Abstract:
The present invention is a method of optimizing radiant and thermal efficiency of a gas fired radiant tube heater. A heat exchange blower receives intake air and delivers intake air through a heat exchanger as pre-heated air to a combustion air blower. The combustion air blower receives pre-heated intake air from the heat exchanger and then provides the pre-heated intake air to a burner for mixing with fuel. The fuel-intake air mixture is burned in the burner thereby producing combustion gasses which are fired into a radiant tube. The exhaust combustion gases pass through the balance of the radiant tube and through the heat exchanger where residual heat is transferred and extracted from the combustion gases to pre-heat the intake air. The turbulators are configured to increase the turbulence within the radiant tube and are placed within the initial 10′ to 30′ of the radiant tube after the burner to increase the tube temperature and the radiation emitted from this section of the radiant tube.

Description:
[0001]    This application claims priority from previously filed US provisional patent application 62/328,045. filed Apr. 27, 2016 by Gjergji File and Kevin Merritt under the title OPTIMIZATION OF GAS FIRED RADIANT TUBE HEATERS. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present concept relates to gas fired radiant tube heaters and more particularly relates to a method of optimizing the radiant and thermal efficiency of gas fired radiant tube heaters. 
       BACKGROUND OF THE INVENTION 
       [0003]    Gas fired radiant tube heaters have become a commercially popular method for providing heat in larger commercial buildings where conventional convective and forced air gas heating systems cannot be implemented efficiently. 
         [0004]    For example larger buildings may have doors, such as service bays, which are constantly being opened and closed resulting in large amounts of air being exchanged constantly throughout the day. Heating these types of buildings using conventional forced air gas or convective type heating systems results in large inefficiencies. 
         [0005]    Under these circumstances radiant heating is the preferred method of providing heat to the building and often the choice is to use gas-fired radiant tube heaters to provide the necessary radiant heating. 
         [0006]    Gas-fired radiant tube heaters have been in commercial use for quite some time now however very little attention has been directed to the radiant efficiency of the heat emission from the heater as well as the total thermal efficiency of the heater. Maximizing the radiant and thermal efficiency of the heater will minimize the operating cost of the system and therefore it is desirable to optimize both the radiant efficiency as well as the thermal efficiency of gas-fired radiant tube heaters. 
         [0007]    It is an object of the present concept to improve the radiant efficiency as well as the thermal efficiency of gas fired radiant tube heaters. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is a method of optimizing radiant and thermal efficiency of a gas fired radiant tube heater. This method includes providing a heat exchange blower to receive intake air and to deliver intake air through a heat exchanger and to further deliver pre-heated intake air to a combustion air blower. 
         [0009]    The combustion air blower receives pre-heated intake air from the heat exchanger, and then provides pre-heated intake air to a burner for mixing with fuel. The burner burns the fuel-intake air mixture thereby producing combustion gasses which are fired into a radiant tube. 
         [0010]    Exhaust combustion gases are passed through the balance of the radiant tube and through the heat exchanger to transfer and extract residual heat from the combustion gases to pre-heat the intake air. 
         [0011]    The method of optimizing radiant and thermal efficiency of a gas fired radiant tube heater preferably includes placing turbulators within the radiant tube. The turbulators are configured to increase the turbulence within the radiant tube and are placed in a region of the radiant tube such that they increase the radiant energy dissipated by the tube by at least 75% measured in kw/m 2 . 
         [0012]    Preferably the turbulators are placed in a region of the radiant tube where the tube temperature is higher than 600° F. such that they increase the tube temperature by at least 100° F. 
         [0013]    Preferably the heat exchanger is a gas to gas heat exchanger. 
         [0014]    Preferably the turbulators increase the turbulence within the radiant tube and are placed in a region of the radiant tube where the tube temperature is higher than 400° F. 
         [0015]    Preferably the turbulators are placed within the initial 10′ to 30′ of the radiant tube after the burner thereby increasing the tube temperature and the radiation emitted from this section of the radiant tube. 
