High-thermal-mass hydronic furnace
Various embodiments of the present invention are directed to a high-thermal-mass hydronic furnace. In one embodiment of the present invention, a high-thermal-mass hydronic furnace includes a firebox, an insulated outer casing, a fluid-transport system, and a draft air-flow system. Fuel input to the firebox is combusted on a high-thermal-mass ceramic refractory. Heat from the fuel combustion is transferred to fluid within the fluid-transport system, via an internal heat exchanger, and circulated to a location external to the insulated outer casing for subsequent distribution to an interconnected heat-delivery system. The draft air-flow system regulates the fuel combustion by controlling the amount of air passing through the firebox.
This application claims the benefit of Provisional Application No. 60/810,609, filed Jun. 2, 2006.
TECHNICAL FIELDThe present invention relates to the field of hydronic furnaces, and, in particular, to a high-thermal-mass hydronic furnace.
BACKGROUND OF THE INVENTIONFor many years, wood-fired furnaces have been used as a relatively simple and inexpensive heat source for residential, commercial, and industrial buildings (“buildings”) of various sizes. Wood-fired furnaces may either be used as a sole heat source, or as a supplemental heat source for other sources of heat, such as oil, natural gas, or electricity. One common type of wood-fired furnace is a hydronic furnace. Hydronic furnaces use a fuel to heat a heat-transfer fluid (“fluid”) that is distributed throughout an area to be heated. For example, water may be heated and distributed to selected radiators located throughout a house.
One type of hydronic furnace is an outdoor wood-fired boiler (“OWB”). An OWB is often a self-standing structure placed within several hundred feet of one or more buildings to be heated. Typically, an OWB is interconnected to each of the buildings to be heated by a number of insulated pipes.
An OWB may be an attractive heating system for some people. In areas where wood is plentiful, an OWB may be a less expensive heating system than heating systems using oil, natural gas, or electricity. Additionally, an OWB may be manufactured with variable-sized combustion chambers in order to accommodate the heating needs of various numbers and sizes of buildings, and to regulate how often fuel needs to be added to a combustion chamber. However, an OWB may also have several drawbacks. Combustion chambers are typically fabricated from steel. A surrounding water tank prevents temperatures in a combustion chamber from reaching the temperatures necessary to completely combust input wood. Consequently, particulates, such as smoke and creosote (“emissions”), are produced during the combustion process and are copiously output from an OWB. Emissions from an OWB sometimes exceed allowable limits in some municipalities and may cause unhealthy, toxic air conditions, as well as unsafe visibility levels. Consequently, a growing number of municipalities have banned the use of OWBs at current emission levels.
In response to developing emission restrictions in certain municipalities, some manufacturers of wood-fired furnaces have introduced systems that use catalytic technologies in an effort to reduce emissions. In a catalyst-equipped wood-fired furnace, exhaust is passed through a ceramic honeycomb element coated with platinum or palladium. Although catalyst-equipped wood-fired furnaces may reduce emissions to levels that are deemed acceptable in many municipalities, a resulting loss in thermal efficiency often results. A loss in thermal efficiency often results in an increase in usage cost. Wood-fired-hydronic-furnace manufacturers, distributors, sellers, as well as people desiring to heat one or more buildings have, therefore, recognized a need for an efficient hydronic furnace that is inexpensive to operate and creates emissions at or below governmentally-acceptable levels.
SUMMARY OF THE INVENTIONVarious embodiments of the present invention are directed to a high-thermal-mass hydronic furnace. In one embodiment of the present invention, a high-thermal-mass hydronic furnace includes a firebox, an insulated outer casing, a fluid-transport system, and a draft air-flow system. Fuel input to the firebox is combusted on a high-thermal-mass ceramic refractory. Heat from the fuel combustion is transferred to fluid within the fluid-transport system, via an internal heat exchanger, and circulated to a location external to the insulated outer casing for subsequent distribution to an interconnected heat-delivery system. The draft air-flow system regulates the fuel combustion by controlling the amount of air passing through the firebox.
Various embodiments of the present invention are directed to a high-thermal-mass hydronic furnace. In one embodiment of the present invention, the high-thermal-mass hydronic furnace (“furnace”) cleanly combusts wood input to a crucible-shaped, high-thermal-mass ceramic refractory in a firebox. Heat produced during the combustion is efficiently transferred to circulating fluid within a fluid-transport system. The heat contained in the fluid may then be distributed to a heat-delivery system.
