Patent Grant
9080777
This invention relates to radiant tube heater systems and in particular to reflectors for radiant tube heaters.
Radiant tube heater systems are well known in the heating industry and are useful for warming large covered spaces such as those found at industrial and manufacturing facilities, air craft hangers and swimming pools. However there are some known and perceived problems with such heating systems including insufficient heat being supplied to areas in which heat is needed and the non-uniform supply of heat to areas along the length of the tubular conduit through which the combustion gases flow.
It is known to provide metal reflectors located directly above the radiant tube and extending the length thereof in order to reflect the heat from the tube downwardly towards the floor area of the building or structure where it is required. A low intensity tube heater generally comprises a burner attached to a steel radiant tube. The sheet metal reflector extends over the top of the tube and also over its two opposite sides. The reflector re-radiates and re-directs infrared heat energy back to the tube, to the floor and to itself and, in this way, it reduces convection losses and directs more radiant heat to the ground or floor where it is needed.
Most known low intensity infrared heaters have a radiant factor of 40% to 45% and a convection heat output of 35% to 40%. Traditional radiant tube systems have an inability to effectively control the convection and radiant outputs. Radiant tube heaters for commercial and industrial space heating systems can have a variety of firing rates ranging from, for example, 45,000 BTU/H to 200,000 BTU/H.
There is a need for better, more efficient radiant tube heaters having an increased radiant factor and an improved heat flux density on the floor area. By improving the efficiency of a radiant tube heater of the aforementioned type, it is possible to reduce fuel consumption while still achieving a comfortable temperature level and it is also is possible to reduce carbon emissions.
According to one embodiment of the present disclosure a reflector for an elongate radiant tube heater having a single tubular conduit through which hot fluid in the form of combustion gases flow comprises an elongate metal reflecting member adapted to extend along the length of the tube heater and to cover the top and two opposite sides of the tube heater in order to reflect and disperse radiant heat waves from the tubular conduit. The reflecting member includes two central panel portions meeting along a longitudinal centerline of the reflecting member and forming an outwardly facing angle ranging between 30 and 100 degrees. A bisector of the angle extends substantially vertically and is vertically aligned with a centerline of the tubular conduit during use of the reflector on the radiant tube heater. The reflecting member has further longitudinal panel sections extending outwardly and downwardly from the central panel portions. The reflector includes a layer of heat resistance insulation extending over an outer surface of the reflecting member.
According to an exemplary version of the aforementioned reflector, the outwardly facing angle formed by the central panel sections ranges between 45 and 80 degrees.
According to a second embodiment of the present disclosure, a radiant heating apparatus for attachment to a burner for burning a heating fuel to produce combustion gases includes an elongate tubular conduit through which the combustion gases can flow and burn, one end of the conduit being connectable to an outlet of the burner. An insulated reflector extends lengthwise along the tubular conduit and is positioned and shaped to reflect and disperse radiant heat waves from the conduit during use of the apparatus. The reflector has a longitudinal centerline located above and spaced from the tubular conduit. The centerline divides the reflector into two half sections. Each half section is formed with at least three longitudinal bends located between the centerline and a respective bottom edge of the half section and dividing the half section into at least four longitudinal reflecting portions including a central portion. This central portion slopes upwardly from the centerline during use of the heating apparatus. The central portions of the two half sections form a central angle ranging between 30 and 80 degrees, this angle facing away from the tubular conduit.
In an exemplary version of the aforementioned heating apparatus, the reflective portions of each half section include a second reflecting portion adjacent a respective one of the central portions and forming a second angle in a transverse plane ranging between 100 and 135 degrees and facing inwardly in the direction of the tubular conduit.
