Abstract:
A flue gas energy transfer system for increasing the efficiency of hot-water and/or steam boiler. They system utilizes an arrangement of heat transfer ducts with damper elements for the selective diverting exhaust combustion gases. The heat transfer ducts transmit the exhaust gases for contiguous contact with the boiler for conductive heat transfer. The heated gases are retained within the transfer duct after boiler shutdown for maintaining an elevated water temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to energy transfer and especially to a heat recovery system for liquid heaters. 
     In particular, the flue gas energy transfer system of this invention concerns a subsidiary heat off-take for boilers. 
     2. Background Information 
     Liquid boilers used for low-pressure steam and hot water installations typically have a pressure vessel divided interiorly into water filed sections, tubes and/or coils. The liquid is separated from a combustion zone by a heating surface. And an oil or gas fired burner produces radiant energy and combustion gases for heating the liquid. The combustion gases are usually exhausted through a flue and discharged into the atmosphere through a chimney. 
     Various systems have been developed for recuperating heat from exhaust combustion gases. For example, U.S. Pat. No. 6,357,396 describes an apparatus for exchanging heat from the exhaust gas to preheat combustion air. A method for utilizing waste heat for heating room air is discussed in U.S. Pat. No. 4,079,885. U.S. Pat. No. 4,938,172 discloses the use of exhaust gases for supplementary heating of hot water. A heat recovery system wherein furnace exhaust gases are used to transfer heat to water coils is shown in U.S. Pat. No. 4,210,102. 
     The above systems however, do not provide subsidiary heating directly to the boiler for improving boiler performance. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly, the nature of this invention concerns a flue gas energy transfer system adapted for hydronic boilers. These boilers frequently include a combustion chamber which generates hot gases for heating a fluid medium contained within the boiler. The combustion gases are discharged, usually through a duct network, for release into the atmosphere. The energy transfer system of this invention recovers the waste heat. This is achieved by an arrangement of dampers, within the duct network. The dampers are selectively actuated for redirecting heated exhaust gases through auxiliary ducts placed contiguous to the boiler wherein heat is conductively transferred from the combustion gases to the fluid contained within the boiler. In accordance with the energy transfer system of this invention, the heated exhaust gases can be retained within the auxiliary duct after boiler shutdown to reduce heat loss. The system of this invention also encompasses automated control for sequencing the operation of the boiler. 
     Having thus summarized the invention, it will be seen that it is a preferred object thereof to provide a flue gas energy transfer system of the general character described herein for improving thermal efficiency of a boiler. 
     Another preferred object of this invention is to provide a flue gas energy transfer system with increased boiler capacity for higher peak loads. 
     A still further preferred object of this invention is to provide a flue gas energy transfer system with decreased demand time for reaching optimum operating temperature. 
     Yet still further preferred object of this invention is to provide a flue gas energy transfer system for maintaining operating temperature for a longer duration after boiler shutdown. 
     Still yet another object of this invention is to provide a flue gas energy transfer system with reduced stack exit temperatures. 
     Yet another preferred object of this invention is to provide a flue gas energy transfer system providing for boiler operation at lower draft pressures. 
    
    
     With these ends in view, the invention finds embodiment in certain combinations of elements and arrangements of parts by which the aforementioned preferred objects and certain other objects are hereinafter attained, all as more fully described with reference to the accompanying drawings and the scope of which is more particularly indicated in the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompany drawings, in which is shown an exemplary embodiment in accordance with the principles of the invention: 
     FIG. 1 is a perspective view of a boiler incorporating the flue gas energy transfer system of this invention; 
     FIG. 2 is a sectional view to an enlarged scale taken substantially along line  2 — 2  of FIG. 1; and 
     FIG. 3 is a schematic drawing illustrating the operation of the heat recovery system. 
    
    
     DESCRIPTION OF THE INVENTION 
     With specific reference now to the figures in detail, it is to be stressed that the particulars shown are by way of example and for the purposes of illustrative discussion of the preferred embodiment of the present invention and is presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt has been made to show structural aspects of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings should make it apparent to those skilled in the art how the invention may be embodied in practice. 
