tantalate, would automatically limit the mesh temperature in the event of a drop in fluid flow due, for example, to a blockage. Mesh failure due to flow restriction may be prevented by the use of a pressure actuated switch which only permits current to be supplied to the mesh when the pressure difference across the mesh faces, caused by the flow through the mesh, exceeds a prescribed value. The material used will depend upon the particular circumstances of use; in particular the nature of the fluid being heated.
The heating element operates by means of I
2
R losses in the conductive strands of the mesh causing the strands to heat up and transfer heat energy to the passing fluid by conduction and convection. The heating element is effective because the fluid stream being heated is divided into many sub-streams each one of which passes through a respective aperture in the mesh. Heating thus occurs as the sub-stream passes through its respective aperture and, in the present invention, these apertures are made small—with typical dimension of the order of 40 to 60 &mgr;m in order to achieve maximum convective efficiency. Convective efficiency is defined by:
η
c
=
actual
heat
transferred
ideal
heat
transferred
Heat transferred is measured in watts. The ideal quantity is achieved when the fluid being heated leaves the heat exchanger at the same temperature as that of the heat exchanger.
This will now be discussed in relation to a fluid passing along a conduit whose walls are heated to thereby transfer heat energy to the fluid. In such a conduit, a thermal boundary layer can be defined immediately against the inside wall of the conduit in which the fluid receives heat purely by conduction from the conduit wall. The process of heat transfer from a wall to a fluid is, at the wall surface, via conduction. This is true within the wall and the fluid. The transfer of heat from the bounding surface throughout the thermal boundary layer is by combined conduction and transport (or movement) of fluid. This latter, combined, process is called convection. It is fairly apparent that, as the fluid progresses down the conduit, the thickness of this boundary layer will increase until eventually it comprises the whole cross section of the fluid. In the ideal heat exchanger (in which the fluid being heated leaves at the heater exchanger temperature), all of the fluid at the exit must be heated so that the thermal boundary layer must extend across the full conduit. To a first order, the rate at which boundary layers grow on a body immersed in a fluid, for a given fluid and a given flow velocity, is fixed. The heat transfer passages in the mesh heater of the present invention are effective because the passage dimension in the flow direction is comparable to the thickness of the boundary layers which grow on the heater elements (wires) and, by this means, the requirement that all, or substantially all, of the fluid passing through each aperture is heated by conduction and/or convection is satisfied. An alternative way of under-standing the high performance of the heater mesh is in terms of the established equation for convective efficiency. The conv
Gillespie David Richard Hugh
Ireland Peter Thomas
Wang Zuolan
Dahbour Fadi H.
Isis Innovation Limited
Volpe & Koenig P.C.
Walberg Teresa
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