Patent Application
20150013668
The present disclosure pertains generally to radiant heaters wherein the primary source of heat transferred from the heater to the object being heated is by radiant heat transfer. More particularly, this disclosure pertains to radiant heaters that are designed to heat large areas of buildings with high ceilings. Such areas are typically found in large manufacturing operations, warehouses or logistic areas, hangers for aircraft where maintenance crews may be working, and so forth.
In these types of applications, it is desirable for the heaters to be located at or near the ceiling. Radiant heat, wherein energy is transferred by the emission of light having wavelengths in the infrared to visible regime, transfers energy from the heater unit to surroundings (floors, people, or other objects) by the absorption of these light particles, also called photons, by said surroundings. Unlike the other common modes of heat transfer, convection and conduction, photons travel through air for long distances without substantially heating the air. Convection is a means of transferring energy by heating a fluid, in this case air, followed by the motion of the fluid, and finally the transfer of energy from the heated fluid to a remote object. Conduction is a means of energy transfer whereby the energy flows through a material by electronic vibrations and motions, from a region of higher temperature to a region of lower temperature. Both convection and conduction are inefficient means of transferring energy over large distances, as they require heating the intervening materials.
A primary advantage of radiant heaters is that energy is transferred from the heater to the objects and in close proximity of objects where heat is desired, without having to heat all of the air between the heater and the objects requiring heat. Inevitably, air is heated when it comes in contact with any object that is at a higher temperature than the air. Air that is hotter than the surrounding air will rise towards the top of the air mass. Therefore, the heat transferred from any hot object or heater to the surrounding air will be substantially wasted in a configuration where the heater is near the ceiling of the building to be heated. In fact, only by heating the entire air mass within a building will objects near the floor of the building be heated by contact with the hotter air. Energy that is expended in heating the air near the ceiling of a high-ceilinged structure is substantially wasted with regards to the task of heating people and objects near the floor of the structure.
A primary goal of the present improved radiant heater is to minimize the transfer of energy from the radiant heater assembly to the surrounding air. The efficiency of the radiant heater can be defined as the fraction of total energy consumed by the heater that is usefully transferred to objects near the floor of the building for which heating was desired. To maximize the efficiency, it is a goal of the radiant heater to both maximize the fraction of energy that is transferred out of the radiant heater via radiation, or photons, as opposed to by convection or conduction, and, it is a goal to maximize the energy that is transferred out of the radiant heater via radiation towards the objects for which heating is intended.
Emission of radiant heat, or energy, from a hot surface is known to be a function of the emissivity of the surface and the temperature of the surface raised to the 4th power. The transfer of energy from one surface to another via the process of radiant heat transfer depends on the emissivity of the each surface, their temperatures, and the solid angle subtended by one surface to the other.
In a radiant heater, the heat created inside the combustion chamber must be transferred out of the chamber which is simply a requirement of the basic physics. The preferred length of the combustion chamber is such that the temperature of the combustion gases that are either drawn out or expelled by an induced or forced draft fan is cooler in comparison to the temperature of the gases where combustion is occurring. Therefore, most of the heat from the combustion is conducted through the walls of the chamber. The outside surfaces of the combustion chamber transfer heat energy by radiation, conduction, or convection.
Previously existing radiant heater designs all utilized combustion chambers that have substantially the same heat conduction from the inside of the chamber to the outside of the chamber in all radial directions along the length of the combustion chamber. This was practical because the most common design was simply a metal tube. Using a radially symmetric tube as a combustion chamber was cost-effective and contained the combustion gases. The good conductivity of the metal minimized the temperature drop from the inside to the outside, so as to maximize the radiant energy emission from the outer surface. However, the radially symmetric tube design made the radiant energy emission radially symmetric from the outside of the tube.
As a result, two detrimental effects occur in the previous designs. First, the emission of radiant energy is equal in all radial directions. Therefore, at least half of the emitted energy initially travels away from the objects and people that are to be heated, including toward the roof of the building for a heater mounted near the ceiling. Second, the total surface area of good conductor on the outside of the combustion chamber and volume are fixed compared with the preferred embodiment of the present radiant heater. Thus, the heat flux per unit area reaching the outside is not optimal, making the temperature lower on the outside for equivalent conducting materials. This reduces the radiant heat emission, which depends on temperature to the 4th power.
Previous radiant heater designs teach a number of ways to attempt to overcome the first deficiency by employing reflectors to redirect the radiant energy emitted towards the roof into useful solid angles. Such designs can be effective at recovering some of the misdirected radiant energy, but all suffer a loss of efficiency. Reflectors are not perfect in the sense that they cannot reflect all radiant energy photons at all wavelengths with equal and very high reflectivity. Because the reflectors absorb some of the energy, they become warm surfaces, not as hot as the outside surface of the combustion tube. Because of the 4th power emission law, the lower temperature reflector surfaces have much reduced efficiency for re-radiating energy they absorb. A second loss of efficiency is that the hot surface area of the combustion tube, including any surface radiating away from the objects to be heated and the area of the warm reflectors, contribute to a much greater heat transfer to the surrounding air by convection. This is very inefficient compared to the much smaller hot surface area of the present radiant heater that is optimally directed towards useful solid angles. The quantity of previous radiant heater designs that depend on optimization of reflectors reinforces the importance and uniqueness of the radiant heater disclosed herein.
