US20080085105A1

US20080085105A1 - Multi-passage electric heater using ceramic foam as a diffuser and method of use

Info

Publication number: US20080085105A1
Application number: US11/522,925
Authority: US
Inventors: Dexter E. Diepholz
Original assignee: Farnam Custom Products Inc
Current assignee: Farnam Custom Products Inc
Priority date: 2006-09-19
Filing date: 2006-09-19
Publication date: 2008-04-10

Abstract

A multi-passage electric heater for heating a gas flowing through the passages uses electric heating elements disposed in the passages for gas heating. One of both of the inlet and outlet of the heater includes a ceramic foam material as a diffuser. The diffuser on the inlet side effectively distributes gas evenly to the passages for heating and insulates against radiated heat loss through the inlet. When located on the outlet side of the heater, a more homogenous outlet stream of heated gas is created, with the outlet diffuser minimizing loss of radiated heat from the outlet side and acting as a heat exchange medium for improved heater efficiency.

Description

FIELD OF THE INVENTION

The present invention is directed to a multi-passage electric heater, and in particular, to an electric heater that employs one or more diffusers made from a ceramic foam material to modify the inlet and outlet flow characteristics of the heater and enhance the heater operation.

BACKGROUND ART

The use of process heaters for heating fluids is well known in the industry. One manufacturer of these types of heaters is Farnam Custom Products of Arden, N.C., with one example being their “heat torch family”. This product as well as other heaters can be seen on their website, www.farnam-custom.com and in company literature. Other examples of this art are demonstrated by product literature published by Farnam Custom Products. Process heaters are used primarily to heat a gas with the most common use being to heat air. The gas is forced through the heater by pressure difference between the heater inlet and the heater outlet. As the gas is directed through the heater its temperature is raised as it comes in contact with the heated internal surfaces of the process heater.
It is well known in the prior art to build process heaters for heating gas that directs the gas stream from the inlet to the outlet through multiple passages, normally a parallel path, within the heater. Within each passage, the gas is heated by means of some heat exchange mechanism such as electric heating elements.
One type of multi-passage electric heater commonly used in the industry is to have separate, individual tubes made of material suitable for the application, e.g., a ceramic material. The tubes are preferably aligned so each is parallel to one another and parallel to the direction of the incoming and outgoing gas flows. The entire group of parallel tubes is contained within an external cover or housing of the process heater assembly. Some means for heating, such as properly configured heating element material powered by an electric current, is used to transfer thermal energy as the gas moves through each tube thereby raising the outlet temperature to a desired level.
A second method is to use a heater core employing a honeycomb configuration having parallel channels for directing the gas from the inlet to the outlet. The honeycombed heater core is made of a material with properties appropriate for the intended use. The most common material is an extruded ceramic. The cross section area of each channel is sized so as to permit an appropriate flow of gas and to accommodate a means of heating. The most common means of heating in these types of heater cores is a properly configured electric heating element wire that when energized will properly heat the gas to a desired temperature. The honeycombed heater core is contained within the outer housing of the process heater.
A typical use of a process heater is to place it in line for heating a gas flowing through a pipe or tube. Other uses may be to use the process heater for directing a hot gas stream in an intended direction to accomplish a given function.
The prior art process heaters for heating gases have significant shortcomings. One ongoing problem with these multi-passage electric heaters is that the exiting gas stream tends to have a non-uniform distribution of temperature and velocity across the opening. This is a result of a non-uniform distribution of the velocity and volume of the gas across the face of the heater inlet. It is desirable to have the heated gas stream to be relatively uniform in velocity and temperature for the most effective use in applications.
Further, the non-distribution of velocities across the cross-section of the inlet fluid stream results in temperature problems further downstream. To solve this problem, certain heaters use metallic screens and/or grids located in the inlet and outlet, which also double as barriers for entry of foreign or unwanted objects. However, these remedies are ineffective as means to yield either uniform temperatures or velocities
The problems with velocity for these types of multi-passage electric heaters create yet another serious shortcoming. As noted in the above description of the prior art, process heaters for heating gas are made up with a collection of paths or passages, for example separate parallel tubes or a body with parallel extruded holes. A means for heating the gas is placed inside the tubes or openings thus raising the temperature during operation.
For proper operation, it is desirable that the velocity and volume flow rate of the gas being heated be uniform over the cross section of the entrance of the heater. If this is not the case, the velocity and thus the volume rate of flow through the tubes or passages will not be uniform. This non-uniform gas flow will result in uneven heating whereby those passages having the lowest velocity or volume rate of flow will operate at higher temperatures than those with the higher velocity or volume flow rate. This is because a lesser fluid flow rate in some tubes or passages results in higher temperatures since the fluid mass to extract thermal energy is less than desirable.
There are deleterious consequences of this type of uneven heating. One already mentioned is that the heated fluid has an undesirable and uneven temperature distribution as it exits the heater outlet and is delivered for its intended use. Other consequences of the uneven heating involve the heater construction. The materials used for the passages and the materials used to generate energy for heating the gas can irreversibly degrade at higher temperatures. Since the ultimate temperature required of the heated fluid is normally high, higher-than-specified temperatures in passages as a result of uneven flow through the heater can have adverse effects on the materials of the heater.
One such undesirable effect will be in that the metallic components of the heater, especially those using electric heating elements, will be degraded and experience a shortened useful life and in some cases premature burnout. This is even more of a problem when the gas is air, given its propensity to oxidize metallic materials. The excessive temperatures will ultimately oxidize the heater materials at a rate greater that for which the product was designed.
Another undesirable effect of the overheating is that ceramic materials, such as the extruded honeycombed heater core or parallel tubes, may operate at sufficiently high temperatures so as to compromise the ceramic material's dielectric properties. This will result in current leakage and deterioration of any electric current carrying parts such as heating elements.
Accordingly, improvements are needed in the field of process electric heaters which employ multiple passages to heat a flowing gas. The present invention responds to this need by improving the construction and operation of these types of heaters through the use of a ceramic foam material, which is utilized at least as flow diffusers for control of the inlet and outlet flow characteristics of the heater.