According to a third embodiment of the present disclosure a radiant tube heater system comprises a single tubular conduit through which hot fluid including combustion gases flow and a plurality of support structures for supporting the conduit at spaced locations along its length. The combustion gases flow from one end to another end during use of the heater. An insulated reflector extends lengthwise along the tubular conduit and is constructed and shaped to reflect and disperse radiant heat waves from the conduit. The reflector has a first longitudinal centerline located above a second longitudinal centerline of the conduit. The first longitudinal centerline divides the reflector into two similar half sections extending in opposite transverse directions from the centerline. Each half section is formed with three longitudinal bends which form obtuse angle facing inwardly towards the conduit. The bends of each half section divide the half section into longitudinal reflecting portions including a central portion that slopes upwardly from the first longitudinal centerline during use of the heater system. The central portions of the two half sections form an outwardly facing angle ranging between 30 and 100 degrees.
In one exemplary form of this tube heater system, the outwardly facing angle is approximately 60 degrees and the reflector is constructed of bent metal sheet and is insulated with ceramic insulation extending over the outer surface of the reflector.
These and other aspects of the disclosed reflectors and radiant tube heater systems will become more readily apparent to those having ordinary skill in the art from the following detailed description taken in conjunction with the drawings.
So that those having ordinary skill in the art to which the present disclosure pertains will more readily understand how to make the subject invention, exemplary embodiments thereof will be described in detail herein below with reference to the drawings, wherein:
Shown in
Optionally, in an exemplary embodiment there is a further bend 32 located at each of the two bottom ends of the half sections. The illustrated bend 32 forms a 90 degree angle, this angle facing generally upwardly and outwardly. The exemplary reflector is made of a highly reflective metal, at least on its inner reflecting surface, two suitable metals being aluminized steel sheet metal and Feran.
A standard exterior diameter of the radiant tube is four inches and the length of the tube varies depending upon the particular job requirements but can arrange for example from 25 to 70 ft or more.
With reference now to
The burner head 40 is mounted within the tube and is adapted for mixing combustible gas and for delivering the resulting mixture into an upstream end section of the tubular conduit as shown. The burner head is generally annular and has a cylindrical inlet portion 46 and a wider cylindrical outlet portion 48. The heater can be provided with natural gas or LPG gas indicated by the arrow G taken from a suitable source and delivered through the gas valve unit 34. Combustion air enters through vents or pores distributed about the periphery of inlet portion 46. The mixture exiting from the burner head 40 is ignited by an ionization electrode 50 so as to produce a long laminar flame that extends substantially the length of the radiant tube. The preferred material for the radiant tube 14 is stainless steel or aluminized steel, at least for an upstream section thereof that surrounds the hottest part of the flame and the burner head. The remaining downstream section can be cold rolled steel.
Both the radiant tube and its reflector 10 can be hung from a ceiling or roof trusses with the use of a series of hangers 62, one of which is illustrated in
In order to develop an improved, efficient reflector for a radiant tube heater, a method has been developed for accurately measuring the heat flux from a radiant tube heater at floor level using a special water-cooled heat sensor. This sensor, its method of cooling and the measuring method for determining the heat flux along the length of a radiant tube heater are explained hereinafter. Using these accurate measurements of heat flux emitted by a radiant tube heater and computational fluid dynamics (CFD) a substantially improved reflector for a radiant tube heater has been developed and one embodiment is illustrated in
