Fluid Fitting and Connector Selection

Fluid line connectors are critical in liquid cooling applications. Selection, installation, and maintenance of a system’s fluid connections are all important in preventing leaks and maintaining system integrity. With so many fluid connector options available, it is often difficult to decide which one is best suited for your application. The two main types of fluid connectors found in liquid cooling applications are fittings and couplings. Part 1 of this article will cover two important factors to consider when selecting fluid connectors. It also will describe the types of fittings most frequently used in liquid cooling applications.

The key in selecting the right fluid connector is understanding your application. Here are some of the questions you should ask:

What is the fluid media? Viscosity and corrosiveness of the fluid must be considered. Understand how changes in the coolant’s viscosity over the operating temperature range can impact the pressure drop across the fluid connectors. Make sure the fluid is chemically compatible with the fluid connector’s wetted materials including any O-rings.

What are the tube or hose size and flow rate requirements? The internal diameter of the fluid path components will have a great impact on pressure drop and fluid velocities. Make sure to account for pressure drop across connectors and check fluid velocities to prevent erosion corrosion.

What are the maximum and minimum system operating temperatures and pressures? Connectors will need to maintain the seal at all these operating points. Consult with your fluid connector supplier for the proper hose or tube wall thickness, surface finish, hardness (durometer for hoses), concentricity, and ovality (tubing only).

Will the system experience vibration, pulsation, or thermal cycling? The seal between the tube or hose and the fluid connector needs to be maintained during these changes in process conditions. Check with your fluid supplier for the proper fluid connector for your application.

How is the connection going to be configured into your application? Common mounting options include pipe thread, in-line, rigid mount, panel mount, or elbow.

What industry standards or other special requirements need to be complied with? Some standards to consider include ISO (International Standards Organization), FDA (Food and Drug Administration), and RoHS (Restriction of Hazardous Substances).

Fittings and couplings are the two main types of fluid connectors commonly used in liquid cooling applications. Both are used to connect cooling loop components such as valves, pumps, cold plates, heat exchangers, hoses, etc. Fittings and couplings are differentiated mainly on how they are used in a system.

A fitting is typically used in applications that do not require the frequent disconnection of equipment or parts, since repeated removal can cause leaks. Fittings are usually inexpensive compared to couplings and come in many different sizes, types, and materials. Fittings also require tools for installation and removal.

A coupling provides a means of quickly connecting and disconnecting a line without a loss of fluid or entrance of air into a system. If equipment needs to be assembled quickly or if it needs routine servicing or repair, then a coupling is a better choice for a fluid connection. For example, equipment designed in modules, such as liquid-cooled chassis used by the military, requires quick disconnect couplings (QDC) in order to be serviced or maintained on the field. (See Figure 1).

Couplings come in a variety of materials, including plastics such as acetal and nylon, which are cost-effective and compatible with a wide range of fluids. Plastic can come in a variety of colors to distinguish between different fluid lines. Metal couplings are typically used in more challenging environments where shock and vibration, higher pressures, weight, temperature variations, personnel safety, and other challenging requirements call for greater durability and strength.

There are many types of fittings used in liquid cooling applications but the more commonly used fittings are: beaded, barbed, NPT tapered pipe threads, SAE straight threads, compression, 37 º AN flare, bite-type, mechanical grip-type and O-ring face seals.

This fitting consists of a straight tube that has a bead around its outside diameter, as shown in Figure 2. An interference fit is used between the internal diameter of the hose and the outside diameter of the fitting to seal the connection. The clamp provides the force to maintain the seal and retain the hose. Beaded tubes are made per U.S. Military Standard MS33660 or per Aerospace Standard AS5131. Proper bead design as well as proper clamp and hose selection are critical in providing a leak-tight connection. Refer to our Fitting and Hose Clamp Selection for Cold Plates and Heat Exchangers application note for more detailed information on beaded fittings.

As with beaded fittings, barbed fittings are used with hoses. Barbed fittings (See Figure 3) are fluid connector devices that have one or more continuous ridges that grip and seal the inside diameter of the hose. Slope and depth of the barb, sharpness of the gripping edge(s), number of barbs, and their spacing, all play a part on how well the fitting grips and seals. Refer to “Don’t be too casual pairing tubes to barbed fittings1” for more detailed information on barbed fittings.

These fittings have tapered internal or external threads (See Figure 3). The seal on these fittings occurs between the flank, crest, and root of the two joining metal surfaces. Because galling and tearing of the joining metal surfaces can occur during installation, it is imperative to apply a lubricant or sealant on the male threads to prevent damage. A popular thread sealant is PTFE (polytetrafluoroethylene) tape. NPT fittings are common fluid connections for cooling systems such as recirculating chillers and for cooling components such as brazed plate liquid-to-liquid heat exchangers. A good reference on how to make reliable pipe thread joints is “Pipe Thread Types and Designations²”.

