1. Field of the Invention
This invention relates to a controller for regulating the air and fuel flow to a burner. The controller utilizes a combustion interface controller having a microprocessor for maintaining the pressure drops of both the air flow and the fuel flow across the burner at desired, optimal rates at each point along the firing range of the burner.
2. Description of the Prior Art
Control systems for regulating the flow of air and fuel to burners and furnaces are well known in the prior art. One of the best known types of these control systems is known as a pressure balanced, constant ratio system. This system operates by balancing the pressures of the air and fuel flow into the burners throughout the firing range of the burners in such a manner that the ratio of the flow rate of the air to the flow rate of the fuel remains at a constant, stoichiometrically optimal value.
Some of the first pressure-balanced ratio systems employed "jack-shafts" which mechanically coupled the air regulating valve and the fuel regulating valve of the system so that when the air valve was set at a different point along the firing range of the burner, the fuel valve automatically mechanically readjusted itself into a position commensurate with the optimal ratio of air flow to fuel flow. Later, mechanical air-to-fuel ratio regulators were developed which worked in conjunction with motor-operated valves in the air conduit. However, such linked valves and mechanical pressure-balanced controls are accompanied by a number of shortcomings. For example, as the mechanical linkage between the air flow and the fuel flow valves wears ad loosens over time, the ability of the system to accurately maintain an optimal air-to-fuel ratio throughout the firing range of the burner diminishes. Similarly, the wear of the diaphragms in the mechanical regulators ultimately impairs the ability of mechanical air-to-fuel ratios to function optimally. Additionally, such mechanical regulators were often inaccurate across every point in the firing range of the burner, even when new. The inaccuracies caused by such wear invariably lead to burning ratios which are less than optimal, and hence fuel-wasting.
To compensate for the inaccuracies which adversely affect such mechanical, pressure balancing controls over time, electronic mass-flow pressure balance controls were developed. These electronic systems generally incorporate flowmeters in both the air and fuel conduits which consist of a calibrated orifice plate mounted in the flowpath of both the air and fuel flows destined for the burner, and a differential pressure sensor which is pneumatically connected across this calibrated orifice plate. The differential pressure sensor transmits an electrical output indicative of the pressure drop across the plate. This electronic output is in turn connected to a microprocessor, which computes the flow rates by calculating the square root of these pressure differential signals. Next, the microprocessor compares these actual air and fuel flow rates with pre-programmed "ideal" optimal ratio set-point rates which have been previously stored in the memory of the microprocessor. The microprocessor then sends signals to motor-operated flow control valves located in both the air and fuel conduits in order to correct any error which it perceives between the actual and set-point air and fuel flows. Some prior art electronic mass-flow pressure balance controls are capable of shifting to a non-stoichiometric "excess air" mode at lower firing rates. Such non-stoichiometric firing rates have been found to increase the heat-producing efficiency of the burner (despite the fact that the resulting air and fuel ratio is not stoichiometrically optimal) because the mixture of excess air and fuel flowing to the burner generates convection currents in the furnace which more effectively and uniformly transfer the heat generated by the burner to the output vent of the furnace.
Despite the superior accuracy that such electronic mass-flow systems have over mechanical-type pressure-balancing systems, certain problems remain. For example, in order for the flowmeters used in such systems to accurately monitor the air and fuel flows destined for the burner, both the inlet and outlet of the orifice plate mounted across the air and fuel conduits must be adjoined to a straight section of conduit at least ten conduit-diameters in length. If such straight lengths of conduit do not adjoin both the inlet and outlet portions of the orifice plate, the flow of the air or fuel through the orifice plate may not have a symmetrical profile across the diameter of the conduit, which in turn will greatly reduce the ability of the flowmeter to relay an accurate flow rate. The requirement that each of the air and fuel sections include a straight section of conduit at least twenty conduit-diameters in length often poses problems when one attempts to retrofit an electronic mass-flow control system onto an older burner. Straight sections having a twenty-diameter length or more may be exist in these older systems, or if they do, such sections may be inaccessible. Hence, the installation of such mass-flow control systems in older burner systems often necessitates the installation of straight sections of conduit in order that the flowmeters necessary for the operation of these systems may function properly. Additionally, the orifice plates of these flowmeters create considerable flow resistances in the air conduit which often necessitates the installation of a new and more powerful air blower which is capable of generating the air flow required at "high fire". Finally, while the accuracy of such electronic mass-flow control systems is generally better than mechanical-type pressure ratio systems, certain inaccuracies are still present even in the best of such systems. Such inaccuracies arise from the fact that the computation of the flow rate is based upon a pressure drop in the air and fuel conduits which is usually considerably upstream of the burner, rather than across the burner itself. Any measured pressure drop upstream of the burner is going to be considerably smaller than the pressure drop across the burner itself. The smaller the pressure drop used to operate the flowmeter, the more difficult it is for the differential pressure sensor to accurately relay differential pressure at the low end of the firing range, which in turn limits the turn-down range of the control system.
Clearly, there is a need for an electronic control system which may be easily retrofitted onto an existing burner system without the necessity of installing straight lengths of conduit in the air or fuel pressure lines, and without replacing the existing blower. Ideally, such a system would be capable of measuring the flow rate of both the air and fuel by accurately measuring the differential pressure drop of the air and fuel across the burner itself, rather than at a point considerably upstream of the burner, in order to extend the potential turn-down range of the system and to reduce the opportunity for inaccurate flow rate measurements to occur. Finally, it would be desirable if such a system was simple and inexpensive in construction, and capable of operating in a hybrid optimum mode consisting of a "splicing together" of various types of optimum modes over the entire firing range of the system.