In the exemplary illustrated reflector, there are the aforementioned three bends 26, 28 and 30 formed in each half section. Located adjacent to the central portion 90 is a second reflecting portion 92. Adjacent this second longitudinal portion and located outwardly there from is a third reflecting portion 94 which, as shown, can be wider than the second reflecting portion. Furthermore adjacent the third reflecting portion and sloping outwardly and downwardly therefrom is a fourth reflecting portion 96 which extends to one bottom edge of the reflector. The first longitudinal bend 26 is formed between its respective central portion 90 and the adjacent second portion 92. The second longitudinal bend is formed between the second reflecting portion 92 and the third reflecting portion 94 while the third longitudinal bend is formed between the third reflecting portion 94 and the fourth reflecting portion 96. In an exemplary version of the reflector, the size of the inner angle at the first longitudinal bend 26 is at least 110. In an exemplary version of the reflector, the inwardly facing angle B ranges between 105 and 140 degrees. Each of the angles at B, C and D are dependent to a degree on the width of the adjacent reflecting portions of the half section. The inner angle C at the second longitudinal bend 28 in the exemplary reflector is at least 150 degrees and in the illustrated exemplary embodiment is 160 degrees. In an exemplary version of the reflector, the angle C ranges between 150 and 170 degrees, the angle C being the angle facing inwardly in the direction of the tubular conduit or radiant tube. In one particular exemplary embodiment of the reflector illustrated in
Turning to the inner angle D formed between the third reflecting portion 94 and the fourth reflecting portion 96 and located in a transverse plane relative to the longitudinal center axis of the radiant tube, an exemplary range for this angle is between 140 and 160 degrees and the illustrated exemplary angle D is 150 degrees. The angle E formed between the horizontal plane and the fourth reflecting portion 96 can vary and depends to a degree on the size of the angles B, C and D. Typically this angle is about 65 degrees. If desired, a short edge flange 100 can be provided along the two opposite longitudinal bottom edges of the reflector. One function of these flanges is to avoid a sharp metal edge at the bottom edges of the reflector thereby making it easier to handle and install.
Although the width of each of the reflecting portions that extends longitudinally along each half section can vary to a degree, based on the diameter of the radiant tube being about 4 inches, an exemplary version of the reflector 10 has a central reflecting portion 90 with a transverse width of at least 1.3 inches while the transverse width of each of the second and third reflecting portions 92, 94 is at least 1.70 inches.
As illustrated in
In general, it is preferred that the radiant tube 14 be not only covered by the reflector over its top side but also on the vertically extending sides of the tube as shown. The illustrated tube heater 14 has a bottom or bottom extremity at 102 and this bottom is aligned approximately with a horizontal plane indicated at P defined by the two opposite bottom edges of the reflector. In the illustrated reflector these bottom edges are formed by the reflector bends at 32. Also shown in
The improved reflector for a radiant tube heater described above is able to provide a better radiant factor based on net calorific value. The reflector for the radiant tube heater has an effect on the radiant factor on the basis of the following factors:
In order to develop and test the above described, improved reflector for a radiant tube heater, it was necessary to develop an accurate system and method for measuring the heat flux generated by the radiant tube heater at floor level.
The rectangular measurement plane indicated at 130 was divided into a grid of squares arranged side by side and drawn or painted on the measurement area. These measurement squares are indicated at 132. The actual size of these squares is dependent upon the horizontal measurements of the heat sensor used for the heat flux measurements, this substantially square sensor being indicated at 134 in
The exemplary heat sensor that was used has a range of 1 millivolt to 1,200 millivolts and measures the local heat flux in one direction with the results being expressed in watts per square meter. The sensitivity of the sensor 134 that was used is 1.1 watts/m2 per 1 MV and it operates in temperatures ranging from −100° F. to 250° F. The DC signal generated by the transducer is conducted to the readout instrument by means of a waterproof cable. Upon obtaining thermal equilibrium with its surroundings, the sensor develops a voltage which is directly proportional to the local heat flux. The principle of operation of this exemplary sensor is that the flow of heat through the transducer creates a minute temperature difference between its surfaces. A multi-element, semi-conductor thermopile consisting of hundreds of Bi/Te elements generates a DC voltage via the Seebeck effect. The resulting signal is directly proportional to the heat flux through the transducer.
