Double Wall Infrared Emitter

1. Field of the Invention

The invention relates to infrared energy emitters having a filament within a tubular envelope, and more specifically to infrared emitters further having an external sheath surrounding the envelope.

2. State of the Prior Art

Infrared emitters provide radiant heat in numerous applications. For instance, they are the preferred heat source for drying paints applied to metal surfaces, including solvent based paints, water based paints, and powder paints. They also provide heat for environmental test chambers and many industrial processes.

Typically, an infrared emitter comprises a slender tubular quartz envelope containing an elongated coiled filament that extends through the envelope and connects to lead-in conductors at opposite ends of the envelope. Infrared emitters may be provided in a variety of designs depending upon the desired wavelength and power density of emitted energy.

Infrared radiation emanates from the filament in all directions in a spherical pattern, and thus the power of the radiant energy decreases in proportion to the square cube of the distance from the emitter. In general, infrared emitters are employed to heat a particular object, such as a car body in a paint curing process. Only the energy which actually strikes the object is transferred to the object as heat energy, and of the energy which strikes the object, a portion will be reflected, a portion will be absorbed, and depending upon the object, a portion may be transmitted through the object. Only the radiant energy which actually strikes the object and is absorbed provides heat to the object. The remaining radiant energy is simply lost to the environment, thereby reducing the overall energy transfer efficiency from the infrared emitter to the object to be heated. Of course, some of the nonabsorbed radiant energy may heat the atmosphere in which the object to be heated resides, and thus be transferred to the object by convection and conduction. However, this effect is typically, either undesired or negligible.

To improve the radiant energy transfer efficiency, the radiant energy leaving the emitter is generally focused in some manner toward the object to be heated. For instance, the infrared emitters are often employed within an enclosed chamber having reflective walls. Thus, energy not directly passing from the infrared emitter to the object and absorbed by the object, continues to be reflected off of the surfaces of the chamber until it strikes the object, escapes from an opening in the chamber or dissipates through inefficiencies in the reflectors. In most applications, more direct focusing of the radiant energy greatly improves the overall transfer efficiency. For instance, in some applications, external elongated reflectors adjacent the infrared emitters focus the emitted radiant energy in the direction of the object to be heated.

In many applications, infrared emitters are used in environments where cleanliness is essential and the heating chamber must be kept free of particulate matter. Flat walls in a heating chamber are much easier to clean and accumulate less dust than walls forming external reflectors for the infrared emitters. External reflectors that are not incorporated into the chamber walls also tend to accumulate dust and are difficult to clean.

A gold reflective coating on the outer surface of the infrared emitter can form an integral reflector. Infrared emitters with reflective gold coatings, used in a chamber with flat reflective walls, improve cleanliness in the heating chamber environment. The flat chamber walls do not tend to accumulate dust and clean easily. Additionally, there are no external reflectors to accumulate dust and be cleaned. An additional advantage of reflective coatings is reduced expense versus external reflectors. Thus, it can be appreciated that the gold reflective coating provides energy efficiency and cleanliness at a reasonable cost, making the gold reflective coating a highly desirable feature. However, the gold reflective coating places certain restrictions upon the infrared emitter design.

The emitter envelope absorbs a small portion of the infrared energy. If present, reflective metal coatings, while highly reflective, absorb a portion of the infrared radiation and become heated. Also, some of the filament's heat transfers to the emitter envelope through conduction and convection to heat the emitter envelope to high temperatures. Air tight end seals at the ends of the filament seal the envelope around the filament. Typically, temperatures above 650.degree. F. damage or destroy the seals, placing a practical upper limit upon the temperature of the envelope. Further, a gold metal reflective coatings may simply vaporize off of the surface of the envelope if heated to too high of a temperature.

External requirements may also affect the temperature requirements of the envelope. For instance, when the emitters are operating in a combustible atmosphere, it is extremely important to keep the envelope operating temperature to a minimum. For instance, the National Fire Protection Association's National Electric Code, which has been adopted by many communities as the local electric code, requires a maximum surface temperature of no more than 329.degree. F. in certain organic dust filled atmospheres. Standard T3 tungsten filament infrared emitters are rated for a 392.degree. F. minimum surface temperature.

Both the wavelength and the power density of the emitted infrared energy affect the envelope temperature, with the power density the most influential factor. Thus, the power density of the emitter is limited by the design of the infrared emitter and by the operating environment. The power density, of tungsten filament infrared emitters is typically 100 watts/lineal inch of filament length. Higher power densities adversely affect the end seals and reflective coatings. Power densities are further limited in many explosive atmospheres.

