Patent Application
20090283514
Particular embodiments generally relate to heating cables.
In cold environments, pipes may transport substances, such as oil, steam, and other process streams, etc. When steam or other process streams are transported through the pipes, the heat from the steam or process stream may help keep the pipes from freezing. However, if the system malfunctions, or if the flow of the process stream stops, and steam is not transported through the pipes, the steam condenses and the pipes may freeze. Accordingly, an electric heater may be used to keep the pipes warm to prevent freezing.
Different long-line heaters, generically called heat tracing products, may be used to keep the pipes warm. For example, all types of heaters are used. However, not all heaters may work well at high temperature. This is especially important when substances are transported at high temperatures in the pipes. Also, if the heater fails, then there is a large likelihood that the pipes may freeze and fail. This is a costly repair for a company and very undesirable. When manufacturing and installing the heaters, mechanical stress may be experienced. This may also damage the heaters and is undesirable.
Particular embodiments generally relate to a heating cable that includes an insulated heating element. The bus wires may be bared in that they do not have an insulation jacket surrounding each of the bus wires. For example, insulation on the bus wires that electrically separates bus wires from heating element is not included. Instead, insulation is included on and surrounds the heating element.
A heating element is includes one or more resistance wires and an insulating layer surrounding the one or more resistance wires. For the resistance wires to electrically contact the bus wires, the insulating layer is bared at a plurality of node areas to expose the one or more resistance wires from the insulating layer. The one or more resistance wires are electrically coupled to the bus wires at the plurality of node areas to create a plurality of resistance zones for generating heat. Because the insulating layer surrounds the resistance wires, it provides added protection for the resistance wires, which protects them from physical damage.
The bus wires may be located in different positions. In one embodiment, a bus wire structure includes a plurality of bus wires and an insulating spacer in between the plurality of bus wires. The spacer keeps the bus wires a certain amount of distance between each other. The heating element is then wrapped around the bus wire structure.
In another embodiment, a spacer is provided in which the heating element is wrapped around it. The bus wires are then positioned on the outside of the spacer and the heating element such that the heating element is in between the spacer and each bus wire.
An additional second insulating layer may surround the heating element and the bus wire structure. A metal sheath also may enclose the bus wire structure and the heating element. Further, a clip may be provided that is configured to wrap around the heating cable at a node to secure the electrical connection between the bus wire and the one or more resistance wires at the node. The clip includes a tab and an aperture, where the tab is inserted through the aperture to exert pressure against the one or more resistance wires to secure the electrical connection to one of the bus wires at the node area.
A further understanding of the nature and the advantages of particular embodiments disclosed herein may be realized by reference of the remaining portions of the specification and the attached drawings.
Different embodiments of heating cables may be used.
Bus wires 102 provide electrical power to heating zones. The bus wires may include round, stranded metal-coated copper conductors, narrow flat bands of copper or other conducting metals, braided copper structures, or other structures that can provide electrical power. In one embodiment, two bus wires 102 are provided and are set parallel to one another. However, it will be understood that any other number of bus wires 102 may be used and can be arranged differently.
A spacer 104 is placed in between the bus wires 102 to electrically isolate the bus wires. For example, spacer 104 may include an insulating material, such as from a glass yarn wrapped around cloth tape made of glass cloth or mica. A wider heating cable may be desirable to provide higher power outputs that can be distributed over a wider and larger surface area of the heating cable.
Bus wires 102 run down the length of heating cable 100 separated by an insulating spacer 104 that keeps the bus wires apart and electrically insulated from each other. Bus wires 102 may be bare bus wires. For example, bare bus wires may not include insulation that surrounds each bus wire. That is, insulation may not immediately cover the outer surface of bus wire 102.
Heater element 106 may be insulated with an insulating layer 220 that prevents heater element 106 from electrically coupling to bus wires 102. Insulation layer 220 electrically separates bus wires 102 from resistance wires of heating element 106. First insulation layer 220 may include layers of glass cloth, braided glass fibers, mica sheets, high-temperature silicon gels and pastes, etc.
As will be described in more detail below, heating element 106 includes one or more resistance wires 202. The resistance wires 202 need to contact bus wires 102 on alternate sides to form an electrical connection with bus wires 102. When a voltage is applied to bus wires 102, heat is generated on resistance wires 202 as current flows through the wires.
