Patent Application
20090044794
Efficient combustion furnaces for heating, ventilating and air conditioning (HVAC) equipment applications are typically provided with a motor driven so-called ventilating or inducer blower which draws air through the combustion passageways of the furnace heat exchanger to improve the efficiency of the combustion and heat transfer processes and to prolong the life of the furnace. Selection of the proper speeds of the inducer blower drive motor for multistage combustion furnaces, in particular, has been a somewhat nettlesome problem. Combustion furnaces which include electronic controls have been developed wherein the inducer blower motor speed is controlled based on opening and closing of pressure switches which measure the pressures developed by the inducer blower at one or more particular points in the air flowpath.
Moreover, so-called learning algorithms have been developed which require setting a blower default speed for multistage furnaces for the low firing rate and high firing rate which is the first speed that the inducer motor will be controlled to when a call for the low firing rate or high firing rate is signaled to the furnace controller. The inducer blower then “learns” a speed based on opening and closing of the pressure switches. Still further, typically, a low speed limit is defined in the control system program to avoid the combustion gas control valve closing prematurely. U.S. Pat. Nos. 6,257,870 and 6,377,426 to Hugghins, et al. and assigned to the assignee of the present invention disclose and claim methods for setting inducer blower operating speeds.
However, for multistage furnaces including, for example, three stage furnaces, it is desirable to maintain the inducer flow pressures above a predetermined setpoint which is particularly important at low firing rates to avoid low combustion gas pressures which could create undesirable combustion characteristics. This occurs because the gas valve output pressure tracks the inducer or system pressures in the aforementioned type of furnace. Still further, it is desirable to simplify the “learning” of the inducer blower motor speeds for respective furnace firing rates in multistage furnaces, including three stage furnaces. In accordance with the present invention, improvements in determining and setting inducer blower speeds and operating pressures have been realized and attendant advantages enjoyed as a result.
The present invention provides an improved method of determining proper inducer or ventilating blower speeds for multistage combustion furnaces for HVAC applications
In accordance with one aspect of the present invention, a method of controlling the inducer or ventilating blower speed and the pressures generated thereby has been developed wherein a control system for the furnace is programmed to provide, initially, a default speed for a medium or intermediate furnace firing rate, for example. A learn routine is provided for the medium firing rate and a multiple of the learned medium firing rate inducer blower speed value is applied to set a blower motor speed for a low firing rate and a high firing rate based on the speed at the medium or intermediate firing rate. These multiples may be based on the realization that there is a substantially linear relationship between properly set inducer blower speed and the flow resistance caused by the venting system connected to the furnace. Thus, the lower inducer blower speed limits and upper speed limits can be defined by this relationship and set as multiples of the learned medium or intermediate firing rate inducer blower speed. Inducer blower speeds determined in this manner allow for proper furnace operation with virtually any type of venting system normally expected to be connected to the furnace.
In accordance with another aspect of the present invention there is provided a method of controlling a combustion furnace, including a multistage furnace and, particularly, a three-stage furnace wherein the furnace control system learns a medium speed for the inducer blower motor to which is applied a multiple or multiplier (less than one and greater than one) to provide inducer blower speeds for a furnace low firing rate and a high firing rate. The multiplier may be different for different models of furnace, but may be provided to the control system for a particular furnace when it leaves the point of manufacture or at a later time. Accordingly, a learning procedure for the inducer blower motor speed at low firing rates and high firing rates is not required and the so-called target inducer blower speeds for low and high firing rates are actually relatively closer to a learned speed than arbitrarily selected default speeds.
In accordance with a further aspect of the present invention a method of determining inducer blower speeds is provided wherein one or more inducer blower speeds for particular firing rates of a furnace may be preset and based on respective multiples or multipliers of another selected motor speed and which multiples may be developed through testing the flow resistance of various lengths and configurations of furnace venting systems likely to be applied to respective different furnace models. The multipliers would possibly be different for different furnace models and could be provided to a particular furnace control system as part of a set of control and operating parameters programmed in the control system directly or on a separate information or “personality” module associated with the furnace prior to or after shipment from the point of manufacture.
Still further, the invention contemplates the provision of a furnace control system and method of operation wherein only a single pressure switch or pressure sensor would be required to properly operate the furnace at different inducer blower speeds.
Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention, together with other important aspects thereof, upon reading the detailed description which follows in conjunction with the drawings.
In the description which follows like elements are marked through the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale and certain elements are shown in schematic or somewhat generalized form in the interest of clarity and conciseness.
Referring to
Air circulated to and from the space 13 is propelled through cabinet 12 by a motor driven air circulation blower 15 disposed within the cabinet, as illustrated in
Control system 26 may include an interface 28 for use by a user or service technician for setting certain control parameters and observing certain operating conditions of furnace 10. Control system 26 includes, for example, a microcontroller 30 for receiving signals from the thermostat 24 and for controlling operation of the blowers 15 and 22 and the fuel flow control valve 18. Suitable pressure sensors or switches, three shown by way of example, are designated by numerals 32a, 32b and 32c in
Combustion products discharged from the furnace 10 are conducted through a vent system or flue pipe 25 suitably connected to the blower 22 and normally having a length sufficient to conduct combustion gases to the exterior of the structure or building in which the furnace 10 is located. Accordingly, the so-called venting system, including the flue pipe 25, may be of various lengths and configurations and may include one or more pipe elements which are curved. Thus, a certain resistance to flow of combustion products would be associated with the configuration of a particular venting system. Accordingly, each furnace design or configuration, including its venting system, would have a set of inducer blower motor speeds corresponding to furnace and venting system flow characteristics and required to produce desired pressures and flow rates through the heat exchanger passages 14a and plenum 14b. Of course, the pressures that the inducer blower 22 is capable of producing in the combustion gas flowpath, including the burner or heat exchanger passages 14a and the plenum 14b, will vary with the speed of the blower and its drive motor 23.
