Fluid Coupling and Connector Selection
Choosing the Right Fluid Coupling for Your Liquid Cooling Application
As there are a plethora of fluid connector options available, it can be difficult to decide which one works best for your application. Liquid cooling fluid connectors typically fall into two categories: fittings and couplings. Part 1 of this article discussed fittings. This part will review the two factors to consider when selecting fluid connectors and will describe the most common types of couplings used in liquid cooling applications.
I. Assessing the Application
Understanding your application is critical in determining which coupling is the best for your liquid cooling system. Some questions you should ask when considering couplings include:
What is the system fluid?
Is the fluid’s viscosity and corrosiveness compatible with the system hardware? Understanding how changes in the coolant’s viscosity over the operating temperature range can impact the pressure drop across the fluid connectors. Check to make sure the fluid is chemically compatible with the fluid connector’s wetted materials including any O-Rings. (Learn more about some of the most common O-Ring compounds
What are flow rate requirements?
This impacts your tube or hose selection and therefore coupling requirements. The internal diameter of the fluid path components will have a great impact on pressure drop and fluid velocities. Account for pressure drop across connectors and check fluid velocities to prevent erosion corrosion. (For more information on erosion corrosion please see our application note “Erosion-corrosion in Cooling Systems“.
What operating range will the system experience for temperature and pressure?
Connectors need to maintain the seal at all these operating points. Ensure that hose or tubing wall thickness, surface finish, hardness or durometer, concentricity, and ovality can withstand expected pressures and temperatures.
Will the system experience vibration, pulsation, or thermal cycling?
Seals between the fluid connector and hoses must withstand these extreme condition changes.
How is the coupler mechanically integrated into the system?
Common mounting options include pipe thread, in-line, rigid mount, panel mount, or elbow.
Do you need to comply with specific industry standards or other special requirements?
International Standards Organization (ISO), Food and Drug Administration (FDA), and Restriction of Hazardous Substances (RoHS) are common standards many applications must adhere to. Sterilization, color coding or labeling, and specific packaging requirements are other common specifications to consider when selecting couplings and connectors.
II. Determining the Type of Fluid Connector
Fluid connectors can be considered either fittings or couplings, which are common hardware components in a liquid cooling system. Both connect cooling loop components such as valves, pumps, liquid cold plates, heat exchangers, hoses, etc. Fittings and couplings are differentiated mainly on how they are used in a system.
Fittings are used when disconnection of equipment is infrequent, since repeated removal can cause leaks. Fittings are relatively less expensive compared to couplings and come in many different sizes, types, and materials. Fittings require tools for installation and removal.
A coupling enables quick connection and disconnection a line without fluid loss or introducing air into a system. A coupling is a better fluid connector for systems that require fast assembly or routine maintenance. For example, modular equipment like a liquid-cooled chassis requires quick disconnect couplings (QDC) in order to be serviced or maintained on the field, especially for military applications (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 also be molded into a variety shapes or include colors to distinguish between different fluid lines. Metal couplings are 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.
Quick Disconnect Couplings Types
There are basically four types of quick disconnect couplings used in liquid cooling applications. They are straight through, single shut-off, double shut-off, and non-spill.
Straight-Through Couplings
The simplest type of coupling is a straight-through coupling (See Fig. 2). These couplings do not have any internal valves to obstruct fluid flow so they provide minimal pressure loss. External manual shut-off valves are required to prevent fluid loss when disconnecting. Straight-through couplings normally have operating pressures of up to 5,000 psi. This type of coupling is typically used in applications where the loss of coolant when breaking the liquid cooling loop can be tolerated.
Single Shut-Off Couplings
Also known as one-way shut-off couplings (See Fig. 3), they consist of a check valve, usually on the female half and no valve on the male mating half. These types of couplings are normally used in applications where leakage or spillage of the fluid on the downstream side of the system is not as important. They are normally installed with the valved half on the pressure side of the circuit to provide automatic shut-off when the coupling is disconnected. Single shut-off couplings are generally suited for working pressures of 60 to 300 psi. (Note that couplings’ maximum pressures depend on design and material.