SUMMARY OF THE INVENTION

A first object of the present invention is an improved multi-passage electric heater.
Another object of the invention is an improvement in the method of using multi-passage electric heaters for heating gas.
Yet another object of the invention is the use of a ceramic foam material as a diffuser for one or both of the inlet and outlet of the multi-passage electric heater.
Other objects and advantages of the present invention will become apparent as the description thereof proceeds.
The invention is an improvement in multi-passage electric heaters having a heater core, an inlet and an outlet, wherein at least some of passages of the heater core each include an electric heater for heating a gas passing therethrough. The improvement of the invention comprises employing a ceramic foam material that is placed over at least a portion of one or both of the inlet and the outlet of the heater. The ceramic foam material can be used for both the inlet and the outlet of the heater, and can cover the inlet and outlet in their respective entireties.
The ceramic foam material can take any shape consistent with the heater inlet and outlet configuration, and can include openings for service wiring. The opening can also account for dead spots in the heater core and provide weight reduction and the like.
The ceramic foam material can be any type of a ceramic foam that would provide diffusing and distribution characteristics for the incoming and outgoing gas stream passing through the heater. As well, the foam can be any type that provides an insulating property and heat retention property to function as a heat exchange medium, particularly when used as an outlet diffuser. One example of such a ceramic foam is a magnesia-stabilized zirconia.
The ceramic foam material can also be mounted to the housing in such a way that the material acts as a stop to minimize movement of the heater core during operation of the heater.
When the ceramic foam material is used as an inlet diffuser, it is preferred to maintain a gap adjacent the upstream face of the diffuser to allow incoming gas to be more readily distributed across the upstream face.
The invention also entails an improvement in the method of heating a gas using a multi-passage electric heater having the heater core, the inlet and the outlet, and passages that contain an electric heater for heating gas passing therethrough. The improvement comprises placing the ceramic foam material over at least a portion of one or both of the inlet and outlet to minimize radiation from the heater and to either distribute/diffuse the gas entering the heater or exiting the heater. The ceramic foam material can be placed over each of the inlet and outlet, and can either cover a portion or the entirety of the inlet/outlet. The ceramic foam material can also be mounted to the housing to cooperate with the housing to minimize the movement of the heater core and any degradation as a result of such movement. Preferably, when the ceramic foam material is employed as an inlet diffuser, a gap is maintained adjacent its upstream end to enhance its ability to distribute the incoming gas to the passages of the multi-passage heater core.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the drawings of the invention wherein:

