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Choosing the Right Fluid Connector for Your Liquid Cooling Application – Part 2: Couplings

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 discussed fittings. Part 2 will review the two factors to consider when selecting fluid connectors and will describe the types of couplings most frequently used in liquid cooling applications.

I. Assessing the application

The key in selecting the right coupling 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. Understanding 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. (For more information on erosion corrosion please see our application note "Erosion-corrosion in Cooling Systems".)
  • 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.
  • 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). Other special requirements could include sterilization, color coding, special packaging, etc.

II. Determining the type of fluid connector

As previously mentioned, there are mainly two types of fluid connectors, fittings and couplings, commonly used in liquid cooling applications. They are both used to connect cooling loop components such as valves, pumps, cold plates, heat exchangers, hoses, etc. It is difficult to provide a strict definition for each, rather it’s easier to describe the differences in how they are used.

A fitting is typically used in applications that do not require the frequent disconnection of equipment or parts at any point during usage, 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.

Figure 1: Liquid-cooled chassis with quick disconnect fittingsFigure 1: Liquid-cooled chassis with quick disconnect coupling

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 also be molded into 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.

This article will only focus on couplings. See Part 1 of this article "Choosing the Right Fluid Connector for Your Liquid Cooling Application – Part 1: Fittings" 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

Figure 2: Straight-through couplingFigure 2: Straight-through coupling

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.)

Figure 3: Single shut-off couplingFigure 3: Single shut-off coupling

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.

Figure 4: Double shut-off couplingFigure 4: Double shut-off coupling

Non-Spill Couplings

Figure 5: Non-spill couplingFigure 5: Non-spill coupling

This variation of a two-way shut-off coupling, also known as flat face 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.