burner vaporizer coil just above the burner grate on a cruban burner
For combustion to occur three ingredients are required. These are fuel, air, and heat (or an ignition source). Additionally all three must be in the proper ratio for combustion to continue. The burner combines these three critical ingredients under controlled conditions permitting continuous combustion to occur. Before the liquid fuel can be burned it must be vaporized. The vaporizer coil, located directly over the burner grate, provides proper vaporization of the liquid fuel prior to burning.
The burner, like the pilot, operates similar to a camp stove. Liquid fuel is heated or vaporized until it turns to gas. The gas is injected into a mixing tube where the gas combines with air. The air-fuel mixture is drawn up through a metal grate where it burns on top of the grate. The action of burning the fuel effectively causes additional air-fuel to be drawn in through the mixing tube so that he process can continue.
The tube used to heat the fuel from a liquid state to a gaseous state is called the vaporizer coil. It is located an inch or so above the burner grate so that it can be heated by the flame. A short length of the vaporizer tube is also routed above the pilot so that it may be kept hot by the pilot when the burner is not firing.
HISTORY OF THE STANLEY BURNER VAPORIZER
A flammable liquid in its liquid state will not burn. It only will ignite when it vaporizes into a gaseous state. All flammable liquids give off vapors that can ignite and burn when an ignition source such as a lighted cigarette or spark is present. How easy it is for a liquid to vaporize can be determined by looking at its flash point. The flash point of a liquid is the temperature at which a particular flammable liquid gives off vapors (vaporizes) and therefore can ignite. The flash point differs for each type of flammable liquid. Kerosene has a flash point of 110 degrees Fahrenheit. Gasoline has a flash point of -40 degrees. This means that at 110 degrees or higher kerosene gives off flammable vapors and can ignite. However, gasoline requires a temperature of only 40 degrees to vaporize to cause an explosion or fire. This means that when the temperature is freezing, gasoline still vaporizes and can cause an explosion and/or fire. At the same temperature, kerosene cannot ignite. Liquids such as gasoline with a flashpoint below 100 degrees are called flammable liquids. Kerosene and other liquids with a flash point above 100 degrees are referred to as combustible liquids.
The vaporizer coil raises the temperature of the liquid fuel to above its flash point so that the fuel is vaporized. Originally Stanley used gasoline for both the pilots and burners. The Stanley’s selected gasoline due to its molecular structure giving it the property to vaporize easily. There were other flammable liquids that they could have selected form but gasoline was plentiful, produced a lot of heat per unit volume burned, and could be easily handled with the simple plumbing hardware available at that time.
As more and more Stanley cars were produced the use of gasoline to fire the burner had to be rethought. Gasoline’s low flash point also made it too hazardous for use with an semi-open flame burner design and the Stanley’s moved to using kerosene to fire the burner. This change necessitated a longer vaporizer coil because of the additional heating needed to vaporize kerosene over gasoline. Thus in 1913 the change to kerosene was made for the burner while the pilot continued to use gasoline for fuel. Another advantage of kerosene fuel is that is provides about 12% more heat per gallon as compared to gasoline. More heat means more steam and faster steaming per gallon of fuel carried by the car.
A 3/8" diameter pipe is commonly used for the burner vaporizer. The proper length and positioning of the vaporizer is important to the proper vaporization of the fuel. If the gas leaving the nozzles is essentially clear it is being properly vaporized. If the gas leaving the nozzles has a foggy appearance then it is considered "wet" in that it contains a high degree of liquid in the vapor. If the gas leaving the nozzles is totally transparent it might be an indication that the gas is too hot and possibly being superheated. Ideally when the vaporization is correct the gas leaving the burner nozzle should have the appearance of a very faint haze.
If the vaporizer tube is too short all of the liquid fuel will not be converted to gas and the result will be a very wet gas flow from the nozzles. This wet gas flow will not only waste fuel it will also cause the burner to back-fire (ignite the air-fuel mixture in the mixing tubes which will quickly damage the mixing tubes and grate). If the vaporizer tube is too long then the gas will become superheated and carbon deposits will form on the inner wall of the vaporizer tube. These carbon deposits will break free and travel to the small orifice of the burner nozzle and plug it up. Generally an optimum length for the vaporizer tube is around eight feet.
Proper positioning of the burner vaporizer coil above the burner grate is also important. The vaporizer coil should be located about an inch or so above the top of the grate so that it receives maximum heating from the fire. As in the case of the length of the vaporizer coil, the position of the vaporizer coil in the flame will contribute to the gas being either wet or superheated. Often if the gas leaving the nozzles is wet or there is a tendency for carbon to choke the burner nozzles the repositioning of the vaporizer coil in the fire will correct the situation and it won't be necessary to lengthen or shorten the burner vaporizer tube.