     Referring now in detail to FIG. 1 of the drawings, there is shown a flue gas energy transfer system  10  in accordance with this invention as applied to a boiler  12 . For the purpose of this illustrative description, the boiler  12  is a commercial hot water boiler fired internally by a gas, oil, or combination gas oil burner (not shown). It should be further understood that although the flue gas energy transfer system  10  of this invention, will be described with reference to a tankless hot water boiler  12 , such as manufactured by Weil-McLain, it finds the application with a wide selection of boilers including water-tube boilers and boilers for generating steam. 
     The boiler  12  is constructed with multiple boiler sections  14  that surround a combustion chamber. The sections  14  contain a fluid medium  15 , such as water, and are connected by piping (not shown) for supplying circulating water within the sections  14 . A rear boiler section  14   a  includes a port  16  for exhausting combustion gases generated in the combustion chamber. The exhaust gases enter at flue collector hood  18  and are vented through a main flue pipe  20 , typically having a circular cross-section, and a stack or chimney  22 , for discharge into the atmosphere. Although this invention is described with a rear exhaust boiler, the system  10  is also adaptable for use with a top exhaust boiler. 
     In accordance with this invention, a set of auxiliary heat transfer ducts  24 ,  24   a  are each connected at a respective end, to the flue collector hood  18 . An opposite end of each of the heat transfer ducts  24 ,  24   a  is connected to a respective branch connector pipe  26 ,  26   a  feeding into the main flue pipe  20 . The auxiliary heat transfer ducts  24 ,  24   a  are preferably rectangular in cross-section and, for example, have a width dimension of 2″ inches, a depth dimension of 8″ and are fabricated of sheet metal or other heat conductive material. The transfer ducts  24 ,  24   a , extend along opposite sides of the boiler  12  in a circuitous path for maximizing the surface area in contiguous contact with the boiler sections  14 . It should be apparent that the duct configuration and circuitous path may be modified for achieving optimum heat transfer; preferably a majority of the span length between the collector hood  18  and branch connector pipe  26  is in surface contact with the boiler  12 . 
     With reference now to FIG. 2, there is shown a sectional view through the boiler section  14  and the auxiliary heat transfer duct  24 . The boiler section  14 , as previous mentioned, surrounds the combustion chamber. An interior wall  28  of the boiler section  14 , interfaces with the combustion chamber and provides a primary heating surface in direct contact with the combustion gases. An exterior wall  30  of the boiler section  14 , provides a secondary heating surface. The walls  28 ,  30  are preferably constructed of steel or cast iron. The water  15  is subjected to heat energy, through radiation and convection as the hot combustion gases (illustrated by the wavy arrows) contact the interior wall  28 . The exhaust gases  17  within the auxiliary heat transfer ducts  24 ,  24   a  provide supplemental heat energy by conduction through the exterior wall  30 . The additional heat decreases the time for the boiler to reach optimum operating temperature. A panel or jacket  32 , fabricated with an insulating material  33 , is placed over the ducts  24 ,  24   a  to reduce heat loss. The heat retention also maintains the boiler operating temperature for longer duration of time after shutdown. 
     The exhaust gas discharge is controlled by a plurality of damper members as will further be discussed with reference to FIG. 3. A flue damper member  34 , is located within the main flue pipe  20 . The damper member  34  is mounted for incremental rotational displacement for controlling passage of exhaust gases and may be manually set or, preferably automatically controlled by a motor  36 . The auxiliary heat transfer ducts  24 ,  24   a  contain a set of intake damper members  38 ,  38   a , preferably controlled by a motor  40  for incremental settings. Another set of adjustable vent damper members  42 ,  42   a  are preferably respectively controlled by a motor  44 ,  44   a . This arrangement provides independent operation of each damper member  42 ,  42   a , for compensatory venting in the event of unbalanced exhaust gas flow. The motors  36 ,  40 ,  44 ,  44   a  are actuated by a control module  46 , such as a microprocessor, or equivalent type controller. The control module  46  also provides for on-off operation of the burner and incorporates safety features through communication with a temperature sensor  48 , a barometric pressure sensor  50  and a smoke detector  52  located in the main flue pipe  20  or the chimney  22  or elsewhere. 
