Which Vacuum Flange Wins? ISO-KF vs ISO vs JIS vs CF Compared

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Building or maintaining vacuum systems can feel like a maze, especially when choosing flanges. Many professionals get confused by the different standards. I will cut through the noise and explain the most common vacuum flange types, their uses, and what to consider.

Choosing the right vacuum flange hinges on understanding the distinct applications and performance capabilities of ISO-KF, ISO (ISO-K/ISO-F), JIS, and CF types. While ISO-KF offers quick assembly for rough vacuum, CF flanges dominate ultra-high vacuum with metal seals, and ISO/JIS serve specific industrial or regional needs.

vacuum flange application guide
From my decade in the vacuum pump industry, I have seen how critical flange selection is. The wrong flange can lead to costly leaks or limit your system’s performance. Let us explore the world of vacuum flanges and help you make an informed decision.

Understanding ISO-KF Flanges: The Quick-Connect Standard?

Are you looking for a fast and reliable way to connect components in your vacuum system? ISO-KF flanges are often the first choice for good reasons, but they have their limits.

ISO-KF flanges, also known as NW or Klein Flange, are the quick-connect workhorse for small to medium vacuum systems, typically operating from rough to high vacuum (10⁻³ to 10⁻⁷ mbar). They use a simple clamp and elastomer O-ring for a leak-tight seal, making them ideal for rapid assembly and frequent system modifications.

KF vacuum flange
In my experience, ISO-KF flanges dominate small to medium vacuum setups due to their simplicity and speed. Instead of bolts, a circular clamp compresses a centering ring and an elastomer O-ring to create a leak-tight seal. This design allows for incredibly rapid assembly and disassembly, which makes them ideal for laboratory environments or production lines where frequent component changes, modifications, or system cleaning occurs. Their ease of use significantly reduces setup and downtime.

Typically, these flanges are made from stainless steel 304 or 316, which provides excellent corrosion resistance, crucial for maintaining vacuum integrity. They come in a standard size range from DN10 to DN50, with DN16, DN25, and DN40 being the most common sizes found on rotary vane pumps and other lab equipment. For instance, a DN25 flange pairs perfectly with a 25.4 mm diameter hose (with a 1.65 mm wall thickness). 

While ISO-KF flanges are a fantastic choice for many applications, it is important to remember their main limitation: the elastomer O-ring. This O-ring material limits the bakeout temperature, typically to around 150°C, which means ISO-KF flanges are generally unsuitable for ultra-high vacuum (UHV) scenarios that require much higher bakeout temperatures to achieve extreme cleanliness.

ISO-KF Flange Key Features and Applications

Feature Description Application Fit
Sealing Mechanism Clamp with elastomer O-ring & centering ring Quick assembly, frequent modifications
Pressure Range Rough to High Vacuum (10⁻³ to 10⁻⁷ mbar) General lab use, pump connections, small chambers
Bakeout Temperature Limited (~150°C) due to O-ring Not suitable for UHV or extreme cleanliness requirements
Common Sizes DN10 to DN50 (DN16, DN25, DN40 popular) Versatile for smaller components and pumps

Exploring ISO Flanges: Are ISO-K and ISO-F Right for Your System?

As your vacuum system grows larger and more permanent, the needs change. ISO flanges come in different forms, each designed for specific heavy-duty challenges.

ISO flanges (LF/LFB/MF) are designed for larger, more permanent vacuum installations, typically starting from DN63. They come in two main subtypes: ISO-K, which uses double-claw clamps for assembly, and ISO-F, which relies on bolted connections, offering robust and reliable sealing solutions for high-vacuum industrial applications.

ISO-vacuum-flange
In my professional experience, ISO flanges are the go-to solution when you are building more substantial or permanent vacuum systems that require greater robustness than KF flanges can provide. They cater to larger pipe diameters and heavier components. The two primary subtypes within the ISO standard are ISO-K and ISO-F, and understanding their differences is vital for proper selection.

ISO-K (Klar flange) systems utilize double-claw clamps that latch into grooves on the tubing. This clamp-based design still allows for relatively quick assembly and disassembly compared to bolted flanges, making them suitable for sections of a large system that might require occasional modification or servicing. They are typically used in high-vacuum setups requiring moderate bakeout temperatures (up to 200°C), slightly higher than KF due to a different O-ring seating.

ISO-F (Flange type), on the other hand, relies on bolted connections with O-rings. This bolted design provides superior strength and vibration resistance, making ISO-F flanges preferred for critical components like large gate valves, pumps, or wall-mounted systems where permanent, vibration-resistant sealing is a top priority. They are common in industrial systems such as semiconductor manufacturing, where absolute reliability and long-term stability are paramount. Both ISO-K and ISO-F start at DN63, with common sizes ranging up to DN500, making them suitable for large-diameter vacuum lines. For example, a DN63 flange is designed to connect to a 50.8 mm diameter hose (with a 1.65 mm wall thickness).