Referring now to
For a particular furnace design and capacity for a three-stage furnace, for example, blower motor speeds for driving blower 22 sufficient to provide required pressures generated by the blower may be predetermined. Moreover, for various configurations of the furnace combustion products venting system, including the vent conduit or pipe 25, for a particular furnace design, the blower speeds for motor 23 may also be determined and which are sufficient to generate the required pressures. With respect to determining pressures, such pressures are normally measured as negative (below atmospheric pressure) in inches of water column. Hence, a high pressure is actually a greater amount of vacuum being pulled by the blower 22 within the plenum 14c or otherwise within the flowpath of ventilation air and combustion gases proceeding through the heat exchangers 14. Since it has been determined there is a linear relationship between the required inducer blower speed for a predetermined amount of pressure generated by the blower 22 and the length or configuration of the vent system, including the vent conduit or pipe 25, a low furnace firing rate speed required of the blower 22 for generating a low firing rate pressure may be determined and the low firing rate speed is related to the medium firing rate speed required of the blower 22 for providing the required pressures at the medium firing rate. Moreover, if a learned medium firing rate inducer blower speed is obtained, then a relationship between the medium firing rate blower speed and the low firing rate blower speed may also be calculated, since it is a multiple of the learned medium firing rate blower speed. Accordingly, by basing the low firing rate inducer blower speed on a learned medium firing rate inducer blower speed which has been learned for a particular furnace installation, a multiplier may be applied to the learned medium speed value to determine the low firing rate speed of the blower 22. Still further, since the position of the control valve 18 and the fuel gas pressure in manifold 17 is correlated with the pressure produced by the blower 22 within the furnace 10, unreasonably low manifold pressures which could create undesirable combustion characteristics are avoided.
In addition to establishing a low firing rate speed of blower 22 and pressures within the heat exchangers 14 produced thereby, the linear relationship between inducer blower speed and the configuration of the vent system, such as the conduit 25, provides for determining the speed of blower motor 23 to produce suitable pressures in the furnace 10 commensurate with a high furnace firing rate and based on the learned medium firing rate blower speed. Moreover, the relationship between the required pressures generated by the blower 22 for a particular firing rate, such as a medium firing rate, and the blower motor speed required to obtain such pressures, may be used to set the inducer blower speeds and attendant pressures for a continuously variable firing rate, based on a table of blower speeds versus vent system effective length for the vent system or conduit 25. This data can be furnished from the module 35 and input to the processor 30 for a particular furnace 10, as previously mentioned.
One preferred method of setting the respective speeds for the inducer blower 22 is indicated in
If, at step 50, the medium firing rate pressure switch is not closed, steps 48 and 50 are repeated until the switch is closed. If the medium firing rate pressure switch is closed at step 46, the process proceeds to step 52 and the speed of blower motor 23 is decremented a minimum predetermined amount and the status of the medium firing rate pressure switch is checked again at step 54. Steps 52 and 54 are repeated until the medium firing rate pressure switch opens. Accordingly, within a relatively narrow range of pressure conditions for the furnace medium or intermediate firing rate, a suitable speed for blower 22 is established and monitored by the processor 30 via the motor control circuit 23a. Once the medium firing rate blower motor speed for blower 22 is established the control system 26 will query the database stored in memories 30a and/or 30b to set the low firing rate blower speed for blower motor 23 at step 56 and then the process may proceed to set the high firing rate speed for blower motor 23 at step 58. The furnace 10 then will continue to run at step 60 while blower motor speed for blower 22 is monitored together with monitoring of the pressure switches 32a, 32b and 32c.
Alternatively, the control system 26 may utilize a pressure sensor in place of plural pressure switches, which sensor continuously monitors pressures in a selected location or locations of the ventilating air and combustion gas flowpath through heat exchangers 14. The pressure settings at which action is taken may be carried out by the control system 26 by monitoring the pressure signal input from such a sensor to the microprocessor 30. For example, the medium firing rate speed of blower 22 could be set based on a limited range of suitable pressures for the medium firing rate. Blower speeds could be incremented or decremented from the aforementioned medium firing rate default speed until the pressure sensed by such a pressure sensor was within the predetermined range.
Still further, the present invention contemplates that a single pressure switch may be used to set the medium firing rate blower motor speed for blower 22 followed by the steps indicated in
Those skilled in the art will recognize that an improved process and system for operating a multistage combustion furnace of the so-called inducer or ventilating type is provided by the present invention. Conventional engineering materials, components and procedures may be carried out to practice the invention. Although a preferred embodiment has been described in detail herein, those skilled in the art will also recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.
