Combustion Catalog | Fives North American

3065 Aspirator Mixer Operating Principles

Aspirator (air-jet) mixers generally operate with low pressure air in the 3 to 20 osi (5 to 35"w.c.) range 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 uniform (high quality) premix. They are often used with a zero governor (atmospheric regulator) which controls fuel/air ratio when the air flow through the mixer is adjusted. To conform to field conditions or to get lower or higher mixture pressures (with more or less suction, respectively), 3065 mixers have interchangeable displacement rods that permit changing the mixer air orifice size. Rods can be changed without "break- ing" the piping. The rod diameter in 32 nds of an inch is stamped on the rod nut. 3065 mixers have a built in pressure tap to measure the air pressure as it enters the mixer and a tap to measure the mix- ture pressure as it exits the mixer. These taps can also be used as points to connect an impulse line for a cross connected ratio regulator control. 3065 mixers are suitable for operation over the entire range of conventional premix burners. Most North American premix burners have a minimum mixture pressure limit of .25"w.c., but depending on piping configuration, 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.

4. The “Air pressure/mixer differential pressure” ratio is important to know (along with fuel type) when picking a mixer size, especially when using a zero governor (atmospheric regulator) for fuel/air ratio control. — For air/natural gas or propane systems (800 Btu/ft3 or more), “the air pressure in osi should be 2X the numerical value of the mixer pressure in inches water column” or a 3.5 to 1 air pressure/ mixture pressure ratio. For example, if the desired mixture pres- sure at a burner nozzle is 4"w.c., then pick a mixer that requires 8 osi (14"w.c.) air pressure at the inlet to the mixer at the desired air flow rate (Note: 1 osi = 1.73"w.c.) — For coke oven gas and manufactured gas “the air pressure/ mixture pressure ratio is 4 to 1". So if the desired mixture pres- sure for a burner nozzle is 4”w.c. the air pressure at the inlet to the mixer at the desired air flow rate should be 16"w.c. (9.3 osi) If the ratio regulator is cross-connected to the mixer with an impulse line, the air pressure/mixture pressure ratio becomes less critical for ratio control. This gives the system designer an option to reduce the system air pressure requirement by choosing a mixer with less pressure drop.

5. The quality (uniformity) of the air/fuel mixture is a function of the percentage of air pressure used for mixing.

The minimum recommended air pressure drop for a cross con- nected system is 33% of the supply pressure. Using less pressure drop across the mixer will result in low quality air/fuel mixing. A minimum air pressure drop of 66% of the air pressure is required to use a zero governor with natural gas. If the ratio regulator is cross connected, it is acceptable to size the mixer so that the air pressure is 2X the mixture pressure. In this case the mixer pressure drop will be 50% of the supply pressure.

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 easiest way to pick the correct mixer for an application is to use the sizing tables in this bulletin. In cases where that is not appropriate, there are a number of factors to keep in mind. 1. “Mixture pressure”: this is the pressure of the premix as it leaves the mixer which is the same as the pressure the burn- er requires upstream of the nozzle, plus piping pressure losses between the mixer and the burner (if any).


3065 mixers are suitable for supplying premix to single or multiple 3/4" pilot tips. The capacity of North American 4021 and 4027 pilot tips is similar to the capacity of a 4651-01-A size pre- mix burner, so use the -01-A data in the sizing tables when sizing for 3/4" premix pilot tips. Consult the pilot bulletin for actual pilot capacities and pressure requirements. The 4031 pilot mixer comes in one size, is simpler than the 3065 and is often preferred for use with a single 3/4" pilot tip. Old pilot systems that use the now obsolete 4035-01 and 4035-02 mixers can be replaced with the 4031 mixer. For better pilot tip reliability, pilot ratio regulators should be cross-connected downstream of the pilot air control valve to a pilot mixture or air pressure tap.

2. “Air pressure”: this is the pressure required to supply air into the mixer.

3. “Mixer differential pressure”: this is the difference between air pressure and the mixture pressure (dp = ap - mp). Besides helping to size the mixer, knowing the mixer differential pressure at the required flow rate, helps in estimating the air flow through the mixer during operation.

Bulletin 3065 page 3

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