Patent Application
20060105279
1. Field of the Invention
The present invention relates to gas burner control and more particularly to feedback control for modulating gas burners.
2. Description of the Prior Art
Gas burners employ a source of gas which passes through a regulator to control the flow emitted through an orifice. A source of air is mixed with the gas and the gas/air mixture is transmitted to a burner where an igniter causes combustion. The resulting flames are thrown past a flame sensor into a heat exchanger that transfers heat to a supply of air directed to the space to be heated. The flow of burning gas/air mixture in the heat exchanger is controlled by a combustion fan at one end. The gas/air flow is proportional to the RPM of the fan which is typically supervised by an air pressure switch. Changes in fan speed cause changes in the amount of heat exchanged and the heat that is directed to the space to be heated may be controlled. However, as the speed of the fan is changed, the ratio of gas to air in the gas/air mixture must also be changed to maintain good combustion and keep efficiency within an acceptable range
It is known that the ratio of gas to air in the gas/air mixture needs to be within certain limits in order to provide good combustion and efficiency. The gas flow may be controlled by an electric modulating gas valve with a gas pressure regulator. Modulating gas burners have been constructed to attempt to obtain the desired gas/air mixture under various conditions but existing modulating gas burners normally rely on open loop control of the gas and air relationship. This leads to two problems: the first is the production tolerance of the modulating gas valve and the second is the tolerance of the combustion air control system.
In
The present invention solves these problems by providing a feedback signal to give an indication of the output level so that the input signal can be adjusted via a closed loop control to achieve the desired output level. In order to detect the output gas flow, we have discovered that the flame ionization signal from a flame sensor, such as mentioned in German Patent P19857238.7 granted Apr. 7, 2000 and that has been used to detect the presence of flame and to provide a shut down of the gas valve if the flame should fail to light or is extinguished after lighting, is also a signal which varies in a predictable fashion with gas flow. By using a predetermined relationship, a controller can monitor the flame ionization level and use it as a feed back signal to adjust the modulation input signal and thus obtain the desired output pressure. However, the flame ionization signal may change with contamination of the flame rod over a period of time so an automatic field calibration should be performed to maintain accuracy.
Accordingly, the present invention also provides calibration by driving the gas valve with a maximum modulation signal which is guaranteed to open the gas valve to a calibrated high pressure setting. The tolerance of the high pressure setting is easily controlled to a tight range of values. The flame is ignited at the high level and the flame ionization signal is recorded. From this high fire flame ionization level, the system determines the flame ionization levels for other output flows. Thus the appliance can be controlled in a narrower pressure tolerance band than could be obtained without this type of feedback control.
Like the gas pressure calibration, the airflow also needs to be automatically calibrated. This is required for the proper accuracy of the gas/air mixture at any point in the modulating range. In the present invention, the airflow is modulated by modulating the fan speed of the combustion air blower to be described. The RPM of the fan is supervised through an RPM sensor. The maximum setting airflow is calibrated by increasing the airflow (by increasing the RPM) until the set point of the pressure switch is reached. This point corresponds to the maximum load or 100% airflow. Now the airflow is calibrated. The airflow from this maximum point is proportional to the RPM of the fan at a certain temperature.
In
Controller 16 also controls the speed of a circulator blower 32 by way of a line 34 and the circulator blower 32 pushes air into a chamber 36 where the heat exchanger 28 is located. Heat is transferred from the heat exchanger 28 to the passing air in chamber 36 to supply heated air, as shown by arrow 40, to a desired heated space. Air from the heated space is also returned to the circulator blower 32 as shown by arrow 42.
The amount of heat transferred to the air 40 is a function of the burning gas/air flow through the snake like tube 30 which, as mentioned, is controlled by the speed of a combustion air fan 46 that receives the gas/air combustion flow from tube 30 and throws the exhaust out of a stack 47. Combustion air fan 46 includes an RPM sensor 46a associated therewith to produce a signal indicative of fan speed on a line 46b to the controller 16.
Also, as mentioned, the gas/air flow is a function of the pressure of the gas/air mixture generated by the combustion air fan 46. A flange, 48 is located at the end of tube 30 and, the pressure difference over flange 48, which could also be a venturi, is sensed using pressure pick up points 49a and 49b on either side thereof. The actual pressures are led to a pressure switch 50 over lines 52a and 52b respectively. Pressure switch 50 is a diaphragm type that, based on the pressure differential on the diaphragm and setting, acts on an electric switch to produce a signal. The signal from pressure switch 50 indicative of switch action is presented to controller 16 over a line 53. The switch action enables the controller to determine the status of the pressure switch 50 and can be a high or low pressure indication due to switch contact being made or not. At each start-up, the airflow must be proven and the RPM of the combustion fan 46 is ramped up until the pressure switch set point is achieved and switch 50 switches. The RPM at this point represents 100% airflow (within the tolerances of the pressure switch 50 set point). From this 100% point, the actual RPM needed can be calculated by:
RPMRequired=Q/Required/QMax*RPM100%
Where Q represents airflow volume.
