November 6, 2003
Mixing oxygen with fuel gases for brazing, cutting, heating, and welding metal has been around since the early 1900s. Oxyfuel processes have remained in use over the years despite the introduction of other metal fabrication processes, such as arc welding, plasma cutting, and laser materials processing. Finding the most economical method for supplying oxyfuel gases requires a basic understanding of the process.
|Photo courtesy of Weldstar Inc.|
In oxyfuel cutting, an oxygen jet, not the preheat flame, performs the cutting. The oxygen jet oxidizes the material and expels the heated material from the kerf. The purpose of the preheat flame is to heat the material to the ignition temperature and support the cutting oxygen jet during the cutting process.
When mixed with oxygen, fuel gases have the ability to produce sufficient temperature for the preheat flame. The preheat flame is divided into the primary (inner cone) and secondary flames (outer cone); the distribution of the heating capacity is a combination of these two flames.
Fuel gas selection depends on the material's thickness and length of cut. You must determine which gas has the capacity to achieve the necessary primary heat (BTUs) and flame temperature (degrees) to make the cut. The total BTU capacity for the fuel gas becomes more important as the thickness and length of the cut increase. Fuel gases with higher BTU per cubic foot and higher flame temperature are best-suited for cutting.
Another primary application for oxyfuel processes is heating. In this process, transferring heat to the workpiece is the major focus. The flame temperature is not as important as in cutting, since the ignition temperature of the material is not required. The BTU rating of the fuel gas is the determining factor. The following chart supplies basic information on commonly used fuel gases and can be used as a guide in fuel gas selection.
Besides performance characteristics—can the gas perform the required process—several other factors need to be considered when choosing a fuel gas:
Once the fuel gas has been selected, you must decide the supply method. Fuel gases are available in individual cylinders, cylinder trailers, or bulk supply. Which supply you choose depends on several factors:
Each fuel gas has supply method limitations. For example, acetylene normally is supplied only in cylinders with capacities from 140 to 400 cubic foot volume per cylinder, while natural gas is best supplied in pipelines only. Choosing fuel gas that cannot be supplied in an economical method is not very cost-effective; determine the methods of supply available for the fuel gas before the final selection.
Applications play a major role in supply selection. Is cutting the only process to be used, or will both cutting and heating processes be utilized? Cutting and heating require different pressures and flows. Even when the same fuel gas is used for both, determine the maximum requirements.
The number of operators you have also can make a difference in the supply method selection. Sufficient pressure and flow rate to handle peak requirements must be available, or the work cannot be performed in an efficient manner. Production slowdown and equipment damage can occur is the proper pressure and flow are not maintained, adding to the cost of the job. To determine the volume of fuel gas required, multiply the number of operators by the largest volume demanded of the process.
All fuel gases have limitations regarding withdrawal rate from the cylinder. These limitations also must be considered when determining the volume of fuel gas supply required. For example, the maximum withdrawal rate of acetylene from a cylinder is limited to 1/7 of the capacity (that is, a 140-cubic-foot cylinder will deliver only 20 cubic feet per hour of product). When the necessary flow rate exceeds a cylinder's limitations, a larger cylinder is required.
If single cylinders cannot supply the required flow, a manifold of the cylinders is recommended. An automatic switchover manifold offers continuous gas flow and notifies you when the cylinders need to be replaced. A centrally located fuel gas supply system offers a safe, economical method of supplying the fuel gas to the work area, removing the cylinders from the work area and eliminating production downtime because of cylinder replacement. If the volume demand is great enough, installing a permanent bulk gas supply may be justified.
The oxygen also must be supplied to complete the oxyfuel process. The same qualifiers for choosing a fuel gas supply system apply, although they are slightly different for oxygen. The pressures and flows required for oxygen are much higher than those required for fuel gas. Each fuel gas demands a specific volume of oxygen to produce the proper flame. You can determine the necessary amount of oxygen pressure and flow by referring to the operator's manual of the equipment to be used.
Once the volume has been determined, the most economical supply method can be selected. Oxygen can be supplied in high-pressure cylinders, liquid cylinders, or permanent bulk installations. If the demand is high, both high-pressure cylinders and liquid cylinders can be manifolded for additional flow requirements.
Although the volume in liquid cylinders is large (average capacity is about 4,500 cubic feet), the withdrawal rate is limited by the cylinder's ability to warm up the liquid product to a gaseous state. Because of this warm-up period, the cylinder's normal flow rate is 350 to 400 standard cubic feet per hour. Thick-plate cutting or heavy heating requirements easily may exceed this flow rate. Additional cylinders or ambient air vaporizers can increase the flow rate.
An ambient air vaporizer is a series of aluminum extrusions welded together. The vaporizer increases the surface area the liquid oxygen is exposed to, allowing for a faster warm-up period. With a vaporizer, the connections should be made to the liquid withdrawal connection on the cylinder, not the gas withdrawal connection. The warm gas then can be delivered to the workstation after leaving the liquid gas supply/vaporizer system. If the flow demand is great enough, a permanent bulk gas supply may be justified.
Oxyfuel processes still are cost effective for many metal fabrication jobs. Selecting the proper fuel gas, gas supply method for the fuel and the oxygen, and gas apparatus can provide a productive system for years to come.