Patent Application
20130264330
This application relates to electrical heaters for process heat exchangers, and in particular to electrical heaters for use with medium voltages.
One type of electrical process heater used in various industries comprises a tank or pressure vessel having an inlet and an outlet for the liquid to flow through. The heater has a heater element bundle mounted in the tank. The heater element bundle has a number of electrical resistance heater elements. Each heater element includes a metal tube, an electrical resistance coil within the tube and embedded in an insulation powder, and an exterior conductor pin secured to the end of the coil. The heater has at least one end located outside of the tank, that end having a header to which each tube is secured. The opposite end may also include a header or the tubes may be U-shaped.
Voltage is applied to the conductor pins to create heat in the electrical resistance coils. Most process heat exchangers operate with three-phase power in the range from about 600 to 640 volts. More recently electrical heaters have been proposed to operate in the range from about 2,400 to 4,160 volts. Although considered a medium voltage for electrical power transmission in general, this voltage creates more demands on the heater elements.
The insulation powder is typically magnesium oxide packed tightly within the tube surrounding the coiled wire. While magnesium oxide provides excellent electrical insulation, it is a desiccant, thus it attracts moisture from the surrounding atmosphere. The penetration of moisture reduces the ability of the insulation powder to insulate.
In the past, heater elements of this nature have been kept in low humidity rooms and/or baked in an oven with their ends open to drive off any moisture. Then, when ready for use, the heater elements are mounted to a header plate and seals are placed over the open ends. For example, a liquid sealant may be poured over the open ends and cured. While these methods work, improving the resistance of the insulation is desirable not only for low voltage heaters but also for medium voltage heaters. The higher voltage is more difficult to insulate, particularly at the exposed end face of the insulation powder, which is subject to moisture penetration. U.S. Pat. No. 7,372,007 shows one type of electrical heater for medium voltages.
An electrical heater has a plurality of heater elements, each of the elements having a metal tube, an electrical resistance wire within the tube, an exterior conductor pin joined to the wire and protruding from an exterior end of the tube, and an electrical insulation powder surrounding the wire within the tube. A cavity is formed within the exterior end of the tube with the exterior conductor pin extending through the cavity. An electrical insulating member has a passage through which the exterior conductor pin extends. The insulating member has a cylindrical shank that is located in the cavity and a cylindrical head protruding from the exterior end of the tube. A layer of epoxy is located between the shank and the powder.
The heater has a header plate having a plurality of apertures. The tubes are joined to the header plate and the exterior conductor pins extend through the apertures. A housing is mounted to an exterior side of the header plate and encloses the exterior conductor pins.
A phase divider may insert into the housing in abutment with the header plate, the phase divider being of an electrical insulating material and having three sections that separate the exterior conductor pins into first, second and third electrical phase groups. The exterior conductor pins within the first group are adapted to be connected to a first phase of a three-phase voltage, the exterior conductor pins within the second group are adapted to be connected to a second phase of the three-phase voltage, and the exterior conductor pins within the third group are adapted to be connected to a third phase of the three-phase voltage.
The sections of the phase divider may join each other at a hub on a longitudinal axis of the housing and extend radially from the hub 120 degrees apart from each other. The phase divider may have a cylindrical wall surrounding and joined to peripheral edges of the sections.
Preferably each of the exterior conductor pins protrudes a same distance from the header plate. Each of the tubes may also have an interior end and an interior conductor pin protruding past the interior end. The interior conductor pins are separated into the same first, second and third groups as the exterior conductor pins. The interior conductor pins within each of the groups are electrically connected to each of the other of the interior conductor pins within the same group. The groups are connected to each other in a Y-configuration.
The end of the shank of the insulating member in contact with the epoxy layer may be flat and perpendicular to the exterior conductor pin. The head of the insulating member preferably has a shoulder that abuts the exterior end of the tube. An annular gap between the exterior conductor pin and the passage in the insulating member may have a layer of epoxy filling the annular gap.
Referring to
In this embodiment, each electrical heater 13 has an exterior portion extending through a wall of tank 11. Each electrical heater 13 has an exterior header plate 15 mounted a selected standoff distance from tank 11. A cylindrical neck 17 is welded to tank 11, and header plate 15 is secured to neck 17. Each electrical heater 13 has a housing 19 secured to the side of header plate 15 opposite neck 17. Each electrical heater 13 also preferably has an interior end cover 21 located within tank 11. Each electrical heater 13 comprises a bundle of parallel heater elements 23.
Referring to
Wire 29 is electrically insulated from tube 25 by an insulation powder 33 such as magnesium oxide. Insulation powder 33 is packed tightly within tube 25 and prevents wire 29 from contacting the side walls of tube 25. The end of insulation powder 33 is recessed within tube 25 a selected distance, creating a cavity 34. The end of insulation powder 33 may be generally flat and perpendicular to an axis of tube 25.
An electrical insulation member 35 having a high dielectric value has a cylindrical shank 37 inserts into cavity 34. The material of insulation member 35 may be ceramic or any suitable high temperature dielectric material. Insulation member 35 has an enlarged cylindrical head 39 with a diameter larger than shank 37 and larger than the inner diameter of tube 25. Head 39 protrudes past the exterior end of tube 25 and has a shoulder 41 that abuts the exterior end of tube 25. Shank 37 has an outer diameter that fits snuggly within the inner diameter of tube 25 and need not form a seal. An axial passage 43 extends through insulation member 35 along the axis of tube 25. Exterior conductor pin 31 extends through passage 43 and is slightly smaller in diameter than passage 43, creating an annulus between. Insulation member 35 has an inner end 45 located within tube 25 that may be flat and perpendicular to the axis of tube 25.
A layer of epoxy 47 is located between the end of powder 33 and insulation member inner end 45. Epoxy 47 also extends into the annulus between exterior conductor pin 31 and the inner diameter of passage 43. Epoxy 47 is uncured when placed in tube 25, then subsequently cured to bond insulation member 35 to powder 33 and tube 25. Epoxy 47 forms a seal against the inner diameter of tube 25 at the end of powder 33, blocking moisture entry. Exterior conductor pin 31 protrudes past insulation member 35 a selected distance.
Referring to
An exterior phase divider 57 formed of electrical insulation material divides the phase groups 55a, 55b and 55c from each other. Phase divider 57 has a cylinder 59 that slides closely into the inner diameter of housing 19. Three web sections 63a, 63b and 63c extend radially outward from a hub 65 and join cylinder 59. Web sections 63a, 63b and 63c are flat plates that separate the phase groups 55a, 55b and 55c from each other, as also illustrated in
The three bus bars 69a, 69b and 69c join each other, as indicated schematically by dotted lines 71a, 71b and 71c , forming a zero voltage neutral point 73. This arrangement defines a Y-configuration for three-phase power. The interior conductor pins 49 within each group 55a, 55b and 55c will receive a different electrical phase of the three-phase power supplied to groups 55a, 55b and 55c on the exterior end. The voltage at neutral point 73 should be approximately zero.
Referring to
While the disclosure has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the disclosure.
