Sizing 3065 Mixers Instructions continued
• For premix burners if only the burner orifice area and required mixture pressure is known, use the mixer throat vs. nozzle area ratio sizing method. The pre-configured mixer/burner combinations shown in Table 1 have a mixer throat area to nozzle area ratio of approximately 1 to 2.5. If that ratio is maintained for new burner sizes, the mixer throat area data in Table 3 can be used to pick the correct mixer size. The mixture pressure to air pressure multiplier should also still be 3.5X for natural gas.
Example E: Select a mixer for burner with 3,000 scfh air flow/ 20"w.c. mixer air pressure/8"w.c. mixture pressure.
- Mixer p available = 20"w.c. - 8"w.c. = 12 w.c. p
Note: the mixer air pressure is 2.5X the mixture pressure, so cross connect the ratio regulator. (20/8=2.5)
— From Chart 1A, pick a 3065-3-S14 or from Table 3A a 3065- 4-18 could be used because it has a similar throat area as the other mixer. Which mixer to choose depends on a number of factors.
Use the table below to pick mixer/burner combinations using the mixer throat vs. nozzle area ratio.
— The 3065-3-S14 has 1½“ pipe connections and the 3065-4-18 has 2" pipe connections.
Mixture Pressure to Air Pressure Multiplier
Mixer Throat Area to
Burner Orifice Area Ratio
— If at any time the system capacity needs to be increased, the capacity of each mixer can be increased by using a smaller displacement rod. — In most cases pick the mixer with the same pipe size as the burner or the existing pipe size if the 3065 is replacing an old mixer. Example F: An existing system uses a 3065-5-17 mixer to feed four (4) 4682-2-A burner nozzles firing natural gas. What mixer is needed if the burners are changed to four (4) 4682-2-B burner nozzles to increase the system capacity? — Because these are standard size Fives North American nozzles, use Table 2. Find the -2-B nozzle row move right and find the (4) nozzle column. Find the 3065-5-12 Mixer from the list. Since the mixers are both the 3065-5 size, only the displacement rod needs to be changed. Example F2: In the example above, if the four (4) 4682-2-A burners are instead changed to four (4) 4682-2-D burner nozzles to increase the system capacity even more, find the correct mixer to replace the 3065-5-17. — Since these are standard size burner nozzles with zero gover- nor ratio control, use Table 2. Find the -2-D burner row, move right and find the 4 burner column. Pick the 3065-6-18 mixer from the list. — Because this new mixer is a 3065-6 it will require 3" air and mixture piping instead of the 2 ½ " pipe used with the existing mixer, but maybe an oversized 3065-5-S will work instead which can use the existing piping. — From Table 3B, the orifice throat area of a 3065-6-18 is (1.040 in2). A 3065-5-S15 is (1.055 in2). So, use the 3065-5-S15 be- cause only the mixer needs to be changed and not the piping.
2.5 X 3.5 X 4.0 X
1 to 2.0 1 to 2.5 1 to 2.9
Example G: Select a mixer to provide premix for a quantity of (12) 4624-01 blast tip burners operating at 6"w.c. and air pressure into mixer = 21"w.c. (using AP = MP X 3.5)
— From Sheet 4624 each 4624-01 blast tip has an open area of 0.131 inches² so, 12 X 0.131 = 1.572"² open area
— From the “mixer throat area/burner orifice area ratio table above (using the 3.5 multiplier row) estimate mixer throat area to be: 1.572 / 2.5 = .629“² — Using table 3, pick a mixer with .629“² open area, both: 3065-4-5 (0.626“²) or 3065-4-S14 (0.635“²) will work, and both have a 2" air inlet and mixture outlet.
• General application notes:
— High air pressure to mixture pressure multipliers generally makes more uniform premix.
— Changing mixer displacement rods, changes the capacity or pressure drop of a mixer.
— For Propane mixer sizing use the Natural gas guidelines.
— Cross connecting the ratio regulator makes picking the optimum mixer size less critical.
— The air flow values shown in the charts and tables in this bulletin are estimations, the actual flow through systems can vary depending on the mixer size and variations in manufacturing. While the orifices are machined, mixer capacity is affected by the roughness of cast surfaces inside the mixer.
Note: Also, check other system components to see if they are suitable for use with the higher capacity.
Bulletin 3065 page 6
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