         [0016]    Preferably the turbulators are placed within the initial 10′ to 20′ of the radiant tube thereby increasing the tube temperature and the radiation emitted from this section of the radiant tube. 
         [0017]    Preferably dilution air is introduced into the combustion gases at the heat exchanger in order to reduce the exhaust gas temperature to below 230° F. The dilution air is provided by the heat exchanger blower. The dilution air is modulated to control the exhaust gas temperature to below 230° F. 
         [0018]    Preferably the exhaust manifold is manufactured from plastic. 
         [0019]    The present method of optimizing radiant thermal efficiency of gas fired radiant tube heaters could further include a condensing section which receives exhaust gas from the heat exchanger for further extracting sensible heat and latent heat of condensation from the exhaust gases thereby cooling the exhaust gases below the dew point temperature such that moisture in the exhaust gases condenses. 
         [0020]    Preferably the condensing section includes a counter current exhaust pipe having an internal exhaust pipe for the flow of exhaust gases outwardly to the atmosphere and having an outer pipe, concentric with the internal exhaust pipe wherein intake air is counter-flowed within the space between the outer pipe and across the surface of the internal exhaust pipe thereby causing condensation and cooling of the exhaust gases within exhaust pipe. 
         [0021]    Preferably the heat exchanger includes a primary heat exchanger and a secondary heat exchanger, wherein the condensing section is included in the secondary heat exchanger which receives exhaust gases from the primary heat exchanger, the secondary heat exchanger cools the exhaust gases below the dew point temperature such that moisture in the exhaust gases condenses. 
         [0022]    The present invention is also a method of optimizing radiant and thermal efficiency of a gas fired radiant tube heater the includes the steps of:
       a) deliver intake air to a combustion air blower;   b) the combustion air blower delivers combustion air to a burner for mixing with fuel and burning the fuel-intake air mixture thereby producing combustion gasses which are fired into a radiant tube;   c) placing turbulators within the radiant tube, wherein the turbulators are configured to increase the turbulence within the radiant tube and are placed within the initial 10′ to 30′ of the radiant tube after the burner thereby increasing the tube temperature and the radiation emitted from this section of the radiant tube.       
 
         [0026]    Preferably the turbulators are placed in a region of the radiant tube where the tube temperature is higher than 600° F. such that they increase the tube temperature by at least 100° F. 
         [0027]    Preferably there is also a damper located after the secondary heat exchanger for venting a selected amount of heated excess air from the secondary heat exchanger for use as convective heating air. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The present concept will now be described by way of example only with reference to the following drawings in which: 
           [0029]      FIG. 1  is a schematic diagram of a gas fired radiant tube heater showing the flow of intake air, combustion air, and dilution air. 
           [0030]      FIG. 2  is a schematic diagram showing measurements of radiant energy taken along the length of a gas fired radiant tube heater fitted with turbulators also referred to as baffles starting at about 14 feet along the radiant tube. 
           [0031]      FIG. 3  is a schematic diagram showing temperature taken along the length of the radiant tube with turbulators installed at both the second and third tube and also along the third and fourth tube. 
           [0032]      FIG. 4  is a chart showing radiant energy measurements taken along the length of a tube for the scenario of low firing rate and the scenario of the high firing rate. 
           [0033]      FIG. 5  is a schematic side perspective view of the heat recovery section in the combustion section of the gas fired radiant tube heater. 
           [0034]      FIG. 6  is a schematic diagram of an alternate embodiment, a gas fired radiant tube heater shown together with a condensing section. 
           [0035]      FIG. 7  is a schematic diagram of an alternate embodiment, a gas fired radiant tube heater shown together with a condensing section. 
           [0036]      FIG. 8  is a schematic diagram of an alternate embodiment, a gas fired radiant tube heater shown together with a condensing section as shown in  FIG. 7 . 
           [0037]      FIG. 9  is a schematic diagram of an alternate embodiment, a gas-fired radiant tube heater show together with a condensing section and excess air outlet. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The present concept shown generally as a gas fired radiant tube heater  100  includes the following major components or sections: combustion section  102 , radiant tube section  104  and heat recovery section  106 . 
