System and methods for modulating gas input to a gas burner

An improved gas appliance having a burner, a gas valve through which the flow of combustion gas to the burner is controlled, and a motor driven blower that supplies combustion air to the burner. The improvement includes means for increasing gas flow through the gas valve as blower speed increases, and decreasing gas flow through the gas valve as blower speed decreases, based on a pressure signal generated independently of combustion air pressure. This improvement allows a constant ratio of gas to air to be maintained in the burner while a combustion flow rate varies dependent on the blower motor revolutions per minute. Thus input pressures of combustion can be controlled at low cost.

FIELD OF THE INVENTION

The present invention relates generally to gas appliances and, more particularly, to controls for gas input to gas appliances.

BACKGROUND OF THE INVENTION

Gas appliances typically include valves for controlling gas input to the appliance's burners. Gas control valves are used in induced draft systems and in forced draft systems with pressure-assist modulation (PAM) to deliver gas to be combined with air for combustion. It is desirable to control gas and air input pressures in order to achieve desired combustion rates in appliance burners. One method of controlling gas input pressure is to electronically modulate gas control valve output relative to the air input pressure, by using a pressure transducer. Such an approach, however, is expensive.

SUMMARY OF THE INVENTION

The present invention in one embodiment is an improved gas appliance having a burner, a gas valve through which the flow of combustion gas to the burner is controlled, and a motor driven blower that supplies combustion air to the burner. The improvement includes means for increasing the flow of gas through the gas valve as the blower speed increases, and decreasing the flow of gas through the gas valve as the blower speed decreases, based on a pressure signal generated independently of the combustion air pressure. In a preferred embodiment, a pump provided on the shaft of the blower motor is driven by the blower motor to generate the pressure signal for controlling the gas valve.

The above-described system allows a constant ratio of gas to air to be maintained to the burner while a combustion flow rate varies dependent on the blower motor revolutions per minute. Thus input pressures to the burner can be simply and reliably controlled at low cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A conventional induced draft combustion system is indicated generally as20in FIG.1. The combustion system20comprises a combustion chamber22having a burner48therein, an air inlet24, and a gas inlet26. A gas valve100in the gas inlet26controls the flow of gas to the burner. A blower30, having an inlet32and an outlet34connected to the combustion chamber22draws the hot combustion gases from the combustion chamber to, for example, the heat exchanger of a residential furnace or commercial heater, thereby drawing air through the air inlet24into the combustion chamber. In a conventional system shown inFIG. 1, increasing the speed of the blower30increases the air flow to the combustion chamber22, but it does not affect the flow of gas to the combustion chamber22. Thus, changes to the blower speed change the air to fuel ratio. Additionally, increasing the speed of the blower30typically increases air flow to the combustion chamber22up to pressures of only about 2.5 inches of water column.

A conventional forced draft PAM system is indicated generally as40in FIG.2. The forced draft system40comprises a combustion chamber22having a burner48therein, an air inlet24, and a gas inlet26. A gas valve100in the gas inlet26controls the flow of gas to the burner. A blower30, having an inlet32and an outlet34between the air inlet and the combustion chamber22pushes air into the combustion chamber, thereby pushing hot combustion gases from the combustion chamber22to, for example, the heat exchanger of a residential furnace or commercial heater. Gas flow is adjusted via a hose line36connecting the blower outlet34and a port110on the gas valve100. In the conventional PAM forced draft system shown inFIG. 2, increasing the speed of the blower30increases the air flow to the combustion chamber and affects the flow of gas to the burner. The blower30, however, produces pressure signals only up to about 2.5 inches of water column. Because gas valves typically operate at pressures above 3 inches of water column for natural gas and at pressures above 10 inches of water column for liquefied petroleum (LP) gas, changes to the blower speed could change the air to fuel ratio when requiring gas valve operation at pressures above 3 inches of water column.

