Stoves and furnaces – Hot-air furnaces – Liquid or gaseous fuel
Reexamination Certificate
2000-07-31
2001-11-27
Clarke, Sara (Department: 3743)
Stoves and furnaces
Hot-air furnaces
Liquid or gaseous fuel
C236S011000, C236S01500E, C137S613000
Reexamination Certificate
active
06321744
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to induced-draft, gas-fired furnaces, and is directed more particularly to an improved induced-draft, gas-fired furnace which is adapted to operate at any of three different firing rates.
An important consideration in the design of multi-stage furnaces, i.e., furnaces which have more than one firing rate, is the quantity of air per unit time which they circulate through the space to be heated at each of those different firing rates. This quantity of air, commonly referred to as the circulating airflow of the furnace, is preferably as low as the heating requirements of the space to be heated permits. This is because lower speeds are associated with quieter operation. Circulating airflows which are relatively lower, but which continue for relatively longer periods of time, are also desirable because they provide greater thermal comfort.
Quietness of operation and thermal comfort, however, are only two of many things that must be considered in designing multi-stage furnaces. Other considerations include the annual fuel utilization efficiency or AFUE of the furnace, the furnace operating cost, and flue gas emission requirements. Still another consideration is the requirement that the heat exchanger of the furnace be protected from “cold spot corrosion” either by maintaining it at a temperature too high for water to condense thereon, or by making it from a corrosion resistant material. Because these requirements conflict with one another, tradeoffs must be made.
Examples of furnace designs which make various kinds of tradeoffs are described in U.S. Pat. No. 4,708,636 (Johnson), U.S. Pat. No. 5,248,083 (Adams et al), U.S. Pat. No. 5,307,990 (Adams et al), and U.S. Pat. No. 5,590,642 (Borgeson et al). The Johnson patent describes a furnace which includes a modulating gas valve and furnace controls that are responsive to a flow sensor that is mounted in the stack. This patent is concerned primarily with maintaining the proper fuel-to-air ratio and a predetermined minimum airflow in the stack, however, and does not take into account the importance of multiple stages and their effect on thermal comfort and noise levels.
The Adams et al 083 patent describes a furnace in which a modulating gas valve is used to control the firing rate of the burner in accordance with the temperature of the heat exchanger, and in which the blower speed is changed as necessary to match the delivery of heat to the heat load on the furnace. The furnace described in this patent, however, uses an adaptive control algorithm which tries to maintain constant on/off times by regulating the blower speed based on previous on/off cycle data for prior heating cycles and is not coordinated with the speed of the induced draft blower. As a result, the efficiency, operating cost, and noise level of the furnace are not optimized.
The Adams et al 990 patent also describes a furnace in which a modulating gas valve is used to control the firing rate of the burner in accordance with the temperature of the heat exchanger. In this furnace, however, the blower speed is continuously adjusted as necessary to maintain a constant differential pressure across the heat exchanger, but is still not coordinated with the speed of the induced draft blower. As in the case of the Adams et al 083 patent, the efficiency, operating cost, and noise level of the furnace again are not optimized.
The Borgeson et al patent describes a furnace in which the state of a modulating gas valve and a variable speed induced draft blower are both varied as necessary to maintain the temperature of a heat exchange medium at a constant value while the blower speed is varied to maintain a constant circulating air temperature. In this case temperature sensors are used and these tend to react in a very unpredictable manner from one installation to the next because they are location and airflow sensitive and do not account for all possible variations.
In an earlier filed U.S. patent application Ser. No. 09/407,052, filed Sep. 27, 1999, now U.S. Pat. No. 6,161,535, which is commonly assigned herewith, and which is hereby expressly incorporated by reference herein, there is described a method and apparatus for increasing the low stage circulating airflow of a furnace without creating conditions that give rise to cold spot corrosion. Generally speaking, this method involves the maintaining of a predetermined relationship between the low stage circulating airflow of the furnace and the low stage combustion airflow thereof, i.e., the rate at which the induced draft blower supplies combustion air to the burner. By maintaining a predetermined relationship between the magnitudes of the combustion and circulating airflows, the temperature at the outlet of the heat exchanger is kept substantially constant at a value high enough to prevent water from condensing thereon and causing cold spot corrosion.
SUMMARY OF THE INVENTION
With the present invention, the use of the airflow relationships described in our earlier application is expanded and applied to a furnace which was originally designed as a two-stage furnace, but which been modified to operate at a new, third and lower firing rate. As modified, the new furnace not only operates at reduced noise levels and provides increased thermal comfort, it also has a reduced electrical operating cost. Advantageously, these improved results are achieved without giving rise to unacceptable excess air levels, and without allowing cold spot corrosion to occur in the heat exchanger.
In preferred embodiments of the invention, these results are achieved by maintaining, between the magnitudes of the combustion (or circulating) airflows that are established at the low and medium firing rates of the furnace, and between the magnitudes of the combustion and circulating airflows that are established at the low and medium firing rates of the furnace, relationships which assure the maintenance of acceptable excess air levels, and the maintenance of either an undiminished total operating cost or a reduced electrical operating cost. Depending on the embodiment, cold spot corrosion is prevented by either maintaining a heat exchanger wall temperature that is high enough to prevent water from condensing thereon, or by fabricating the heat exchanger from a corrosion resistant material, such as stainless steel.
The furnace of the invention includes a gas flow control assembly which, in all preferred embodiments, includes a main gas valve that has a closed state and two open states and a throttling valve that has two open states. These valves are connected in series between the gas supply and the burner, thereby assuring that the overall rate at which gas is supplied to the burner is dependent on the states of both valves. Because these valves are separate but connected in series, they may be actuated in predetermined combinations of states, with different combinations of states corresponding to different respective ones of the three desired firing rates of the furnace, or to a medium stage ignition state that initially allows the establishment of a gas flow rate high enough to permit ignition to occur, and then reduces the gas flow rate for steady state operation at low stage. The use of a relatively low gas flow rate during steady state low stage operation, in combination with other features of the present invention, allows the furnace to have a low operating cost, while providing reduced noise levels and increased thermal comfort.
The furnace of the invention may also, however, use a gas flow control assembly which includes either a single modulating or three stage gas valve, i.e., a valve specifically designed to provide three different firing rates, or three independent valves connected in parallel. Because furnaces which use gas flow control assemblies of the latter types can be made to operate in generally the same way as the gas flow assembly of the preferred embodiment, they will be understood to be within the contemplation of the present invention. Because such gas flow control assemblies a
Dempsey Daniel Joseph
Thompson Kevin Dale
Carrier Corporation
Clarke Sara
Wall Marjama & Bilinski
LandOfFree
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