GB2573742A

GB2573742A - A heating element

Info

Publication number: GB2573742A
Application number: GB201806180A
Authority: GB
Inventors: Newlove Paul; Brown James
Original assignee: Exheat Industrial Ltd
Current assignee: Exheat Industrial Ltd
Priority date: 2018-04-11
Filing date: 2018-04-16
Publication date: 2019-11-20
Also published as: GB201806180D0; GB201805981D0

Abstract

A heating element 30 for a process heater comprises an electrical resistance wire embedded in insulating material and housed in a metallic sheath. The heating element may have an elongated ‘U’ shape and includes a first section or heating zone 30A operating with a first thermal flux and a second section or heating zone 30B operating with a second thermal flux which is lower than the first thermal flux. An inactive or unheated section 30C may also be provided. Varying the thermal flux along the length of the heating element may be achieved by varying the wire diameter or pitch or by the use of more than one wire in one of the sections or zones. A process heater may include one or more such heating elements housed in a vessel with a tubular interior through which fluid can flow.

Description

Heating Element

Technical· Field of the Invention

Embodiments of the present invention relate generally to elements for process heaters and in particular to variable heat distribution elements. The heaters are particularly suitable for use in hazardous environments or for heating flammable hazardous materials.

Background to the Invention

Process heaters are well known and typically refer to enclosed devices that heat raw or intermediate materials during an industrial process. Process heating refers to the heating of process streams, in which the material of which the stream is comprised is usually in fluid form. Such heaters comprise a range of process flow heaters including flameproof electric heaters or electric line heaters, some of which are certified for use in hazardous areas to avoid any ignition sources. High temperature applications reach 100s of degrees Celsius.

Some known process heaters consist of a tubular shell arranged for the flow of gas to be heated, the shell enclosing multiple heating elements to transfer heat to the gas flowing past the heating wire, electrical connections for the heating elements, a gas feed and a gas outlet. The heating element can comprise a metallic tube, inside which electrical heating wires are encased or housed. The heating elements are supported inside the shell in a segmental or rod-type baffle assembly 12 to prevent flow-induced vibration and hot spots. The process flow is baffled in the heater to optimise transfer of heat from the element to the process material. Pitches of the baffled section can be up to 600 mm. Elements can be fixed to a flange 13 by way of welding or using a compression fitting.

Usually the heating wires are in the form of fine wires which are wound in a spiral configuration the cross-section of which is very much smaller than the tube cross-section, the wires having current passing therethrough and being thereby resistively heated. The electrical energy converted into heat by the heating wire depends on the available electrical voltage and the resistance of corresponding heating wires, in which respect to achieve desired resistance values, the length of a spiral-wound wire can be correspondingly adapted or a plurality of corresponding heating wires can be connected in parallel or also in series.

It is desirable with process heaters to increase heating power. However, there are limitations to the achievable surface temperature, therefore increasing the number of heating elements leads to increased overall size of the process heater. Accordingly, the known structure of process heaters places a constraint on the overall size, with a minimum diameter required for the metallic tubes. Furthermore, such heating elements are costly to produce due to the design complexity and high costs of materials required. It would be desirable therefore to provide a more compact heater which is cheaper to manufacture.

It is to these problems, amongst others, that embodiments according to the present invention attempt to offer a solution.

Summaiy of Invention

In an independent aspect, there is provided a heating element for a process heater, the heating element having a first section operating with a first thermal flux and a second section operating with a second thermal flux which is lower than the first thermal flux.

The heating element has at least two sections providing different heat distribution density. In other words, the heating element provided is a variable heat distribution element.

Such a heating element is suitable for hazardous environments, heating fluids to a higher temperature part way along its length as well as closer to the end.

Advantageously, a reduced number of heating elements can be used in an assembled process heater, resulting in a process heater with a reduced diameter and reduced costs for materials used in the elements and shells. Furthermore, the footprint of the heater is reduced and product sustainability is increased.

The process heater can be used in applications reaching 100s of degrees Celsius, for example, with a high element sheath temperature (e.g. above 350°C). A process temperature increase of above 100°C can be achieved, preferably within a single pass.

It will be appreciated that the terms first and second sections may be used interchangeably, and thus, in alternative embodiments, the second thermal flux may be higher than the first thermal flux. It will also be appreciated that the heating element of any one of the preceding claims, may comprise further additional, heated section(s).

In a dependent aspect, the heating element comprises at least one U-shaped metallic tube with a curved end and comprising an electrical wire arranged within insulating material inside said tube, wherein said second section includes the curved end of said tube.