         [0039]    Air for mixing with fuel  114  at fuel inlet  124  is provided by two blowers connected in series namely heat exchange blower  108  and combustion air blower  110 . 
         [0040]    Fresh intake air  130  is received at cold air intake  131  of heat exchange blower  108 . There the intake air stream may be split into two streams namely optionally dilution air stream  134  and heat exchange stream  136 . The air moving through heat exchange stream  136  passes through heat exchanger  132  where it is preheated by combustion gases  116  to provide preheated intake air  112  to the second blower namely, combustion air blower  110 , which provides the preheated intake air to mix with fuel for combustion purposes. 
         [0041]    Combustion air blower  110  receives preheated intake air  112  from heat exchanger  132  and is mixed with natural gas  122  which enters through fuel inlet  124  and is combined at burner  120  to produce combustion  140  thereby producing combustion gases  116 . Combustion gases  116  travel along radiant tube  118  of radiant tube section  104  and more particularly travel along a first tube  142 , a second tube  144  through elbow  150  onwards through third tube  146 , fourth tube  148  where the combustion gases  116  pass through the heat recovery section  106  more particularly heat exchanger  132 . 
         [0042]    By utilizing preheated intake air  112  one can increase the temperature of combustion gases  116  being emitted out of the combustion section  102  therefore increasing the temperature of radiant tube  118  particularly along first tube  142  and second tube  144 . In practice first, second, third and fourth tubes generally speaking are approximately 10 feet in length each. 
         [0043]    Radiant energy efficiency is proportional to the radiant tube temperature to the fourth power. In other words the higher the tube temperature the greater the amount of usable radiant energy which is given off by the radiant tube. The radiant energy increase emitted by the radiant tube  118  falls off dramatically as temperature drops below approximately 600° F. Therefore any modifications that can be made to increase temperature in the areas of radiant tube where the temperature is above 600° F. will greatly increase radiant energy efficiency of the gas fired radiant tube heater  100 . 
         [0044]    Therefore the inventors have found that by placing turbulators  160  also known as baffles into the interior of radiant tube  118  at approximately the point in the radiant tube where the temperature is falling off and approaching 600° F. namely shown in this example at approximately the 15 foot mark to approximately the 30 foot mark the increased turbulence provided by the turbulators  160  increases the radiant tube temperature as shown in  FIGS. 3  and thereby increases dramatically the amount of radiation or radiant energy being emitted by the radiant tubes as depicted in  FIG. 2 . 
         [0045]    Historically turbulators and/or baffles have been utilized more predominantly in the later sections of the radiant tube namely in the third and fourth tube  146  and  148  as depicted in  FIG. 1  in order to increase the temperature of the radiant tube in those sections. 
         [0046]    The inventor has found that the increase in radiant efficiency by placing turbulators in for example the fourth tube is minimal. The amount of radiation emitted by the fourth tube  148  is greatly diminished since the temperature is well below 600° F. in this section. 
         [0047]    The inventor has found however that by placing turbulators  160  also known as baffles earlier along the length of the radiant tube namely in the second tube  144  one is able to increase the temperature in the radiant tube to dramatically increase the radiant energy emitted by the tube from for example approximately 12-13 kW/m 2  to approximately 23-24 kW/m 2  at the same point along the length of the tube. 
         [0048]    Looking at  FIG. 2  and  FIG. 3  for example at the 15 foot mark the temperature increase due to the use of the turbulator is from about 750° F. to approximately 950° F. or approximately a 200° F. increase in temperature which results in a radiant energy increase from approximately 12 kW/m 2  to 23 kW/m 2  or over 75% increase in the emission in radiant energy at that point along the tube. 
         [0049]    Looking now at approximately the 18 foot point along the radiant tube one will see that the temperature increase is from approximately 700° F. to 800° F. and the radiant energy emitted has increased from approximately 9 kW/m 2  to approximately 14 kW/m 2 . 