FIG. 1 is a perspective view of a heater according to the invention;

FIG. 2 is a side view of the heater of FIG. 1;

FIG. 3 is sectional view of the heater housing of the heater of FIG. 1;

FIG. 4 is a perspective view of an exemplary ceramic foam material for use as a diffuser in the heater of FIG. 1.

FIG. 5 is a view of a portion of the diffuser of FIG. 4 in more detail.

FIG. 6 is a partial schematic view of portions of the heater of FIG. 3 enlarged to show more detail.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention offers significant advantages in the field of process electric heaters employing multiple passages for heating of a flowing gas. That is, the use of a ceramic foam material as either or both of an inlet and outlet diffuser of the heater improves gas flow to the heater as well as gas exiting the heater. Better heating is achieved, and heater performance is enhanced with the insulating properties of the ceramic foam minimizing radiation losses and allowing good heat exchange, particularly at the downstream side of the heater. The ceramic foam diffusers can also be used to secure the heater core of the heater to minimize its degradation through movement during heater operation.
FIGS. 1-3 show one embodiment of the invention with the heater designated by reference numeral 10. The heater includes a heater housing 1 that surrounds a heater core 3. The heater core 3 in this embodiment is a ceramic honeycombed body having a plurality of passages 5, each containing an electric resistance heating wire 7. The resistance wires 7 in FIG. 2 are shown schematically as being located in the passages 5 of the heater core. The wires 7 would extend to a terminal box 60 for connection to a power source and control as typically done in these types of process heaters. Since this connection is well known in the art, it is not illustrated in FIG. 2.
The terminal box 60 is mounted to the housing 1 and is intended to house the wiring, terminals, etc., for electrical operation of the heater. These components are generally designated by the reference numeral 66 in FIG. 2. Since these components are well known for these types of process heaters, a further description is not deemed necessary for understanding of the invention. The terminal box 60 also includes an access 61 to accommodate wiring into and out of the box 60 and a tube 63 that provides a passageway for a gas to be directed to the heater core 3. The gas can be any type that needs to be heated, but is air in most instances. The tube inlet 65 is best seen in FIGS. 2 and 3. FIG. 3 also shows thermocouple wires 67 that extend from an outlet end 9 of the heater 10 to the inlet end 11 and beyond. The thermocouple wires 67 monitor the temperature of the gas exiting the heater 10.
Referring now to the heater housing 1, it includes a flange 13 that extends from an outer surface 15 of the housing. The flange includes openings 17 that allow the flange 13 to be attached in a desired location, e.g., in a duct or pipe, for heating of gas passing therethrough. It should be understood that the flange is optional and other means for mounting the heater 10 is a desired location for operation can be employed.
The heater housing 1 is also shown with two ceramic foam diffusers, one serving as an inlet foam diffuser 19 and the other as an outlet foam diffuser 21. The inlet diffuser 19 is positioned across the inlet of the heater core 3, with the outlet diffuser 21 positioned across the outlet of the core 3.
The ceramic foam inlet diffuser 19 functions as a very effective gas diffuser, whereby it evenly distributes the gas being introduced by tube 63 across a given cross section of the heater core 3 with minimal pressure drop, compared to other means such as baffles, screens, plates, etc. This assures even gas distribution among the heater coils, reducing the chance of localized hot spots, thereby enhancing the life of the heater through lower overall resistance wire temperatures for a given output gas temperature. At higher output temperatures, the ceramic foam inlet diffuser 19 also captures some of the infrared radiation energy that would normally be lost through the inlet. This provides an opportunity to reintroduce the captured radiation energy into the incoming gas, thereby improving the efficiency of the heater.
The ceramic foam inlet diffuser 19 is also beneficial in that it provides a barrier to the heater core, thus protecting entry of unwanted objects into the heater core.
The ceramic foam outlet diffuser 21 provides some of the same advantages as the inlet diffuser 19 along with others. The outlet location of the diffuser 21 increases the mixing and turbulence of the output gas stream so as to create a more homogenous outgoing stream of gas. This creates more uniformity in the outgoing stream in terms of both temperature and velocity, thereby improving the effect of the heater.