Vaporizer coils are exposed directly to the heat of the fire and therefore should be made of material which will not only withstand the extreme temperature but also will not deform under the 140 PSIG pressure of the fuel entering the vaporizer coil. A vaporizer is generally constructed of 1/4" pipe. Heavy-walled seamless, Schedule 80, 304 stainless steel pipe is often chosen today as the material of choice for making a vaporizer tube.
To held distribute the heating effects of the vaporizer tube on the fuel flowing through the coil a stranded length of wire is inserted in the end of the vaporizer coil. This length of wire should fill at least 2/3rds of the length of the pipe although some Stanley owners have cable the full length of their vaporizer coil. The outside diameter of the vaporizer wire should be very nearly the inside diameter of the vaporizer pipe. Type 316 Stainless Steel Wire Rope, 7 X 19 Strand Core, 3/16" diameter is often chosen for this application. To prevent fraying the ends of the cable are TIG/MIG welded together making sure that the weld doesn't increase the overall diameter of the wire so that the wire still will slide freely into the vaporizer pipe.. For Stanley burners the cable is generally inserted in the nozzle end of the vaporizer tube while for Cruban burners the cable is inserted in the fuel supply end of the vaporizer tube.
If the proper length of vaporizer tube is used and it is positioned optimally in the fire there will be little carbon build-up on the interior of the vaporizer tube and on the surfaces of the wire. However, in time carbon will collect and the wire will need to be pulled from the vaporizer tube. The burner nozzles are also removed to allow for cleaning of the vaporizer tube. The tube can then be flushed with fuel (by briefly opening the fuel valve) to wash it clean of loose carbon deposits (this is obviously done when the car is cold and there is no flame around). Air can be used to blow the fuel out of the vaporizer tube before the wire is replaced.
Some owners have chosen to connect a steam line, often referred to as a "steam enema", to the fuel supply line for the purposes of cleaning the vaporizer tube. Before blowing down the car the vaporizer tube wire is removed along with the burner nozzles. A steam supply valve is opened and the steam rushing through the vaporizer tube quickly cleans it. Once the car has cooled the wire can then be reinstalled along with the nozzles.
Another cause of vaporizer carbon build-up is a leaking steam enema valve. If the steam enema is leaking the steam rapidly cools the superheated fuel before it vaporizes and carbon black is formed. The carbon black deposits quickly plug the vaporizer and a lack of fuel flow results. An easy tell-tail that the steam enema valve is leaking is a continued wisping of what appears to be vaporized fuel exiting the burner nozzle(s). When the burner is shut down using the burner valve on the car's dash, any residual vaporized fuel remaining in the vaporizer coil should bleed off in a minute or so and the burner nozzles should not display any "leakage". If there is a continued flow of "gas" from the nozzles 5 minutes after the burner is shut down then checking to see if the leakage ignites will determine if the fuel valve is leaking or the steam enema valve is leaking. In either case prompt corrective action is needed before the car is operated.
Occasionally some bad fuel can cause the vaporizer to actually choke nearly solid with carbon and other deposits and block the flow of fuel. In a like manner if the vaporizer is improperly positioned and superheats the gas such that carbon is readily formed, the vaporizer tube can become choked. When this occurs it is necessary to remove the burner nozzles and pull the vaporizer wire (pulling the wire can be difficult if the carbon buildup is especially bad). The burner should be dropped from underneath the boiler and the vaporizer tube removed to an outdoor area for burning out.
Using an oxygen-acetylene torch (a propane torch or similar isn't hot enough) the end of the vaporizer tube is heated cherry red for a three to four inch length (be careful not to melt the tube!). When the tube is glowing red hot turn off the acetylene flow to the torch and place the nozzle end of the torch into the end of the vaporizer tube so that the oxygen will flow through the tube. The oxygen flow will cause the carbon to burn vigorously and you may observe a shower of sparks and burning bits of carbon jetting out the opposite end of the vaporizer tube. For really bad carbon deposits one can watch the progress of the burning carbon as it will heat the vaporizer tube cherry red at the flame boundary within the pipe and the cherry red zone will moved along the length of the pipe. When all sparks and flame cease coming from the vaporizer tube it is an indication that all the carbon has been burned out. The vaporizer coil can be allowed to cool and then reassembled to the burner and the burner returned to the underside of the car.
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