We currently have access to a 4-20ma signal being used for data logging from a gas pressure transducer. The gas pressure to the burner varies from 8"H20 to a high of 13.5 at full fire. The roast master prefers to manually control the firing rate himself throughout the roast. He would prefer to not have to change the speed of the combustion air mixture dc blower every time a change is made in firing rate. Using the 4-20ma loop as an input, what would be the best way to have the fan track the burner gas pressure producing a variable dc output to the 12 volt dc blower? DC voltage at it's slowest speed is 5 volts, highest speed is at 10 volts. The fan draws 4 amps at 10 volts. Coffeeal

 

You could use a mixing valve and leave the blower speed constant at it's maximum output, or near that if the process doesn't require the maximum output. You tune the valve to adjust the intensity of the flame and you can use any sort of control you want. The valve is operated by a motor with a mechanical linkage between both the gas train and the air system and it is variable position using typically 4-20 mA. You can use a pot or install thermocouples and a UDC and operate off setpoint (or manual if is still the preference).

 

The usual way of controlling flame intensity is by adjusting the gas pressure by piloting the gas regulator directly from the variable combustion air pressure. I dont know if this would help you but if you tube directly from the combustion air at the burner nozzle to the gas regulator ambient side, and adjust your regulator to fully on, the air pressure will automatically adjust the gas to an equal or preset ratio. The other way is to use your 4-20mA signal and condition it with resistors to give you your 5-10vDC output to the blower. Hope this helps.

 

The best way to control combustion in a premix air-gas system is to mantain air-ratio constant through the entire firing range. To accomplish this you usually controll the air, not the gas.

A temperature controller in the oven will determine the power needed (in a 4 to 20 m-a. signal) that should be fed to a butterfly control valve moved by a motor thar accepts a 4 to 20 mA. signal, or to a variable speed drive for the DC motor to vary the air pressure.

The variable pressure of the air is then fed to the mixer and through an "impulse air piping", usually 1/4" n.p.t. or similar pipe, to a gas regulator or gas proportinator valve (Eclipse, North American, Kromschroeder, etc). The output pressure of the gas will equal the air pressure mantaining a constant air-gas ratio to ensure good combustion from low to high fire.

If you e-amil us a schematic of the system, the amount of gas used by your oven and the pressure and volume developed by your blower, we can advise you the whole picture.

Alejandro Sanchez
ALTEK Combustion Controls
[email protected]


On October 22, 2003, Allen Bartee wrote:
> We currently have access to a 4-20ma signal being used for data logging from a gas pressure transducer. The gas pressure to the burner varies from 8"H20 to a high of 13.5 at full fire. The roast master prefers to manually control the firing rate himself throughout the roast. He would prefer to not have to change the speed of the combustion air mixture dc blower every time a change is made in firing rate. Using the 4-20ma loop as an input, what would be the best way to have the fan track the burner gas pressure producing a variable dc output to the 12 volt dc blower? DC voltage at it's slowest speed is 5 volts, highest speed is at 10 volts. The fan draws 4 amps at 10 volts. Coffeeal <

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