Abstract:
One embodiment of the present invention is a boiler for boiling water to produce steam, having (1) an evaporating section comprising (a) a combustion chamber for burning fuel with air and generating hot flue gas, (b) an evaporating heat exchanger around the combustion chamber for exchanging heat between the flue gas and water to produce the steam which exits the boiler; and (2) a condensing section comprising (c) a condensing heat exchanger for exchanging heat between the hot flue gas from the combustion chamber and a low-temperature water return having a temperature below approximately 100° F., generating flue gas condensate, which leaves the boiler, wherein the low-temperature water return is heated by the hot flue gas in the condensing heat exchanger before entering the evaporating heat exchanger for additional heating. The disclosed vacuum condensing boilers make vapor vacuum steam more efficient and economical for industrial, commercial, and home applications.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority from U.S. Ser. No. 14/025,292, filed on Sep. 12, 2013, entitled “VAPOR VACUUM HEATING SYSTEMS AND INTEGRATION WITH CONDENSING VACUUM BOILERS,” issued on Apr. 22, 2014 as U.S. Pat. No. 8,702,313, which itself is a non-provisional of and claims the benefit of U.S. Ser. No. 61/702,533, filed on Sep. 18, 2012, entitled “Condensing boiler and vapor vacuum heating system combo,” the entireties of both of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to vapor vacuum condensing boilers and their designs for use with vapor vacuum heating systems. 
     BACKGROUND OF THE INVENTION 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Existing positive low-pressure steam heating systems provide simple and reliable techniques for heating in a wide variety of industrial, commercial, and residential applications. Water (as a liquid) heated in a boiler becomes steam (a gas), which then rises through the feeder pipes (conduits) and condenses in radiators, giving off its latent heat. Radiators become hot and heat up objects in the room directly as well as the surrounding air. Steam is traditionally delivered under a low pressure of up to 2 psig at 218° F. in order to improve boiler safety and efficiency. Additionally, steam at lower pressure moves faster, contains less water, and doesn&#39;t create boiler low water problems. The boiler creates the initial steam pressure to overcome friction in the feeder pipes. 
     An existing steam system can be converted to a vapor (steam) vacuum system by operating the steam system under 5-10 inches of Hg vacuum. Although there are some efficiency gains, the conversion of a steam system into a vacuum system results in an increased maintenance cost due to additional vacuum equipment, condensate pumps, and electricity usage. In existing vacuum systems, steam traps are utilized in which condensate is separated from steam, sucked by a vacuum pump, and returned into the system by a water pump. Steam trap usage is also a major maintenance, repair, and replacement problem. Few new vacuum systems have been installed in the last fifty years due to high installation and maintenance costs. 
     Existing steam (vapor) systems are robust and reliable but have multiple problems, including high installation costs, noise, uneven heat distribution, and control difficulties. Therefore, many worn out steam systems are being retrofitted into hot water heating systems. However, such retrofits are very expensive because the boiler and the old plumbing have to be replaced which requires significant demolition of building internals. Alternatively, the level of building destruction is much less for conversion of a steam into a vacuum system. Therefore, a low-cost and efficient vacuum system would be an advantageous alternative for steam system retrofits as well as for new heating system installations. 
     In order to boost energy efficiency, modern hot water condensing boilers absorb the latent heat of water vapor from the flue gas. The recommended temperature of the water return (supply into boiler condensing section) is below 100° F. in order to condense most of the water from the flue gas. In reality, this temperature is at 140° F. or above for most of the heating season in order to deliver enough heat into the building. As a result, benefits of condensing mode usage are lost. Another problem of hot water condensing boilers is limited temperature of supply water. The typical temperature drop through a hot water heating system is 20° F., and therefore for condensing boilers, supply water temperature is limited to 120-160° F. At such low temperatures, the energy value of delivered heat is less than in a regular hot water system. This results in hot water condensing boilers that operate as traditional boilers with their condensing section inefficient for most of their operating time, eliminating the energy saving benefits of condensing boilers almost entirely while still having their high capital costs. 
     The temperature of condensate return in existing vacuum systems is either equal to the temperature of vapor rising through the same pipe or slightly lower in two pipe systems. The high temperature of condensate return is considered an inherent feature of the system and never challenged. Steam and vacuum systems are never used with condensing boilers, and therefore no steam or vacuum condensing boilers exist. Accordingly, as recognized by the present inventor, what are needed are vacuum condensing boiler designs for use with vapor vacuum heating systems. 
