Multiplaz-3500 Evaluation, Part 05: Comparing the Multiplaz Cutting Torch
DISCLAIMER!
Let me emphasize that I will not be able to tell you whether the Multiplaz-3500, or any other piece of equipment, will be a good investment for you. Only you can decide that. My intent is to provide as much factual information as I can about the Multiplaz-3500 so that others in our company can make an informed decision about that. The company has no objection to my sharing the information with you as long as I leave their name out of it and make it clear that I am not endorsing any particular product.
DISCLAIMER!
One of the more difficult questions that comes up when doing an evaluation of this kind is “What standard(s) does one use to compare the various aspects of performance?” For some properties, such as size, weight, voltage, power, and temperature, the standard of measurement is well defined. For others, there is no agreed upon standard, and so different manufacturers use the same words to describe performance that is quite different. What usually results is that evaluators make lists of the properties and performance of comparable units and then try to compare the units in some way. Or, alternatively, they may just present a table of results for comparable units and then let the viewer do the comparative ranking, depending upon what properties or performance aspects are the most important to them. In the case of the Multiplaz-3500, there is only one other unit that I know of that I would consider comparable in design, and that is the Fronius TransCut 300.
Another alternative is to compare units not on the comparability of design, but on the comparability of performance doing the job for which they are intended. That is, compare units that are intended to perform a certain operation, say cutting 6 mm mild steel sheet. The idea of using performance standards (rather than static standards like output current or rate of flow or type of technology) for comparison is one that has gained popularity in the last few decades, and for good reason. One needs to look at comparable units for the same intended purpose before one can decide if the unit in question is a good investment.
Another useful approach is to ask what properties (or combination of properties), from a scientific standpoint, would be expected to affect the performance and then to look at those properties for a given unit. This requires that one first understand the science involved in the operation of the unit. Hence, we will dive into a little (only a little) of the physics and chemistry of what I will loosely call “flame cutting” of metals.
We all know that if we heat them to a high enough temperature, most metals will melt, that is, change from a solid to a liquid. If some force is then applied to the melted metal, the metal can be made to flow. In order to “flame cut” metal, one wants to use some kind of hot flame (whether generated by electrical arc or by chemical reaction) to melt a line of metal (usually sequentially) through its entire thickness and then to somehow force the melted metal out from between the non-melted metal on each side. That “somehow force the melted metal out from between the non-melted metal on each side” is usually done by blowing on the melted metal with a gas stream.
In order to blow the melted metal out of the way, one must overcome the surface tension that tends to hold the melted metal to the unmelted metal. The surface tension can be decreased by increasing the temperature of the melted metal. This requires putting more energy into the workpiece and also results in a larger Heat Affected Zone (HAZ) because the rest of the workpiece is also getting hotter. One can also overcome the surface tension by blowing on the melted metal with more force. However, unless the gas stream used is heated sufficiently, the gas will tend to cool the melted metal, making it harder to blow out. So what one wants is a stream of gas that is hot enough to melt the metal all of the way through and that is forceful enough to then blow the melted metal out. Ideally, one would have independent control of the heat input and the force of the gas.
The earliest methods of “flame cutting” used chemical reactions (e.g., fuel gas + oxygen gas) along with either (1) a high enough flow of the fuel and/or oxygen gases themselves, or (2) a supplementary stream of gas (typically air) to “blow” the melted metal out of the workpiece. With the advent of practical electrical arcs, such as the carbon electrode arc, the second approach was used. In either case, the basic approach is “melt the metal all of the way through and blow it out of the way”. The problem that arises with the use of chemical reactions is that the heat input and the force of the gas are directly related. So as one increases the heat input, the force of the gas also increases in roughly the same proportion. Thus, the ratio between the energy available to melt the metal (and the amount of metal melted) and the force available to blow the melted metal out can only be changed significantly by changing the design of the oxy-fuel cutting torch nozzle and over the decades the design of cutting torch nozzles has been optimized about as much as is possible. However, except for ferrous metals which react with the oxygen gas and change the heat/force ratio, an oxy-fuel torch does not give a satisfactory cut in most metals.
