What really limits the station
A 400 A machine with a duty cycle of 60 percent will not give you 400 A for 6 minutes if the torch is rated for 250 A. The limit of a station is always the weakest element of the current path, and at high currents that element is the torch.
A gas cooled torch carries heat away through the current cable and the flow of shielding gas. A liquid cooled torch has a separate circuit of coolant that takes the heat from the neck and from the cable. There is no single limit of amperes above which liquid becomes obligatory. The rating of a torch is given by the manufacturer separately for every gas, duty cycle and process, and it is those four numbers together, not the current of the power source alone, that say whether the torch will take your shift.
If you first want to understand the number on the rating plate itself, read what a 60 per cent duty cycle at 400 A means (guide in Polish).
How to read a torch data sheet
Below are data from the Abicor Binzel catalogue for the MB EVO series, given to EN 60974-7. We take this series as an example, because the manufacturer shows all four variables in one table. Your torch may have other values; check its data sheet.
Gas cooled torches
| Model | Rating in CO2 | Rating in the mixture M21 | Duty cycle | Wire diameter |
|---|---|---|---|---|
| MB EVO 15 | 180 A / 150 A | 180 A / 150 A | 35 / 60 percent | 0.6 to 1.0 mm |
| MB EVO 25 | 230 A | 200 A | 60 percent | 0.8 to 1.2 mm |
| MB EVO 24 | 250 A | 220 A | 60 percent | 0.8 to 1.2 mm |
| MB EVO 26 | 270 A | 240 A | 60 percent | 0.8 to 1.2 mm |
| MB EVO 36 | 320 A | 290 A | 60 percent | 0.8 to 1.2 mm |
Liquid cooled torches
| Model | Rating in CO2 | Rating in the mixture M21 | Duty cycle | Wire diameter |
|---|---|---|---|---|
| MB EVO 240 D | 300 A | 270 A | 100 percent | 0.8 to 1.2 mm |
| MB EVO 401 D | 400 A | 350 A | 100 percent | 0.8 to 1.2 mm |
| MB EVO 401 | 450 A | 400 A | 100 percent | 0.8 to 1.2 mm |
| MB EVO 501 D | 500 A | 450 A | 100 percent | 1.0 to 1.6 mm |
| MB EVO 501 | 550 A | 500 A | 100 percent | 1.0 to 1.6 mm |
The catalogue values refer to a power source with a characteristic to VDE, described by the formula U = 14 + 0.05 x I. The manufacturer adds two reservations that change the account more than the choice of model: in pulsed welding the rating has to be lowered by 35 percent, and with liquid cooled torches the recommended power of the cooling unit is about 800 W and after the end of welding the circuit is to run for at least four more minutes.
Four variables, not one
- Gas: MB EVO 36 takes 320 A in CO2 and 290 A in the mixture M21. The same neck, a difference of 30 A. With liquid cooled torches the difference reaches 50 A.
- Duty cycle: MB EVO 15 has two numbers, 180 A at 35 percent and 150 A at 60 percent. Giving the value 180 A alone, without the duty cycle, means nothing.
- Pulse: 290 A from MB EVO 36 in the mixture is, after subtracting 35 percent, about 188 A in pulse. Even the liquid cooled MB EVO 401 drops from 400 A to about 260 A.
- Alternating current in TIG: the rating for AC is given by the torch manufacturer separately. Do not convert it from the value for direct current with a coefficient of your own, because the share of each polarity depends on the AC balance you set, and in modern power sources you adjust that over a wide range. If the data sheet gives only direct current, ask the manufacturer for the value for AC and for your balance.
Hence the conclusion for the station: do not ask from how many amperes liquid is needed. Write down your gas, the working current, the required duty cycle and whether you weld with pulse, and then find in the data sheet a torch that has those four things at once.
How much arc time the move to liquid gives
Take two torches from the table. MB EVO 36 at 60 percent gives 6 minutes of arc in every 10 minutes. MB EVO 401 at 100 percent forces no break, so the theoretically available arc time grows from 6 to 10 minutes, that is by 66.7 percent. How much of that you turn into metres of weld depends on the duty cycle of the power source, the preparation of the joints and the breaks of the welder, so treat it as an upper limit, not a promise.
Three cases from the workshop
200 to 220 A: a gas cooled torch is enough
A metalworking shop welding frames from 40x40x3 sections and sheet up to 6 mm works at 160 to 220 A in the mixture. The runs are short, 100 to 300 mm, with breaks to reposition the part. The actual arc time is 1.5 to 2.5 hours a shift.
A gas cooled torch of the MB EVO 24 class, that is 220 A in the mixture at 60 percent, covers this working point. It is lighter, cheaper, it has no coolant hoses and there is nothing in it to lose tightness. A cooling unit would be another device to service with no gain. If the same workshop moves to pulse, the data sheet tells you to go down by 35 percent, that is to about 143 A, and then you have to reach for a stronger torch or for liquid cooling.
260 to 300 A: the limit at which this still works
A workshop welding channel sections and sheet of 8 to 12 mm works at 260 to 300 A. In the mixture the strongest gas cooled torch of this series gives 290 A at 60 percent, so you work at the edge or beyond the data sheet. In pure CO2 the same torch has 320 A, so there is a reserve, but the gas is chosen by the technology, not by the torch.
A gas cooled torch used close to its rating heats up strongly in the neck. The handle becomes unpleasant after an hour, and the contact tip wears faster. Work is possible if the runs are 300 to 600 mm with a break between them for cleaning and setting, but every extra metre of run moves the account towards liquid cooling.