     Therefore, it would be an advancement in the state of the art to provide vacuum condensing boilers. It is against this background that various embodiments of the present invention were developed. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, one embodiment of the present invention is a boiler for boiling water to produce steam, comprising (1) an evaporating section comprising a combustion chamber for burning fuel with air and generating hot flue gas, and an evaporating heat exchanger around the combustion chamber for exchanging heat between the flue gas and water to produce the steam which exits the boiler; and (2) a condensing section comprising a condensing heat exchanger for exchanging heat between the hot flue gas from the combustion chamber and a low-temperature water return having a temperature below approximately 100° F., generating flue gas condensate, which leaves the boiler, wherein the low-temperature water return is heated by the hot flue gas in the condensing heat exchanger before entering the evaporating heat exchanger for additional heating. 
     Another embodiment of the present invention is the boiler described above, further comprising fins adapted to enhance the exchange of heat between the flue gas and evaporating water. 
     Another embodiment of the present invention is the boiler described above, wherein the counter-current heat exchanger contains one or more passes. 
     Another embodiment of the present invention is the boiler described above, wherein the counter-current heat exchanger contains two or more passes. 
     Another embodiment of the present invention is the boiler described above, wherein the boiler is utilized with a vapor vacuum system. 
     Another embodiment of the present invention is the boiler described above, further comprising an array of short thick wall heat pipes. 
     Yet another embodiment of the present invention is the boiler described above, wherein the heat pipes comprise closed-end tubes with a working fluid under vacuum. 
     Other embodiments of the present invention include methods corresponding to the boilers and systems described above, as well as methods of operation of the boilers and systems described above. Other features, utilities and advantages of the various embodiments of the invention will be apparent from the following more particular description of embodiments of the invention as illustrated in the accompanying drawings, in which like numerals indicate like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a thermal efficiency of a condensing boiler system as a function of return condensate temperature. 
         FIG. 2  illustrates a physical interpretation of the Farber-Scorah Boiling Curve. 
         FIG. 3  illustrates a schematic of an embodiment of a vacuum condensing boiler according to one embodiment of the present invention. 
         FIG. 4  illustrates a schematic of another embodiment of a vacuum condensing boiler according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is merely exemplary in nature and is in no way intended to limit the present disclosure, application, or uses. 
     In order to boost energy efficiency, modern hot water condensing boilers (“CB”) absorb the latent heat of water vapor from the flue gas. Recommended temperature of water return temperature (supply into boiler condensing section) is below 100° F. in order to condense most of the water (see  FIG. 1 , adapted from T. H. Durkin, “Boiler System Efficiency,” ASHRAE Journal, vol. 48, July 2006, p. 51). In reality, water return temperature is at 140° F. level for most of the heating season in order to deliver enough heat into building. As a result, benefits of condensing mode usage are lost. Another problem with hot water condensing boilers is limited temperature of supply water. Typical temperature drop through hot water heating systems is 20° F., and therefore CB supply water temperature is limited to 120-160° F. At such low temperatures, the energy value of delivered heat is less than in regular hot water systems. 
     The temperature of condensate return in traditional vacuum single-pipe systems is either equal to the temperature of vapor rising through the same pipe, or slightly lower in two pipe systems. The high temperature of condensate return is considered an inherent feature of traditional vacuum and steam systems and is never challenged. The present inventor has recognized that the lowered temperature of condensate return can be used with new and improved vapor vacuum condensing boilers, which would improve system efficiency. 
     Herein are presented several designs for condensing vacuum boilers that can be utilized with a low temperature vapor vacuum system, as described in related U.S. Pat. No. 8,702,313 entitled “VAPOR VACUUM HEATING SYSTEMS AND INTEGRATION WITH CONDENSING VACUUM BOILERS,” issued on Apr. 22, 2014 to the inventor of the present application. Since the various vapor vacuum system embodiments described therein allow integration of condensing boilers for the first time, vacuum condensing boilers are desirable. Accordingly, embodiments of various vacuum condensing boiler designs are described herein. Various condensing boiler designs are envisioned within the scope of the present invention, and the particular condensing boiler designs are not intended to limit the scope of the present invention as one of ordinary skill in the art would envision multiple modifications and combinations of the design concepts illustrated herein. 
     A vapor vacuum system can be used in any building and/or dwelling as needed. For the purposes of the descriptions herein, the term “building” will be used to represent any home, dwelling, office building, and commercial building, as well as any other type of building as will be appreciated by one skilled in the art. For purposes of this description, “steam” and “vapor” are used interchangeably. “Single-pipe” and “one-pipe” are used interchangeably and refer to systems with a single pipe used for both feeding vapor to the radiators and returning condensate. “Two-pipe” and “double-pipe” are used interchangeably to refer to systems in which a separate pipe is used to return condensate from the pipe used to feed the vapor to the radiators. As used herein, “closed-loop,” “closed loop,” and “closed system” are used interchangeably to mean an essentially closed vacuum system and piping with essentially air-tight connections and negligible leakage. The term “steam system” shall refer to positive pressure steam systems, usually operating at up to 2 psig, whereas the terms “vapor vacuum system,” “vacuum system,” “vapor vacuum heating,” and “VVH” shall refer to negative pressure steam systems operating with at least 5 inches Hg vacuum. 
