LIQUID-BACKED GAS-FIRED HEATING SYSTEM

Abstract: A liquid-backed gas-fired heating system (10) is disclosed which utilizes an infrared burner module (12) having a helical coil wrapped concentrically thereabout. The helical coil located in the radiant zone and con­ vective zone of the module (12) receives heat from the hot products of combustion and transfers the heat to a circulating fluid, which in turn transfers the heat through a fan coil (14) to the space to be heated.

Description: This invention relates generally to a heating system and, more particularly, to a heating system which utilizes an infrared burner module having a heat exchanger coil wrapped around a radiant burner. In heating systems for homes and commercial buildings, central furnaces to heat a space all operate on the same general principle. Air for a space to be heated circulates around a closed system, and is heated either as it passes through a heat exchanger in contact with a burning fuel, or as it passes in contact with a secondary fluid which has been heated by a burning fuel. Since burning the fuel results in the production of noxious combustion gases having exhaust temperatures which can exceed 260°C (500°F), it is necessary to exhaust the combustion gases through a chimney or flue to the atmosphere. These systems are relatively inefficient due to the high exhaust temperatures of the flue gases, and costly due to the construction of the necessary flue or chimney. An indirect fired furnace, one in which the air being heated is not contacted directly by the combustion gases generated, is generally used in both forced air systems and hydronic systems. A forced air system consists primarily of a heat exchanger having combustion chambers arranged in relation to the flow of air to be heated such that fuel is introduced at the lower end of the chamber where a combustion flame causes heat to be generated. The heat rises through a series of internal passages before exiting through an upper end of the combustion chamber into the flue or chimney. Simultaneously, circulated space air to be heated passes around the outside of the heat exchangers to absorb heat through conduction and convection. A hydronic system consists primarily of a firebox having a heat exchanger therein. The heat exchanger is in a closed loop for continuously circulating fluid, e.g. water, from the heat exchanger to a remote radiator located in the space to be heated. However, this system is also relatively inefficient and expensive due to the combustion gas temperatures at the outlet of the firebox, the cost of the chimney, and the delay in heating the mass of liquid in the closed loop. In an attempt to increase the efficiency of a heating system condensing furnaces were implemented. In condensing furnaces exhaust combustion gases are used to preheat the space air prior to this air contacting the primary heat exchanger. Thus, in this type of furnace exhaust temperatures are reduced but corrosive condensates are formed as the exhaust combustion gases are reduced below their dew point. Another drawback of known heating systems is the large size of the furnace. The large furnace structure is the result of the need for large heat exchange surface areas to transfer the heat from the combustion products of the ribbon burner to the space air or hydronic fluid. Still another drawback of known heating systems is the use of only air or water as the heating medium. Another problem in space heating installations is the annoying low frequency noise called combustion roar caused by products of combustion within the furnace. This is especially true where the furnace is connected to a duct of the heating system which tends to amplify the noise. Still another problem in previously known heating systems, as a result of the combustion process used in these systems, is that a furnace normally generates gaseous combustion products which include oxides of nitrogen (NO x ) which are vented to the atmosphere as flue gases. It is desirable to limit these oxides of nitrogen emissions since NO x is considered a pollutant, and combustion systems for gas-fired furnaces sold in certain geographical areas must meed strict NO x emission standards. A heating system according to the precharacterizing portion of independent claim 1 is disclosed in GB-A-2 160 967. The known heating system has a single row coil. An upper portion of the coil is juxtaposed a radiant burner while a lower portion of the coil is juxtaposed a plug located centrally at the bottom of the coil. The plug contributes to the turbulent flow and forces the combustion gases to flow around the bottom turns of the coil. In DE-B-2 215 188 there is disclosed a heating module having a multiple coil heat exchange tubing. The object of the invention to provide an improved heating system that is quiet, compact, and efficient and that will, to a large extent, overcomes the problems noted in the prior art. According to the invention, to achieve this, there is provided a heating system for heating a space comprising a closed fluid flow loop means having a heat transfer