The present invention overcomes these and other limitations by providing an external sheath of quartz or other highly transmissive material about the infrared emitter envelope, with a reflective metal coating applied to the outer sheath.

An infrared energy emitter according to the invention comprises a longitudinally extending filament and a tubular enclosure of infrared energy transmitting material enclosing the filament. A longitudinally extending outer tubular sheath of infrared energy transmitting material ensheathes the tubular enclosure and is provided with a reflector. The outer sheath is spaced apart from the inner tubular enclosure thereby allowing the infrared emitter to run at high power densities while maintaining a relatively cool outer surface temperature.

In one embodiment of the invention, the reflector has a semicircular cross-sectional shape. Advantageously in accordance with this invention, energy is reflected back onto the filament thereby reducing emitter power requirements.

In accordance with one particular aspect of the invention, the space formed between the outer sheath and inner enclosure is provided with openings and may be ventilated to further reduce the outer surface temperature of the infrared emitter and enhance its ability to operate at high power densities. In one particular embodiment, fluid conductive filters are provided at each end of the sheath to filter cooling fluid passed through the space.

The invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a front elevational view of an infrared emitter according to the invention;

FIG. 2 is a detailed sectional view of an end portion of the infrared emitter of FIG. 1;

FIG. 3 is an end view of the infrared emitter of FIG. 1:

FIG. 4 is a sectional view taken along line 4--4 of FIG. 1;

FIG. 5 is a sectional view taken along line 5--5 of FIG. 2; and

FIG. 6 is a perspective sectional view taken along line 6--6 of FIG. 1.

FIG. 1 illustrates an infrared emitter 10 according to the invention which emits electromagnetic radiation in the infrared (IR) portion of the spectrum. The infrared emitter 10 generally comprises an elongated inner element 12 received within a tubular outer sheath 14. The inner element 12 comprises an elongated tubular envelope 16 of quartz or silica, preferably quartz, and an incandescent helically coiled tungsten wire filament 18 extends coaxially within the envelope. A pinch seal 20 closes each end of the envelope 16, and lead-in conductors 22, each having a thin foliated intermediate portion 24, extend longitudinally into the envelope 16 through the pinch seal 20 and connect to the filament 18.

Infrared emitters, such as represented by inner elements 12 are commercially available. A more detailed description of the inner element 12 can be found in U.S. Pat. No. 2,864,025 to Foote et al., incorporated herein by reference.

Commonly, infrared emitters are divided in two broad categories: short wave emitters having a wavelength of 0.9 to 2.3 microns and employing a tungsten filament, and medium wavelength emitters having a wavelength of 2.3 to 4.5 microns and employing a nichrome filament. The wavelength of the energy emitted by an infrared emitter depends upon the temperature of the filament. For instance, a tungsten filament heated to 4,000.degree. F. will emit radiant energy with 75% of the energy emitted in a band width ranging from 0.9 microns to 1.5 microns, with a 1 micron peak wavelength. In contrast, a nichrome filament heated to 1600.degree. F. will emit radiant energy in a band width having its peak at 3 microns. Although the inner element 12 is described with respect to a tungsten filament 18 operating in the short infrared range, it will be appreciated that the principles of the invention may be applied to nichrome and other filaments operating in any portion of the infrared spectrum.

Turning to FIGS. 2 and 5, stranded lead wires 26 crimp onto the lead-in conductors 22 with the aid of crimping strips 28. The crimping strip 28 comprises a short strip of crimpable metal formed into a loop. The lead wire 26 is received within the loop, and the loop is flattened so that the crimp strip extends radially away from one side of the lead wire 26. The portion of the crimp strip 28 receiving the lead wire 26 is placed adjacent to the lead-in conductor 22. The lead wire 26 is wrapped over the lead-in conductor 22 and held thereto by a portion of the crimp strip 28 wrapped about a terminal end 30 of the lead-in conductor 22.

The lead-in wires 26 extend from the lead-in conductor 22 coaxially through a tubular steatite ceramic spacer 32 and out of the open ends of the outer sheath 14. Cup-shaped stainless steel retainer caps 34, having a cylindrical wall 36 extending from a circular end wall 38, fit over the ends of the outer sheath 14. The inner diameter of the retainer cap cylindrical wall 36 slightly exceeds the outer diameter of the outer sheath 14, and a high temperature gasket material 40 fits therebetween and attaches the retainer cap 34 to the outer sheath 14. An L-shaped retainer clip 42 attaches to the retainer cap 34 and comprises a radial leg 44 parallel with and affixed to the retainer cap end wall 38, and also a return leg 46 spaced apart from, yet axially aligned with, the retainer cap cylindrical wall 36. The retainer clip 42 fits within a standard connector (now shown) and aids in providing the proper rotational orientation of the infrared emitter 10 within the connector.