Because heating element 106 is insulated, that is, the resistance wires 202 are insulated, first insulation layer 220 is bared at certain node areas to expose a portion of the resistance wires. If heating element 106 is wrapped around the bus wire structure, the bared areas of heating element 106 allow the resistance wires to contact bus wires 102 at alternating positions. For example, heating element 106 contacts one bus wire and is then wrapped around to contact the other bus wire, and so on. This allows heating element 106 to electrically couple to bus wire 102. The baring of bus wires 102 allows electrical connections to be made at any point along the length of bus wires 102. This is in contrast to an insulated bus wire in which node areas would need to be formed to expose the bus wires. Although it is described that heating element 106 is wrapped around bus wires 102, it will be understood that other configurations may be appreciated, such as heating element 106 may be a mesh.
Second insulation layer 108 is wrapped around the heating element 106 and bus wire structure to electrically insulate heating element 106 from the metal sheath that encloses it. Second insulation layer 108 may include layers of glass cloth tapes and mica/glass cloth tapes, or other suitable high temperature insulation materials.
Metal sheath 109 encloses the outside of the bus wire structure and heating element 106. Metal sheath 109 may protect the bus wire structure and heating element 106 from moisture ingress. Metal sheath 109 may be corrugated to allow flexibility. Accordingly, metal sheath 109 may afford an appropriate amount of mechanical and chemical protection to the bus wire structure and heating element 106. Materials used for metal sheath 109 may include stainless steel, incoloy alloys, inconel alloys, high-temperature aluminum, and other chemically-resistant steels. Other embodiments of metal sheath 109 may include a tape that is seam-welded on one side or both sides, a tape that has been slightly corrugated before welding, a tube, a slightly-flattened tube, a corrugated tube, and a slightly-flattened corrugated tube.
In one example, bus wires 102 are substantially flat. A flat bus wire creates a structure that is more round than oval (using stranded or round bus wires 102 cause a more oval shape to be formed). The round shape sometimes allows the structure to be inserted in metal sheath 109 easier in the field.
Spacer 104 may be various cross-sectional shapes, such as rectangular, square, circular, etc. Spacer 104 may be described as being substantially rectangular but it will be understood that substantially rectangular may be a four sided polygon. Spacer 104 may be made with an insulating material, such as from glass yarns wrapped around cloth tape made of glass cloth or mica. Spacer 104 may be different dimensions. In one embodiment, spacer 104 can be produced in larger dimensions to create a wider heating cable, which may be desirable to provide higher power outputs that can be distributed over a wider and larger surface area of the heating cable.
Heating element 106 may include an insulating core and one or more resistance wires wrapped around the core in a helical manner. Also, heating element 106 may just include the resistance wires without the core. Other variations of heating element will also be described below. Further, any number of heating elements 106 may be used to provide redundancy.
Heating element 106 is placed on an outer surface of spacer 104. For example, heating element 106 is wrapped around spacer 104 in a helical manner. Heating element 106 is placed on spacer 104 before bus wires 102 because bus wires 102 are placed on the outside of heating 106 and spacer 104.
Bus wires 102 provide electrical power to heating zones. In one embodiment, two bus wires 102 are provided and are set parallel to one another. However, it will be understood that any other number of bus wires 102 may be used and can be arranged differently.
The desired resistance to include in a node may be measured as heating element 106 is wrapped around spacer 104. The mechanical and physical arrangement of heating element 106 may be inspected before placing bus wires 102. This allows any quality issues to be addressed at an early stage of manufacturing of heating cable 100. For example, heating element 106 is wrapped around spacer 104 and can be inspected for any quality issues. The inspection is made before hiding it with bus wires 102 thus making it easy to notice imperfections or quality issues.
Bus wires 102 are then placed on the outside of heating element 106. For example, if spacer 104 is rectangular in shape, a first bus wire is placed on a first side of the rectangle and a second bus wire is placed on a second side. This forms a substantially rectangular structure. Also, the first and second bus wires may be placed on the top and bottom or spacer 104 to form a more circular structure. Bus wires 102 are placed such that heating element 106 is in between spacer 104 and each individual bus wire.