The use of an electrical arc with a separate gas stream makes it possible to independently control the heat input (i.e., the rate at which the metal is melted) and the force of the gas used to blow the melted metal out. This allows one to adjust the ratio of these two parameters in order to optimize the cutting of the metal. With the discovery of the mechanically-constricted plasma arc, it became possible to have very high heat input in a very small focused spot, so that a very narrow cut (kerf) could be readily achieved. The technology of what are usually just called “plasma cutters” has developed enormously in the last 55 years and the air plasma cutter is now the preferred technology for “flame cutting” of most metals. The general design of the air plasma cutting torch (including cathode material, nozzle material, orifice size, and operating conditions) has been developed to the point that the torches are almost the same for all of the manufacturers. By adjustment of the plasma arc current one can control the heat input to the metal. By use of the air pressure (flow rate) to the torch one can control the force of the gas stream. The force available to blow the melted metal out increases with the ratio of the flow rate of the air to the heat input to the torch (amount of metal melted). Typical values are 25 to 45 g(air)/min per kW(max rating) in the arc. (See the table below.) At less than the maximum power into the torch, the ratio of gas flow rate to arc power (amount of metal melted) increases and the kerf typically becomes narrower and cleaner.
The Multiplaz-3500 (water) and the Fronius TransCut 300 (water plus 10% to 25% ethanol) units use the evaporation of a liquid to produce the gas for both the plasma and for the blowing out of the melted metal. The evaporation of the liquid occurs in the torch head. Both units have the same inherent problem as the oxy-fuel cutting torch in that the heat produced (the amount of metal melted) and the force of the gas to blow the melted metal out are directly related. And, just as in the oxy-fuel cutting torch, the heat/force ratio is higher for ferrous metals that react with the high-temperature oxygen in the plasma. This makes it harder to blow the melted metal out of the kerf, so for a clean cut more gas force is required. The situation with plasma cutters is different from the oxy-fuel torch in that the very high heat input in a very small focused spot allows the efficient cutting of almost any electrically-conductive material, provided that the gas force is sufficient to blow the melted metal out. For metals with low surface tension of the melted metal, plasma cutters are particularly efficient.
A look at the last column in the table below shows that the ratio of (gas mass flow rate / maximum power into the plasma arc) is similar for the Multiplaz and the Fronius units, but both have a much lower ratio (10 to 20 times lower) than the air plasma cutters. Measurements of the Multiplaz cutting rates for various thicknesses of metals confirm that the force of the gas to blow the melted metal out is marginal. (See Part 04 of this Evaluation.) This is especially true for mild steel where the additional heat from the iron reacting with the hot oxygen gas in the plasma makes the rate of melting too fast for the gas to blow out. The heat of the plasma jet itself is more than adequate to melt through the metal and it does so very well. But the force of the gas is not adequate to blow the metal out and so the melted steel tends to hang as a large drop(s) under the kerf due to surface tension. Then a layer of cooling metal tends to fuse the bottom edge of the kerf back together as the plasma moves forward. The thickness of the fused layer (~1/32”) seems to be almost independent of the thickness of the steel (1/16” to 1/4”) and it is readily removed in a second pass. It appears that the performance of the Multiplaz cutting torch is about what would be expected, given the technology used.
It is possible to increase the gas flow rate by evaporating more water with a separate additional heater, but other considerations may make it impractical, especially for the Multiplaz cutting torch. In the Multiplaz torches, the liquid is stored in the torch. The amount is 60 mL (60 g) of water. At a maximum mass flow rate of 4 g/min, the torch can operate for 15 minutes and then must be refilled. With the Fronius unit, 1.5 liters (~1500 g) of fluid is stored in the power supply unit and pumped to the torch. At a maximum mass flow rate of 8 g/min, that torch can operate for about 180 minutes before refilling. If the mass flow rate were increased by a factor of 2 or 3, then the operating times would be only 5 to 7 minutes for the Multiplaz torch and about 60 minutes for the Fronius torch. The small useful time would probably make the Multiplaz cutting torch impractical, but for the Fronius torch the time does not seem unreasonable, as long as refilling is simple and inexpensive (not true at the moment). There are also other technical and safety concerns:
(a) If the high pressure steam is generated in the power supply unit, it must be transported to the torch through tubing larger than what is used now for the liquid water. The tubing will need to be about the size of what is used for compressed air in a typical air plasma torch. What if the tubing springs a leak and steam sprays out? This is a much more serious problem than if room temperature compressed air leaks out.
(b) If a second (presumably resistance) heater is to be in the torch, then the torch will be larger and heavier. If the heater is to be independently controlled, there will need to be additional electrical leads.
(c) There will be a time lag in the heating and cooling of the auxiliary heater, no matter where it is placed. This may or may not be a problem.
