Although the two terms can sometimes be used interchangeably, there is a difference between "Pipe" (also called nominal) size and "Tube" size and how it's measured. Tube size is normally the actual outside diameter, or O.D. Nominal pipe size is always 1/8" larger in actual outside diameter (O.D.) than the standard/stated size designation. So, 1/4" (nominal/pipe) is actually 1/4+1/8= 3/8 O.D. — and visa-versa. This can present some confusion, specifically when trying to match up components, unions, fittings and pipe of varying sizes — not to mention the fact that the terms pipe and tube are often used interchangeably. The important difference to know is whether the tube/pipe or fitting you are using is measured "nominally"? Or by O.D.? Then make adjustments accordingly.
There are several grades of copper pipe and tube, but the most common you might find for our use are:
K is thickest and heaviest and most expensive, but fine to use where the weight would not be as much a consideration. (Usually stamped green)
L/ACR is thick, but lighter than K,fairly heavy walled. Good all-around general use and commonly found. (Usually stamped blue)
M is thinner, lighter, and also cheaper. Thinner copper tends to heat/cool faster. Is good for our purposes, but particularly for liebigs where you don't want the weight, and the thin walls make good temperature gradients. It's also much easier to solder/braze in larger sizes. (Usually stamped red)
DWV which is for "drain and vent" (non-pressurized) purposes, is thinnest walled and cheapest. It typically only comes in rigid pipe in limited size range from 1¼ inch to 8 inches. (Usually stamped yellow)* DWV is fine for our purposes, and can save you a bundle when dealing with larger diameter sized pipe as well on the fittings. It sometimes has a max temp rating printed on the side of the pipe. This is actually a pressure rating which has nothing to do for our use. You can ignore it.
The above are usually offered in hard temper/rigid (straight lengths usually 5' or 10') or Soft (coil) *with the exception of DWV which is only available in rigid pipe and limited sizes. Soft coil sizes of the grades above may include both O.D. and I.D./nominal measurements on the package. As mentioned, you can solder either one (hard or annealed/soft) and use standard copper fittings on them. Just be sure to get your soldered copper fittings in the same nominal size as your pipe. If you are using 1" nominal pipe, use 1" fittings, etc. If you are using compression fittings, you'll want to match the outside diameter of your pipe/tube. If you don't have O.D., just remember to buy the compression fittings 1/8" larger than the pipe size you are using. eg: 1/2" nominal pipe, use 5/8" compression fitting.
Types K, L, M are designated by ASTM standard (nominal) sizes, with the actual outside diameter always 1/8-inch larger than the standard size designation. Each type represents a series of sizes with different wall thicknesses. All inside diameters depend on tube size and wall thickness. As a result, when mixing pipe sizes/types together, it's usually a better fit, tighter joint, to use a reducer between them, rather than trying to fit one inside the other — although that can sometimes work as well depending on your needs.
Copper tube for air-conditioning and refrigeration field service (ACR) is normally designated by actual outside diameter (O.D.) but may also have nominal size or internal wall thickness listed as well. 1/4" OD coil of this type is typically used to make reflux condenser coils. The more you work soft copper, the stiffer it gets. You can anneal or re-anneal any copper (pipe/tube), including those listed above, by heating it to red, then cooling it. This will soften the area and make it more pliable again. This can be done several times as needed. Any of the above copper can be used. The important thing to remember is how its sized vs whatever fittings you wish to use. (ie. whether your fitting is measured in the same OD size, or whether it's from nominal size). Just make sure you adjust for any sizing differences as needed.
Here's a size chart that may help:
Fittings are sized nominally, and are generally designated by their type and ends. The most basic types of ends for a fitting are "C" (socket), FTG (pipe size), Mips (Male pipe threads on outside of fitting end), Fips (female pipe threads on inside of fitting). Standard C socket ends will accept the designated nominal pipe size. FTG, same as pipe size, will fit inside another C or socket fitting..handy when you want to join to different fittings but don't want to use pipe in between. There are many different types of nominal sized fittings to choose from.... couplers with stops, coupler with no stops, reducing fittings, elbows, caps, etc. These fittings will work fine with O.D. spec'd copper tube, just remember to adjust for the size difference by subtracting 1/8" from your starting O.D. size, to find the corresponding nominal sized fitting. Compression couplings/fittings are normally measured O.D. So, if you are using one of these with nominal I.D. pipe, be sure to add 1/8" to accommodate nominal sizing differences (ie., 1/2" nominal pipe is actually 5/8" O.D.).
(Hat tip to Husker and member ozone39)
