The oil furnace |
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What is fuel oil?
Number 2 fuel oil is a product of distillation of crude oil. It has the highest BTU content of all oil except crude oil itself. The content is 140,000 to 144,000 BTUs per US gallon. It is much the same as Diesel fuel. This fuel is used in burners that pressurize the fuel and atomize it for easier combustion. Number 1 fuel oil is somewhat more volatile, (that means it gasses off easier) than #2 fuel oil. It is used in oil pot burners. #1 fuel oil has 138,000 to 140,000 BTUs per gallon.
When fuel oil became popular for space heating, the first burners developed were called "pot burners". These burners used #1 fuel oil because of its higher volatility. The fuel is introduced to the bottom of a cylinder and is manually lit. As the fire burns, it evaporates more fuel to be burned. Air is added from the sides of the cylinders to make the burn complete. These burners were simple in design and did not necessarily need any electrical power to operate. However, they had low thermal efficiency and required considerable maintenance. Below is a video about the pot burner.
The oil fired pressure burner.
To increase thermal efficiency of the oil burn, the pressure burner was developed. This burner used high pressure (around 100#) to atomize fuel oil and a high voltage transformer to ignite the atomized oil. The burners consist of a motor to drive the high pressure pump, a fan mounted to the motor shaft to provide combustion air, the transformer, the gun assembly with the atomizing nozzle and a flame safety control to shut the burner down if the flame was not established. Below a video gives a quick overview of the burner parts.
Below is video disassembling a Beckett burner to show the parts.
The oil furnace nozzle.
Because oil is a liquid, it must be changed to a gas before it can be burned. To do this, we use a nozzle. The nozzle is supplied high pressure (100# plus) oil. The first part of the nozzle is the filter. Most filters are made of sintered bronze. The outlet hole is very small and this last filter is quite important to keep the nozzle clean. The hole at the end of the nozzle determines the size or gallonage. Inside the nozzle, there is a part that induces a swirling of the oil. The video below shows these parts.
When the nozzle is in operation, it sprays the oil in small droplets. These droplets are heated by the flame itself to become a gas to continue the burn. The smaller the droplets, the more efficient the burn. This is where the pressure comes in. The video below gives images of a nozzle operating at different pressures. You can see the larger droplets at lower pressures, with them disappearing at higher pressures.
When the burner is lit, the flame is directed into a combustion chamber where combustion is completed. The video below shows how the flame looks from above.
The amount of air supplied to the flame is extremely important to the efficient operation of the oil furnace. If there is too little air supplied, the flame will be sooty and dirty looking. The soot will deposit on the heat exchanger, reducing the heat exchanger tube size sometimes to the point that the heat exchanger gets plugged. The soot is also a very good insulator and reduces heat transfer. Surprisingly, a sooty flame does not reduce the heat available from the combustion as much as would be thought.
Too much air is even worse for the oil flame. As is shown by the video below, increasing the air supplied to the flame reduces the flame size. What is happening here is the excess air cools the flame. Remember, the droplets in the spray must be vaporized before they can be burned. The flame provides the heat for that vaporization. Also if the flame is cooler, the lower temperature difference between the flame and the air surrounding the heat exchanger reduces the heat extraction efficiency. There is balance here that is necessary for the complete combustion of the oil.
Too much air is even worse for the oil flame. As is shown by the video below, increasing the air supplied to the flame reduces the flame size. What is happening here is the excess air cools the flame. Remember, the droplets in the spray must be vaporized before they can be burned. The flame provides the heat for that vaporization. Also if the flame is cooler, the lower temperature difference between the flame and the air surrounding the heat exchanger reduces the heat extraction efficiency. There is balance here that is necessary for the complete combustion of the oil.
In the video below, we cover how the flame looks with low and high air supplied. It also shows where the adjustment is on this burner and demonstrates how a very small adjustment can have a fairly large effect on the flame.