Turning to FIG. 3, the lead wire 26 extends through an aperture 48 in the center of the retainer cap end wall 38, and attaches to the retainer clip 42 or retainer cap 34 in a conventional fashion, as by spot welding. The retainer cap end wall 38 also has a breather hole 50 for ventilating an interior space 52 between the inner element 12 and outer sheath 14 (see FIG. 2). Alternatively, the retainer cap 34 may be formed of a conductive porous stainless steel or other metal, such as employed in fuel filters in some carburetors for internal combustion engines. Preferably, such a porous metal filters particles above 10 microns.

Ventilation of the inner space 52 may be allowed to occur naturally as through the normal circulation of air in the operating environment of the infrared emitter 10. Alternatively, a cooling fluid such as air or other nonconductive fluids, may be forced through the inner space 52 for an enhanced cooling effect upon the inner element 12 and outer sheath 14. The forced cooling fluid may comprise a nonconductive liquid.

Turning to FIGS. 4 and 6, a reflective coating 54 is applied to an outer surface 56 of the outer sheath 14. A reflective coating applied to an infrared emitter, such as coating 54, typically is less than a thousandth of an inch thick, as described in U.S. Pat. No. 3,804,691 issued Apr. 16, 1974 to Trivedi. The reflective coating 54 extends longitudinally substantially in register with the filament 18, and circumferentially about the outer sheath 14 and thus creates a longitudinal window 58 along one side of the outer sheath 14 not covered by the reflective coating 54. Radiant energy from the filament 18 reflects off of the reflective coating 54 back into the infrared emitter 10, and the window 58 disperses infrared radiation through a focused solid angle 60. The magnitude of the angle 60 depends upon the predetermined radial width of the window 58, which is established according to the requirements of a particular application for the infrared emitter 10 and may vary from less than 1.degree. to nearly 360.degree.. Typically, an angle 60 of 90.degree. provides good dispersion for heating large objects. The reflective coating 54, thus, directs the radiation from the infrared emitter 10 in a desired direction to improve the efficiency of the infrared emitter 10.

Infrared radiation radiates in all directions from the filament 18 which lies along the central axis of the outer sheath 14. Thus, radiation emanating out to the reflective coating 54 tends to be reflected directly back onto the filament 18, thereby raising its operating temperature and decreasing the infrared emitter 10 power requirements.

By placing the reflective coating 54 on the outer sheath 14, higher maximum power densities may be achieved. Typical prior single tube tungsten filament infrared emitters, having a gold reflective coating, have maximum power densities of 40 to 50 watts/lineal inch. The infrared emitter 10 may be designed with a maximum power density of approximately 600 watts/lineal inch. The increased distance of the reflective coating 54 from the filament 18 achieved by placing a coating on the outer sheath contributes greatly to the higher maximum power density of the infrared emitter 10. The insulating effect of the inner space 52 reduces the temperature of the reflective coating 54 and thus also increases the allowable maximum power density of the infrared emitter 10 without damaging the reflective coating 54.

Even higher power densities may be achieved by ventilating the inner space 52 with a cooling fluid as previously described. Forced cooling in this manner also cools the inner element envelope 16 so that the temperature of the pinch seal 20 will not exceed 650.degree. F. In an explosive atmosphere, forced cooling with a cooling fluid in the space 52 maintains the outer temperature of the sheath 14 within acceptable limits, even at high power densities. The double walled infrared emitter 10, thus provides a significant advantage over prior single walled emitters.

When a mercury vapor is placed within the inner element envelope 16, the infrared emitter 10 will also emit ultraviolet (UV) radiation. The filament 18 will heat and excite the mercury vapor atoms, causing them to release UV radiation. In some paint curing processes a photo-initiator in the paint aids in curing the paint in the presence of UV radiation. The infrared emitter 10 with mercury vapor would obviate the requirement for additional UV emitters in such a process.

In most instances, however, UV radiation from the infrared emitter 10 is undesirable as it can be harmful to personnel. The filament 18, although emitting primarily in the infrared spectrum, emits a small amount of UV radiation in all types of infrared emitters. Since quartz absorbs radiation in the UV spectrum, the quartz outer sheath 14 acts as a UV filter and aids in reducing trace UV radiation.

While particular embodiments of the invention have been shown, it will be understood, of course, that the invention is not limited thereto since modification may be made by those skilled in the art, particularly in light of the foregoing teachings. Reasonable variation and modification are possible within the foregoing disclosure of the invention without departing from the scope of the invention.