         [0050]    In other words a small increase in temperature at the higher temperatures namely in the 15 to 25 foot range of the radiant tube results in a dramatic increase in radiant efficiency due to the increased radiant energy emitted by the radiant tube  118 . 
         [0051]    Once the combustion gases  116  reach the heat recovery section  106  there is still substantial temperature within the combustion gases  116  namely about 435° F. temperature as shown in  FIG. 1 . 
         [0052]    Simply exhausting the combustion gases  116  at this temperature will result in poor thermal efficiencies since a great deal of the heat will simply be sent out as exhaust gas  170 . 
         [0053]    The inventor has found that in order to improve the thermal efficiency of the gas-fired radiant tube heater  100  one can recover the residual heat from combustion gases  116  using a heat exchanger  132  in order to preheat the intake air  130  which will be provided for combustion in burner  120  in the combustion section  102 . 
         [0054]    In other words intake air  130  passes through heat exchanger  132  to create preheated intake air  112  having a temperature of about 210° F. which ultimately increases the temperature of the combustion gases  116  fired into radiant tube  118  thereby improving the overall temperatures of the radiant tube  118  and improving the radiant efficiency of gas fired radiant tube heater  100 . 
         [0055]    In other words by recuperating residual thermal heat from combustion gases  116  one can convert these into increased temperature of combustion gases  116  which ultimately are converted into increased radiant energy efficiencies of gas fired radiant tube heater  100 . 
         [0056]    In other words thermal losses which normally would be experienced by the exhaust gas  170  can be converted into radiant energy increases at the radiant tube  118 . 
         [0057]    It is beneficial to be able to limit the exhaust gas temperatures to 230° F. or lower in order to be able to use synthetic or plastic exhaust manifolds  172 . 
         [0058]    If one is able to keep the exhaust gas temperature below 230° F. one is able to utilize corrosion resistant plastic and/or synthetic materials for the construction of the exhaust manifold  172  thereby minimizing the highly corrosive conditions produced by the exhaust gases  170 . 
         [0059]    The intake air  130  which is provided to heat exchange blower  108  therefore is split into two steams namely dilution air stream  134  and heat exchange stream  136 . 
         [0060]    Dilution air stream  134  is cold fresh intake air  130  provided by heat exchange blower  108  and is injected directly and mixed with combustion gases  116  travelling down through radiant tube  118 . 
         [0061]    By controlling the amount of dilution air one is able to control the residual exhaust gas temperature thereby ensuring that it remains under 230° F. 
         [0062]    The reader will note that in  FIG. 1  for example by introducing enough dilution air  174  in dilution air steam  134  one is able to lower the exhaust gas temperature to 200° F. thereby bringing it under the threshold to be able to use very corrosion resistant plastic and synthetic materials for the exhaust gas manifold. 
         [0063]    In summary the inventor has found that the use of turbulators  160  or baffles located in regions of the radiant tube where it is possible to obtain a temperature increase of 100° to 200° into the temperature range of between 700° F. and 1,000° F. provides for the greatest increase in radiant energy emissions by the radiant tube  118 . In other words by placing the turbulator baffle  160  closer to the hottest parts of the radiant tube namely in second tube  144  just after first tube  142  thereby maintaining the tube temperature as high as possible for as long as possible to provide the greatest release of radiant energy and therefore increase the radiant energy and efficiency of the overall system. 
         [0064]    Secondly the inventor has found that it is beneficial to recover residual thermal energy by recuperating waste heat from the combustion gases  116  in the fourth tube or the exit section of the gas-fired radiant tube heater  100  by preheating the intake air. This in turn will increase the temperature of the combustion gases  116  leaving the burner  120  thereby increasing the temperatures of radiant tube  118  thereby increasing the release of radiant energy and increasing the radiant efficiency (up to 7%) and the thermal efficiency (up to 5%) of the system. In other words by utilizing the heat exchanger  132  one can convert waste thermal energy into increased radiant energy and thereby convert thermal losses into increased radiant energy. Thermal efficiency measurements were carried out under CSA standards. Radiant efficiency improvements were measured as increases in radiant efficiency relative to prior art or unmodified, conventional radiant heaters. Those skilled in the art recognize there is no accepted world standard for measuring absolute radiation energy efficiencies therefore the best practice at this point in time is to measure relative improvements or differences. 
