KF, CF or ISO — which vacuum flange should I use?

Pressure range decides it, and size decides the rest. KF for rough and high vacuum up to small sizes, ISO for the same pressure range in larger bores, CF when you need ultra-high vacuum or bakeout. Everything else — cost, assembly time, whether you need tools — follows from that choice rather than driving it.

KF (also written NW or QF) is a quick-release flange: two identical flanges, a centring ring carrying an O-ring, and a clamp you tighten by hand. It assembles in seconds without tools, which is why it dominates on roughing lines, gauges and anything reconfigured often. The elastomer O-ring sets its limit — practically, down to the 10⁻⁷ mbar decade with good practice, and no bakeout. Sizes run from DN10 to about DN50; above that the clamp force is no longer adequate.

ISO takes over where KF runs out of size. ISO-K uses claws or collars around a grooved flange, ISO-F uses a bolted flange with through holes; both seal on a centring ring with an elastomer O-ring, so their vacuum capability is the same as KF — the difference is mechanical, for bores from DN63 up to DN1000 and beyond where a quick clamp cannot hold.

CF (ConFlat) is a different principle: a knife edge on each flange bites into a soft copper gasket, forming a metal-to-metal seal. That removes the elastomer entirely, so there is no permeation and no outgassing floor, and the joint can be baked to 450 °C. The costs are real: you need bolts and a torque sequence, a fresh copper gasket for every assembly, and the flanges are heavier and more expensive. Use it where the pressure or the bakeout requires it, not by default.

The mistake that costs the most time: mixing standards without planning the adapters. A system that runs CF on the chamber and KF on the roughing line needs a CF-to-KF adapter at a defined point, and buying one after the pipework is cut is how a build loses a week. Decide the boundary between standards at design time and buy the adapter with the rest.

Two details worth specifying rather than assuming: the centring ring material for KF and ISO — with or without a filter mesh, and which elastomer for your process gas — and the bolt material for CF, since a stainless bolt in a baked joint needs the right grade and lubricant to come apart again afterwards.