Flanges & Connections — How Components Join
Estimated time: 20–25 minutes
Learning Outcome: Recognise the three main vacuum flange standards (CF, KF, ISO); identify common hardware components (clamps, centering rings, bellows). Competency: M04-COMP-01, Indicators M04-IND-01.02, M04-IND-01.03
Orient
Vacuum systems are assembled from individual components — chambers, pumps, valves, gauges — connected by standardised flanges. The flange standard determines the seal type, the connection method, and the vacuum performance achievable.
Knowing which flange you're looking at tells you a lot about the system's intended performance range.
Core Content: Three Flange Standards
KF (Klein Flansch / Small Flange) — also called QF or NW
What it is: A quick-connect flange system using elastomer O-ring seals. Sizes designated by nominal bore: KF16, KF25, KF40, KF50.
How it connects: Two flat KF flanges meet face-to-face. A centering ring (a metal ring holding an O-ring) sits between them. A clamp (hinged or wing-nut style) squeezes the flanges together, compressing the O-ring.
Performance range: Rough vacuum to moderate high vacuum (~10-7 mbar with care). Standard for forelines, roughing connections, and general plumbing.
Key feature: Fast assembly and disassembly — one clamp, no bolts. This is the "quick connect" of vacuum.
On R1-A: The foreline connection between R1-V-ISO and R1-L-FL, and the connection from R1-L-FL to R1-P-RP, likely use KF-style fittings. Quick to assemble, adequate for rough vacuum performance.
ISO Flanges (ISO-K and ISO-F)
What it is: Larger flanges for higher-throughput connections. ISO-K uses clamps (like oversized KF); ISO-F uses bolt circles. Elastomer O-ring seals.
How it connects: Similar to KF but for larger port sizes (DN63 and above). ISO-K uses a chain clamp or toggle clamp around the outside. ISO-F uses bolts through a fixed bolt circle.
Performance range: Similar to KF — rough to moderate high vacuum. Used for large pumping ports and chamber connections where high conductance (Lesson 3 concept) requires large diameter.
Key feature: High conductance. Large-diameter ISO flanges provide the wide openings needed to preserve pumping speed between the chamber and high-throughput pumps.
CF (ConFlat) — Metal-Sealed
What it is: A precision flange system using copper gaskets and knife-edge sealing surfaces. The gold standard for high-vacuum and ultra-high-vacuum.
How it connects: Two CF flanges have knife-edge grooves machined into their faces. A copper gasket sits between them. Bolts are tightened in a star pattern, driving the knife edges into the copper to form a permanent metal-to-metal seal.
Performance range: High vacuum through ultra-high vacuum (10-10 mbar and below). Required for any system that will be baked out above 200°C.
Key feature: Metal seal — zero permeation, negligible outgassing, bake-out compatible. The trade-off is single-use gaskets and slower assembly.
Checkpoint — What You've Gained So Far
You can now recognise all three flange standards (KF, ISO, CF) and explain their sealing mechanisms. The comparison table below puts the trade-offs side by side so you can identify which standard fits which application.
Flange Comparison
| Feature | KF | ISO | CF |
|---|---|---|---|
| Seal type | Elastomer O-ring | Elastomer O-ring | Metal (copper) gasket |
| Connection | Single clamp | Clamp or bolts | Bolt circle (star pattern) |
| Assembly speed | Fast (seconds) | Moderate (minutes) | Slow (minutes, careful torquing) |
| Reusable seal | Yes | Yes | No (new gasket each time) |
| Vacuum range | Rough to ~10-7 mbar | Rough to ~10-7 mbar | High to UHV (10-10 mbar) |
| Bake-out | Limited (~150°C) | Limited (~150°C) | Up to 450°C |
| Typical sizes | KF16–KF50 | DN63–DN630 | CF16–CF300+ |
Common Hardware Components
Beyond flanges, vacuum systems use several standard hardware items:
Centering rings: Metal rings that hold the O-ring centred between KF or ISO flanges. They also set the correct gap between flanges for proper O-ring compression.
Clamps: Hinged, wing-nut, or chain-type devices that hold flanges together. KF clamps are single-piece; ISO-K uses chain clamps for larger diameters.
Bellows: Flexible metal tubes (usually stainless steel) that allow movement between connected components. Used for vibration isolation (between pump and chamber), alignment adjustment, and thermal expansion compensation.
Couplers and adapters: Transition pieces between different flange standards (KF-to-CF, ISO-to-KF) or different sizes (KF25-to-KF40). Every adapter introduces a restriction — keep transitions in mind when thinking about conductance (M03).
Viewport and feedthrough flanges: Specialised flanges with windows (viewports) for visual inspection or with electrical/mechanical feedthroughs for sensors, heaters, or motion.
Visual Reference: Flange Standards Photo Comparison
The photographs below show each of the three flange standards in assembled and detail views. When examining these images, focus on the connection method (clamp versus bolt circle), the seal element (elastomer O-ring versus copper gasket), and the relative complexity of each assembly.
[VIS-M04-003] Textbook Reference
See Basic Vacuum Practice, Ch. 6, pp. 158–159
O-ring seal cross-sections — groove geometry, compression ratios, and sealing principles for elastomer and metal seals
Notice the progression in complexity from left to right: KF assembles in seconds with a single clamp, ISO-K takes slightly longer with its chain clamp, and CF requires careful bolt torquing in a star pattern. This complexity trade-off directly mirrors the performance progression — from rough vacuum convenience (KF) to UHV capability (CF).
Video Reference: Flange Assembly & Seal Inspection
The following video demonstrates how a KF connection is assembled with a centering ring and clamp, and how a CF flange joint is prepared with a copper gasket and star-pattern bolt torquing. Watch for the handling precautions — note how the technician avoids touching sealing surfaces with bare hands, and how the CF gasket is inspected for scratches before installation.
After watching, consider how the handling discipline shown in the video connects to the contamination principles from Module 2. A fingerprint on a copper gasket or an O-ring sealing surface introduces hydrocarbons that outgas under vacuum — the assembly technique itself is part of maintaining system cleanliness.
What You Can Now Do
By the end of this section, you can:
- Recognise KF, ISO, and CF flanges and describe their sealing mechanisms
- Explain why CF flanges are required for UHV applications (metal seal, bake-out compatible)
- Identify centering rings, clamps, bellows, and adapters and describe their functions
- Explain the trade-offs between flange standards (speed vs performance vs cost)
- Consider how adapters and transitions affect conductance