Although initial heat flux measurements were taken by placing the sensor directly on the floor, it was found that the millivolts readings fluctuated significantly at a selected location because the floor acts as a heat storage reservoir, and once it is heated, the floor will give off heat by radiation to the surroundings. The sensor was later tested by mounting it on a small wood panel but again some fluctuations in the readings at the selected location were observed. This difficulty was overcome by modifying the sensor so as to provide cooling by circulating water through the lower or bottom part of the sensor at a substantially constant temperature by means of a pump. In order to provide for water cooling, the aforementioned heat sensor was modified by the addition of grooves and channels adjacent its bottom side through which water can circulate. Two water nipples were added to the sensor so as to provide an inlet and outlet for the water and these nipples were attached to plastic hoses.
This set-up is illustrated schematically in
For purposes of heat flux measurement, it was also necessary to measure accurately the surface tube temperature of the radiant tube heater and type K thermocouples were used for these measurements, these thermocouples being indicated at 150 In
In addition to measuring the surface temperature along the length of the radiant tube, it is also necessary to measure the ambient temperature of the air in the vicinity of the tube. The ambient temperatures were monitored by three thermostats placed along the length of the radiant tube heater, namely in the region of the first tube section located adjacent the burner, the middle section and adjacent the outlet or distal end of the radiant tube. In one exemplary set up for heat flux measurements the first thermostat was positioned about three feet away from the burner and the hot end of the radiant tube, the second thermostat was located five to seven feet away from the middle of the radiant tube while the third thermostat was about three feet from the distal or outlet end of the radiant tube. The three thermostats were used to calculate an average temperature which was then used to establish a boundary condition for the CFD software simulation of the heat flux measuring process. (see below)
In order to measure the voltage induced by the heat sensor 134 at each location on the grid, a voltmeter was used. An exemplary voltmeter that can be used is a Fluke-289, a precise and calibrated voltmeter having an accuracy within 10 to 15 millivolts and a precision of 1 microvolt. The readings from this voltmeter were taken after the surface tube temperature of the radiant tube heater reached a steady state. It was found that the steady state can easily be obtained from one half to one hour from burner start up. The achievement of this steady state condition was ensured by the above described taking of measurements of the radiant tube surface temperature and checking to confirm that the measurements did not change with time. The millivolts readings were allowed to fluctuate within 2% according to the manufacturer's specifications but the readings rarely fluctuated more than 3% of the average reading's value. If the fluctuations were very large and continued for a relatively long time, the measurements were stopped and the sources of error were investigated. It was found that possible sources of error in the heat flux readings include a change of ambient temperature, people passing close to the measurement area, environmental radiation, and excessive noise in the area of the measurement squares. To eliminate the possible effect of dust and debris on the tube and reflector, a vacuum cleaner and gauze were used to clean and wipe the tube and reflector twice a week. To avoid any fouling or scaling inside the grooves/channels of the heat flux sensor 134, in the hoses or in the pump, filtered water was used and changed daily. Any windows in the measurement area were covered with shutters to avoid sunlight hitting the enclosed area.
Calculation of Heat Flux Employing CFD Software
In order to validate the heat flux measurements taken using the above described measuring method computational fluid dynamics (CFD) software was used to compute the theoretical heat flux on a floor area corresponding to that used for the actual heat flux measurements. In order to use this software a number of parameters pertaining to the tube-reflector system were determined. One of these considered as an operating variable for the computer program was the height of the RTH above the floor area which is set initially at 100 inches corresponding to the actual height of the RTH using the measurement method described above. Maximum average values of numerical simulation results of the tube-reflector assembly were determined for heights of 14 feet and 18 feet and these values are set out in Tables 1 and 2 below. In the CFD numerical study, the effects of minor parts of the RTH such as clamps, screws, wire hangers and hanger plates were eliminated. This study used seven interpolation functions (see below) each for a respective one of seven 10 foot sections of the radiant tube and generated by Table 3D Curve software and these functions were used to approximate the tube temperature along each 10 foot length. The numerical results of heat flux were calibrated with the experimental data which was affected by slight changes of ambient temperature, material emissivity, environmental radiation and local meshing settings.
