Troubleshooting plasma cutting cooling systems
Increase uptime and parts life by keeping cool
What is torch coolant?
Torch coolant is a working fluid composed of de-ionized water and a solute to depress the freezing point. Where there is no risk of freezing, many shops use plain de-ionized water. De-ionized water is used because it is free of conductive ions that can contribute to deterioration of the cooling lines and hard starting of the plasma torch. Figure 1 is a diagram of the internal coolant passages in a plasma torch. The coolant is similar to the agents used in an automotive cooling system with an important distinction: radiator antifreeze has materials to clog up leaks. These are not suitable for plasma systems.
The cooling system
A typical plasma cutting cooling system consists of a motor, pump, cooling lines, torch, flow switch, heat exchanger and reservoir (see Figure 1). Motors 1⁄3 to 1⁄2 hp are standard. Usually the service of a motor is long unless there are constrictions in the system causing the motor and pump to work harder.
Rotary vane pumps are typically used in plasma systems. These have an adjustable bypass screw that increases or decreases the operating pressure and flow of the pump. Bearings in these pumps wear out. The coupling between motor and pump can break. The vanes inside the pump can become worn down until the pump will no longer develop pressure.
Cooling lines carry coolant to and from the plasma torch. These usually contain the main DC power cables as well. Water-cooled power cables prevent the multi-stranded copper wire from overheating. In mechanized applications, cooling lines are usually routed through flexible power track or festooned above the cutting machine. They are subject to leaks from cracked or cut hoses, melted holes, damaged connectors, etc. Constructions are also common, particularly in the return hose from the plasma torch. Debris from parts failures may accumulate in the torch or in the return lead, restricting flow. The copper power cables can also break down from constant flexing allowing filaments of copper to clog up the ends of the hoses. Constrictions in the leads cause reduced flow and increased wear on the pump and motor.
The torch is usually the major constriction in any plasma cutting cooling system. The internal water passages are small in order to increase the velocity of the coolant and maximize heat exchange. Most high amp electrodes are hollow-milled on the inside to create a post of copper around the emitting element. A water tube in a torch extends over this post and forces coolant along the post and against the back wall of the electrode to remove the heat (see Figure 2). It is critical that the torch and coolant remain free of contamination to allow proper flow through these passages.
Flow switches are designed to prevent catastrophic failure of the torch and parts in the event of low coolant flow. Brass block plunger type devices are typically used with a microswitch that must be satisfied for the system to run. Flow switches fail mechanically or electrically: either the plunger can stick or the switch components can fail. Filters: most systems use a particular filter similar to most commercially available water treatment filters. Five-micron paper filters or deionizing filters are standard. These should be changed every few months or whenever a drop in flow or pressure occurs.
Most systems use some type of heat exchanger with radiator fins and fan to remove heat from the working fluid. Some systems acutally chill the coolant using a refrigeration unit. The most common failure in these simple systems is fan motor burnout.
These are typically equipped with a level indicator or float switch. The tank should be checked daily to make sure there is adequate coolant. Low coolant levels can cause air to be introduced into the coolant stream, which reduces cooling. If the system is interlocked, low coolant may cause intermittent or total shut down of the system. Particulate may accumulate in the bottom of the tank. This should be flushed out and removed. Usually the returning flow of coolant can be seen through the filler cap of the coolant reservoir.
Checking the coolant stream: If the coolant pressure or flow appears to fluctuate, check the coolant stream as it enters the top of the reservoir–it should be clear. If it appears milky white then air is probably getting into the system–usually because of a low level in the reservoir. Add coolant over a two-minute period, while the pump is running, to keep the coolant at the recommended level. If this does not correct the milky white conditions there may be a loose connection on the inlet side of the pump that allows air to aspirate into the system. A common problem with rotary vane pumps is that the acorn nut and sealing gasket are sometimes left off after an adjustment is made to the output pressure. The acorn nut and gasket must be in place to prevent air from entering the system.
Checking the coolant path: If the pressure or flow readings are below the manufacturer's recommendation the complete coolant path must be checked. First, check the filter located just before the pump. Next, check the pump itself. Over time pumps will have to be adjusted to compensate for wear. Adjustment procedures vary with the type of pump and should be detailed in the instruction manual. Finally, inspect all hoses, leads and fittings to ensure everything is properly tightened and there are not any kinked or broken hoses.
Checking the coolant flow rate–"the bucket test": To verify the actual flow through the system, perform this simple test. Remove the coolant return lead from the reservoir. Start the pump. Collect the returning flow of coolant in a clean container for a determined period of time (usually 1⁄2 to 1 minute), then shut off the pump. Measure the volume and convert this to gallons/ liters per minute/hour. Compare the findings to the manufacturer's recommendation.
Checking Coolant flow rate–flow meter: An even better way to ensure proper coolant flow is to install a water flow meter just before the return line to the coolant reservoir. A simple 0-4 gpm flow tube (under $50) works well. This is cheap insurance against parts or torch failure.
Check coolant conductivity/ resistivity: Over time coolant can begin to ionize and/or become contaminated with copper particulate or other conductive material. This can cause hard starting of the plasma torch because of the high voltage electricity used to initiate a pilot arc can be dissipated through the cooling water. A conductivity meter can be used to check the coolant. Coolant resistivity should be less than 10 microOhms or greater than 10,000 Ohms/cm resistance.