         [0065]    As an additional benefit to the use of the heat exchanger  132  it was found that one is able to reduce the temperature of the exhaust gas  170  to below 230° F. by splitting the air intake into two streams that is exiting the heat exchange blower  108  namely into a dilution air stream  134  and into a heat exchange stream  136 . The dilution air stream  134  is air which would be injected into the combustion gases  116  to bring the residual temperature of the exhaust gas  170  down to below 230° F. in order that one could use a highly anti-corrosive material such as plastic for the exhaust manifolds  172 . 
         [0066]    Referring now to  FIG. 6  which shows an alternate embodiment namely gas-fired radiant tube heater  200  which includes a condensing section  202  which is shown schematically in the figure. Other than the addition of condensing section  202  gas-fired radiant tube heater  200  contains all of the same components as gas-fired radiant tube heater  100 . 
         [0067]    Gas-fired radiant tube heater  200  includes radiant tube section  104 , heat recovery section  106  which includes a heat exchange blower  108  and condensing section  202  with a counter current exhaust pipe  204 . 
         [0068]    The condensing section  202  includes a counter current exhaust pipe  204  having an internal exhaust pipe  220  for the flow of exhaust gases outwardly to the atmosphere and having an outer pipe  240 , concentric with the exhaust pipe  220  wherein intake air  130  is counter-flowed across the surface of the exhaust pipe  220  thereby causing cooling and condensation of the exhaust gases  170  within exhaust pipe  220 . 
         [0069]    The exhaust gas  170  exits out of the heat recovery section at under 230° F. and is further cooled by intake air  130  to the point where it condenses and liquid is collected at a siphon  206  and drained away. In this manner intake air  130  is preheated through the counter current exhaust pipe  204  such that further increases in efficiency are accomplished since the pre-heated intake air  112  is yet further increased in temperature prior to reaching the combustion air blower  110 . 
         [0070]    Therefore by using condensing section  202  one can further increase the thermal and radiant efficiencies since one is able to further increase the tube temperatures and take advantage of recuperating the waste heat of the exhaust gas  170 . 
         [0071]    Additionally there are installation advantages in that only one aperture is needed through the wall or through the roof line  208  as shown in  FIG. 6  for both the intake and exhaust gas conduit. 
         [0072]    Referring now to  FIGS. 7 and 8  which shows yet another alternate embodiment, a gas fired radiant tube heater shown generally as  300  which also includes a condensing section  302  which is internally located adjacent to the primary heat exchanger  308 . Gas fired radiant tube heater  300  is in most aspects the same as radiant tube heater  100  except for the addition of a secondary heat exchanger  310 . 
         [0073]    Rather than having a counter current exhaust pipe  204  as shown in  FIG. 6  one would move the condensing section within the building next to the primary heat exchanger which in the first embodiment  100  above is identified as the heat recovery section  106 . 
         [0074]    In the gas fired radiant tube heater  300  as shown in  FIGS. 7 &amp; 8  the heat recovery section  306  includes the primary heat exchanger  308  which is equivalent to the heat exchanger  132  plus a secondary heat exchanger  310  which is a condensing section  302  and has a condensation drain  312 . 
         [0075]    In this case it is likely that dilution air is no longer necessary in that the temperature is brought below 230° F. within the secondary heat exchanger  310  namely the condensing section  302 . 
         [0076]    The heat exchange blower  108  is now positioned in such a manner that it will blow air through both the primary heat exchanger as well as the secondary heat exchanger as schematically depicted in  FIG. 7 . The flow of intake air  130  is shown in dashed lines in  FIG. 7 . 