fluid flowing therethrough; means for circulating said heat transfer fluid through said closed fluid flow loop means; heating module means for transferring heat to said heat transfer fluid flowing through said closed fluid flow loop means, said heating module means including a tank enclosure, an infrared burner disposed within said tank enclosure, said burner connected to a supply for a mixture of vaporous fuel and air, helical heat exchange tubing disposed within said tank enclosure and in heat exchange relationship with said infrared burner, said heat exchange tubing having an inlet and an outlet and being juxtaposed to said infrared burner for absorbing heat from said infrared burner to heat the fluid flowing through said tubing; and a fan coil connected in said closed fluid flow loop means downstream of said heating module means for transferring heat from said fluid to the space to be heated, characterized in that said helical heat exchange tubing comprises multiple coils including an outer coil and an inner coil, said inlet being connected to said outer coil, said outlet being connected to said inner coil and said outer coil being in contact with the tank enclosure, that said tank enclosure includes a condensate level control means to maintain a predetermined condensate level in said tank enclosure, and that said helical heat exchange tubing is juxtaposed generally along the entire length thereof to said infrared burner. Thus, there is provided a liquid-backed gas-fired heating module which will limit NO x emissions to a desired level without reducing the combustion efficiency of the system. The modular heater has a radiant burner which efficiently transfers heat from the hot products of combustion to a circulating fluid in a heat exchanger coil wrapped around the burner. By having the outer inlet coil now in contact with the outer housing or tank enclosure the housing is kept cool since the liquid in the outer coil row is about 21°-38° C (70°-100° F), or lower, Still further, by maintaining a certain condensate level in the module also the bottom of the module is kept cool. Also, the helical coil rows can be arranged in parallel flow for two-phase liquids, e.g. refrigerants, to prevent high superheated vapor, while the coils are arranged in counter flow for single phase liquids, e. g. water, for maximum efficiency. Thus, the fluid can be a variety of liquids, e. g. water, water glycol, refrigerant 22, 12, 500, 113, or 11. Preferably, the helical heat exchanger coil has a prime or smooth surface in the radiant zone because of the better absorption of radiant heat and the good surface emissivity to absorb radiant heat and a coated prime tube in the convective zone to protect the tubing from acidic condensate. For a better understanding of the invention, its operation and specific advantages attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention. In the accompanying drawings forming a part of the specification, and in which reference shown in the drawings designate like or corresponding parts throughout the same, Figure 1 is a schematic diagram of a hydronic heating system embodying the present invention; and Figure 2 is an elevation view, partly broken away, of the liquid-backed gas-fired heating module of the present invention having a two-row coil arranged in parallel flow. Figure 3 is another embodiment of the liquid-backed gas-fired heating module of the present invention having a three-row coil arranged in counter flow. Referring now to Figure 1 there may be seen a schematic view of a residential heating system 10 using a liquid-backed heating module 12 for supplying energy to the system. The heating system 10 includes heating module 12, remote fan coil 14, and a liquid pump 16. The remote fan coil 14 may typically be a plate fin heat exchanger or a radiator having air flowing therethrough in the direction of the arrow. Radiators are generally used in hydronic systems in which the air in the space to be heated is circulated by gravity. In a conventional forced air system air is drawn from the room through cold air ducts and, after passing around the heat exchanger in the furnace, is discharged through hot air ducts. Normally, a fan is incorporated in the furnace to provide forced circulation of the air between the furnace and the rooms or other space to be heated. In the present invention the fan coil 14 is connected to the discharge of the heating module 12 by a pipe section 22. The fluid in the heating system circulates from the fan coil 14 through pipe section 24 under the pressure of the liquid pump 16 and is returned to the heating module 12 through pipe section 26. An expansion tank 28 is connected to pipe section 26 to take up the volume of expanded liquid and to dampen any pressure surges in the heating system. As illustrated, the heating module 12 includes a gas line 30 having a regulator 32 for supplying fuel to the module. Further, air is supplied to the module through line 34. The air/fuel mixture is burned on the infrared burner, described hereinafter, and the flue gases are discharged from the