For both the Multiplaz and the Fronius torches, there appears to be enough power available to evaporate more water. It takes about 43 watts to convert 1 gram of water per minute from a liquid at 20 degrees Celsius (68 F) to steam at 100 degrees Celsius (212 F), and about 37 more watts (about 80 watts total) to raise the temperature of the steam to 1000 degrees Celsius. The maximum total power into the plasma arc is 2.5 kW for the Multiplaz torch and 3.0 kW for the Fronius torch. Of this power, approximately 320 watts and 720 watts, respectively, are now used in making 4 grams per minute and 8 grams per minute, respectively, of steam. To double the mass flow rate of gas out of the nozzle, an additional 320 watts and 720 watts, respectively, would need to be used to make additional steam. NOTE THAT THIS ENERGY IS NOT LOST FROM THE PLASMA. The rate of energy flow out of the nozzle is essentially unchanged. Some of it is merely converted to increase the force (more correctly, the momentum) of the gas coming out of the nozzle.
If it is all so simple, why hasn’t Fronius done it? Sufficient energy is certainly available from the arc to increase the mass flow rate. The problem is how to efficiently use it to produce more steam. This is a general problem, not only a problem in this context. The efficient production of steam has been an engineering problem for hundreds of years, and the crux of the problem centers around the transfer of heat from a solid surface to a liquid, particularly a liquid at its boiling point. Further complicating the problem is the nature of heat transfer from the plasma arc. Obviously, if the transfer were very efficient, the inner core of the plasma arc wouldn’t be able to sustain a temperature of 8,000 to 20,000 degrees Celsius.
What all that means in terms of the Multiplaz and Fronius torches is that, at least with the present designs, it is not possible to transfer heat from the plasma through a solid surface and into the liquid water at a rate that is fast enough to produce more steam. Until that problem is solved (or an auxiliary source of heat is used to make steam), the water plasma torch will always have a very low ratio of (gas mass flow rate / maximum power into the plasma arc). That low ratio will limit the usefulness of such a cutting torch in most applications.
to be continued
larry lee
TABLE (The posting software insists on removing tabs and extra spaces, so I have added colons and underscores to try to make it more readable.)
Plasma Fluid, Mass Flow Rate, and Maximum Arc Power for Some Plasma Cutters
Plasma Cutter: Plasma: Mass___: Max arc: Duty: Mass flow/
Model Number: fluid__: flow___: power__: cycle: Arc power
Model Number: used__: (g/min): (kW)___: (%)__: (g/min/kW)
Multiplaz-3500: Water: 4: 2.5: 100: 1.6
TransCut 300: Water +: 8: 3.0: 35: 2.7
AirForce 250ci: Air (Int): 37: 1.3: 35: 28
Spectrum 125C: Air (Int): 41: 1.3: 35: 31
Tomahawk 375: Air (Int): 96: 2.3: 35: 43
Powermax 30: Air (Ext): 119: 2.5: 50: 48
Spectrum 375 X: Air (Ext) 175: 2.8: 35: 63
Tomahawk 625: Air (Ext): 96: 3.8: 35: 25
Cutmaster 42: Air (Ext): 109: 3.8: 40: 29
PCM-750i: Air (Ext): 142: 5.3: 40: 27
Powermax 45: Air (Ext): 204: 5.9: 50: 34
Tomahawk 1000: Air (Ext): 156: 6.2: 50: 25
Multiplaz 7500: Air (Ext): 300: 6.6: 100: 45
Powermax 85: Air (Ext): 227: 12.2: 50: 19
Multiplaz 15000: Air (Ext): 576: 13.0: 100: 44
Powermax 105: Air (Ext): 260: 16.8: 70: 16
CLARIFICATION
Replace the sentence
"Obviously, if the transfer were very efficient, the inner core of the plasma arc wouldn't be able to sustain a temperature of 8,000 to 20,000 degrees Celsius."
with
"Obviously, if the radial heat transfer were very efficient, the inner core of the plasma arc wouldn't be able to sustain a temperature of 8,000 to 20,000 degrees Celsius. It is the axial flow of the hot plasma that transfers most of the heat."
CORRECTION
"720 watts" should be "640 watts".
larry lee
Replace the sentence
"Obviously, if the transfer were very efficient, the inner core of the plasma arc wouldn't be able to sustain a temperature of 8,000 to 20,000 degrees Celsius."
with
"Obviously, if the radial heat transfer were very efficient, the inner core of the plasma arc wouldn't be able to sustain a temperature of 8,000 to 20,000 degrees Celsius. It is the axial flow of the hot plasma that transfers most of the heat."
CORRECTION
"720 watts" should be "640 watts".
larry lee