As with the inlet diffuser 19, the ceramic foam outlet diffuser 21 provides an insulating effect against the infrared radiation energy generated by the heater core. Radiation energy that would normally be lost from the outlet end of the heater is now absorbed by the ceramic foam outlet diffuser. With this absorbed energy, the outlet diffuser 21 functions as a heat exchanger as well as a diffuser for the gas exiting the heater. Moreover, the ceramic foam material provides a very efficient heat exchange surface by virtue of its numerous passageways and high surface area. The outlet diffuser 21 also acts as a barrier to the heater core 3, thus minimizing the chance of shock by contact by contact with the resistance wires of the core by a worker and entry of unwanted objects into the core.
One example of a ceramic foam material for use as the inlet and outlet diffusers 19 and 21 is a magnesium-stabilized zirconia material, available from Steele Corporation of Hendersonville, N.C. and known as their PRZ ceramic foam filter, see http://www.selee.com/steel.htm. It is believed that this foam is well suited for its use as a diffuser, insulator, etc., due to its high temperature properties, density, pore size, etc. It should be understood that other ceramic foams would work as equally as well as the PRZ filter, including others made from different ceramic compositions other than zirconia, providing that they would be able to provide the necessary diffusion and distribution of incoming and outgoing gas and insulating properties, and not create an appreciable pressure drop that would affect the overall gas flow stream.
FIG. 4 illustrates an exemplary inlet diffuser 19, having an opening 22 to allow for the passage of wires between the housing 1 and terminal box 60. As explained above though, the foam diffuser could be used without any openings as the outlet of the heater, or even the inlet if another heater construction allowed for a different manner of connecting the wiring associated with the heater core to a power source. FIG. 5 shows a more detailed view of the ceramic foam diffuser 19 and its internal structure.
The specific arrangement and mounting of the two ceramic foam inlet and outlet diffusers with respect to the heater core and housing could vary as long as the inlet diffuser covers at least a portion of the heater core inlet or the outlet diffuser covers at least a portion of the heater core outlet. In certain instance, the inlet diffuser 19 or outlet diffuser 21 could have openings such that the entire heater core is not covered with the diffuser. For example, the inlet diffuser 19 may need to provide a passageway for lead wires of the resistance heating wires of the core to the terminal box 60 as shown in FIG. 4. In other instances, it may be possible to use an outlet diffuser 21 with openings for weight reduction or to align with dead spots in the heater core 3, e.g., the outlet diffuser would abut the heater core so that openings in the diffuser would directly align with dead spots in the heater core.
In a preferred assembly and referring now to FIGS. 2 and 6, the inlet and outlet diffusers 19 and 21 are arranged in the heater housing 1 in a particular combination with other components besides the heater core 3. The housing 1 is equipped an outlet end lip 23, which serves as a stop to retain the outlet diffuser 21 in place. A mica washer 25 or some other spacer adapted for high temperature use is inserted in the housing 1, and then the outlet diffuser 21 is placed therein. The mica washer 25 provides a cushioning effect between the rigid housing lip 23 and the somewhat brittle ceramic foam outlet diffuser 21 so as to minimize damage to the diffuser 21. While one mica washer 25 is shown, more than one could be employed if so desired.
A second mica 27 washer is placed adjacent the outlet diffuser 21 to provide a clearance between the outlet diffuser 21 and a downstream end 29 of the heater core 3. The clearance is needed in instances wherein the heater core 3 has resistance wires, shown as coiled or spiral wires 7′ that may extend beyond the downstream end of the core 3. The washer 27 also provides a cushion so that the ceramic honeycombed core 3 is not in direct contact with the ceramic foam outlet diffuser 21. While washers are disclosed, other types of spacers could be used. In addition, if a space is not need at the downstream end of the heater core, the washer 27 could be eliminated. Likewise, the washer 25 is optional as well, and the ceramic foam diffuser could abut the lip 23. The cushioning effect of the washers may not be needed if the ceramic foam diffusers or heater core do not show a tendency to break down when in contact with each other or the core.
