Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2000-12-08
2002-11-19
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S529000, C219S549000, C338S208000
Reexamination Certificate
active
06483087
ABSTRACT:
The invention relates to a heater element, a heater and a process for producing the heater element and heater. In particular, the heater comprises an electrically conductive fabric layer laminated between two layers of glass fiber-reinforced thermoplastic films. The heater further comprises bus bars and electrical leads, and it is produced by consolidating the layers of film and fabrics into a single sheet heater. The heater can be produced having variable geometry, electrical supply voltage and power. The heater of the invention is more durable than prior art heaters because it is able to withstand more mechanical, chemical, ultraviolet radiation and other environmental stresses than prior art heaters.
BACKGROUND
Laminate or film heater of the prior art have been made using metal foil, wires and electrically conductive fabrics laminated together using resins in between the layers to bond the integral layers together. Heaters manufactured from wire and foil have been used in industry for some time. In particular, such wire and foil heaters when used, for example, in the aerospace industry for deicing structures such as airplane wings and jet engine inlets, suffer from fatigue failure while in use, which considerably shortens their life when compared to fabrics. In addition, foil heaters, in particular, are expensive to produce and lack flexibility.
Because of their flexibility, light weight and even heat distribution characteristics, prior art laminate fabric heaters have been preferred in the art for many applications over metal foil heaters. In the aerospace industry, for example, fabric heaters are used for deicing structures such as airplane wings, jet engine inlets and antenna dishes, in the buildings industry for heating solid structures such as floors, countertops, pipes and tanks; in the food industry for heating food receptacles and in shipping industry and marine structures for preventing biofouling. See for example, U.S. Pat. Nos. 5,344,696; 5,925,275; 5,932,124; 5,942,140; 5,954,977; 5,966,501; 5,981,911.
Laminate fabric heaters of the prior art have been made with woven or non-oven fabrics containing fibers, which are electrically conductive fibers such as carbon fibers, and non-conductive fibers such as polyester. Non-conductive fibers for use in heaters are usually coated with a metal so that they can conduct the current via the metal coat, or the fibers are dispersed in a resin containing conductive particles, such as carbon black or iron particles. Conductive fibers can also be coated with a metal to improve their conductive properties.
Carbon fibers consolidated into a random, non-woven fabric or veil have been used in the art for shielding against electromagnetic interference. Conductive fabrics used in heaters for deicing and anti-icing aerospace structures are disclosed in, for example, U.S. Pat. No. 5,344,969 to Hastings et al. discloses an integrally bonded laminate that is used to thermally control a surface of an aircraft to which the laminate is bonded. The patent describes that the use of fabrics have numerous advantages over prior methods for deicing and heating airplane wings; for example, the conductive fiber is of low weight, and or permits nominal intrusion in terms of aerodynamics; it is easy to handle compared to wire and foil, and most notably, it allows the even distribution of heat. These factors contribute to a more efficient use of energy. Deicing and anti-icing aircraft applications necessitate an extreme in terms of product requirements. Because aircraft operate on very limited electrical resources and extreme atmospheric conditions, a system must be efficient as well as robust to provide protection. The patent also discloses, however, that the laminated fabric heater is manufactured using adhesive resins to bond the laminating layers together.
U.S. Pat. No. 4,534,886, to Kraus et al., discloses an electrically conductive web composed of a non-woven sheet of conductive fibers and non-conductive fibers. The sheet is saturated with a dispersion containing conductive particles and is then dried. The Kraus et al. heater element is used primarily in heating pads. The patent also discloses that the fabric heater layers are laminated together using an epoxy resin.
U.S. Pat. No. 5,925,275 to Lawson discloses an electrically conductive composite heating assembly. The assembly has an electrically conductive non-woven fiber layer laminated between layers of fiberglass and other dielectric material. The assembly further has an abrasion resistant outer layer. The heater element is used on aerospace structures as an ice protection system to withstand the repeated mechanical stress and thermal cycles encountered in extremely harsh aerospace environments. This patent also discloses that the fabric heater is manufactured using adhesive resins to bond the layers of the heater assembly.
Conductive fabric heaters disclosed in UK Patent Application No. 2,285,729 to Gerrard, are manufactured by baking a woven polymeric fabric to extreme temperatures in a multi-step process. The resultant fabric heater is limited by flexibility in resistance goals and furthermore lack variability of its Temperature Coefficients of Resistance. For example, the patent discloses that the fabric heaters are primary useful for low voltage source operation.
While laminated fabric heaters made using adhesive resins have been used with some success in the art, a disadvantage of using these type of heaters is due to the adhesive resins used. Adhesive resins used in making or bonding such laminated heaters must be cured and the process becomes time consuming and dangerous due to the toxicity of the materials involved. In addition, while adhesive resins are widely used to make laminated fabric heaters, the heat output from these type of heaters over a period of time dries the resin, leading to cracking of this layer and ultimately, the heater delaminates and loses function. Therefore, the art always seeks to develop new heaters or to improve the existing heaters. while adhesive resins are widely used to make laminated fabric heaters, the heat output from these type of heaters over a period of time dries the resin, leading to cracking of this layer and ultimate, the heater delaminates and loses function. Therefore, the art always seeks to develop new heaters or to improve the existing heaters.
In addition, heater designs as mentioned in Kraus, Hastings and Lawson suffer from expensive production methods and low operating temperatures. Machine setup is difficult to modify for different applications especially due to the use of epoxy and other resins, which have problems difficult to overcome; such as cost, shelf life, operating temperature and chemical limitations, long curing cycles and toxicity precautions. The addition of carbon black particles, polyaramid fibers (1), conductive adhesives (3), and multistage layer processing (2) contribute to the complexity and therefore the heater cost while limiting service temperature, suitability for complex designs, and high volume—low cost production.
SUMMARY OF THE INVENTION
The invention is directed to a laminated fabric heater element, a heater and a process for manufacturing the heater element and heater. The heater of the invention has many advantages over prior art heaters in that it is thin, flexible, produces more uniform temperature, has high fatigue life, and can be mass produced at less costs. In addition, the heaters of the invention can be operated at voltages ranging from millivolts to about 600 volts from either alternating current or direct current power supplies.
Specifically, the heater element comprises a consolidated electrically conductive fabric layer, two bus bars, and two thermoplastic layers; wherein each bus bar is contacting opposing edges of the conductive fabric layer and the consolidated electrically conductive fabric layer and the bus bars are sandwiched between the thermoplastic layers forming a single sheet.
The electrically conductive fabric layer of the laminated fabric heater of the invention may be selected f
Gardner Alan D.
Miller Andrew J.
Parkin Jeff R.
Rolls John A.
Fuqua Shawntina T.
Thermion Systems International
White & Case LLP
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