Fluid Coupling & Connector Selection

Choosing the Right Fluid Coupling for Your Liquid Cooling Application

Fluid line connectors are essential to liquid cooling loops. Engineers must consider selection, installation, and maintenance of a system’s fluid connections to prevent leaks and ensure system longevity. 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 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.

This article focuses on couplings. See Part 1 of this article: Fluid Fittings & Connector Selection for details on how to choose the right fittings for your application.

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.