The heater core 3 is then placed inside the housing 1, preferably with a ceramic felt 31 or other insulating material surrounding its outer periphery. The felt 31 provides both a cushion between the rigid metal housing 1 and the rigid ceramic core 3 as well as some insulating properties. The core could be placed in the housing without another configuration than a felt for insulating and or spacing purposes or without any felt or other structure if so desired.
After the core 3 is inserted into the housing, an x-brace 33 is inserted into the housing to center the heater core 3. The x-brace has two perpendicularly disposed legs 35 and 37, each leg having a finger 34 designed to slide into the gap 36 between the housing 1 and heater core 3 for centering purposes. The x-brace 33 also has openings 39 to accommodate the wiring servicing the resistance wires 7′ of the heater core. While an x-brace is shown, any type of brace that would provide a space for the wiring as well as secure the heater core in place could be employed. For example, the brace could engage openings in the heater core itself rather than to engage its periphery as done with the fingers.
Following insertion of the brace 33, the inlet diffuser 19 is placed in the housing 1. The terminal box 60 is then attached using the box plate 67, the flange 35 on the housing 1 and nut and bolt fasteners 69. In a preferred embodiment, another mica washer or other spacer 37 is positioned between the inlet diffuser 19 and box plate 67 to form a gap 39. The gap 39 allows gas traveling through the tube 63, see arrows “A” and “B” to spread out across the upstream face 41 of the inlet diffuser 19 for better distribution than if the end 66 of the tube 63 were to be placed in contact with the inlet upstream face 41. While not shown, the inlet diffuser 19 has an opening in it as depicted in FIG. 4 to allow for the lead wires servicing the resistance wires of the heater core to connect to the necessary components of the terminal box 60.
Another advantage of the arrangement of the inlet and outlet diffusers 19 and 21 in the FIG. 2 embodiment is that the movement or creep of the heater core and its resistance wires 7′ in the heater core 3 is minimized, and this results in a minimizing of deterioration in the heating coil life and performance. That is, the diffusers 19 and 21 effectively sandwich the heater core to keep it relatively stationary in use.
In the inventive method of heating, a typical multi-passage heater is modified by the use of the ceramic foam material as one or both of an inlet and outlet diffuser for the heater. The diffusers can be mounted to the heater in any fashion, with the inlet diffuser functioning as a diffuser, barrier, insulator, and heat exchanger. Similarly, the diffuser when used at the outlet of the heater also functions as a distributor and homogenizer of the outgoing gas, heat exchange media, and barrier. The inlet and/or outlet diffusers can also be employed in conjunction with the housing surrounding the heater core to minimize movement of the heater core and its attendant degradation.
The use of the ceramic foam diffuser as a diffuser for the heater is illustrated above for both the inlet and outlet of the heater. In certain applications as noted above, it may make sense that the ceramic foam diffuser only needs to be employed in the inlet or the outlet to achieve acceptable flow distribution of the incoming or exiting gas. While the ceramic foam diffuser is shown with a circular shape and uniform thickness, the segment could be made of any shape and thickness that would fit with a particular heater inlet or outlet. For example, the segment could be in the form of a semicircle or the like, or have one or more openings in it to create a desired inlet or outlet flow for the heater or accommodate other heater structure. The thickness of the diffuser could also vary depending on the particular heater configuration.
Although a ceramic honeycombed structure is exemplified as the heater core, the heater core can be any configuration of multiple passages with electric heaters associated therewith for use in combination with ceramic foam material as inlet and/or outlet diffusers. The multiple passages forming the heater core can be separate ceramic tubes that are grouped together so that the gas flows through the individual tubes. The form of the electric heating can also vary as known in the art. In certain honeycombed heater cores, spiral resistance wires can be employed, but any type of resistance wire or other electric heating means could be used as part of the heater core.
As such, an invention has been disclosed in terms of preferred embodiments thereof which fulfills each and every one of the objects of the present invention as set forth above and provides a new and improved multi-passage electric heater and method of use.
Of course, various changes, modifications and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.