The present disclosed radiant heater is comprised of a combustion chamber constructed primarily of two different materials in two different regions, an insulating portion and a conductive portion, each with unique properties to optimize heat transfer from the enclosed flame in a desired direction. The combustion chamber is a long, tubular structure within which combustion occurs. However, the length of the chamber is such that combustion does not occur along the complete length of the chamber. The combustion chamber contains the flames of combustion at the beginning of the chamber and serves to entrain the combustion gases until combustion is complete and the gases have cooled before they exit the end of chamber. The tubular shape of the combustion chamber refers to the fact that the chamber length is long compared with its cross-sectional dimensions, but tubular does not connote or limit the cross-sectional shape to a circle or ellipse in this description.
The insulating portion of the combustion chamber is a good insulator that does not conduct heat efficiently, such as a ceramic material, while the conductive portion is a good heat conductor, such as a metallic material. The outside temperature of the surface of the insulating portion is low compared to the conductive portion. By utilizing a material that is a good insulator, only a small fraction of the total heat energy will be conducted through this region to reach the outer surface. The outside surface of the conductive portion can be treated to have a high emissivity to enhance the radiant energy emission. Examples of these treatments include mechanical surface treatments such as sand blasting, bead blasting, or any media blasting which increases surface area. Chemical treatments include treatments wherein a layer of different chemical properties is created by conversion of the surface to oxides, commonly black oxides with better “black body” properties. Alternatively, a metallic surface layer such as aluminum or plating of a different alloy could be used for the treatment, usually for the purpose of preventing oxidation or rusting of the surface when low carbon lower cost steel is used for the lower temperature return tube. Any suitable treatment for enhancing radiant energy emission can be utilized. The insulating portion together with the conductive portion form an enclosed conduit in which combustion of an air-fuel mixture takes place. The hot gases of combustion can travel through the conduit until the gas temperature drops to a minimum practical level as a result of heat transfer to the surface of the conductive portion.
The nature of radiant energy emission from a surface is such that the shape of the surface will alter the direction and pattern of energy emitted, analogous to how a flashlight can focus the light or a conventional light bulb without a reflector will cast a diffuse light. One unique aspect of the radiant heater is that the shape of the conductive portion's surface can be altered from concave to flat to convex to other variable shapes to modify the radiant energy pattern to suit the application. Similarly the insulating portion of the combustion chamber and hot gas conduit can be shaped to compliment the desired shape of the radiating surface. The two materials can be layered or overlapped, or they can be fitted together in a manner that minimizes overlap. Certain embodiments described herein are directed to configurations of the combustion chamber.
In embodiments of the radiant heater, the volume of the combustion chamber can be modified such that it is larger or smaller while retaining the same radiating surface area. This allows for decreasing the flame and hot gas velocity to optimize heat transfer along the length of the conduit. Additionally, in embodiments of the radiant heater, the insulating portions and conductive portions of the combustion chamber wall need not remain the same along the complete length of the combustion chamber. The walls of the combustion chamber in regions beyond the extent where combustion is occurring, where the internal temperature of the gases is significantly lower, and therefore the total heat transfer through the combustion chamber wall is lower, may have different proportions of insulating circumferential portion to conducting circumferential portion, or even altered cross-sectional shape, as is desired to alter either heat transfer characteristics or costs of the combustion chamber wall materials.
The radiant heater described herein achieves greater heating efficiency than previous radiant heaters used to heat objects or people remote from the heater assembly, particularly when the distance between the heater and the object is very great. Modifying the outside surface of the combustion chamber significantly improves the net efficiency of radiating energy to the desired objects and people. It is important to recognize that the present radiant heater itself radiates heat energy substantially in useful directions compared to previously existing radiant heaters that require reflectors to re-radiate heat energy from combustion chambers that do not radiate heat energy substantially in useful directions.
The current improved radiant heater design includes a combustion chamber that is made up of a substantially tubular conduit. The conduit is comprised of conduit walls enclosing an inner combustion space. A circumferential portion of the conduit walls extending transversely along the length of the conduit is an insulating portion and a remaining circumferential portion of the conduit walls is a conductive portion. The conducting portion of the conduit walls has a substantially higher external temperature than the external temperature of the insulating portion of the conduit walls when combustion occurs within the inner combustion space, releasing substantial heat energy inside the combustion chamber, and temperatures within the conduit have reached steady-state.
Alternate preferred embodiments of a combustion chamber for use in a radiant heater design are illustrated in
Conductive portion 17 may be configured in a variety of shapes to control the direction of heat emission, as seen in
Heating of air or gases within inner combustion space 15 results in a substantial transfer of heat energy to the inner surfaces of conduit 12. Because the heat energy passes more effectively through conductive portion 17 to its outer surface, there is increased radiant heating through conductive portion 17 and reduced radiant heating through insulating portion 16. The increased radiant heating through conductive portion 17 occurs in a desired direction depending on the shape of conductive portion 17 and does not require the use of reflectors, nor does it involve convection or conduction. In
An embodiment of a radiant heater 100 is shown in
In radiant heater 100, insulating housing 120 surrounds insulating portion 117, while conductive portion 118 is not contacted by insulating housing 120. Radiant heater 100 also includes control box 130. Control box 130 provides heated air or gases into heated conduit branch 112 and receives cooled air or gases from cold return conduit branch 114. Control box 130 may include a burner box to heat air or gas for passage through heated conduit branch 112 and a fan to expel the cooled air or gas by forced or induced draft from cold return conduit branch 114.
The design of radiant heater 100 is such that by the time air or gases return to control box 130 through cold return conduit branch 114, they are completely or nearly completely cooled. Thus, as much heat as possible has passed through conductive portion 118 of combustion chamber 110. Varying the shape, length, and volume of combustion chamber 110 allows for optimization of the radiation pattern, flow rate, and heat transfer.
By way of example, without limitation, one preferred embodiment of a radiant heater uses a conduit 15 to 22 feet in length and uses a D-shaped combustion chamber such as that shown in