Double Shut-Off Couplings
Double shut-off couplings (See Fig. 4), also known as two-way shut-off couplings, have a check valve on both the male and female halves. They are used in applications where downstream leakage or spillage is undesirable. This type of coupling is generally capable of much higher pressures than single shut-off couplings. Double shut-off couplings can support applications with pressures up to 10,000 psi.
Non-Spill Couplings
This variation of a two-way shut-off coupling, also known as flat face, dry break, or non-spill coupling (See Fig. 5), is a two-way shut-off coupling designed for applications where any leak or spillage poses a risk of contamination. The internal valve configuration prevents any loss of fluid upon disconnection and minimizes air entry when connecting. This type of fitting usually has operating pressures of up to 5,000 psi.
As with the fittings we discussed in Part 1 of this article, there are many coupling options available, so it is very important to understand your application requirements in order to have a reliable and serviceable liquid cooling system. To ensure you choose the right fluid connector for your application, it is best to work closely with your coupling or liquid cooling components partner early in the design process.
Fluid Fitting and Connector Selection
Choosing the Right Fluid Connector for Your Liquid Cooling Application – Part 1: Fittings
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.
I. Assessing the Application
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).
II. Determining the Type of Fluid Connector
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.
Fitting Types
Beaded Fittings
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.
Barbed 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.
NPT (National Pipe Thread) 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 Designations”.
SAE (Society of Automotive Engineers) Threaded Fittings
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.
Compression Fittings
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.
Flare Fittings
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.”
Bite-Type Fittings
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.
Mechanical Grip-Type Fittings
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.
O-Ring Face Seal Fittings
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.
Fitting and Hose Clamp Selection for Cold Plates and Heat Exchangers
Determining the Best Way to Prevent Flexible Hosing Leaks
The selection of the proper fittings and hose clamps for cold plates and heat exchangers is critical to a reliable liquid cooling system. It can prevent unnecessary downtime and equipment damage. Based on over 50 years experience with scores of fittings and clamps in hundreds of applications, we have found two options which have proved to be superior. Below we discuss the use of beaded fittings and the advantages and disadvantages of the two preferred clamp options.
When a system requires flexible tubing, the beaded tube fitting has been specified almost universally, especially in critical applications such as computer, medical, laser and automotive where operation must be 100% leak free. The bead, a 0.035″ +/-0.003″ ridge for 0.375″ OD tubing (See Figure 1), prevents the clamp and hose from slipping off the tube; the bead does not prevent leaks, however.
The clamp prevents leaks at the joints. The clamp sealing performance, defined as the static pressure and tightness, depends on several factors:
- The material and design of the hose
- The compatibility of the hose bore to the connecting part
- The correct selection and installation of the clamp
- Various mechanical factors such as pressure, temperature, and vibration
Boyd recommends two types of clamps, the ear/crimp clamp and the worm or gear clamp. The ear clamp, shown below in Figure 2, is a metal band that is crimped with a special tool to form a leak-free connection. The worm clamp, Figure 3, consists of a notched band with a screw mechanism to loosen or tighten the clamp.
Each clamp has advantages and disadvantages as determined by your application. See Table 1 for a comparison of features.