module through flue pipe 36 by induction fan 38. If, however, the pressure drop across the module is too high for an induction fan, a forced draft fan may be used upstream of the module. Ignition device 40 is a conventional furnace control and will not be described in detail herein. In a conventional control, a spark ignition system is used to ignite the air/fuel mixture and a flame sensor is used to sense whether combustion actually occurs. The control system, for example, may be a Honeywell Model S87D Direct Spark Ignition System. As best shown in Figure 2, the heating module 12, comprises a housing 20 with an infrared burner located centrally therein. In this embodiment air is supplied through air line 34 and fuel is supplied through regulator 32 and gas line 30 such that the air and gaseous fuel are 100% premixed, thus, no secondary combustion occurs. A heat exchange means 19 is located in spaced relation to the infrared burner 18 to receive heat from the infrared burner. The heat exchange means in the form of a two-row helical coil has fluid, e.g. refrigerant flowing therethrough which absorbs heat from the infrared burner 18 and transfers this heat to the space to be conditioned by way of the remote fan coil 14. The refrigerant enters the inner coil through inlet 21 and flows down through the inner coil and up through the outer coil 15 and out outlet 23. This parallel flow of a two-phase liquid prevents high superheat of the vapor. Moreover, the heat exchange means 19 is heated by both radiation and convection heat transfer from the infrared burner 18. In Figure 3 the adjacent coils of a three-row helical coil are closely spaced to form a plurality of walls. In this embodiment the fluid enters the outer coil 15 through inlet 21 and exits outlet 23 in the inner coil 17. Thus the combustion gases will flow along the inner wall of the inner coil 17 and then down between the inner and middle coil walls and up between the middle and outer coil, to be discharged by the induction fan 38 to atmosphere. Thus the coil is arranged in counter flow for single phase fluids, e.g. water, for maximum efficiency. The helical coil is juxtaposed generally along the entire length thereof to the infrared burner 18. The outer coil 15 of the present invention is in direct contact with the housing 20 to keep the housing cool, since the fluid in the outer coil is generally 27-38°C (70°-100°F). Further, as shown in Figure 3 condensate 27 collects at the bottom of the module to keep the bottom of the module cool. The level of the condensate 27 is maintained by trap 41. Moreover, the acidic condensate necessitates a protective coating 29, e.g. a layer of polypropylene, to protect the prime or smooth surface of the coils. Prime tube gives better absorption of radiant heat than finned tube and also coats easier.

Claims: Heating system for heating a space comprising a closed fluid flow loop means having a heat transfer fluid flowing therethrough; means (16) for circulating said heat transfer fluid through said closed fluid flow loop means; heating module means (12) for transferring heat to said heat transfer fluid flowing through said closed fluid flow loop means, said heating module means (12) including a tank enclosure (20), an infrared burner (18) disposed within said tank enclosure (20), said burner (18) connected to a supply for a mixture of vaporous fuel and air, helical heat exchange tubing (19) disposed within said tank enclosure (20) and in heat exchange relationship with said infrared burner (18), said heat exchange tubing (19) having an inlet (21) and an outlet (23) and being juxtaposed to said infrared burner (18) for absorbing heat from said infrared burner (18) to heat the fluid flowing through said tubing (19); and a fan coil (14) connected in said closed fluid flow loop means downstream of said heating module means (12) for transferring heat from said fluid to the space to be heated, characterized in that said helical heat exchange tubing (19) comprises multiple coils including an outer coil (15) and an inner coil (17), said inlet (21) being connected to said outer coil (15), said outlet (23) being connected to said inner coil (17) and said outer coil (15) being in contact with the tank enclosure (20), that said tank enclosure (20) includes a condensate level control means (41) to maintain a predetermined condensate level in said tank enclosure (20), and that said helical heat exchange tubing (19) is juxtaposed generally along the entire length thereof to said infrared burner (18). Heating system according to claim 1, characterized in that said helical heat exchange tubing (19) includes a smooth outer surface. Heating system according to claim 1, characterized by an induction fan means (38) for drawing products of combustion through said tank enclosure (20). Heating system according to claim 1, characterized in that said helical heat exchange tubing (19) comprises three coils.

Inventors: FRIEDMAN, JAMES N., RIPKA, CHESTER D.
Applicants: CARRIER CORPORATION
Application year: 1988
Main classification: F24H1
IPC codes: F24H1, F24H8, F24D3