Because heating element 106 is placed on spacer 104 before bus wires 102, the length of resistance wire that is wrapped around spacer 104 may be measured. The desired resistance in the zone may then be measured before baring first insulation layer 220 on heating element 106 at points where zone boundaries are created. An accurate forming of node areas 110 may then be created where the zones are desired.
Metal sheath 109 encloses the outside of the bus wires and heating element 106. Metal sheath 109 may protect the bus wire structure and heating element 106 from moisture ingress. Metal sheath 109 may be corrugated to allow flexibility. Accordingly, metal sheath 109 may afford an appropriate amount of mechanical and chemical protection to the bus wires 102 and heating element 106.
Further embodiments of heating cable 100 may be disclosed in U.S. patent application No. ______, entitled “HEATING CABLE WITH A HEATING ELEMENT POSITIONED IN THE MIDDLE OF BUS WIRES” and U.S. patent application No. ______, entitled “HEATING CABLE”, both of which are filed concurrently and incorporated by reference in their entirety for all purposes.
Heating element 106 may be insulated with a fibrous structure or the like. For example, an insulation layer 220 is shown that surrounds resistance wires 202 and insulating core 204. Insulating layer 220 may be bared at a plurality of node areas 110.
At spaced-apart intervals, the plurality of node areas 110 are formed by removing insulation 220. The conductive resistance wire 202 is exposed over a short distance. In
Although bare bus wires are described, it will be understood that bus wires 102 may be partially insulated. For example, certain parts of bus wire 102 may be insulated and other parts may be bared. The bared parts correspond to areas where heating element 106 has node areas allowing electrical connections to be made.
Step 414 places a spacer in between the plurality of bus wires. This electrically separates the plurality of bus wires and forms a bus wire structure.
Step 416 provides a heating element 106 comprising one or more resistance wires 202 and an insulating layer 220 surrounding the one or more resistance wires 202. The insulating layer 220 is bared at a plurality of node areas 110 to expose the one or more resistance wires 202 from the insulating layer 220. The plurality of node areas 110 may be formed by removing insulation from heating element 106 to expose the one or more resistance wires 202. The heating element 106 may be formed by wrapping the one or more resistance wires around an insulating core.
Step 418 places the heating element 106 such that the one or more resistance wires are electrically coupled to the bus wires to one or the other bus wires at the plurality of node areas to create a plurality of resistance zones for generating heat. Bus wires 102 may be on the outside or inside of heating element 106 as shown in
Step 420 places a tab around the heating element and bus wire structure to secure the electrical connection to one or the other of the bus wires at a node area in the plurality of node areas. Placing of the tab may be optional.
Step 422 places a second insulation layer around the bus wire structure and the heating element. Step 424 then places a metal sheath around the second insulation layer.
Particular embodiments provide many advantages. For example, the resistance of a zoned heating element can be measured before wrapping it around bus wires 102 thus making sure that the correct heat output may be achieved in the finished heater. Also, providing insulation layers over the fine-gauge resistance wire 202 protects the fine-gauge wire from physical damage.
Also, during the insulation process, the finished heating cable is easier to terminate bus wires 102 connected to a power supply since they are already bared. For example, there is no need to strip away the bus wire insulation to connect them to the voltage supply as would be needed if each bus wire were individually insulated.
Different embodiments of heating element 106 will now be described. These embodiments describe redundancy schemes that may or may not be used. Heating element 106 may include an insulating core and one or more resistance wires wrapped around the core in a helical manner. In another embodiment, multiple heating elements 106 may be wrapped around the bus wire structure. For example, two heating elements 106 may be wrapped around the bus wire structure concurrently without touching each other. This may form a redundant design where two heating elements 106 are connected at intervals along the length of a zone where the insulating layer 220 has been bared. This provides additional redundancy.
The insulating core may be a tape, such as a cloth tape made up of a glass material. The tape may be flat and a certain width, length, and height, such as tapes from ¼ to ½ inch width. The cloth tape is folded over to form insulating core 204. As will be described in more detail below, the tape when folded over is somewhat stiff and exerts an outward force because the tape wants to open up again. The tendency to open up maintains an outward force on resistance wire 202. Because resistance wire 202 is wound around insulating core 204, resistance wire 202 is kept taut and tight and is not able to move around or slip around insulating core 204. Thus, different sections of resistance wire 202 are prevented from touching each other.
