Flexible tubing is a very handy fluid conveyance option in liquid cooling systems. The ability to snake tubes into tight areas and avoid other components, without the need to do complex, high-precision tube bending of rigid tubes, makes system integrators’ lives easier during installation and maintenance. But how do these flexible tubes interface with pumps, radiators, cooling plates, and other liquid cooling system components? This post describes the common considerations and mechanisms of flexible tube fluid fittings, as summarized in the graphic below.
Barbed fittings are a very common technique for attaching fluid hoses. A flexible hose with an inner diameter equal to the barb fitting size slides over the barb to create a fluid seal. Although in name the inner diameter of the tube matches the barb fitting size, in actuality the fluidic barb has a slightly larger outer diameter than the tubing inner diameter (e.g. a 3/8” barb fitting may have an actual outer diameter of ~0.415”). This causes the flexible tubing to deform around the outer diameter of the barbs, causing an inward reaction force from the deformed tubing to contain low fluid pressures. To sustain a higher interior pressure, hose clamps are often attached to the exterior of the flexible tubing to provide a stronger inward radial force and prevent leaks.
Push-to-connect fittings are a very quick method of creating liquid seals with flexible and rigid tubing. A hose with an outer diameter equal to the push-to-connect fitting size is inserted into the push-to connect fitting body. In the interior of the push-to-connect fitting are two major features to create and sustain the seal: 1) an o-ring gasket, which is radially compressed between the inserted tube outer diameter and the surrounding fitting body, and 2) a grasping ring, which grabs the tube with plastic tines to prevent from being axially ejected. Push-to-connect fittings are typically for lower pressure liquid cooling applications and may drip fluid or incorporate air when installed, but provide an easy tool-free installation option in frequently changing systems.
Compression fittings cause a permanent alteration in the tube during the installation process, but provide a higher pressure sealing option. A flexible or rigid hose with an outer diameter equal to the compression fitting size is integrated into an assembly of a compression nut, a flared ferrule, and a flared and threaded fitting body. As the compression nut is tightened on the fitting body threads, force is transmitted onto the ferrule, causing it to pinch and deform the exterior of the tubing. This produces an outward reaction force from the deformed tubing to produce a strong sealing force. This method requires tubing with a high durometer to provide adequate outward force. Alternatively, a plastic or metal insert may be added to low durometer tubing to provide the stiffness required for the compression installation to be effective.
Permanent Attachment Fittings
In a final, non-specific case, some flexible tubing options are “permanently” attached to a threaded fitting with which to interface with other parts of a liquid cooling system. Of course, with the right tools, anything can be deconstructed. But in its intention, these plastic tubing options are not meant to be separated from their inherent fittings in the same way that barbed, push-to-connect fittings, and even compression fittings are. This could be done using a variety of techniques, whether some sort of interference fit with a metal sleeve, adhesive or epoxy, or combining a solder or weld with another metal part to create a rigid bond with the neighboring interface.
The table below provides a summary and compatibility chart of some different flexible tubing and fitting types. This may serve as a quick reference table for choosing the appropriate flexible tubing or fitting attachment mechanism for an upcoming liquid cooling application.
Table 1: Summary of flexible tube fitting options for liquid cooling systems.  Compression fittings may be used with lower durometer tubing if a high stiffness insert is added.  This pressure rating corresponds to the fitting interface ONLY, and is situation and material dependent. Often the tubing chosen will fail before the fitting. Be sure to evaluate all factors in each situation carefully to avoid system failures and safety hazards.
 “About Plastic and Rubber Tubing” McMaster-Carr, www.mcmaster.com/tubing
 “Hose and Flexible Tubing” Swagelok, www.swagelok.com/downloads/webcatalogs/en/MS-01-180.PDF