Fuel/Air Ratio Control
Using a zero governor (atmospheric regulator)
Zero governors work by maintaining atmospheric pressure at their outlet by modulating gas in proportion to the amount of suction generated by air flowing through an aspirator mixer. To develop enough suction the air pressure must be 3.5 to 4X of the mixture pressure. Because it only requires suction from the mixer to pull the fuel into it, this control method is ideal when only low pressure gas is available. Since most locations have sufficient gas pressure, cross connecting the regulator has become more popular.
Zero Governor (Ratio Regulator)
3065 Mixer
Burner
Vent Connection
Air Valve
Figure 3. Typical ratio control system with zero governor ratio regulator
Using a cross-connected ratio regulator
There are a number of reasons to cross-connect the ratio regulator feeding a 3065:
Gas Line
— It maintains better ratio control if the combustion chamber pressure varies or is different than the room pressure. For example, premix burners used as pilot tips work better when cross connected. — By cross connecting the regulator it is not necessary to maintain a 3X to 4X air pressure to mixture pressure multiplier. Besides potentially reducing the air pressure requirement, this reduces the need for precise burner mixer sizing. The impulse line can be connected to the air or mixture pressure taps. The gas pressure at the ratio regulator inlet should be set to the impulse line pressure plus the pressure drop across the regulator. If the gas pressure is set higher than necessary, the air/fuel ratio adjustment will lose resolution and get touchy. Cross connecting to the mixture pressure provides slightly better ratio control resolution than using the air pressure tap, but there are advantages to using the air pressure tap for the impulse.
Zero Governor (Ratio Regulator)
3065 Mixer
Burner
Air Valve
Impulse Line on Mixture Pres. Tap
Figure 3b. Typical ratio control system with zero governor ratio regulator cross-connected to the mixture pressure.
Gas Line
Zero Governor (Ratio Regulator)
3065 Mixer
Burner
Impulse Line on Air Pres. Tap
Air Valve
— Applications where having an air/fuel premix in the impulse line and regulator case is objectionable.
Figure 3c. Typical ratio control system with zero governor ratio regulator cross-connected to the air pressure.
— It leaves the mixture tap free to be used for a premix test tip for tuning the ratio setting.
— The air pressure tap is always in the same relative position. Because of the way the smaller 3065’s are made the rotation position of the mixer tap can vary. Consult Bulletin 7218/7219A, Instructions 7218-2, and the Practical Pointers book for additional information and set-up instructions for ratio regulators.
Bulletin 3065 page 4
Made with FlippingBook flipbook maker