SAE straight threaded fittings are designed to provide retention via the threads. They do not provide a metal-to-metal seal like NPT fittings. Sealing is provided by an O-ring, generally located at the base of the male thread. (See Figure 4). This type of threaded fitting offers advantages over an NPT connection in that maintenance, accessibility, and remake of the fitting are significantly easier. It also offers an advantage over compression fittings that are typically tightened within a higher, yet narrower torque range, which makes it easier to strip the threads and crack or distort fitting components and cause leaks. The rubber-to-metal seal provides a “feel” when the operator tightens the fitting. O-ring fittings tend to be more expensive than their all-metal counterparts and care must be taken when installing the fitting so that the O-ring does not get damaged or fall out of the groove. Selecting the wrong O-ring or reusing one that has been deformed or damaged can cause leaks.

A compression fitting is comprised of three components: a threaded nut, body, and ferrule or olive (see Figure 5). When the nut is tightened, it compresses the ferrule, causing it to conform to the circumference of the tube. To work properly, the ferrule must be oriented correctly. One advantage of this type of fitting is that no special tools are required during assembly. Some of the disadvantages are that it comes in limited materials (brass or copper) and can handle minimal pressure compared to flare, bite-type, or mechanical grip type fittings (see below). A compression fitting is not recommended for applications having vibration, thermal cycling, or other dynamic forces.

A flare fitting is also made up of three components: a nut, sleeve, body, and rigid tube with a flare, as shown in Figure. 6. Metal-to-metal sealing takes place as tightening of the nut draws the fitting into the flared end of the tubing. This type of fitting typically can handle higher pressures than a compression fitting and it requires tooling to flare the tube-end in preparation for installation. Improper flaring of the tubing can cause axial cracks on thin or brittle tubing. Care must be taken when cutting the tube, since poorly designed tube cutters or ineffective hacksaws will create an uneven sealing surface.

There are several types of flare fittings. There is the 45º JIC flare, which is often used in lower pressure applications such as fuel lines and HVAC applications. A commercial grade 37º flare fitting, better known as a 37º JIC flare fitting, is manufactured to SAE-J514/ISO 8434-2 standard and uses a UNS (Unified Thread Standard) class 2A/2B straight thread. Another type of 37º flare fitting is the 37º AN flare fitting, which is manufactured to MIL-F-5509 per AN (U.S. Air Force/Navy standard).

Although these two 37º fittings are compatible with each other, the 37º AN flare fitting uses a tighter UNS, Class 3 tolerance on the threads, allowing a 40% increase in fatigue strength. This is why 37º AN flare fittings are about three times more expensive than similar SAE/JIC 37º flare fittings. Since one cannot visually determine one from the other, AN 37º flared fittings are marked with an MS or AN marking per MIL-P-5509D, and can also be differentiated by the way they are specified in writing. For example, they might be specified as “AN fitting: ½-20 UNJF-3B, SAE/JIC fitting: ½-20 UNF-2B.”

Similar to other compression fittings, a bite-type fitting has a threaded nut, body, and ferrule(s). On a single ferrule fitting (See Figure 7), the leading edge bites into the surface of the tubing to achieve holding ability. The seal is made on the long, deep surface between the ferrule and the internal taper. Typically, bite-type fittings are of single ferrule design. On a two ferrule design, the first ferrule provides the sealing and the second ferrule provides the retention. The spring-like action of the ferrule(s) during installation compensates for the variations in tubing material and hardness, as well as the thickness of the wall tube and temperature variations. This provides a leak-tight fluid connection for an extensive range of applications.

A mechanical grip-type fitting is comprised of a threaded nut, body, and two ferrules. The difference with the two ferrule bite-type fitting discussed previously is that a mechanical grip-type uses the back ferrule to spring load the front ferrule as it seals by coining the surfaces of the tubing and coupling body (see Figure 8). Another difference over the bite-type fitting design is that break and remake of this fitting after installation can be better accomplished without damage to either the fitting components or the tubing.

An O-ring face seal fitting consists of a threaded fitting body with O-ring groove, O-ring, threaded nut, and sleeve or tailpiece. The fitting assembly seals when the tailpiece, which is permanently brazed or welded to the tube, compresses the O-ring on the face of the threaded fitting body as the nut is threaded onto the external threads on the fitting body. (See Fig. 9). This fitting is a “zero clearance” system because you do not need to spring or pull the tubing in order to seat the fitting or purge the system. This tube fitting can be disassembled and reassembled many times. Simply replace the O-ring and tighten to manufacturers’ recommended torque. The O-ring conforms well to sealing surface irregularities. These types of fittings are recommended for high vibration applications because the O-ring absorbs shock better than any metal-to-metal sealing system.

With so many fitting options available, it is critical to understand your application as well as the frequency of connection and disconnection required. The reliability and serviceability of your system depends on the fitting selected. To ensure you choose the right fitting for your application, it is best to work closely with your fitting or liquid cooling components partner early in the design process.

1. Brown, Jim, Colder Products Company, "Don't Be Too Casual Pairing Tubes to Barbed Fittings", Medical Design, September 2007, page 28.

2. Colder Products Company, "Pipe Thread Types and Designations"

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