Fluid feedthroughs are designed to transmit fluids into high and ultrahigh vacuum environments or pressurized  systems through a hermetic seal. Generally, fluid feedthroughs are passed through the walls of vacuum systems to cool the entire system.  Specific transmission materials include air, water, process gases, and liquid nitrogen.  

There are two main types of fluid feedthoughs: those designed for general service and those for cryogenic applications. General service fluid feedthoughs are designed for use with liquids or fluids that are at or above room temperature. These devices are usually of single walled construction and transmit water or gases as the cooling medium.  General purpose feedthroughs are available as weldable, or thread, nut or vacuum flange-mounted units for ease of installation.

Cryogenic service feedthroughs are designed to pass coolants, most notably liquid nitrogen, through the vacuum system. Due to the extreme thermal gradients cryogenic fluid feedthroughs are design with dual walls and coaxial tube construction.  The coaxial cavity between these tubes is on the vacuum side of the feedthrough assembly and provides a thermal barrier that reduces condensation and ice buildup on the atmosphere side of the assembly.  A buildup of frost or ice could easily compromise the system, clogging the pipes and allowing the machinery being served to heat up unchecked; or worse, a catastrophic release of liquid nitrogen.

When determining which of the available fluid feedthroughs would be best for a given application, there are a number of factors to consider beyond whether general service or cryogenic service is needed.  The most important consideration is the vacuum level that will be required for the application, and which fluid feedthroughs are rated to function within it.  Additionally, one must consider the diameter of the tubing that will be used and the diameter of the holes though which the feedthrough will be mounted.