02-07-2014, 02:52 PM
MODEL FOR OPERATING COSTS OF PLASMA CUTTING
MODEL FOR OPERATING COSTS OF PLASMA CUTTING.doc (Size: 448 KB / Downloads: 57)
Abstract
Operating costs of plasma cutting should form the basis for evaluating its profitability. Acceptable cut quality, increased traverse speed and lower cost per meter of the cut together with cheaper equipment and possibility of cutting various materials assure wider application of this procedure. Optimizing a plasma cutting operation based on operation cost is typically a trial-and-error process that is usually inspired in recommendations given by manufacturers of plasma cutting tools and consumables. The operating costs for plasma cutting are presented in this paper
INTRODUCTION
The plasma-arc process had its origin almost 70 years ago. In 1941 the U.S. defence industry was looking for better ways of joining light metal together for the war effort and, more specifically, for the production of airplanes. Out of this effort, a new welding process was born. An electric arc was used to melt the metal, and an inert gas shield around the arc and the pool of molten metal was used to displace the air, preventing the molten metal from picking up oxygen from the air. This new process "TIG" (Tungsten Inert Gas) seemed to be a perfect solution for the very specific requirement of high-quality welding.
By 1950, TIG had firmly established itself as a new welding method for high-quality welds on exotic materials. While doing further development work on the TIG process, scientists at Union Carbide's welding laboratory discovered that when they reduced the gas nozzle opening that directed the inert gas from the TIG torch electrode (cathode) to the work piece (anode), the properties of the open TIG arc could be greatly altered. The reduced nozzle opening constricted the electric arc and gas and increased its speed and its resistive heat. The arc temperature and voltage rose dramatically, and the momentum of the ionised and non-ionised gas removed the molten puddle due to the higher velocity. Instead of welding, the metal was cut by the plasma jet.
PLASMA CUTTING
Plasma cutting is a process that is used to cut steel and other metals of different thicknesses (or sometimes other materials) using a plasma torch. In this process, an inert gas (in some units, compressed air) is blown at high speed out of a nozzle; at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. The plasma is sufficiently hot to melt the metal being cut and moves sufficiently fast to blow molten metal away from the cut.
The characteristics of the plasma jet can be altered greatly by changing the gas type, gas flow rate, arc current, arc voltage and nozzle size. For example, if low gas flow rates are used, the plasma jet becomes a highly concentrated heat source ideal for welding. Conversely, if the gas flow rate is increased sufficiently, the velocity of the plasma jet is so great that it ejects molten metal created by the hot plasma arc and cuts through the workpiece.
COST OF PLASMA CUTTING
How to calculate cost of operation and establish metrics for improvement? There are many costs associated with a mechanized plasma-cutting machine beyond the capital equipment purchase. There are general overhead costs, maintenance costs, service call charges, gas costs, consumable and torch costs, and electricity charges. The plasma-profiling machine is also likely to have a host of auxiliary equipment that may also be considered: material handling equipment, environmental control equipment, safety gear etc. The labor component for plasma cutting may include machine operators, helpers, maintenance personnel, secondary operation workers and others. The intent of this article is to review the most significant variables affecting annual cost of operation and to establish metrics for improvement.
In typical plasma cutting operations there are four major ongoing costs: cost of power, gas, cost of consumables, and cost of labor.
OERATING COSTS
The major power consumer in a cutting machine is the DC power supply. Most of the energy consumed by the system is put directly to work on the material in a very hot energy-dense arc. To get a rough idea of the power consumption of plasma system is multiply the amperage output by the average operating voltage. To calculate kilowatts of input consumed, multiply by a power supply efficiency factor of around 85%. Example an 80A plasma system has an average operating voltage of about 100V. This means the power supply puts out 6.8 kW (8kVA x 0.85 = 6.8 kW).
To arrive at daily or yearly power consumption multiply times the average up-time or arc-on time in a day. Arc-on time is the amount of time actually spent cutting over a given time interval. This can be measured by a pierce and arc-on time counter, or calculated from programming distances and speeds and daily throughput. Arc-on time will vary with material type and thickness, size of cut pieces, material handling, machine speed, torch height control speed, and many other factors. Most shops average about 55% actual arc-on time. That means in a given 8-hour shift only 4.4 hours are spent cutting. In the year we have 1144 hours are spent cutting (260 days).
CONCLUSION
Plasma cutting is a process that is used to cut steel and other metals. Here are some recommendations for optimizing plasma cutting machine to lower cost of operation and increase productivity