The external portion 213 of the fluid-transport system 214 includes a first aquastat 308 and a second aquastat 310. The first aquastat 308 controls the furnace 200 by controlling the air flow into the firebox 208, via the draft air-flow system 220. The first aquastat 308 monitors the temperature of fluid contained within the fluid-transport system 214 and controls the damper (226 in
The air intake system 222 includes a number of air-input apertures, such as air-input aperture 402, the damper 226, and the damper actuator 224. The air-input apertures extend through the rear face 212 of the outer casing 202, through the refractory (not shown in
A firebox is where a fuel source, such as wood, is combusted to create heat. When wood ignites, the temperature may accelerate to a temperature of approximately 500° F., at which point the wood begins to breakdown chemically and emit gases. The emitted gases may combust causing the temperature to accelerate to approximately 110° F., at which point the solid wood begins to combust. When the combustion is able to continue at a temperature at or above 110° F., a complete combustion may occur. In a complete combustion, the combustion may continue until all of the solid wood and the emitted gases are consumed. When the combustion is unable to continue at a temperature at or above 1100° F., an incomplete combustion may occur. In an incomplete combustion, unconsumed solid wood and emitted gases may be vented by a furnace as one or more types of particulates, such as smoke and creosote. In an OWB, the surrounding water tank prevents the combustion chamber from sustaining a temperature high enough for complete combustion to occur. Thus, a relatively large amount of emissions may be output from an OWB. Conversely, in one embodiment of the present invention, a firebox for a furnace contains a ceramic refractory with a mass of at least 140 pounds per cubic foot of firebox, or a “high-thermal-mass ceramic refractory,” that is able to withstand sustained temperatures in a firebox at or above temperatures obtained during the combustion of wood and accompanying gases so that a complete combustion may be obtained.
In addition to enabling the attainment of sustained high temperatures, a ceramic refractory may also be used to store heat when a furnace is not in use. Heat stored in a high-thermal-mass ceramic refractory may support automatic re-firing of fuel input to a firebox for several days after the previous combustion. Storing heat in a high-thermal-mass ceramic refractory may also be safer than storing heat in a water tank, such as with an OWB, because a high-thermal-mass ceramic refractory does not store heated water in a contained space. Large quantities of heated water may create a pressure build-up that may potentially lead to an explosion when a relief valve fails.
Combustion efficiency is a measure of how well a furnace converts input fuel to useful energy. A high-thermal-mass ceramic refractory promotes an increase in combustion efficiency by enabling the attainment of temperatures high enough to support the complete combustion of input fuel, such as wood. As discussed above, a high-thermal-mass ceramic refractory needs to include at least 140 pounds of ceramic refractory per cubic foot of firebox. Thus, more than one thousand pounds of ceramic refractory are needed for an 8.5 cubic foot firebox. In one embodiment of the present invention an 8.5 cubic foot firebox contains a high-thermal-mass ceramic refractory that is four to six inches thick and weighs 1300 pounds. When a high-thermal-mass ceramic refractory of 1300 pounds is used in an 8.5 cubic foot firebox, tests have shown the attainment of a combustion efficiency of approximately 96%.
Thermal efficiency is a measure of the rate at which heat exchange surfaces transfer heat to a transfer medium, such as from air to water. Thermal efficiency is typically measured as a ratio of British Thermal Unit (“BTU”) output of hot water to BTU input of fuel. The use of high-thermal-mass ceramic refractory may increase thermal efficiency by enabling prolonged high temperatures in a firebox. Additionally, the shape used for the high-thermal-mass ceramic refractory may affect thermal efficiency by channeling air flow, as discussed below, with reference to
Thermal efficiency may also be affected by minimizing the amount of heat loss. Accordingly, insulation may be used to minimize the amount of heated air passing through an outer casing for a furnace. In one embodiment of the present invention, three different types of insulation are used in various locations around the furnace to minimize heat loss. Ceramic fiber blankets may be used to line an outer casing and be positioned in locations exposed to direct flames. Additionally, several different types of mineral wool may be used to line a firebox to form an insulated layer between a high-thermal-mass ceramic refractory and an outer casing. Insulation may be attached to surfaces using heat-resistant metals, such as welding insulation to an outer casing using iron washers. In one embodiment of the present invention, when a crucible-shaped, high-thermal-mass ceramic refractory is used and a firebox and outer casing are insulated, tests have shown the attainment of a thermal efficiency of approximately 87%. In addition to increasing thermal efficiency by including insulation, safety is increased as well because the outer surfaces of a furnace are maintained at a temperature that typically does not burn an individual touching the outer surface of a furnace.