The term tube is to be broadly interpreted in and ultimately defines only a hollow space for receiving the wire therethrough. In that respect the cross-section over the length of the tube does not even have to be constant and can vary across different sections of the heating element.

In a reverse flow situation, it will be appreciated that the first section is included in the curved end of said tube.

In an alternative embodiment, instead of a U-shape tube, the heating element comprises a single straight metallic tube and an electrical wire arranged within insulating material inside said tube. The single straight metallic tube is typically referred to as a cartridge type element, which is essentially the same as the U-shaped tube but without the curved portion (without the U turn).

The first section is longer than the second section or the other way round.

In a dependent aspect, the heating element comprises an additional, an unheated section

In a dependent aspect, the insulating material comprises magnesium oxide (MgO) or similarly suitable insulating material.

In dependent aspects, the length of the element is from 0.5m to 10m, preferably from 0.5m to 4m. In dependent aspects, the heating element has a maximum diameter of 6 to 22 mm, preferably 12.5 mm. This advantageously provides a compact heating element.

In dependent aspects, the heating element has a pitch of 10 to 200 mm, preferably 24 mm.

In dependent aspects, the second thermal flux is 10-90 % of the first thermal flux. In alternative embodiments, the first thermal flux is 10-90 % of the second thermal flux.

In a dependent aspect there is provided a process heater comprising at least one heating element according to any one of the preceding claims.

In a dependent aspect, the process heater comprises two or more heating elements, wherein the respective first and second sections of the heating elements are respectively aligned to form at least two sections of the process heater with different heat distribution.

Brief Description of the Figures

Examples of the present invention will now be described with reference to the accompanying drawings, where:

Figure 1 schematically shows a process heater;

Figure 2 schematically shows heating elements with variable heat distribution (dual flux element design);

Figures 3A shows exemplary design specification parameters for the single flux element design shown in Figure 4;

Figures 3B shows exemplary design specification parameters for the dual flux element design shown in Figure 5;

Figure 4 schematically shows a process heater with multiple heating elements of single flux distribution; and

Figure 5 schematically shows an equally powered process heater with multiple heating elements of variable heat distribution (dual flux element design).

Detailed Description of the Invention

Turning firstly to Figure 1 a heater, generally referenced, 1 has one or more heating elements 30 housed in a vessel with a tubular interior (pipe 2) in which fluid (liquid or gas) can flow through. In this example, an electric line heater 1 has a housing in the form of a tubular shell 2 encasing multiple heating elements 30. The diameter of the pipe or tubular shell 2 can range from approximately 10 cm to 2.4 m for example. The heater 1 can have a length between 0.5m to 10m for example, preferably around 4m.

An element 30 can comprise 80/20 nickel-chromium resistance wire embedded in insulating material and housed in a metallic sheath, such as nickel alloy, copper/copper alloys, aluminium, steel, stainless steel.

It will be appreciated that the resistance wire can be made of a suitable resistance material. The wire can be wound in a spiral configuration, with coils being electrically insulated from themselves and the surroundings with insulating material of good thermal conductivity. Preferably, the insulating material is made of an easy-flow powder, Magnesium Oxide (Magnesia or MgO), which has a density of 2.35-2.45 g/cm³. Using Magnesia to insulate the wires enables the manufacturing process of the wires.

The heating element 30 has an elongated ‘U’ shape, with an outer diameter of 6 to 22mm, preferably around 12.5 mm. The element pitch, as indicated in Figure 2 may be between 10 to 200mm, preferably 24 mm.

Heating elements 30 may operate on a pre-determined supply voltage and at a predetermined duty. Advantageously, each element can operate at two or more heat flux densities (watt densities) to provide a variable heat flux, in sections 30A and 30B, respectively as shown in Figure 2. An inactive or unheated section (30C) is also provided to be suitable for use in a hazardous area. Preferably, for a given process flow, the watt densities do not exceed a process driven maximum temperature.

By using a plurality of watt densities, a higher duty per element can be achieved. The higher duty reduces the required number of heating elements in the heater. Further, where possible the heater size can be reduced which increases the velocity at which the process passes through the heater. Increasing the velocity means higher watt densities can be used to achieve the same maximum surface temperature of the element.

Elements 30 are sealed to ensure that the electrical performance is protected against the ingress of moisture to the element internals. The seal also provides electrical clearances to allow operation in a hazardous area. In this example, elements are electrically terminated in a suitable hazardous area enclosure, with two conductive pins 50A, 50B as shown in Figure 2. The resistant wire is electrically connected to the conductive pins 50A, 50B.

