Self-regulating heater cable with buffer layer
Embodiments of the invention provide self-regulating heater cables utilizing substantially solid polymeric buffer layers surrounding the heating elements having improved heat transfer efficiency as well as improved reliability and endurance. The assembly includes first and second power supply wires configured to carry electrical power and separated by a solid spacer, a substantially solid electrically-insulating buffer layer in thermal contact with the heating element, and a cable jacket including a polymeric outer surface and an inner metallic sheath surrounding the buffer layer and in thermal contact with the buffer layer. The buffer layer includes a polymeric material having a thermal conductivity greater than air at standard temperature and pressure and surrounds the heating element, power supply wires, and spacer.
1 . A self-regulating heater cable assembly, the assembly comprising:
first and second power supply wires configured to carry electrical power and separated by a solid electrically insulating spacer;
a self-regulating heating element in electrical contact with the first and second power supply wires and converting electric current into thermal energy when the first and second power supply wires are energized, the heating element being spirally wound around the first and second supply wires and the spacer, and having a spacing that creates gaps between consecutive windings of the heating element;
a conductive ground layer that couples the heater cable to electric ground, the ground layer surrounding and physically separated from the heating element and the first and second supply wires;
an outer jacket surrounding the ground layer; and
an electrically-insulating buffer layer disposed between the ground layer and the heating element and in thermal contact with the heating element, the buffer layer being extruded over the heating element to result in filling of the gaps between the windings of the heating element, the buffer layer comprising a polymeric material and having a thermal conductivity greater than air at standard temperature and pressure, and the buffer layer being porous and incorporating at least one of voids and pockets containing trapped gasses.
2 . The assembly of claim 1 , wherein the buffer layer further comprises one or more particulate additives disposed within the buffer layer so that the thermal conductivity of the buffer layer is greater than a thermal conductivity of the polymeric material; and
wherein at least one particulate additive comprises one of alumina, boron nitride, carbon black, magnesium oxide, sand, silica, and glass.
3 . The assembly of claim 1 , wherein the buffer layer is applied to the assembly by pressure extrusion.
4 . The assembly of claim 3 , wherein the melt point of the polymeric material is below a maximum operating temperature of the heating element, the assembly further comprising an electrically insulating, thermally conductive inner jacket surrounding the heating element, the first and second supply wires, and the spacer, and physically separating the ground layer from the heating element.
5 . The assembly of claim 4 , wherein the inner jacket is applied to the assembly by vacuum extrusion, the inner jacket partially filling the gaps and contacting the heating element, and wherein the buffer layer is applied within the gaps to completely fill the gaps.
6 . The assembly of claim 5 , wherein the inner jacket surrounds the buffer layer.
7 . The assembly of claim 4 , wherein the buffer layer is applied in the gaps and fills the gaps, and wherein the inner jacket is applied over the buffer layer by vacuum extrusion.
8 . The assembly of claim 1 , wherein at least a portion of the buffer layer is applied to the assembly by vacuum extrusion.
9 . The assembly of claim 8 , wherein the buffer layer comprises:
a first buffer layer comprising a plurality of fibers disposed in the gaps and filling the gaps and spirally wound around the spacer and the first and second supply wires adjacent the heating element; and
a second buffer layer applied by vacuum extrusion over the spacer, the first and second supply wires, the heating element, and the first buffer layer.
10 . The assembly of claim 1 , wherein the buffer layer comprises a plurality of fibers disposed in the gaps and filling the gaps and spirally wound around the spacer and the first and second supply wires adjacent the heating element.
11 . The assembly of claim 10 , further comprising an electrically insulating, thermally conductive inner jacket surrounding the heating element, the first and second supply wires, and the spacer, and physically separating the ground layer from the heating element.
12 . The assembly of claim 11 , wherein the inner jacket is applied to the assembly by vacuum extrusion.