Controller 16 produces a speed control signal to combustion air fan 46 by a line 54 to cause the desired airflow to be maintained and sets and controls the required RPM for the required load. The load requirement at any point depends on the deviation of the sensor inputs to the controller 16, its set point and the control algorithm. The sensor inputs are shown in
The airflow must match the gas flow at any point in the control range. That is, the predetermined gas/air ratio at a certain firing rate (between low rate and 100% rate) is equal to the actual rate within the tolerance range. Full capacity represents 100% airflow and 100% gas flow. It is clear that in this linear one-to-one gas/air relation, 40% airflow matches with 40% gas flow for a good combustion at low rate. (40% of full rate is considered to be a “low rate”.) The controller 16 can also work with a predetermined offset (in air or gas). Any predetermined offset will depend on the specific application for which the invention is used and controller 16 will have an appropriate mathematical function, the transfer function, stored therein so as to produce the offset. For example, to prevent condensation in the heat exchanger 28, it may be desirable to run the combustion in burner 22 at a higher excess air flow rate for low fire conditions than at high fire conditions. The desired offsets can be easily included in the controller 16. As mentioned above, we have found that the output of flame rod 26, when properly installed in the flame, is a predetermined function of the gas pressure and may thus be used to control the operation of modulating valve 14. The predetermined function can be as simple as a linear function where Desired Flame Current=K×(Firing Rate+Offset). Controller 16 uses the Desired Flame Current as a set point for a feedback control loop, using Measured flame Current as its input, that controls the valve setting to maintain the Desired Flame Current.
As also mentioned, the output of the flame rod 26 can change with time and thus, the output should be periodically calibrated to assure accuracy is maintained. This calibration is performed by driving the modulating valve to the maximum open condition and measuring the signal from the flame rod. Then, the pressures at various smaller openings can be accurately predicted from the maximum flow signal because the calibration will modify the “K” and the “Offset” in the above equation.
One method by which the flame current can be calibrated is to read the actual flame current while the valve is fully open. At this point the outlet pressure of the gas valve is controlled via the internal regulator having a fixed set-point, therefore, the firing rate is well known. The flame current value is read as Current Full Fire by the controller.
Current Full Fire is then used to calculate K Calibrated:
K Calibrated=(Current Full Fire−Offset)/Full Firing Rate
This K Calibrated and/or the Current Full Fire are saved in memory for future use by the controller.
The current at other firing rates is now calculated by:
Desired Flame Current=K Calibrated*Firing Rate+Offset
A second method can calibrate the Offset value and the K value if the valve has two regulated pressure settings. The full fire current is measured as above. The valve is then activated at a regulated low fire point where the pressure is again controlled to a known pressure. An additional current is measured at the low fire rate as Current Low Fire. The calibrated K term is calculated as:
K Calibrated=(Current Full Fire−Current Low Fire)/(Full Fire Rate−Low Fire Rate)
Offset Calibrated=Current Full Fire−K Calibrated*Full Fire Rate
The current at other firing rates is now calculated by:
Desired Flame Current−K Calibrated*Firing Rate+Offset Calibrated.
Should the Offset need to be calibrated on a valve with only one regulator setting, it may be possible to develop an empirical function that relates change in Offset to change in K. The controller will find K Calibrated as in the first method and then calculate Offset from Offset=Empirical Function (K Calibrated). The empirical function will likely vary for each burner and flame rod combination.
It is thus seen that we have provided a modulating gas burner system that is more accurate than prior systems with the use of less expensive modulating gas valves. This has been accomplished by a closed loop feedback system and by utilizing the existing flame rod to provide gas pressure signals in addition to flame-out condition signals and by providing for calibration of the flame rod as it may change with time. It will be obvious that the system described for a furnace control may also be used for other gas burner control systems such as water heaters and boilers. Also, the various components described in connection with the preferred embodiment may have alternate equivalent components. For example, various kinds of igniters and differential pressure detectors, air movers and the like may be used in the present invention without departing from the spirit and scope of the present invention. Accordingly, we do not wish to be limited to the specific disclosures used in describing the preferred embodiment.