The heating element 30 shown in the example of Figure 2 has a length (L) equal to the distance between a first end 40 and a second end 50, which is 3912 mm. The heating element 30 has a first section 30A providing a heat flux density of 7.6 W/cm² and a second section 30B (shorter than the first section in this example), the second section providing a heat flux density of 5.8 W/cm². The firstand second sections combined form the ‘active’ portion of the heating element. A third section, 30C, positioned next to the second end 50 of the heating element does not provide a heating flux and is thus referred to as ‘inactive’. The unheated section 30C in this example is 722 mm.

Varying the heat flux along the length of the heating element can be achieved in a number of ways. For example, the wire diameter can be varied by stretching the wire before coiling, the wire pitch can be varied, more than one wire can be used in one of the sections.

With the variable heat distribution element, a high element sheath temperature (e.g. of up to 900 °C) can be achieved, with a process temperature increase of up to several hundred °C.

Figures 3A (single flux) and 3B (the equivalent using dual flux) show exemplary design specification parameters for element designs respectively shown in Figures 4 and 5. For comparison, Figure 4 schematically shows a process heater with a single flux design (see Figure 3A), whilst Figure 5 shows the process heater with a dual flux design as shown in Figure 3B.

Figure 5 schematically shows a process heater 10 with a bundle of heating elements 30, with heating elements in the yz plane shown in Figure 5. As with the example above, each one of the 63 active heating elements 30 has two active sections 30A, 30B and an inactive section 30C by the end adjacent an instrument box 11. The heating elements 30 are supported inside a shell 2 of the process heater (not shown) by a plurality of element supports/baffles 12 to induce turbulent flow which in turn reduces hot spots. The heating elements 30 are normally arranged in parallel, with their respective sections being aligned as shown in Figure 5 to form the three sections (two heated, one unheated) in the yz plane. All of the heating elements are normal to the xz plane and it will be appreciated that any number of heating elements can be bundled in this plane in the x direction as well as the z direction, each having two or more aligned, matching heated sections and at least one inactive section. In other words, the shape on the bundle can vary, although it is usually of an overall circular cross section to fit the tubular shell.

In this example, the length of the process heater elements is 3912mm, the first active/hot section 30A is 1455 mm, the second active/hot section is 854 mm, and the unheated length of each element is 772 mm. The bundle diameter is 330 mm. Figures 3B shows exemplary design specification parameters for the dual flux element design.

In preferred embodiments, the heat distribution difference between the two active sections is up to 40%, heating element diameters are between 8 to 22 mm and the heating element has a maximum length of 5m.

It will be appreciated that a plurality of heating elements or process heaters can also be connected axially one after the other. Optionally, the process flow may be in an opposing direction. Accordingly, the high and low density sections may be swapped around.

It will be appreciated that the term “comprising and its grammatical variants must be interpreted inclusively, unless the context requires otherwise. That is, comprising should be interpreted as meaning including but not limited to.

Moreover, the invention has been described in terms of various specific embodiments. However, it will be appreciated that these are only examples which are used to illustrate the invention without limitation to those specific embodiments.

Claims

1. A heating element for a process heater, the heating element having a first section operating with a first thermal flux and a second section operating with a second thermal flux which is lower than the first thermal flux.

2. A heating element according to claim 1, the heating element comprising at least one Ushaped metallic tube with a curved end and comprising an electrical wire arranged within insulating material inside said tube, wherein said second section including the curved end of said tube.

3. A heating element according to claim 1, the heating element comprising a single straight metallic tube and an electrical wire arranged within insulating material inside said tube.

4. A heating element according to any one of the preceding claims, wherein the first section is longer than the second section.

5. A heating element according to any one of the preceding claims, further comprising an unheated section.

6. A heating element according to any one of claims 2 to 5, wherein the insulating material comprises MgO.

7. A heating element according to any one of the preceding claims, wherein the length of the element is between 0.5m and 10m, preferably between 0.5m and 4m.

8. A heating element according to any one of the preceding claims, having a maximum diameter of 6 to 22 mm, preferably 12.5 mm.

9. A heating element according to any one claims 2 to 8, having a pitch of 10 to 200 mm, preferably 24 mm.

10. A heating element according to any one of the preceding claims, wherein the second thermal flux is 10-90 % of the first thermal flux.

11. A process heater comprising at least one heating element according to any one of the preceding claims.

12. A process heater according to claim 11, wherein the process heater comprises two or more heating elements, wherein the respective first and second sections of the heating elements are respectively aligned to form at least two sections of the process heater with different heat distribution.