Table 1: Clamp Comparisons of Features
| Ear Clamp | Worm Gear Clamp |
| Superior clamping performance | Standard clamping performance |
| Circular, uniform clamping pressure | Some have flat spot under tightening mechanism |
| Shows tampering | Does not show tampering |
| Not reusable | Reusable |
| Special tool required for installation; hand tool or pneumatic tool (recommended) | Standard screwdriver required for installation |
| Can be difficult to install (crimp) in tight places | Can be installed in tight places |
Ear Clamps
Ear clamps provide superior sealing because the connection cannot loosen over time. Since ear clamps have a low spring constant, Hooke’s Law dictates that ear clamps maintain a greater force after hose relaxation. They maintain a circular geometry after tightening which proves more reliable. Installation is simple, the ear clamp slips over the hose fitting joint and requires a pneumatic or hand tool to tighten. We strongly recommend the pneumatic tool because it insures consistent pressure applied around the fitting and greater consistency in a production environment. Also any tampering is visible with the ear clamp.
The drawback of the ear clamp is that it can only be used one time and it requires a crimping tool for field replacement. The crimping tool can be bulky and difficult to use in tight places. These three disadvantages limit the ear clamp’s serviceability.
Worm Clamps
The worm clamp has its own set of issues. Some designs have a flat spot below the screw mechanism that can result in uneven pressure around the fitting. However, many worm clamp vendors have new designs that eliminate the flat spot. In addition, worm clamps tend to be stiff so a slight hose relaxation can result in a significant loss of force. In the past the notches for the screw mechanism would cut into the hose and could be a potential leak source. Again, new designs include a smooth inner surface.
The chief advantage is greater system flexibility and serviceability. The worm clamp is easily replaced in the field with just a screwdriver. And thanks to its wide diameter range, the same clamp can be used with a variety of hose sizes. The correct choice of materials and the availability of many band sizes make the worm clamp ideal for low and medium pressure applications.
Hose Clamp Selection Conclusions
Let your performance requirement guide you. In the end, the best fitting/clamp combination will be determined by your performance goals. If leak integrity is paramount and high cooling liquid pressures are involved, the ear clamp is appropriate in most cases. When easy field maintenance is the key factor and liquid coolant pressures are low to medium, the worm clamp/beaded fitting combination will serve you best. If you still have questions about your specific application, consult your clamp manufacturer. With the proper fitting and clamp, your liquid cooling loop will provide years of leak-free operation.
Choosing Quick Disconnect Couplers
Considerations for Selecting the Right QD for your Application
In order to select an optimal quick disconnect, start first with a thorough understanding of your assembly, application and operating environment.
Temperature:
- What are the minimum and maximum operating temperatures? Based on selected coupler material, common temperatures range from -40º F to 200º F.
Pressure:
- Know the operating pressure range the quick disconnect will be exposed to. Confirm that the quick disconnects maximum pressure rating will not be exceeded by your application.
Media:
- What fluid will be used in the system? Check the fluid’s viscosity and corrosiveness; confirm chemical compatibility of the fluid with the quick disconnect system.
Shutoff Options:
- There are a variety of shut off options. Consider what shut off options meet the needs of your application: single-sided, double-sided, non-spill, automatic, or integral shutoff valves.
Flow:
- Know your application’s required pressure drop and gallons per minute (GPM). In your flow calculations, consider full system impact including the effect of shutoff valves and tubing connections.
Tubing:
- There are a wide variety of tubing sizing, both inner and outer diameter. Know what size is specified in your application and confirm chemical compatibility with selected fluids.
Tubing Connections:
- Know your tube connection type. Are you using hose barb, compression fittings, push-to-connect or some other less common termination style?
- Know your tube connection size. Properly match tube inner diameter for hose barbs. For push-in fittings, properly match outer diameter. For compression fitting, match both tube inner diameter and outer diameter.
Mounting Options:
- How have you designed your quick disconnect to fit and function within your custom application? There are a wide variety of options to mount your quick disconnect: pipe thread, panel mount, in-line or elbow.
Special Requirements:
- Do you have additional special requirements? Cleanliness requirements or sterilization? Specialty material specifications like NSF approved or USP Class VI approved? Do you have custom finishing requirements like special packaging, color coding, keying or lot traceability?
Most importantly when specifying a quick disconnect consider serviceability, replacement availability and processes as well as ease of installation.