The present invention is a system and method whereby the fuel gas flow rate is automatically adjusted with changes in the blower speed to substantially maintain the air to fuel ratio despite changes in the blower speed. The system includes a gas valve shown generally as100in FIG.3. The gas valve100is similar to conventional gas valves, except for the provision of a port for receiving pressure signal from the blower, as described in more detail below. As shown inFIG. 3, the gas valve100comprises a body101having an inlet102, an outlet104, and a flow path106therebetween. There is a main valve118adjacent the outlet104. The main valve118comprises a valve seat120, and a valve stem122, which is controlled by a diaphragm124, and biased closed by a spring126. The diaphragm124defines an upper chamber128and a lower chamber130in the valve100. The relative pressures in the upper and lower chambers128and130determine the position of the valve stem122relative to the seat120, and thus whether the flow path106in the valve100is open or closed.

A control conduit132, selectively closed by a control valve134operated by a control solenoid136, extends to a regulator138. A passage140has a port142opening to the control conduit132, and a port144opening to the lower chamber130. Thus, when the control valve134is open, the inlet gas pressure is communicated via conduit132and passage140to lower chamber130, which causes the stem122to move and open the main valve118.

The regulator138includes a valve seat146and a diaphragm148that seats on and selectively closes the valve seat146, and which divides the regulator into upper and lower chambers150and152. There is a spring154in the upper chamber150on one side of the diaphragm148. The relative pressures in the upper and lower chambers150and152determine the position of the diaphragm148relative to the valve seat146, and thus the operation of the regulator138. A screw adjustment mechanism158compresses the spring154and adjusts the operation of the regulator138. A passage160has a port162opening to the lower chamber152of the regulator138, and a port164opening to the upper chamber128of the valve. When the regulator valve is open, i.e. when the diaphragm148is not seated on valve seat146, the inlet gas pressure is communicated via passage160to the upper chamber128, tending to equalize the pressure between the upper and lower chambers128and130, and close the main valve118.

A secondary valve166, comprising a valve seat168, a valve member170, and solenoid136, is disposed in the flow path106between the inlet102and the main valve118. The secondary valve166also closes the gas valve100, acting as a back up to the main valve118.

In accordance with this preferred embodiment, the regulator138includes a port174that communicates with the upper chamber150for receiving a pressure signal from a blower-driven pump as further described below. The pressure signal on the port174changes the operating point of the regulator. When the pressure signal from port174increases the pressure in the upper chamber150of the regulator, the regulator valve closes passage160, tending to increase the opening of the main valve118. When the pressure signal from the port174decreases the pressure in the upper chamber150of the regulator, the regulator valve closes less readily, keeping passage160open, and tending to close the main valve. Thus the port174provides feed back control, increasing gas flow with an increase in blower speed, and decreasing gas flow with a decrease in blower speed.

In accordance with this invention, the pressure signal is preferably created by the operation of the blower motor. In the preferred embodiment, a pump is provided on the shaft of the blower motor. Rotation of the blower motor shaft operates the pump, and the outlet pressure of the pump is substantially proportional to the speed of the blower motor.

A pump adapted for use with the present invention is indicated generally as200inFIGS. 4 through 9. The pump200comprises a housing202having a one-way air inlet204and an air outlet206. A diaphragm208in the housing202is operated by the reciprocation of a shaft210, which in turn is driven by cam212. The cam212is operatively connected to shaft of the blower motor. The pump200has a socket214for engaging the shaft of the blower motor. Thus the pressure generated by the pump changes with the speed of the blower motor.

An induced draft combustion system constructed according to the principles of this invention is indicated generally as300in FIG.10. The combustion system300is similar in construction to system20described above, and corresponding parts are identified with corresponding reference numerals. The combustion system300comprises a combustion chamber22having a burner48therein, an air inlet24, and a gas inlet26. A gas valve100in the gas inlet26controls the flow of gas to the burner48. A blower30connected to the combustion chamber draws the hot combustion gases from the combustion chamber22to, for example, the heat exchanger of a residential furnace or commercial heater, thereby drawing air through the air inlet24into the combustion chamber.