     An attractive feature of the vapor vacuum heating system is advanced heat transfer conditions. Heat transfer coefficients in the boiler are changed by orders of magnitude depending on temperature differences between the wall and boiling temperature of the saturated liquid (see  FIG. 2 , physical interpretation of the Farber-Scorah Boiling Curve, adapted from FIG. 5.1 in M. L. Corradini,  Fundamentals of Multiphase Flow,  1997; see also FIG. 6.14 in P. K. Nag,  Heat and Mass Transfer , 2nd Ed., 2007). Hot water boilers work in the least efficient regime of interface evaporation (pure convection). Furthermore, in hot water systems, the “bubbles” regimes, which have the highest heat transfer coefficients, are avoided because the hot water circulation worsens in the presence of the vapor phase. Conversely, in a vacuum system, heat transfer occurs instantly in the most efficient “bubbles” regime because water boils at lower temperatures. Therefore, the required heat exchange area can be reduced significantly not only in the boiler evaporative section, but also in the boiler condensing section. 
       FIG. 3  illustrates a schematic of a vacuum condensing steam boiler with a single pass down flow configuration according to one embodiment of the present invention. Two- and three-pass apparatus may be used as well. High temperature flue gas from a burner  306  evaporates water in a boiler cylindrical evaporating section  301  and then flows down into a condensing section  302  along a spiral tube heat exchanger  305  filled with condensate return from the radiators (radiators not shown). Air  308  and fuel  309  are supplied from the boiler top; an air blower  307  is utilized to start the system. Cold condensate  312  from radiators enters into the spiral tube heat exchanger  305  from the bottom of the boiler and rises up due to hot water&#39;s lower density, boils, and exits the boiler as vapor phase  313 . Condensate  312  from the radiators periodically returns into the boiler through a back flow valve  304  when the boiler stops and the system pressure equalizes. To avoid a sharp decrease in the heat transfer in the evaporating section due to transition into film boiling, fins  303  are provisioned to direct vapor phase outward from the heat exchange area in the evaporating section  301  of the boiler. Flue gas  310  leaves the boiler bottom through an exhaust line, while flue gas condensate  311  is removed from the boiler bottom through a separate line. 
     In one alternative embodiment of the vacuum condensing boiler, an array of short thick wall heat pipes can be utilized in the condensing section instead of the spiral tube heat exchangers, as shown in  FIG. 4 . High temperature flue gas from a burner  406  evaporates water in a boiler cylindrical evaporating section  401  and then flows down into a condensing section  402 . Heat pipes  405  are threaded through the inner wall of the condensing section  402 . These heat pipes have no wick capillary structure; instead, they comprise short, closed-end tubes with a working liquid under vacuum (water can be used as a working liquid in some embodiments). The condensing section  402  comprises two semi-cylinders  408  connected to the evaporating section  401  by lines  409  that can be taken apart for the heat pipes&#39; inspection and replacement. Although the tips of these heat pipes  405  will be exposed to corrosive flue gas, the condensing section  402  would still be functional if the walls of one or several heat pipes fail. Air  410  and fuel  411  are supplied from the boiler top; an air blower  407  is utilized to start the system. Cold condensate  414  from the radiators enters from the bottom of the boiler and rises up due to hot water&#39;s lower density, boils, and exits the boiler as vapor phase  415 . Condensate from the radiators periodically returns into the boiler through a back flow valve  404  when the boiler stops and the system pressure equalizes. To avoid a sharp decrease in the heat transfer in the evaporating section  401  due to transition into film boiling, fins  403  are provisioned to direct the vapor phase outward from the heat exchange area in the evaporating section of the boiler. Flue gas  412  leaves the boiler bottom through an exhaust line, while flue gas condensate  413  is removed from the boiler bottom through a separate line. 
     In some embodiments of the present invention, vacuum condensing boilers having multiple passes designs are possible according to the principles of the present invention. Proposed in  FIGS. 3 and 4  were designs of vacuum condensing boilers with single-pass flue gas flow. Like hot water condensing boilers, two- and three-flue gas passage designs can be utilized for the purpose of compact design and efficiency. Instead of using a single-pass flow of flue gas from top to bottom as shown in  FIG. 3 , flue gas flows in multiple passes from top to bottom, and back to the top, as it exchanges heat with the condensate return. Such multiple pass embodiment can increase the efficiency of heat exchange and provide for an even more compact design. 
     While the methods disclosed herein have been described and shown with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form equivalent methods without departing from the teachings of the present invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations is not a limitation of the present invention. 
     While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the present invention.