3065 Aspirator Mixer Operating Principles
PICKING THE AIR PRESSURE/MIXTURE PRESSURE MULTIPLIER
Aspirator mixers are used to create a uniform air/fuel mixture for burners that require premix. They normally operate with low pressure air (below 35"w.c.) and use the venturi effect to pull fuel into an air stream. This creates shear between the fuel and air flows that efficiently mixes them to make a high-quality premix. To ensure that the proper amount of fuel is mixed with the air, a ratio regulator is installed upstream of the mixer’s gas inlet. Normally the regulator is cross connected to the air or mixture pressure. If only very low-pressure gas is available, a zero governor (atmospheric regulator) must be used. A very wide range of 3065 mixer sizes with interchangeable displacement rods is available. Changing the rod size changes the mixer air orifice area and is easy to do without breaking piping. Built-in pressure taps are included at the inlet and outlet of every 3065. They can also be used as points to connect an impulse line for a cross connected ratio regulator. Knowing the pressures needed at these tap locations is important in the process of picking mixer sizes. — The pressure at the outlet of the mixer is the “Mixture Pressure” (mp). This is the pressure (plus any piping pressure losses) of the premix as it enters the burner. This pressure is based on the burner requirement.
North American 3065 mixers and many premix burners were designed in a time when many customers only had access to low pressure Natural gas, so zero-governor atmospheric regulators were the best option to control the air/fuel ratio. To ensure enough suction to pull the gas into the mixer the air pressure had to be approximately 3.5X the mixture pressure. The classic “rule of thumb” was the air pressure in osi should be twice the numerical value of the mixer pressure in inches water column. For example, if the desired mixture pressure at the burner nozzle was 4"w.c., the air pressure (ap) should be 8 osi (13.8” w.c.). Once sufficient gas pressure became widely available, most premix ratio regulators were cross connected. This eliminates the need for as much suction from the mixer, so a lower air pressure to mixture pressure multiplier can be used which allows for lower system air pressures. Using a low air pressure to mixture pressure multiplier like 2X allows for a lower system air pressure, but the result can be lower quality premix where the air and gas dœs not fully mix as quickly. adding extra distance between the mixer and the burner gives the air and gas more time to mix better. A high air pressure to mixture pressure multiplier, like the “classic” 3.5X produces a very uniform premix which allows the burner to be mounted closer to the mixer. See the Mixer Installation section in this bulletin for the minimum recommended straight pipe runs between the mixer and burner. Air pressure to mixture pressure multipliers below 3.5X can be used when application experience shows acceptable performance. Fortunately, it’s easy to change the characteristics of an installed 3065 mixer by replacing the displacement rod. Increasing the rod diameter will lower the capacity if the air pressure is maintained or increase the air pressure to mixture pressure multiplier if the air pressure is increased to match the old capacity. Using a smaller rod diameter dœs the opposite.
Air Pressure Tap (ap)
Mixture Pressure Tap (mp)
Gas Inlet
Interchangeable Displacement Rod
Body Throat
Burner Exit Port
Air Inlet
Figure 2. Typical 3065 Mixer & Premix Burner
— The pressure at the inlet of the mixer is the “Air Pressure” (ap). To get uniform air/gas mixing it’s important for the air pressure entering the mixer to be 2X to 4X that of the mixture pressure. Generally, the higher the “multiplier” the faster the air and gas mix together. — The “mixer differential pressure” ( p) is the pressure difference between the air pressure and the mixture pressure, p = ap – mp. Mixer differential pressure is used to estimate the air flow through the mixer. Mixers should be sized so that the mixer differential pressure ( p) is at least 1⁄3 the air supply pressure (ap). Most North American premix burners have a minimum mixture pressure limit of .25"w.c., but depending on the piping configura- tion, often must be set higher to prevent flashback. This means the low fire air pressure setting at the mixer inlet is usually above 1"w.c. Follow the North American piping guidelines to reduce the risk of flashback at low fire and flame lift-off at high fire.
Bulletin 3065 page 3
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