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What is the best plasma torch coolant?

April 22, 2013

Why does a plasma torch need coolant?

Every plasma cutting system uses some sort of fluid to cool the torch, and prevent the electrode and nozzle from melting due to the high temperature of the plasma arc. On smaller torches, typically anything up to 100 Amps, that fluid can be air, and it can be the same air that is used as the plasma gas. But as the current gets higher, a separate cooling liquid is required to carry away all of the heat.

You would think the biggest source of heat in the system would be the plasma cutting torch, but it's usually the power leads, which consist of a flexible braided metal cable inside of the cooling hoses that connect to the torch body. Because those cables conduct a large amount of electrical current in a relatively small cross-section, they generate a lot of heat that has to be removed to prevent the hoses from melting.

Inside the torch body, the electrode is a big heat source. The plasma arc attaches to the face of the electrode, so a lot of power is passing through a small metal part. To keep it from melting, cooling fluid circulates against the back side of the electrode.

After cooling the torch, the coolant is slightly warmer than it was when it entered the torch. If the coolant is just pumped around and around in a closed circuit, it will keep getting warmer and warmer until it can no longer cool the torch. To keep the coolant temperature low, it is pumped through either a cooler or a chiller.

A “cooler” is typically made up of a simple radiator with a fan blowing air through it. As the coolant flows through the radiator, heat is conducted out of the liquid and into the metal radiator, then conducted into the moving air and carried away. A “chiller” refers to a system that uses a refrigerant, compressor, and heat exchanger to dramatically reduce the temperature of the coolant. Whether using a cooler or a chiller, the torch coolant is the same.

In either case, the coolant is pumped from a reservoir at relatively high pressure, so that sufficient flow rate can be maintained through the small passageways inside the torch, even through long hoses. Insufficient coolant flow will let the torch overheat.

Plasma torch cooling water - Click for larger viewHow a plasma torch is cooled

Inside the plasma torch, the first thing the coolant hits is the back of the electrode. The opening for coolant to flow against the back of the electrode is very thin, so that it moves through at high speed and carries away heat more efficiently. This is the tightest spot in the torch, and the point at which there is the biggest pressure drop in the coolant system.

After cooling the electrode, the liquid circulates back up into the torch body, and then out through a different passageway so it can cool the nozzle. Swirling coolant around the outside of the nozzle helps extend nozzle life.

The coolant then exits the torch and returns to the cooler.

What types of coolant are available?

There are a number of different types of plasma torch coolant available, from many different manufacturers. Most plasma equipment manufacturers offer their own brand of coolant, and there are also several after-market brands. They all use either ethylene glycol or propylene glycol mixed with distilled water as the main ingredients. Most mixtures range from 25 to 50% glycol (75 to 50% water), although there is at least one coolant that has no glycol at all. They offer freeze protection to anywhere from +12º F to as low as -35 F.

Plasma coolant freezing temperature by content - Click for larger view

The glycol additives reduce the freezing temperature. The higher the glycol content, the lower the freezing temperature. But the glycol additive also has an adverse effect, it reduces cooling efficiency, which shortens consumable life.

CautionCaution: Plasma Torch Coolant is Clear! Some torch coolants contain dyes. These coolants may work fine in TIG or MIG torches. However, these should never be used in a CNC plasma cutting machine, where the temperature and voltage is much higher. Colored coolant has known to solidify, creating a jelly-like substance that will clog your torch and water cooler.


Who makes the best torch coolant?

Which of the many different plasma torch coolants is best? I’d like to suggest that it is best to stay with the coolant supplied by the plasma system’s manufacturer. It’s true that the system manufacturer doesn’t actually mix their own coolant; that is contracted out to a chemical supplier who will mix and bottle the coolant according the plasma system manufacturer’s specs. So any off-the-shelf coolant should work, right? The problem with that theory is that sometimes there is more to the equation than meets the eye.

Some plasma coolant mixes will have other trace ingredients, which may or may not adversely affect your plasma system. These could be included as a preservative, or to prevent algae or bacterial growth. Sometimes a coolant may be manufactured to loose specification tolerance. For example, the bottle of coolant may say 30% glycol, but the specification by which it was manufactured might have been 30% +/- 10%. If the actual chemical supplier was located in a foreign country, and had sub-standard quality control, the resulting mix that you get might be significantly outside of that range.

Since the manufacturer of the plasma system is the one who has to deal with any warranty issues related to their equipment, they are the ones who will have the best experience related to what works in their plasma systems, and what doesn’t.

Which is the best coolant mix to use?

So which is the best one to use? The answer depends primarily on the working environment in your shop. Here is what you need to know:

Pure water is the best coolant! That’s right, when it comes to conducting heat away from the electrode and discharging it through a radiator, pure water does the best job. And the better you cool the electrode, the longer it will last. The coolant mix can have a huge impact on consumable life. Changing from a high percentage to a low percentage of glycol in the coolant can increase consumable life by 30% or more!

But plain ‘ole water tends to freeze pretty easily (and there are other reasons NOT to use plain water). So you have to you use a mixture with a low enough freezing temperature to make sure it never freezes. If your shop ever gets down to sub-zero temperatures, but your plasma coolant only protects down to +12º F, it’s not going to end well. The damage caused by a hard freeze will put a machine completely out of commission, and require replacing a lot of hoses, fittings, and expensive plasma control components. That’s why the most important thing is to avoid freezing.

The next consideration is consumable life. So the best plasma coolant to use in your application is the one with the highest percentage of water that still provides adequate freeze protection in your shop environment. 

 

Posted in Cutting Systems , Tagged with Coolant, Plasma

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