The use of glass cloth tape also enables different width heating elements 106 to be made easily. For example, additional cloth tape may be wrapped around to form a thicker or thinner insulating core 204. By providing a different width insulating core 204, greater lengths of resistance wire 202 may be used per foot of heating element 106. For example, a thicker insulating core 204 allows more resistance wire 202 to be wrapped around it per linear foot. This may be important when more resistance wire is desired per zone. Different combinations of spacing pitch of the wrapping of heating elements give different resistances and power output of the heating cable depending on applied voltages, as will be described in more detail below. Accordingly, flexibility is provided using the cloth tape in addition to providing an outward force to tightly wind resistance wires 202 around insulating core 204.
After wrapping resistance wires 202 around insulating core 204, heating element 106 then wraps around the bus wire structure as shown in
Resistance wire 202 may contact bus wire 102 at nodes 110. This provides an electrical connection between resistance wires 202 and bus wires 102. When a voltage is impressed on bus wires 102, resistance wire 202 generates heat. For example, current can flow through resistance wires 202. In between the zones, heat is produced on resistance wires 202.
The zone length of zone heaters using fine gauge resistance wire as a resistance element depends on the overall resistance between nodes 110. This depends on the resistance per unit length of resistance wires 202, its length within zones, and the amount of heat desired and voltage applied to bus wires 102. If a fine gauge resistance wire is about 42 AWG (0.0025 inch diameter), the resistance is about 100 ohms/foot of length, a length of fine gauge wire to produce 10 watts/foot of heater at 240 volts AC is necessarily very long (wire length=240*240/10*100=57.6 feet of fine gauge wire). Particular embodiments provide this length of fine gauge wire into a shorter length of heater. By wrapping resistance wires 202 around insulating core 204 to form heating element 106, and then wrapping heating element 106 around the bus wire structure or bus wires 102, shorter zone lengths are provided. This is because the length of resistance wire needed in a zone is shortened by wrapping the resistance wire around insulating core 102 and then wrapping heating element 106 around the bus wire structure. For example, a zone length may be about 1 or 2 feet using particular embodiments. By providing shorter zone lengths, if a zone is cut, only a small part of the pipe may not be heated. Also, by wrapping heating element 106 helically around the bus wire structure, more resistance wire is used within a zone and may produce more heat.
Accordingly, resistance wire 202 can be wound around the glass cloth fabric such that the length of resistance wire 202 is several times the length of the insulating core. Resistance wire 202 may be wound around insulating core 204 and wound around another insulating core 204 to produce an even greater length of resistance wire and this process may be repeated again and again. Resistance wires 202 may be sewn into glass cloth fabric in a zigzag fashion. Also, resistance wires 202 can be woven into glass cloth fabric and then that glass cloth fabric can be cut on a bias to produce angled redundant long resistance wire paths between bus wires.
Particular embodiments also provide redundancy within zones using heating elements 106, as long as the resistance wires and or the heating elements are electrically connected in some way within that zone. Thus, redundancy can be provided using resistance wires 202 and/or heating elements 106. For example,
Further, as seen in
In
If a good electrical connection is not made at nodes 110, then electrical contact may be disconnected physically. Also, if a good connection is not made, nodes 110 may become higher in contact resistance over time under the high temperature conditions during the use of the heating cable. High contact resistance at node 110 leads to poor electrical contact and/or voltage drop at that point that could destroy the contact and/or resistance wire at node 110 over time.
The many wraps of resistance wires 202 around insulating core 104 in heating element 106 and the long length of bus wires causes resistance wire 202 to contact bus wires 102 in many spots at each node 110. Using clip 500, the node may be encased and resistance wire 202 is held with firm physical contact onto bus wire 202.
Clip 500 provides many advantages of making electrical and physical contact over node 110. A wide area can be covered using clip 500 where resistance wires 202 touch bus wires 102. Further, the entire area of node 110 may be contacted to make contacts with all the resistance wires 202 that are contacting bus wire 102 in node 110.