The temperature of the radiant tube was taken at a steady state condition and this temperature acted and served as boundary conditions for the simulation code. Although the flow simulation software can accommodate data from a few points, because the data points were in the order of 100 or more, it was necessary to use an analytical function. The obtained data was fed into the Table 3D Curve program in order to generate the corresponding interpolation functions. The experimental readings based on the above mentioned heat flux measuring method employing a heat flux sensor were compared to the numerical results produced by the CFD software for radiant tube height at 100 inches. The comparison between the experimental data and the generated numerical values produced by the analytical functions showed a definite correlation with the correlation percentage being between 97 and 98%.
The interpolation functions for a 200K Btu/H radiant tube heater (70 foot tube length) were determined to be the following:
Function 7:
(−434.6960678210+28.2928932179*z+94.0109695683*theta−0.2640692641*z^2−1.8256318715*theta^2−1.1093418259*z*theta).
It should be understood that function 1 is used for the first 10 ft length of the radiant tube and each of the subsequent functions is used for respective one of the following six ten foot sections of the tube.
The problem of determining leaving and net radiant heat fluxes is solved using a discrete Monte Carlo method. This numerical method solves the following radiative transfer equation (RTE) in steady state:
The first term of the above equation represents the spatial distribution of the radiant intensity, I, and the subscript λ is to designate that each quantity in the RTE is taken as a function of the wavelength. The variables κ and θ represent the medium absorption and extinction coefficients. μ, ξ, and η are the directional cosines that describe the direction of the radiant intensity. φ is the scattering phase function which is equal to 1 in isotropic scattering. By numerically solving the above RTE equation, one can find the radiant intensity, I, at any point, wavelength, and direction in the enclosed area. This approach does not require calculation of view factor which is cumbersome in some cases. The above RTE does not have an analytical solution for most cases because of the complicated directional and spectral nature of thermal radiation exchange between solid objects of various complex 3D shapes.
The Monte Carlo approach was used to solve the above equation numerically. This approach uses computational mesh cells containing faces approximating the radiative surfaces. The cells are joined in clusters by a special procedure that takes into account the face area and the angles between the normal to the surface and the face in each partial cell. The cells intersected by boundaries between radiative surfaces of different emissivity are considered as belonging to one of these surfaces and cannot be combined in one cluster. The Monte-Carlo approach has been used in the CFD flow simulation to reduce computational time and minimize the computer memory requirements.
After trial and error, an environmental temperature of 85° F. and an ambient temperature of 68° F. were adopted. The environmental temperature is an approximate value of the average wall temperature surrounding the tube reflector-assembly. Emissivities for different tubes, sensor and reflector material were also determined. The first two tube sections, each 10 feet in length, were assumed to act as black bodies and thus to have an emittance of 1. The emittance of the third and fourth tube sections was assumed to be 0.94 and the emittance of the last three tube sections was taken as 0.76. The total sensor area was split into three parts A, B and C with part A having the same emissivity as the first two tube sections, part B having the emissivity of the third and fourth tube sections and part C having the emissivity of the last three tube sections. These settings were determined by trial and error. Metallic surfaces have higher emittance at higher temperatures than at lower temperatures. There is a steep temperature gradient along the 70 foot radiant tube heater used in carrying out the present method, the temperature decreasing from a peak of 1,150° F. in the first tube section to only 300° F. approximately at the last 10 foot tube section. The first two tube sections, each 10 feet in length, have the highest emittance due to the radiant tube having the highest tube temperature along this portion, while the last three tube sections exhibit the lowest emittance due to their relatively low tube temperature. As indicated, the third and fourth tubes have an emittance of 0.94, which falls between 1 and 0.76.
