Ensuring your air compressor is sized adequately for your plasma cutter and its intended usage.
Sizing a compressor for a plasma cutter depends on a couple of things:
1. The minimum flow/pressure requirements of the plasma cutter you intend to use. The manufacturer can provide the specs and they can vary from model to model. A small Hypertherm Powermax30 will use about 4.5 cfm (cubic feet per minute) at 90 psi inlet pressure. A Hypertherm Powermax45 uses 6 cfm at 90 psi. If you have a small compressor with exactly the same cfm rating as the plasma, then the compressor will run continuously when you are plasma cutting; the size of the air compressor tank will allow the plasma to run for a few seconds (or minutes if the tank is huge) until the pressure drops a bit (usually to about 90 psi), then the compressor will run to try to maintain the pressure.
2. How much cutting are you planning to do? If you are a weekend hobbyist just cutting an occasional piece of steel for a project, then a compressor with a cfm rating slightly higher than the plasma will likely be adequate. If you are doing a lot of cutting, cutting thick plate (same air consumption but slower cut speeds = longer cut time) or are using the system to earn a living, and then size the compressor at 1.5 to 2 times the plasma system requirement.
Another note on air for plasma systems and all air tools: Compressors take air at atmospheric pressure and increase the pressure and store it in a tank. Humidity in the air is often condensed in the tank and in the airlines, more so during humid weather. Drain your compressor tank often, and if moisture in your air lines is at all noticeable, it is a good idea to add additional filtration to the air system that absorbs or removes moisture.
How to be sure the air for the plasma system is free of moisture and particulates.
Most air plasma systems from reputable manufacturers have an onboard particulate filter and many also have a coalescing filter with an auto drain that will remove some moisture from air systems. For most low use situations (hobbyists and light industrial users) the onboard filter dryer is adequate. There are many situations however that will require additional steps are taken to ensure moisture does not affect performance of the plasma cutter, often these steps are needed when the system is used in extremely humid environments, or during those few weeks of the year when the hot summer weather brings humid conditions.
Moisture that forms in air lines has a tendency to condense into larger drops when the air pressure decreases as it is entering the plasma torch. When these drops enter into the high temperatures (as much as 20,000 degrees F.) in the plenum of the torch, they immediately break down into oxygen and hydrogen, which alters the normal chemical content of air in the torch. These elements will then dramatically change the plasma arc which causes the torch consumables parts to wear very quickly, altering the shape of the nozzle orifice and dramatically affecting cut quality in terms of edge squareness, dross formation, and edge smoothness.
As a minimum with air systems that contain moisture be sure to drain the receiver (tank) on the air compressor at least daily. If water comes out of the bottom petcock valve of your tank, then there are moisture particles in your air lines. Draining the tank regularly will minimize the moisture and the above effects, but will not eliminate it. In order to ensure that the air receiver tank stays as moisture free as possible, install an automatic drain system that rids the tank of moisture with every pressure building cycle.
The next minimal steps if moisture is found in the receiver would be to install a particulate filter that is designed to either trap water through absorption (the filter element literally absorbs water and must be changed after it is near saturation). This type of filter (a popular one is the MotorGuard M80 filter canister with its replaceable elements) should be mounted at least 20 feet downstream of the compressor, allowing the heat generated by the compressor to cool a bit as this will help to condense the water droplets and trap them in the absorption filter. Another type of moisture trap is of the coalescing (kind of a centrifugal filter that spins the heavier moisture and drains it through an automatic drain) type typically used in many automotive paint spraying applications.
If the plasma system is used at medium to high duty cycles, for production use, or is in a climate with extreme humidity, then higher technology steps may need to be taken in order to remove moisture from the air system adequately. A refrigerated air dryer, as its name implies, uses a small refrigeration circuit that cools the compressed air to its dew point. This cool air then more easily displaces the moisture into a coalescing moisture trap with an auto drain. There also are a variety of desiccant filter systems that will adsorb (yes that is spelled correctly!) moisture through a chemical process, drying the air in the system very effectively.
I always refer plasma chuting shops that are looking for air drying answers to the same companies that help condition air systems for automotive body and paint shops, as the air flow and dryness requirements for paint spraying applications are very similar to plasma cutting needs. Here is a nice schematic of a complete shop plumbing system as found on the Sharpe Air Systems website. It may be a bit overkill for a small plasma cutting operation, however it correctly shows methods of plumbing that include drop, legs for draining the system, shows that the clean dry air is best sourced at the highest points in the plumbing system, as well as showing best locations for filters, regulators and drying equipment.
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*Image courtesy of Sharpe Air Systems*
Determining if your plasma system is receiving adequate inlet pressure and flow from the air system.
Every plasma system has input minimum and maximum air or gas flow pressure specifications. Most plasma systems also have a choice of consumable parts for cutting at different amperage levels for different thickness materials. To determine if the plumbing system leading up to the inlet of the plasma is adequate follow these steps:
In your plasma system operators’ manual look up the inlet gas pressure range, this will be found in the gas specification section. As an example, the Powermax85 air plasma system specifies a range from 85 to 135 psi air pressure at the inlet.
Install a pressure gauge right at the inlet to the plasma system that will accurately measure the proper inlet pressure.
Install the largest amperage set of consumables for the plasma system. Example: with a Powermax85 install the 85 amp shielded consumables. Activate the system so that air is flowing at the torch, set the cutting gas flows or pressures according to the cut charts in the operators’ manual. (Note that the cut gas flow/pressure is not the same as the inlet has flow/pressure).
Observe the newly installed gauge at the inlet to the plasma system. You will notice the pressure on this gauge is higher when there is no gas flowing at the torch; this is static pressure and it should be no higher than the maximum pressure specification from your operators manual (Powemax85 maximum is 135 psi). When the air is flowing the gauge reading will drop, this being dynamic pressure. If the gauge drops below the minimum requirement (Powermax85 is 85 psi), then there is a restriction in the plumbing system that will affect operation.
Summary: A system with a properly sized gas or air plumbing system will show a minimal pressure drop between static and dynamic pressure as measured right at the inlet to the plasma system. If the drop is excessive, then one of the following is suspect:
- Hose or pipe inside diameter is too small.
- Hose or pipe length is too long.
- Too many fittings. Each fitting on a plumbing system creates a small pressure drop.
- Filters or moisture/oil traps are incorrectly sized or are clogged/saturated.
By Jim Colt | Hypertherm Inc.