The contact between bus wires 102 and resistance wires 202 should be a good both electrically and physically. The connection should be able to withstand high temperature and remain in good contact upon mechanical stress and cycling between low and high temperatures. The connection between resistance wires 202 and bus wires 102 can be made in various ways. For example, only physical contact may be provided between resistance wires 202 and bus wires 102 by wrapping heating element 106 around the bus wire structure. In one example, the folded glass tape may exert the outward force, which may provide a better electrical connection between resistance wires 202 and bus wires 102. For example, the outward force may cause resistance wires 202 to physically stay against bus wire 102. In the example shown in
Accordingly, particular embodiments provide good mechanical and electrical contact between heating element 106 and bus wires 102 at nodes 110. This contact is maintained for design lifetime of the heating cable under mechanical and temperature extremes during the use of the heating cable.
As shown in
In
Particular embodiments provide redundancy and reliability. For example, redundancy is provided in which resistance wires may be broken but alternate paths are provided such that the connection is not lost between zones. Also, good contact is provided at nodes due to a clip that holds resistance wires firm to bus wires 102 at nodes 110. Also, shorter zone lengths are provided because resistance wires 202 are wrapped around insulating core 204, which then is wrapped around a bus wire structure. Thus, longer lengths of resistance wire may be wrapped around in a zone thus resulting in shorter zone lengths.
Accordingly, particularly embodiments reduce the danger of non-heated lengths of zones for a particular element that is being heated, such as a pipe. Redundancy, reliability, and shorter zone length provide a better heating cable.
In one embodiment, metal sheath 109 may be removed. A tape, such as glass fiber-mica tape, may be wrapped around heating element 106 and the bus wire structure. A metal braid layer then encloses the glass cloth insulation and then a high temperature resistant polymeric jacket encloses the outer braid layer. The braid layer provides electrical protection and can be grounded and provides mechanical protection for the heating cable. The polymeric jacket material can withstand a long-term high temperature environment.
An example will now be discussed but it will be understood that other examples will be appreciated. Two heating elements 106 of medium length are wrapped in a co-rotated manner between a node 110-1 on one bus wire 102-1 to a node 110-2 on another bus wire 102-2. There may be two electrical circuits, made by inserting ties between the heating elements, connecting heating elements 106 at one-third points between nodes 110. The heater produces 20 watts/unit length at 120 volts AC. By Ohm's Law, the total resistance between nodes is 720 ohms, each of the three sections having resistance of 240 ohms and producing 6.67 watts. The current flow through the heater is 0.278 amps.
If resistance wire 202 on each heating element 106 is made of 38AWG resistance wire with a resistance of 48 ohms/feet of wire length, then 16 feet of resistance wire is needed between nodes 110. If this resistance wire is wrapped around bus wires in a conventional zone heater configuration, then the zone length of the heater would be about 4 feet. However, particular embodiments may achieve a zone length of 1.33 feet by wrapping resistance wire 202 around insulating core 106. If two parallel resistance wires 202 are used, then the zone length may be doubled.
If one resistance wire 202 in one section of a heating element 106 is broken, then that section has resistance of 480 ohms and the other two sections still have resistance of 240 ohms each, and the sections are in series. Since total resistance is now 160 ohms, the current flow is 1.56 amps. The overall power output of the heater is now 15 watts, distributed as 7.5 watts in a section where the wire is broken and 3.75 watts in each of the other two sections. Though one resistance wire 202 has been broken, heat is still produced in all sections of a zone.
The above example is only an example and can be extended to additional redundant resistance wires 202 or heating elements 106 in parallel, as well as more electrical circuit ties between resistance wires 202. With increased parallel resistance wires 202, the distance between nodes 110 increases, however the inclusion of an increased number of electrical circuit ties 402 between resistance wires 202 decreases the effective zone length of the heating cable. This can also apply to the counter-rotated wrapped resistance wires 202 which also contain redundancy and for which power output reduction on a break in the wire is minimal.
Although the description has been described with respect to particular embodiments thereof, these particular embodiments are merely illustrative, and not restrictive. For example, heating cable may be used to provide heat to a number of different structures and is not limited to pipes.
It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
Thus, while particular embodiments have been described herein, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of particular embodiments will be employed without a corresponding use of other features without departing from the scope and spirit as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit.