Claims

1. In a multi-passage electric heater having a heater core, an inlet and an outlet, wherein at least some of passages of the heater core each include an electric heater for heating a gas passing therethrough, the improvement comprising a ceramic foam material placed over at least a portion of one or both of the inlet and the outlet of the heater.

2. The heater of claim 1, wherein the ceramic foam material is placed over both the inlet and the outlet of the heater.

3. The heater of claim 1, wherein the heater inlet and outlet are circular in shape, and the ceramic foam material is in the form of a disk.

4. The heater of claim 1, wherein the ceramic foam diffuser covers an entirety of the inlet or the outlet or both.

5. The heater of claim 2, wherein the ceramic foam material covers an entirety of the inlet or the outlet or both.

6. The heater of claim 1, wherein the ceramic foam material is mounted to a housing of the heater to cooperate with the housing as a stop against movement of the heater core during operation.

7. The heater of claim 1, wherein the ceramic foam is a magnesia-stabilized zirconia ceramic foam.

8. The heater of claim 2, wherein the ceramic foam material is mounted to a housing of the heater to cooperate with the housing as a stop against movement of the heater core during operation.

9. The heater of claim 1, wherein the heater further comprises a gas inlet tube for directing incoming gas to the heater core, the ceramic foam material being positioned between the gas inlet tube and heater core as an inlet diffuser, the inlet diffuser further positioned to form a gap between an upstream face of the inlet diffuser and an outlet of the gas inlet tube to facilitate distribution of incoming gas to the upstream face of the inlet diffuser.

10. In a method of heating a gas using a multi-passage electric heater having a heater core, an inlet and an outlet, and wherein at least some of passages of the heater core each include an electric heater for heating gas passing therethrough, the improvement comprising placing a ceramic foam material over at least a portion of one or both of the inlet and outlet to minimize radiation from the heater and to either distribute the gas entering the heater or exiting the heater.

11. The method of claim 10, wherein the ceramic foam material is placed over each of the inlet and outlet.

12. The method of claim 10, wherein the ceramic foam material is placed over an entirety of either the inlet or the outlet or both.

13. The method of claim 11, wherein the ceramic foam material is placed over an entirety of either the inlet or the outlet or both.

14. The method of claim 10, wherein the ceramic foam material is mounted to a housing of the heater so as to cooperate with the housing as a stop against movement of the heater core during heater operation.

15. The method of claim 10, wherein the ceramic foam material is used as an inlet diffuser, and further forming a gap adjacent an upstream face of inlet diffuser in the heater to facilitate distribution of incoming gas to the upstream face of the inlet diffuser.