The external portion 213 of the fluid-transport system 214 includes a temperature gauge 608 for monitoring the temperature of fluid in the fluid-transport system 214, the first aquastat 308 and the second aquastat 310 for controlling operation of the furnace (200 in
During operation, the fluid-transport system 214 needs to be able to continuously circulate fluids while withstanding temperatures of approximately 2000° F. In one embodiment of the present invention, a fluid-transport system is fabricated from American Society for Testing and Materials Grade A36 mild steel plate and Schedule 40 steel pipe, using a combination of gas metal arc welding, gas tungsten arc welding, laser or waterjet cutting, and precision pipe bending.
The interconnected heat delivery system may be a heat-delivery system exclusively for the furnace 200, or the heat-delivery system may be part of an existing heating system to which the furnace is interconnected as one of multiple possible heat sources. In an alternate embodiment of the present invention, a fluid-transport system does not include a flat panel heat exchanger. Instead, a heat-delivery system is a direct part of the fluid-transport system. When a heat-delivery system is included as part of a fluid-transport system, a larger circulating pump may be necessary to accommodate the additional distances traveled by fluids within the fluid-transport system.
The expansion tank 216 is interconnected to the fluid-transport system 214 in proximity to the circulating pump 616. The expansion tank 216 accommodates thermal expansion of fluid within the fluid-transport system 214 and supplies additional fluid to the fluid-transport system 214 when the fluid level falls below a predetermined level. In one embodiment of the present invention, the expansion tank 216 is open to the atmosphere. Thus, the fluid-transport system 214 may avoid becoming pressurized when, for example, an excessive amount of fuel is combusted, the circulating pump fails, the damper actuator fails, and/or the amount of fluid in the fluid-transport system 214 falls below a level needed to circulate fluid.
Selective safety measures may be used in connection with operation of a furnace. In one embodiment of the present invention, a fail-safe damper actuator is used to control damper movement. When an interruption to the furnace power supply occurs, the damper is placed in a closed position so that airflow into the firebox is vastly reduced; thereby causing any current combustion to cease when denied an air supply. In another embodiment of the present invention, a low-water cutoff switch is installed that causes the damper actuator to place the damper in a closed position when the level of fluid in the fluid-transport system falls below a predetermined level. In yet another embodiment of the present invention, the probes for the aquastats are positioned in an immersion well that is mounted below the prime level of the fluid in the fluid-transport system so that, in the event of a low fluid level, the aquastat probes remain immersed in fluid.
A furnace may be placed indoors or outdoors. Thus, a furnace may be placed in many possible locations, including a room in a building to be heated, a nearby shed, a garage, a basement, or other location. A flue is often interconnected to an exhaust vent for creating a draft to maximize thermal and combustion efficiency and for relocating emission dissipation to a location away from high-use areas. Accordingly, a furnace may be positioned in or around an existing chimney. However, a flue may be built specifically for a furnace at other desired locations.
Additional modifications within the spirit of the invention will be apparent to those skilled in the art. For example, the size and shape of many individual parts may be altered. The size of the firebox may be adjusted to accommodate various amounts and sizes of input wood. The lengths and diameters of various parts of the draft air-flow system may be adjusted to accommodate the creation and maintenance of a draft for various sizes of fireboxes and for the combustion of various types of fuel sources. The lengths and diameters of the fluid-transport system may be adjusted to accommodate various sizes of fireboxes and the combustion of various types of fuel sources. Additionally, the flow rate used within a fluid-transport system may be adjusted to improve thermal efficiency and accommodate various heating needs. Various types and amounts of insulation may be used to improve combustion efficiency and improve safety. In various embodiments of the present invention, wood has been used as an example of a combustible fuel source. However, other fuel sources may be used as well, such as various types of biomass, including switchgrass, hemp, corn, poplar, willow, sugarcane, and other types of biomass.