         [0077]    Referring now to  FIG. 9  which shows yet another alternate embodiment, a gas fired radiant tube heater shown generally as  400  which also includes a condensing section  302  which is part of secondary heat exchanger  310  which is internally located adjacent to the primary heat exchanger  308 . Condensing section  302  further includes excess air outlet  402 . Gas fired radiant tube heater  400  is in most aspects the same as gas fired radiant tube heater  300  except for the addition of an excess air outlet  402 . 
         [0078]    As in the embodiments depicted in  FIGS. 7 and 8  condensing section  302  is located next to primary heat exchanger  308 , however could also be positioned other locations. 
         [0079]    Heat recovery section  406  includes the primary heat exchanger  308  and secondary heat exchanger  310 . Secondary heat exchanger  310  includes condensing section  302  and has a condensation drain  312 . 
         [0080]    Heat exchange blower  108  is positioned such that it will blow air through primary heat exchanger  308  as well as secondary heat exchanger  310  as schematically depicted in  FIG. 9 . The flow of intake air  130  is shown in dashed lines in  FIG. 9 . 
         [0081]    Intake air  130  enters through blower  108  and passes through secondary heat exchanger  310  where it is heated to become initial preheated air  410 . Initial preheated air  410  continues to primary heat exchanger  308  where it is further heated and becomes preheated intake air  112  before entering combustion section  102 . Excess air is released through excess air outlet  402  as heated excess air  408 . Heated excess air  408  can be used to provide convective heating in addition to the radiant heating. In other words the heated excess air  408  can be used for additional heating within a building housing the radiant heater  400 . 
         [0082]    Excess air outlet  402  may be a damper  404  or an orifice located between secondary heat exchanger  410  and primary heat exchanger  308  to control flow of initial preheated air  410  to primary heat exchanger  308 , and release a selected amount of heated excess air  408 . 
         [0083]    Exceeding temperature limits in the combustion air blower  110 , combustion section  102  or radiant tube  118  can result in damage to the equipment and significant reductions in equipment life span. Opening damper  404  allows a greater amount of heated excess air  408  to be released from excess air outlet  402 . Closing damper  404  reduces the amount of heated excess air  408  that is released. In this manner one is able to balance the system to provide enough combustion air to the burner  120  at a maximum feasible and safe temperature. 
         [0084]    It should be apparent to the reader that it is possible to modulate damper  404  for use with a two-stage burner or modulated firing rates. 
         [0085]    The reader will note that the use of two blowers namely heat exchange blower  108  and combustion air blower  110  provides for the ability to control much more closely the air requirements of a gas fired radiant tube heater  400  since the heat exchanger blower  108  can be sized to provide enough air to overcome the pressure drop that one sees across heat exchangers  308  and  310  which in many instances can be sizable since high efficiency heat exchangers tend to result in relatively large pressure drops. 
         [0086]    Two blowers mounted in series namely, heat exchanger blower  108  and combustion air blower  110  gives one the flexibility to be able to precisely control air movement through the heat recovery section  406  and air movement through the combustion section  102  both of which are critical to obtaining high thermal efficiencies as well as high radiant efficiencies. 
         [0087]    The use of two blowers divides the maximum pressure peaks or spikes in the system in approximately half as each blower contributes approximately half of the total pressure and air movement through the system. The efficiency of combustion air blower  110  is improved by the use of heat exchange blower  108  by providing positive or neutral air pressure to the inlet of combustion air blower  110 . Heat exchange blower  108  and combustion air blower  110  can operate under more efficient conditions as opposed to a single blower which in the described embodiments would encounter very inefficient blower operating parameters to achieve the desired flows. 
         [0088]    Use of two blowers in a gas fired radiant tube heating system provides greater flexibility in control of air pressures, air flows and air temperatures at the heat exchangers and burner, allows for a more compact design compared to the use of a single, larger blower and is an overall less expensive option in terms of blower cost, maintenance and replacement compared to a single, large blower. 
         [0089]    It should be apparent to persons skilled in the arts that various modifications and adaptation of this structure described above are possible without departure from the spirit of the invention the scope of which defined in the appended claim.