How to select a gas supply system for a thermal cutting machine?
Cylinders, Liquid Tanks, or Bulk Tanks?
Selecting the best type of gas supply system for a CNC shape cutting machine will depend primarily on how much gas you will use, but there are some other factors that might affect your decision also. For the most common gases used on a thermal cutting operation (Oxygen, Nitrogen, Argon, etc) There are three main types of gas supply systems:
- High pressure cylinders
- Portable cryogenic liquid tank
- Bulk cryogenic systems
The biggest factor is going to be how fast you use the gas. Cylinders hold the least amount of gas, but because it is a gas, it will not boil off and vent, so a cylinder of gas can be stored and used over a long period of time. With a liquid tank, the liquid slowly boils off and builds pressure in the tank, eventually reaching the point that it has to vent off to relieve the pressure. So if you don’t use it fast enough, you will eventually loose all the gas in your liquid tank, which is lost money!
Here are the typical cut-off numbers that you should keep in mind:
- A cylinder is about 300 Cu. Ft., so a 12-pack of cylinders is about 3,600 Cu. Ft. If a cylinder pack will last more than a week, then that is probably the right system for you. If you go through more than one 12-pack in a week, then you should look into a portable liquid tank.
- A portable liquid tank is about 4,500 Cu. Ft. If you don’t use that much gas in a week, then it will likely be venting off and wasting gas – especially when sitting idle over the weekend.
- If you are going to be using more than 20,000 to 25,000 Cu. Ft. per month (more than one portable liquid tank per week), then you’d be better off with a bulk cryogenic system.
Typically high pressure cylinder gas will have the highest price per Cu. Ft., portable liquid cylinders will cost less, and then bulk liquid system will be the least expensive per unit of gas.
Advantages & Disadvantages
High Pressure Cylinders
The high pressure cylinder source is the typical system for the small user.
- Low installation cost - only have to find a suitable-safe location and provide a regulator and hoses.
- Highest cost per Cu. Ft. of gas
- Lease/rental cost of cylinders
- Cost of frequent change out of cylinders
- Cost of standby cylinders
- A manifold system is inexpensive
- Can provide high gas flow for a short period of time
- 5 to 10% of the gas in each cylinder that cannot be used, since a cylinder can’t be used all the way down to “zero” pressure.
Portable Liquid Tanks
The portable liquid gas tank is suitable for a wide range of users.
- Lower cost per Cu. Ft. than high pressure cylinders.
- Low installation cost – only have to find a safe location and provide regulator and hoses.
- Lease/rental cost for the tanks
- Liquid tanks have a maximum withdrawal rate that may be as low as 500 CFH, so a heat exchanger may be required
- There will be some venting of the gas during long periods of downtime, such as over a weekend.
- A spare liquid tank should be kept on standby to reduce downtime, unless you are located very close to a distributor who will give quick response in case of an empty tank.
- Some portable liquid tanks have gauges that indicate the quantity of gas remaining, but operators seldom check, and often cuts are lost in mid-cut when the tank goes empty.
Bulk Cryogenic Systems
For the larger users, with highest gas consumption, the least expensive gas is that supplied by a bulk cryogenic system.
- Lowest overall cost per Cu. Ft. of gas.
- Higher installation cost – these systems are almost always placed outside of a building, and the location must meet various codes. You must provide a concrete pad that meets the supplier’s specifications, and permanent piping to route the gas to the cutting machines.
- The larger the tank the higher the lease cost associated with it.
- Refilling may require several days to schedule.
There are other limitations and specifications that you should know about as well, so be sure to ask about CGA, AWS, and OSHA regulations when you talk to your local gas distributor.
Selecting the best gas supply system for your installation is not difficult. As long as you have the above information in mind when you talk to your gas supplier, you should be off to a good start.