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.