|
|
|
©Neil A McRae 2002
|
OK I know it is a rare lamp that has a knob to turn which will alter the mixture in a pressure lamp burner. However the ratio between fuel and air is critical and even minute changes in this ratio can alter the performance of a lamp. This critical ratio is around 4% fuel and 96% air. In pressure lighting, air is sucked into the cone of vaporised fuel emitted from a small orifice under pressure. The cone of gaseous fuel creates a vacuum which sucks in air and then blows this combination of fuel-gas and air into a mixing tube. In general the suction so generated will gather much more air than is required for complete combustion so the air intake has to be restricted in some way in order to achieve the correct ratio of fuel to air. This restriction is achieved in various ways in different designs of burner. Lamps like Coleman and Tilley discharge the fuel jet into an enclosed venturi tube or tubes which draw air from a remote part of the lamp. Coleman air tubes bend down to the base plate of a lantern and Tilley are horizontal tubes to the outside of the lantern hood but both achieve air restriction through a fixed length and diameter of tubing. Burners of the Petromax type discharge the fuel into the atmosphere but within a short distance the fuel cone enters a venturi tube which cuts off any excess of air. In most burners this restriction is fixed and cannot be altered by an operator, or can it?
A few manufacturers of older lamps made burners with methods of altering the air volume. One of the best known is the shutter provided on Aladdin model PL1 lanterns where one of the two air tubes can be closed off completely. This shutter has to be closed if the lantern is used with gasoline but open when used with kerosene. This is because of the difference in calorific value of the two fuels. However this has no bearing on what I want to look at here. In burners where the orifice is inside the air tubes such as Coleman and Tilley types the position of the orifice inside the tube is critical. Generally the orifice will be set to about half way into the tube. Tilley types can be disregarded here as the venturi is constructed in such a way that the burner has an internal shoulder sitting directly on the vaporising tube and the position is therefore fixed. Wear on this shoulder will slightly alter the position over many hours of use but this is not something that can be corrected except by replacing the burner. What I am concerned with is alterations and adjustments that may be made inadvertently when a restored lamp or lantern is re-built. So when a lamp has been dismantled and rebuilt you need first to take a look at the design and decide if the position of the gas orifice relative to the air intake can be altered in any way. In most, but not all, Swedish burners it cannot and neither can it in most English models. However in Coleman and Petromax types it can, and getting this wrong WILL affect the performance of the lamp.
In a Petromax there is however some adjustment which can be made to the position of the U shaped venturi within the inner hood, so although the distance can be altered, the solutions are different in the two lantern types. I will discuss them separately for convenience although what is true for Coleman is also true for Petromax if not as important.
Coleman valves are normally fixed into the fount with a tapered brass thread, factory fitted with a thread sealer and screwed in very tight. In general the distance the thread enters the fount will be near enough equal in different lanterns of the same model so the relative heights of the gas tip and venturi are fixed. However when replacing a valve it is possible to achieve a gas tight seal over a considerable range and, depending on the sealer used, the valve will often be higher than originally designed. "Not by much?" I hear you say. Well that depends on what "much" means. Coleman valve threads have a 27 tpi pitch so one full turn is about one twenty seventh of an inch or 0.037" If a Teflon tape sealer is used the valve can be sealed maybe two full turns or more higher than it should be. So the height can be altered by almost a tenth of an inch. Given that the air tubes are generally around a quarter inch Dia which is 0.25" then even one turn difference is 14% of the width of the air tube. Two full turns is getting on for 30% and bearing in mind the gas tip should be in the middle of the air tube, two turns difference puts the discharge point almost out of the tube. So the height of the valve is actually very critical indeed. Even a quarter turn can make a difference to the air to fuel ratio. When rebuilding a Coleman most people do not turn the valve in as far as it was originally because that is a high torque and a tape sealer will often mean an even tighter joint. This results in a high proportion of rebuilt lanterns with a rich mixture because the air volume is reduced. A rich mixture means a yellow and reduced power light. The solution is to measure the height of the valve in the fount before removing it for service, and then making sure it goes back in the same place. Measurement may be difficult but a simple count of the turns and recording of where the valve knob was relative to the filler cap or pump will ensure you know where to screw it back to. This of course assumes it was right in the first place and not too high because of some previous owner getting it wrong.
The problem in Petromax lantern types is easier of solution because there is nearly always some adjustment possible of the location of the U shaped venturi tube within the inner hood. The valve height is therefore less likely to be critical. Most lanterns of this type provide a gauge to determine the gap between the gas orifice and the lower end of the venturi. Often this guage is part of the spanner (Wrench) which incorporates a gap measure. In some of these lanterns the venturi is held in place with a locking plate screwed tight to the tube. In use this device often works lose and the venturi then drops and reduces the air gap. It is therefore important to regularly check the air gap to ensure it remains correct. However in some of this type of lantern the venturi is screwed to the inner hood by a pair of locating nuts on the tube just above the mantle carrier. This type do not often work loose although a check now and then still pays dividends.
So altering the mixture in a lamp is possible in more cases than most people think and badly adjusted lamps will not perform well. There are of course other reasons for burners performing badly which include things like widened gas tips, partial blockages of air tubes, incorrect mantle sizes, restricted fuel supply, and air leaks into burners, to mention but a few. However when restoring a lantern it is as well to look at the design, to see if the mixture can be adjusted, before dismantling. It is also worth looking into when you have a lamp or lantern that does not seem to work well even though it is all squeaky clean and fitted with a new generator and such.
Well yes, sometimes it can. Petromax type burners often have a small adjusting screw set into the venturi tube just above the opening. This operates a flat narrow butterfly valve inside the venturi which can interfere with the gas flow by turning it to present either the flat face or the edge to the direction of flow. It is arguable whether this is actually of any practical use but it does permit some adjustment to the air volume and therefore it is possible to compensate to a certain extent for changes in atmospheric pressure, for instance due to altitude. A lot of lamps like Petromax which use a U shaped venturi tube also permit some alteration to the position of the venturi which will alter the air gap or distance between the orifice and the open end of the venturi.
Coleman and Petromax lantern types are completely different in design and appear to have little in common but they do have one common design feature which can be altered to affect the mixture. In both types the venturi is fixed in position vertically by the body of the globe cage and this in turn is a fixed distance from the top of the fount due to the dimensions of the globe cage support collar or base rest. In a Coleman the air tubes are nearly always fixed to the base plate directly and in a Petromax the inner hood, which contains the venturi, sits directly on the top rim of the globe cage. So the position of the part of the lantern to which the air tubes are fixed cannot be adjusted relative to the fount.
OK so I can hear you saying "What is the point of all this" After all you can dismantle a lantern and then all the bits go back where they came from so it has to be right doesn't it? Well actually no they don't, and it took me a while to realize this, so it is not immediately obvious. We have established that the venturi is a fixed distance from the fount; well so is the generator but not for the same reason. The generator is fixed to a valve unit which is screwed directly into the fount. In a Petromax lantern the valve unit has a lead washer as a seal and whilst there will be little difference it is possible to alter slightly the thickness of this seal and that change in dimension will alter the relationship between the gas orifice and the venturi. In a Coleman type this is much more important.