ISO Flange Types: K vs. F Comparison

Type Sealing Mechanism Typical Use Case Pros Cons
ISO-K Double-claw clamps with O-ring Moderate bakeout, occasional disassembly Faster assembly than bolted, larger sizes than KF O-ring limits UHV, less rigid than bolted
ISO-F Bolted connections with O-ring Permanent installations, high vibration, gate valves Superior rigidity, vibration resistance Slower assembly/disassembly, O-ring limits UHV

Delving into CF Flanges: Why Are They the UHV Champion?

When extreme vacuum is needed, common flanges just will not cut it. CF flanges are specifically engineered for the most demanding environments, but they come with their own set of requirements.

CF (ConFlat) flanges are the undisputed champion for ultra-high vacuum (UHV) systems, achieving pressures below 10⁻⁹ mbar. They utilize a specialized knife-edge design that bites into soft copper gaskets, creating a metal-to-metal seal capable of withstanding extreme bakeout temperatures up to 450°C, crucial for minimizing outgassing.

CF-vacuum-flange
If your application demands ultra-high vacuum (UHV) or extreme cleanliness, CF (ConFlat) flanges are not just a choice, they are a requirement. Unlike KF or ISO flanges that rely on elastomer O-rings, CF flanges achieve their superior sealing through a unique metal-to-metal contact. Each flange has a sharp knife-edge (or two, depending on the type) that precisely bites into a softer, oxygen-free high-conductivity (OFHC) copper gasket. This creates a highly robust, hermetic seal that is virtually leak-proof.

The capability to withstand high bakeout temperatures is a primary reason for CF flanges’ dominance in UHV. Components in UHV systems must be heated to hundreds of degrees Celsius (up to 450°C for CF) to drive off adsorbed gases and moisture from their surfaces. Elastomer seals would simply degrade at these temperatures, releasing contaminants. CF flanges, typically made of stainless steel 304 or 316, can handle these extreme bakeouts, ensuring minimal outgassing and achieving the lowest possible pressures. CF flanges are available in a range of sizes from DN10 to DN250. 

While OFHC copper is the standard gasket material, Viton or PTFE gaskets can be used for lower-temperature applications if UHV is not strictly required. However, for true UHV performance, copper gaskets are mandatory and must be replaced after each disassembly to maintain seal integrity.

CF Flange Characteristics for Ultra-High Vacuum

Feature Description UHV Impact
Sealing Method Metal knife-edge compressing soft copper gasket Creates hermetic, virtually leak-proof seal
Pressure Range Ultra-High Vacuum (below 10⁻⁹ mbar) Essential for most demanding vacuum applications
Bakeout Capability Withstands up to 450°C Enables extreme cleanliness by removing adsorbed gases
Gasket Replacement Copper gaskets must be replaced after each use Ensures consistent high-integrity sealing

How Do JIS Flanges Compare to Other Standards?

Beyond the mainstream ISO and CF standards, JIS flanges offer another option, particularly if you are integrating with Japanese-manufactured equipment. Understanding their place is important.

Japan’s JIS B 2290 standard defines VF (flat) and VG (gasketed) flanges, emphasizing modular design and offering reliable seals from 10⁻² to 10⁻⁸ mbar. While less common globally than ISO or CF, JIS flanges are crucial for compatibility with specific Japanese industrial equipment and can be found in both aluminum and stainless steel variants.

My experience with diverse vacuum systems has shown me that global standards vary, and JIS flanges are a prime example. The Japanese Industrial Standard (JIS B 2290) covers “VF” (flat) and “VG” (gasketed) flanges. These flanges use interlocking grooves and typically employ copper gaskets for robust and reliable seals. Their design emphasizes modularity, often making them useful in systems requiring flexible configurations or those built with a modular approach.

JIS flanges are effective across a pressure range from 10⁻² to 10⁻⁸ mbar, placing them in the rough to high vacuum categories, similar to the broader ISO range. They are available in materials like aluminum or stainless steel, depending on the specific thermal and corrosion resistance needs of the application. Aluminum variants, for instance, are lighter, which can be advantageous for portable vacuum setups or systems where weight is a concern. While not as universally adopted as ISO-KF or CF flanges, JIS flanges shine in specific niches.

They are particularly relevant when working with Japanese-manufactured equipment or systems that require compatibility with legacy industrial tools following this standard. Knowing about JIS flanges ensures you can correctly integrate components when faced with non-ISO or non-CF systems.

JIS Flange Characteristics and Comparisons

Feature Description Comparison to ISO/CF
Standard Japanese Industrial Standard (JIS B 2290) Regional standard, less global than ISO/CF
Types VF (flat) and VG (gasketed) Offers modularity similar to some ISO principles
Pressure Range 10⁻² to 10⁻⁸ mbar Covers similar range to ISO, lower than CF
Materials Aluminum or Stainless Steel Similar material options to ISO, wider use of Aluminum
Primary Use Japanese-manufactured equipment, legacy systems Niche market, specific compatibility needs

Final Thoughts

Choosing the right vacuum flange is about matching the flange to your system’s specific vacuum level, flexibility needs, and operational environment. Whether it is the quick convenience of ISO-KF, the heavy-duty reliability of ISO, the ultra-high performance of CF, or the niche compatibility of JIS, each flange has its ideal application.

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