8

The Gas-Oxygen Torch

8.1TheDynamics of theGas-Oxygen Torch

8.1.1Types of Gas-Oxygen Torches

There areoperations in thelaboratory that require more flame heat than a Bunsen or Fisher burner can provide. Forexample, if youwish to fire-polish the end of a chipped glass tube or "seal off' (also called "tip off) a sample for NMR study, you will need to usea gas-oxygen torch.

A Bunsen burner burns methane (lab gas) and the oxygen available in air (=21%) and cannot provide sufficient energy to effectively work borosilicate glass, which makes upmost of the glassware used in a laboratory.* To obtain more energy you must use a flammable gas (methane or propane), pure oxygen, anda torch that is specifically designed for this type of use. There aretwotorch designs for gas-oxygen combustion, one is known as a premix torch and the other is known as a surface-mix torch.

The designations premix and surfacemix refer to the region of thetorch where (in relation to the tip)thegasandtheoxygen aremixed before combustion. A premix torch design combines thegasandoxygen in theneck of the torch before they reach the tip.A surface-mix torch keeps the gas and oxygen separated until they are mixed at the moment of combustion. Both types of torches are represented in Fig. 8-1. In addition to these designs, there are larger table models (of both designs) typically used by glassblowers. These table models are impractical for laboratory use because they are not portable and cannot be used for operations such as sealing off tubes.

Superficially, the torches look similar. However, there are distinct advantages and disadvantages toboth. The premix design is less expensive, and because it has one flame emitted from thetip(see Fig. 8.2)it is possible to obtain precise heating

•Typically, the first opportunity that one hasto do anyglass work is making an eye dropper or bending a tube with a Bunsen burner. This is possible because the glass used wassoda-lime glass. Most of the glassware used in thelaboratory is borosilicate glass, which hasa higher working temperature than soda-lime glass. Formore information on thedifferent types of glass used in thelaboratory, see Sec. 1.1.5.

477

Fig. 8-1 Surface-mix and premix torch designs. The illustration of the surface-mix torch is based on the Sharp Flame Hand Torch made by the Bethlehem Apparatus Company, Inc. (Hellertown, PA 18055), reproduced with permission. The illustration of the premix torch is based on the National Torch made by Premier Industries (Fridley, MN 55432), reproduced with permission.

in one specific location. On the other hand, although it is possible to control the size and character of the flame to some degree by controlling the gas and oxygen flow, radical flame size changes require changing the removable tip with one of five different available sizes. In addition, the premix torch's flame is louder than the surface-mix torch.

Probably the biggest disadvantage of a premix torch is the characteristic of the torch to pop loudly when attempting to change the flame size or when turning the flame off. This popping sound will occur if the rate of gas combustion is faster than the gas flow rate from the tip. If the gas speed is too slow, the flame can enter the neck where premixed gas and oxygen are ready to instantaneously combust (explode) with a pop. This noise invariably does more damage to your nerves than anything else. If your torch has popped, turn off both oxygen and gas needle valves*, then open the oxygen valve until you hear a loud hissing noise for several seconds. Finally, close the oxygen valve and relight the torch. To prevent the pop, maintain steady and full gas and oxygen flow rates. For more of poping prevention, see Sec. 8.1.3.

*Be sure to close the gas valve before fully opening the oxygen valve as the [greater] oxygen pressure could force oxygen into an open gas valve and into the gas line where potential damage could occur if a "pop" is created.

Gas is released from long thin

tubes.

Fig. 8.2 The various tips of the surface-mix and premix torches.

When using the premix torch to work on borosilicate glass, use a gas-oxygen tip design, not a gas-air tip* (both tip designs are represented in Fig. 8.2). Using a gas-air tip with oxygen will cause a great deal of popping and may cause damage to the tip or torch. The gas-oxygen tip comes in a variety of types and sizes: " 1 " is the smallest size, and "5" is the largest. A size "3" tip is a good, general-size tip for most needs around the lab.

The surface-mix torch (see Fig. 8.2) has a nonremovable, one-piece tip with multiple holes, or channels, that run the length of the neck. The oxygen passes to the tip using the area surrounding the gas tubes. This design prevents any mixing of the oxygen and gas until they meet at the tip, where combustion takes place. These torches are impossible to pop, and glass tubing from sizes 4 to 35 mm can all be heated without changing the tip (there are no tips to change). The surfacemix torch cannot be used as a gas-air torch.

In both torches, different flame sizes are obtained by controlling the amount of gas and oxygen. However, in the surface-mix design, you do not need to change tips to achieve the same range of flame sizes that are capable with the premix torch. On the other hand, the premix torch can achieve significantly smaller, and/ or more concentrated, narrower flames than the surface-mix torch. The premix torch's flame has a cooler hole in the center of the bushy flame, while the surfacemix's bushy flame is more uniform throughout (see Fig. 8.3). Some people like the subtle heat variations that are possible in the premix design, while others prefer the uniformity of the surfacemix. All in all, the decision of which type of torch to use (surfacemix or premix) is based on the personal likes and dislikes of the user. For most laboratory uses, either is sufficient.

The connection between the gas hose connection of the torch and the gas source can be made using Tygon, an amber latex, or any similar tubing used to connect

*To use a premix torch with a gas-air tip, connect the hose connection for the green knob to a house air supply, or a tank of compressed air, rather than to the regulator of an oxygen tank.

Suriacemix Premix

Fig. 8.3 Surface-mix and premix design flame characteristics.

gas lines in the lab (or a propane tank).* (The advantages and disadvantages of amber latex versus Tygon tubing are discussed on page 290.) The oxygen hose connection can also be connected using Tygon, a heavy-wall amber latex (V4 in. x '/g in.), or any similar tubing used to connect a regulator attached to an oxygen tank.1^ Open any needle valve on the regulator fully to control the gas flow from the needle valve on the torch, not from the regulator. Paired tubing is available where one tube is red or orange and the other tube is green. This tubing can be handy to know which tube is carrying gas (the orange or red) and which is carrying oxygen (green). Laboratory gas pressure is typically so low that there is essentially no chance for gas pressure to blow the tubing off the torch. If the gas is from a propane tank, the pressure may be sufficiently great to potentially cause the tubing to be forced off. The same potential exists with the oxygen tube. However, if the oxygen regulator is set for about 5-10 lb. of delivery pressure, this is extremely unlikely. Regardless, to ensure safety, either a screw clamp (see Fig. 1.11) can be placed on the torch barbs (hose connections) or some nichrome wire can be wrapped around the barb and secured tightly with a pair of pliers.

8.1.2 How to Light a Gas-Oxygen Torch

Although it may seem straightforward, lighting a gas-oxygen torch seems to provide a challenge for a surprisingly large number of people. Probably the biggest mistakes in torch-lighting are either (a) not allowing enough time for the gas to reach the torch tip or (b) flooding the torch region with too much gas. The former is similar to turning on the shower water but not waiting for the hot water to reach

*Acetylene should not be used on glass because it is a dirty gas and can leave undesirable deposits. tBefore opening the main tank valve, set the outgoing pressure of the oxygen regulator to zero, then bring the pressure up to 5-10 lb. Too much pressure on the hoses can blow them off the hose connections or cause the tubing to expand like a balloon.