The emittance of the aluminized steel that was used to carry out the heat flux measurements was taken as 0.09 and the source temperature for the burner was estimated to be around 220° F. The effect of solar radiation was excluded because the heat flux measurements, according to the present method, were taken in an area where the windows were covered by shutters. It was also assumed that the environment did not scatter or absorb thermal radiation from the RTH which is a valid assumption if the atmosphere is not very humid. Using these assumptions, the above equation was reduced to the following:
Symmetry was used in the computational domain dividing it into two equal parts. The actual heat flux measurement results using the above described method showed that the maximum heat fluxes moved symmetrically to the two edges of the measurement plane having a width of 65 inches. This was translated in the software by taking into account in the calculations the two outermost sloping surfaces of the reflector 10 as shown in
Theoretical heat flux measurements using the above mentioned interpolation functions developed by CFD software were determined and are set out in Table 1 below. This table sets out the theoretical heat flux measurements for a radiant tube heater located 100 inches above the floor area and the table provides maximum, average and minimum measurements. The assumed firing rate for the burner for these calculations was 200 K and the transverse coverage area was set at 25 feet. The indicated heat flux amounts are for part A of the total sensor area, this part corresponding to the floor area below the first two tube sections (each assumed to be 10 feet in length).
The existing reflector results in Table 1 assume a reflector shape as shown in
In addition to this existing reflector configuration, Table 1 shows the numerical calculated heat flux measurements for various reflectors with different outer central angles, these reflectors having multiple bends on both sides of their center line. The calculated results are shown for reflectors having a central outer angle of 140, 160, 180, 120, 100, 80, 60, and 30. It will be seen from Table 1 that the calculated maximum heat fluxes for reflectors having a center angle ranging between and including 30 degrees and 80 degrees are substantially higher than the maximum heat flux reading for the existing prior art reflector having a maximum calculated heat flux of 760. The highest calculated maximum heat flux reading is for a reflector having a center angle of 60 degrees wherein the calculated heat flux is at least 1001.
The angle F is formed by two central panel portions 182, 183 which meet at the centerline 18 of the reflector.
Turning to the simulated metal reflector illustrated in
Turning now to the simulated reflector 230 illustrated in
Table 2 below shows the calculated numerical heat flux measurements for various reflectors positioned at three different heights above the floor, namely 100 inches, 14 feet and 18 feet. Calculated measurements are shown for maximum, minimum and average heat flux measurements. The column entitled Existing Reflector is based on a reflector design such as that shown in
In the case of a burner having a low firing rate, such as 60,000 BTU/H, the average heat flux significantly decreased to 176 BTU/FT2 which is below an acceptable heating level for most heater applications. Therefore, a burner having a firing rate of 200 BTU/FT2 is desirable for a radiant tube heater located 100 inches or more above floor level.
As is well understood in the art, radiant tube heaters can be installed at different heights in a building depending upon the heating requirements and the height of the ceiling in the building. The actual heat flux measurements that were conducted using the above described equipment were conducted at a height of 100 inches. The 100 inch height is indicated in the schematic drawing of
Generally speaking, the radiant tube energy which strikes a wall such as the wall 162 on the right side of
The improved reflector construction in accordance with the present disclosure represents a substantial improvement over known reflectors for radiant heating tubes. The use of the improved present reflectors can result in substantial savings of heating costs and indirectly can reduce the emission of greenhouse gases created by the operation of radiant tube heaters. Moreover the present reflectors can be manufactured at little or no additional costs compared to known reflectors for such heaters.
The CFD calculations described above verified the actual heat flux readings. The tube temperature measurements taken at the steady state acted and served as boundary conditions for the assimilation code. The correlation between the experimental data measurements and the generated CFD values is between 97 and 98% which establishes the validity of the testing procedures described above.
Shown in the computer drawings of
While the present invention has been illustrated and described as embodied in various exemplary embodiments, e.g., embodiments having particular utility in radiant heating applications, it is to be understood that the present invention is not limited to the details showed herein, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the disclosed systems and reflectors can be made by those skilled in the art without departing in any way from the scope of the present invention. For example those of ordinary skill in the art will readily adapt the present disclosure for various other applications without departing from the scope of the present invention.
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