Combustion system with high turn down ratio

Abstract

A low NO x combustion system for reducing the production of nitrogen oxides in its emissions while allowing unusually high turndown ratios in operation is shown. The system includes an improved burner which has a firing head with an outlet end which can be mounted on a sidewall of a heat exchanger such as a boiler. The burner includes a fan, a windbox, a burner drawer assembly and a gas manifold. The windbox has an internal scroll-shaped passageway which is contoured to improve air flow from the fan to the firing head. An improved air straightener and diffuser head in the burner drawer assembly cooperate with improvements in the air supply system to provide improved combustion characteristics and a higher than usual turndown ratio for the burner assembly.

1 . A low NO x  combustion system, the combination comprising: 
 a heat exchanger having sidewalls defining a closed interior containing a medium to be heated;    a burner assembly having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the heat exchanger;    fuel supply means, fluidly coupled to the burner, for conveying a combustible fuel to the burner;    air supply means, fluidly coupled to the burner, for conveying combustion supporting air to the burner;    igniting means, positioned adjacent said outlet end of the burner, for igniting the combustible fuel to thereby heat the medium contained in the heat exchanger; and    a windbox provided as a part of the air supply means connecting with the firing head, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway, an inlet and an outlet openings, and wherein air from the air supply means travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the air supply means to the burner, and wherein the scroll-shaped passageway is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner.    
   
   
       2 . The combustion system of  claim 1 , wherein the fuel supply means utilizes natural gas as a primary combustible fuel.  
   
   
       3 . The combustion system of  claim 1 , wherein the fuel supply means utilizes oil as a primary combustible fuel.  
   
   
       4 . The combustion system of  claim 1 , wherein the heat exchanger is a fire tube boiler or furnace.  
   
   
       5 . The combustion system of  claim 1 , wherein the firing head includes a burner drawer assembly located within a head extension which connects the windbox to the sidewall of the heat exchanger, the burner drawer assembly including a diffuser, a pilot and an air straightener all carried on a longitudinal support tube which extends perpendicular to a back plate, and wherein the air straightener comprises a single plate having a length and a width, opposing side edges and opposing planar surfaces, the plate being mounted on the longitudinal support tube along a selected opposing side edge thereof.  
   
   
       6 . The combustion system of  claim 5 , wherein the longitudinal support tube comprises an oil gun tube, the oil gun tube being slidably received within an opening provided in the back plate, whereby the position of the air straightener can be varied longitudinally by sliding the oil gun tube within the opening provided in the back plate, the oil gun tube also being rotatable with respect to the opening provided in the back plate.  
   
   
       7 . The combustion system of  claim 5 , wherein the firing head diffuser provides directional control of combustion air for mixing and combustion stability, the diffuser comprising a manifold plate having a first annular wall region which forms a collar-like region about a central opening, the manifold plate also having a second annular wall region having a series of overlapping fins which are separated by slits which provide a swirling action to combustion air passing through the diffuser, and wherein a plurality of gas orifices are located about an outer periphery of the second annular wall region for conveying and communicating natural gas outward and into a combustion region of the combustion system.  
   
   
       8 . The combustion system of  claim 1 , wherein the air supply means includes a combustion air fan which attaches to the windbox, whereby the windbox routes combustion air form the fan to the firing head, and wherein the air supply means also includes an air damper which communicates with an inlet to the combustion air fan, the air damper comprising a box-like enclosure which houses a single damper blade, the damper blade upper and lower longitudinal sealing edges and opposing side edges, the upper and lower longitudinal sealing edges and opposing side edges being provided with sealing strips which provide ease in adjustment for leakage.  
   
   
       9 . The combustion system of  claim 5 , wherein the head extension connects to a cylindrical gas manifold which, in turn, connects to the heat exchanger sidewall, the gas manifold having radial gas ports used to direct gas fuel to the burner outlet, at least selected ones of the radial gas ports having gas spuds installed therein to improve the distribution of the gas, and wherein the gas manifold holds the outlet end of the burner assembly, the gas manifold having an outer face which is protected from flame temperatures at the burner outlet by a refractory front plate.  
   
   
       10 . The combustion system of  claim 9 , wherein a ceramic blanket is used between the outer face of the gas manifold and the refractory plate to further prevent the transfer of heat.  
   
   
       11 . A low NO x  combustion system for firing a Scotch Marine boiler, the combination comprising: 
 a boiler having sidewalls defining a closed interior containing a fluid medium to be heated;    a burner assembly having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the boiler;    fuel supply means, fluidly coupled to the burner assembly, for conveying a combustible fuel to the burner assembly;    a combustion air fan, fluidly coupled to the burner assembly, for conveying combustion supporting air to the burner assembly;    a pilot, positioned adjacent said outlet end of the burner assembly, for igniting the combustible fuel to thereby heat the fluid medium contained in the boiler; and    a windbox connecting the combustion air fan and the firing head of the burner assembly, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway, an inlet and an outlet openings, and wherein air from the combustion air fan travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the windbox to the burner assembly, and wherein the scroll-shaped passageway is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner assembly.    
   
   
       12 . The combustion system of  claim 11 , wherein the firing head includes a burner drawer assembly located within a head extension which connects the windbox to the sidewall of the boiler, the burner drawer assembly including a diffuser, a pilot and an air straightener all carried on a longitudinal support tube which extends perpendicular to a back plate, and wherein the air straightener comprises a single plate having a length and a width, opposing side edges and opposing planar surfaces, the plate being mounted on the longitudinal support tube along a selected opposing side edge thereof.  
   
   
       13 . The combustion system of  claim 12 , wherein the longitudinal support tube comprises an oil gun tube, the oil gun tube being slidably received within an opening provided in the back plate, whereby the position of the air straightener can be varied longitudinally by sliding the oil gun tube within the opening provided in the back plate, the oil gun tube also being rotatable with respect to the opening provided in the back plate.  
   
   
       14 . The combustion system of  claim 13 , wherein the firing head diffuser provides directional control of combustion air for mixing and combustion stability, the diffuser comprising a manifold plate having a first annular wall region which forms a collar-like region about a central opening, the manifold plate also having a second annular wall region having a series of overlapping fins which are separated by slits which provide a swirling action to combustion air passing through the diffuser, and wherein a plurality of gas orifices are located about an outer periphery of the second annular wall region for conveying and communicating natural gas outward and into a combustion region of the combustion system.  
   
   
       15 . The combustion system of  claim 14 , wherein an air damper communicates with an inlet to the combustion air fan, the air damper comprising a box-like enclosure which houses a single damper blade, the damper blade upper and lower longitudinal sealing edges and opposing side edges, the upper and lower longitudinal sealing edges and opposing side edges being provided with sealing strips which provide ease in adjustment for leakage.  
   
   
       16 . The combustion system of  claim 15 , wherein the head extension connects to a cylindrical gas manifold which, in turn, connects to the boiler sidewall, the gas manifold having radial gas ports used to direct gas fuel to the burner outlet, at least selected ones of the radial gas ports having gas spuds installed therein to improve the distribution of the gas, and wherein the gas manifold holds the outlet end of the burner assembly, the gas manifold having an outer face which is protected from flame temperatures at the burner outlet by a refractory front plate.  
   
   
       17 . The combustion system of  claim 16 , wherein a ceramic blanket is used between the outer face of the gas manifold and the refractory plate to further prevent the transfer of heat.