320 to 400 A with a long arc time: liquid is what is left
A plant welding beams and brackets from sheet of 15 to 25 mm works at 320 to 400 A in the mixture, with runs of 1 to 3 metres, several layers on one joint. The arc time reaches 5 hours a shift.
At this point the catalogue of gas cooled torches simply ends: the strongest one has 290 A in the mixture at 60 percent. Torches for 350 to 400 A in the mixture are given by the manufacturer only as liquid cooled, and at 100 percent duty cycle. On top of that comes the comfort of the welder, because a liquid cooled torch at the same current is cooler and thinner in the handle than its gas cooled counterpart.
What it costs at the start
| Item | Net price, indicative |
|---|---|
| Gas cooled torch 3 m, class 250 A | 200 to 450 PLN |
| Gas cooled torch 3 m, class 320 A | 300 to 650 PLN |
| Liquid cooled torch 4 m, class 400 to 500 A | 800 to 1,800 PLN |
| External cooling unit with a trolley | 2,500 to 5,000 PLN |
| Coolant, a pack of about 10 l | 120 to 250 PLN |
In machines of the 400 A class and above the cooling unit is sometimes part of the set or a module slid under the power source, so its cost may already be included. Check this in the list of the set with every machine, because this is an item of a few thousand PLN. We write out the contents of the set with every unit in the category used MIG/MAG.
A customer scenario: when it pays back
Below is the account for one workshop, with the assumptions written out. This is not a rule, only an example for you to put your own numbers into. The workshop works 4 hours of arc a day at 350 A in the mixture, 20 days a month, which gives 80 hours of arc a month. We compare a gas cooled torch pushed to the limit of its data sheet and a liquid cooled torch chosen with a reserve.
| Monthly item | Gas cooled torch at the limit | Liquid cooled torch |
|---|---|---|
| Contact tips | 60 pieces, 240 PLN | 10 pieces, 50 PLN |
| Gas nozzles | 4 pieces, 160 PLN | 1 piece, 45 PLN |
| Necks and liners | 100 PLN | 25 PLN |
| Complete torch, spread over months | 100 PLN | 60 PLN |
| Coolant | 0 PLN | 15 PLN |
| Consumables in total | 600 PLN | 195 PLN |
| Stoppages to change tips | 3 hours, 450 PLN | 0.5 hour, 75 PLN |
| Consumables and stoppages in total | 1,050 PLN | 270 PLN |
The cost of the coolant, 15 PLN a month, already sits in the row of consumables and in the total of 270 PLN. Do not add it a second time. The difference between the columns is 780 PLN net a month.
Energy and maintenance, counted separately
Outside the table there are two items that concern only the liquid variant. The first is the power drawn by the cooling unit: check the draw on its rating plate, usually of the order of 0.2 to 0.5 kW, multiply it by 80 hours of work a month and by your price of energy. The second is the maintenance of the circuit: a change of coolant and an inspection once a year, spread over 12 months. Both together usually give several dozen PLN a month; in this example we assume 50 PLN.
After subtracting them, 730 PLN net of saving a month is left. The investment is a cooling unit for 3,500 PLN plus a liquid cooled torch for 1,200 PLN minus 500 PLN that you do not spend on a gas cooled torch, that is 4,200 PLN. On these assumptions the payback comes after about 6 months.
The figures are an example and depend on how many tips you really burn. Put in your own: count how many packs of tips you buy in a month and how many times a day the welder leaves the station for a new nozzle. That second item is sometimes larger than the first.
Keeping the circuit so that this works
A cooling unit has three things to watch: the level of the coolant, its state and the flow through the heat exchanger. Change the coolant roughly once every 12 months, or earlier if it darkens and leaves a deposit. Used coolant loses its anti corrosion properties and starts to block the channels in the neck of the torch.
Do not top it up with plain water or with automotive coolant. Coolants for welding have a lowered electrical conductivity, which matters with a torch under voltage. Once a quarter blow through the fins of the heat exchanger and check that the fan of the cooling unit turns freely. After a longer weld leave the cooling unit running for a few more minutes, because the torch manufacturer gives this as a condition for keeping the rating.
When not to buy liquid cooling
- Arc time below 2 hours a day and a current that fits in the data sheet of a gas cooled torch: the running costs are too small for anything to pay back.
- Mobile work and assembly: a cooling unit is another piece of equipment to carry, and the coolant hose is sensitive to damage.
- A workshop not heated in winter: the coolant has to have an anti freeze property, and during standstills the circuit has to be protected.
- A spot station for tacking: short arc strikes do not heat the torch enough for liquid to have anything to take away.
In those four situations a gas cooled torch chosen with a reserve against the data sheet is cheaper to keep and simpler to handle. A reserve of rating for your gas and your duty cycle matters more here than the type of cooling.
What to do before you spend money
Check whether your machine has a socket and control for a cooling unit, because not every design supports this. In machines with a cooling unit socket there is also a flow sensor, which blocks welding when there is no coolant and protects the torch. If the machine cannot be extended, a cheaper route is sometimes to change the power source rather than add an external cooling unit.
Machines prepared for liquid cooling you find in the categories new MIG/MAG and Migatronic, for which we are an authorised distributor and service. If you want to count it first on your own job, take a set with a cooling unit from rental for a month and compare the use of tips.
We will count the payback on your figures
Give us the gas from the cylinder, the working current, the required duty cycle, the information about pulse and the monthly use of tips, and we will prepare a summary for your station together with the choice of torch and cooling unit. Call or write, and if the machine needs its circuit checked first, book it in our service workshop for a cooling inspection.