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The foregoing descriptions of specific embodiments of the present invention are presented for purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents:
Claims
1. A high-thermal-mass hydronic furnace comprising:
- an insulated outer casing;
- a firebox within the insulating outer casing for combusting input fuel, the firebox including a ceramic refractory;
- a draft air-flow system connecting the firebox to the environment external to the outer casing, the draft air-flow system including an air intake system for bringing air into the firebox and an exhaust vent for removing air from the firebox; and
- a fluid-transport system interconnecting the firebox to the environment external to the outer casing, the fluid-transport system containing a circulatable heat-transfer fluid.
2. The high-thermal-mass hydronic furnace of claim 1 wherein the ceramic refractory is crucible-shaped.
3. The high-thermal-mass hydronic furnace of claim 1 wherein the firebox includes at least one hundred forty pounds of ceramic refractory per cubic foot of firebox.
4. The high-thermal-mass hydronic furnace of claim 1 wherein the insulation in the outer casing includes one or more of mineral wool; and
- a ceramic fiber blanket.
5. The high-thermal-mass hydronic furnace of claim 1 wherein the air intake system includes
- a number of air-input apertures in the high-thermal-mass ceramic refractory and outer casing;
- a movable damper positioned over top of the air-input apertures on the outer casing; and
- a damper actuator for moving the movable damper.
6. The high-thermal-mass hydronic furnace of claim 5 wherein the fluid-transport system includes a first aquastat for monitoring the temperature of the circulatable heat-transfer fluid and transmitting signals to the damper actuator.
7. The high-thermal-mass hydronic furnace of claim 5 wherein the movable damper may be placed in one of
- an open position wherein the air-input apertures are unobstructed; and
- a closed position wherein the air-input apertures are obstructed.
8. The high-thermal-mass hydronic furnace of claim 7 wherein the draft air-flow system utilizes a draft to draw air into the firebox when the damper is in an open position and to vent air from the firebox out the exhaust vent.
9. The high-thermal-mass hydronic furnace of claim 1 wherein the exhaust vent is interconnected to a flue.
10. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes an internal heat exchanger positioned above the firebox.
11. The high-thermal-mass hydronic furnace of claim 10 wherein the circulatable heat-transfer fluid in the internal heat exchanger becomes heated in response to the heat produced by fuel combusting in the firebox.
12. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes a circulating pump for circulating the circulatable heat-transfer fluid in a continuous loop through the fluid-transport system.
13. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes a flat panel heat exchanger for transferring heat from the circulatable heat-transfer fluid to an interconnected heat-delivery system.
14. The high-thermal-mass hydronic furnace of claim 1 wherein the fluid-transport system includes a second aquastat interfaced with a second heating source for coordinating usage between the high-thermal-mass hydronic furnace and the second heating source.
15. The high-thermal-mass hydronic furnace of claim 1 wherein the exhaust vent is positioned in proximity to the air-input apertures for pre-heating air input to the firebox.
16. The high-thermal-mass hydronic furnace of claim 1 wherein the exhaust vent is positioned in proximity to the fluid transfer system after the circulatable heat-transfer fluid has passed the flat panel heat exchanger, but before the circulatable heat-transfer fluid has reached the internal heat exchanger for pre-heating the circulatable heat-transfer fluid input to the internal heat exchanger.
17. The high-thermal-mass hydronic furnace of claim 1 wherein the combusted fuel is one of
- wood; and
- biomass.
18. A method for heating a room, the method comprising:
- providing a high-thermal-mass hydronic furnace, the high-thermal-mass hydronic furnace including a firebox within an insulated outer casing, the firebox having a ceramic refractory, and a fluid-transport system interconnecting the firebox to the environment external to the outer casing, the fluid-transport system containing a circulatable heat-transfer fluid;
- igniting fuel input to the firebox;
- transferring the heat from the combusting fuel in the firebox to the circulatable heat-transfer fluid;
- transferring the heat from the circulatable heat-transfer fluid to an interconnected heat-delivery system.
19. The method of claim 18 wherein the firebox includes at least one hundred forty pounds of ceramic refractory per cubic foot of firebox, the ceramic refractory configured into a crucible shape.
20. The method of claim 18 wherein the high-thermal-mass hydronic furnace further includes a draft air-flow system connecting the firebox to the environment external to the insulated outer casing.
Type: Application
Filed: Jun 1, 2007
Publication Date: May 8, 2008
Inventor: Richard J. Richings (Shoreline, WA)
Application Number: 11/809,781
International Classification: F24H 1/40 (20060101); F24D 3/00 (20060101);