In system300, a pump200is mounted on the shaft of the motor of the blower30. The outlet206(shown inFIGS. 4-9) of the pump200is connected to the port174in gas valve100via line302, to adjust the operation of the regulator with changes in the blower speed, thereby tending to maintain the air to fuel ratio as the blower speed changes. The pump outlet pressure is generated independently of, and can exceed, the combustion air pressure generated by the blower30. Thus an adjustable bleed orifice310of the line302is used to adjust the pump pressure signal to the gas valve100. Thus the pump200, line302, orifice310and port174operate as a controller that increases the flow of gas through the gas valve100as the blower speed increases, and decreases the flow of gas through the gas valve100as the blower speed decreases, based on a pressure signal substantially proportional to drive shaft revolutions of the blower motor.

A differential pressure switch320between the air inlet24and gas valve outlet104is configured to sense both gas flow and air flow into the combustion chamber22. When a predetermined difference in gas flow and air flow is sensed, the switch320cooperates, for example, with a system300ignition or blower motor control (not shown) to shut down the system300. Thus an automatic shutoff is performed if, for example, lint accumulates in the air inlet24in such amounts that the predetermined difference in gas and air pressures is detected.

A PAM combustion system constructed according to the principles of this invention is indicated generally as400in FIG.11. The combustion system400is similar in construct to system40, described above, and corresponding parts are identified with corresponding reference numerals. The combustion system400comprises a combustion chamber22having a burner48therein, an air inlet24, and a gas inlet26. A gas valve100in the gas inlet26controls the flow of gas to the burner48. A blower30between the air inlet and the combustion chamber pushes air into the combustion chamber, thereby pushing hot combustion gases from the combustion chamber22to, for example, the heat exchanger of a residential furnace or commercial heater. In system400, a pump200is mounted on the shaft of the motor of the blower30. The outlet206(shown inFIGS. 4-9) of the pump200is connected to the port174in gas valve100via a line402, to adjust the operation of the regulator with changes in the blower speed, thereby tending to maintain the air to fuel ratio as the blower speed changes. The pump outlet pressure is generated independently of, and can exceed, the combustion air pressure generated by the blower30. Thus an adjustable bleed orifice410of the line402is used to adjust the pump pressure signal to the gas valve100. Thus the pump200, line402, orifice410and port174operate as a controller that increases the flow of gas through the gas valve100as the blower speed increases, and decreases the flow of gas through the gas valve100as the blower speed decreases, based on a pressure signal substantially proportional to drive shaft revolutions of the blower motor.

A differential pressure switch420between the blower outlet34and gas valve outlet104is configured to sense both gas flow and air flow into the combustion chamber22. When a predetermined difference in gas flow and air flow is sensed, the switch420cooperates, for example, with a system400ignition or blower motor control (not shown) to shut down the system400.

It is apparent from the foregoing that the relationship between inches of pump outlet pressure and RPMs of the blower motor is substantially linear, and that the pump200is capable of generating pressures exceeding typical blower generated combustion air pressures of up to 2.5 inches of water column.

The above system and method provide for maintaining a constant ratio of gas to air going to a furnace while varying a combustion flow rate dependent on blower motor revolutions per minute. Because the pump200generates a pressure signal dependent on the blower motor speed, gas flow can be modulated without sensing or sampling combustion air pressure. The pump can be configured with gas valves that operate at pressures above, below and including two inches of water column. More specifically, the pump can provide pressures of up to fourteen inches of water column. Thus the pump produces pressures sufficient for use in gas appliances having burners using either natural or LP gas, and also is inexpensive to manufacture. Thus input pressures of combustion can be controlled at low cost.

Other changes and modifications may be made to the above described embodiments without departing from the scope of the present invention, as recognized by those skilled in the art. Thus the invention is to be limited only by the scope of the following claims and their equivalents.