Function of AFG burner motor "wheel".

rgar98

I'm Confused about the purpose of motor blower wheel. If movement of combustion air over fire to breech and out chimney is due to draft differences, then what is the function of the wheel that is attached to the drive motor?
Richard

EBEBRATT-Ed

It take about 1700 cubic feet of air to burn 1 gallon of oil. That's a lot of air. Most (but not all) boilers and furnaces run with a slight negative pressure in the combustion chamber. If the burner fan puts in more air than the chimney can draw out this air pressure would turn positive and combustion fumes would enter the building. There are some boilers that can run with positive pressure in the firebox and are designed that way

The burner fan has to deliver enough air for combustion and the burners job is to mix that air with the atomized oil spray. This requires turbulence and energy in the blast tube which draft can not provide on its own.

In the past there were some burners made, for example pot type burners that had no fan, and horizontal rotary burners where 80-85% of the air was from the chimney draft and 15-20% was from the primary air fan

Air control is important to the burners efficiency so those earlier have been somewhat disregarded as they were in some cases not as efficient.

But some of the old vertical rotary burners like Timken and Torridheat had excellant efficiencies

Steamhead

That blower wheel moves the air through the slots on the combustion head. It takes a certain amount of static air pressure to do this. The result is the air and oil spray are thoroughly mixed, and the hot combustion products are moved through the boiler.

There were a few natural-draft oil burners in the 1920s and 1930s, but they didn't work too well. That's why we now have burners that move their own air.

rgar98

Thanks, Ed
If all that air is needed, and most of it is supplied by the burner fan, then why are the small draft negative pressures (over fire, breech) so critical?

EBEBRATT-Ed

@rgar98

You want to control the draft over the fire at -.01 to -.02 "wc in most cases. You don't want excess heat going up the chimney so you keep the draft as low as possible. Thats what a barometric damper in the flue pipe does because the amount of draft varies with the outdoor temp

Like I said earlier some equipment is designed to run "forced draft" with positive combustion chamber pressure and no barometric damper

mattmia2

In the more basic sense, negative draft is necessary to make sure the products of combustion go out the vent instead of inside the house since the combustion chamber isn't airtight so if there is not negative draft the products of combustion will leak out those areas.