     The operation of the energy transfer system of this invention will now be described. When the boiler  12  is in the “off” mode, each of the respective damper members  34 ,  38 ,  38   a ,  42 ,  42   a  is in a closed position with the respective flow paths through the main flue pipe  20  and the auxiliary transfer ducts  24 ,  24   a  being blocked. If the boiler  12  is to be operated in an alternate mode, without utilizing the auxiliary heat transfer ducts  24 ,  24   a , then the damper member  34  is placed in the “open” position (shown in broken-line); the damper members  38 ,  38   a ,  42 ,  42   a  remain in the “closed” position (shown in solid-line), and the exhaust gases exit through the main flue pipe  20 . 
     When the automatic control mode is operational, the control module  46  will respond to a heating load “demand” by initiating a signal to actuate the burner. After a burner flame is sensed, a command signal will be transmitted to actuate the motor  36  for opening the damper member  34 . The control module  46  will also send a command signal to actuate the motors  40 ,  44 ,  44   a  for opening the respective damper members  38 ,  38   a ,  42 ,  42   a ; simultaneously the motor  36  will receive a signal to close the damper member  34 . This will establish a flow path (illustrated by the arrows) for the exhaust gases  17 , through auxiliary transfer ducts  24 ,  24   a , the respective branch connector pipes  26 ,  26   a  and the main flue pipe  20 , When there is no longer a “demand” for heat, as detected by the control module  46 , the burner is shutdown and the damper motors  40 ,  44 ,  44   a  close the respective damper members  38 ,  38   a ,  42 ,  42   a  thus retaining the hot exhaust gases  17  within the auxiliary transfer ducts  24 ,  24   a.    
     By way of example, the water  15 , within the boiler sections  14 , was found to reach an operational temperature of 150° F. within 1 minute from receipt of a heat “demand”. The same boiler, without the energy transfer system of this invention, required a duration of 3 minutes to reach the same operational temperature. Furthermore, after the burner shutdown the temperature continued to rise to 170° F. within the next 3 minutes, whereas the boiler operating without the system of this invention, reached a maximum temperature of only 165° F. after burner shutdown. The accelerated rate of increase in water temperature provides for a shorter operating cycle resulting in energy savings and less pollution. 
     A comparison of stack temperatures was also investigated and it was determined that with the system of this invention, a maximum stack (chimney) exit temperature of 140° F. was reached 1 minute after receipt of heat “demand” (at boiler operational temperature) and that the stack temperature dropped to 80° F., 5 minutes after burner shutdown. The boiler operation without the system of this invention reached a stack temperature of 190° F. in 3 minutes of heat “demand” (at boiler operational temperature) and after burner shutdown the temperature dropped to 100° F. The stack temperature is directly proportional to heat loss; the lower stack temperature with this system is indicative of improved thermal efficiency. 
     In view of the above results it should be apparent that the energy transfer system of this invention will effect a reduction in burner fuel consumption for the reason that the optimum water temperature can be reached in a shorter time frame and the temperature can be maintained for a longer time duration after boiler shutdown thus providing improved boiler performance, energy conservation, and a cost reduction in operation. 
     It is also within the scope of this invention to monitor and regulate stack temperature, as by use of temperature sensor  48  and by automatically adjusting the setting of the damper members  34 ,  38 ,  38   a ,  42 ,  42   a  for example, by using a feed-back loop. The draft control is also automatically maintainable by a feed-back loop from the pressure sensor  50 . The detector  52  will transmit a signal to the control module  46  when unsafe smoke conditions arise. The module  46  will activate an alarm and simultaneously shutdown the burner. An automatic burner re-start e.g. after 2 minutes, can be programmed in the system. 
     It should also be noted that the energy transfer system of this invention can be used as a retrofit to existing boilers or integrated as part of a new boiler installation. Thus it should be seen that there is provided a flue gas energy transfer system which achieves the various preferred objects of this invention and which is well adapted to meet conditions of practical use. Since other various possible embodiments might be made to the present invention or modifications might be made to the exemplary embodiment set forth above, it is to be understood that all materials shown and described in the accompanying drawings are to be interpreted as illustrative and not in a limiting sense.