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Heat Leak - Explaining the Numbers
Malaria Vaccine Dewar
Pipe Layout Template

Heat Leak - Behind the Numbers

Heat leak is a term used to compare different types of cryogenic equipment and insulation. Like other specifications, heat leak can be useful to compare components and different manufacturers of those components. But what do those numbers really mean?
 
The term heat leak (vs. heat transfer) is used since cryogenic liquids are more concerned with the heat gained. For cryogenic piping it is often expressed in BTU/hour per foot or Watts/meter, expressing the amount of energy transferred through the external surface area. The units may be left off and often the cryogen and temperatures are not reported at all. That can create challenges when comparing between manufacturers!
 
Small differences really don’t matter when comparing foam-covered or uninsulated pipe to vacuum-jacketed pipe (VJP) because VJP is so much better. If you're comparing smaller differences though, make sure they have similar conditions. Heat leak (transfer) varies depending on the cryogenic fluid temperature, exterior temperature, surface area, and of course the thermal conductivity of the materials used. Even when comparing 1” pipe with the same internal diameter, different assumptions for the temperatures (for example liquid nitrogen versus liquid helium) will change the reported heat leak value.
 
Real world values will also vary. Ambient air conditions have little effect on VJP, but dramatically effect foam or non-insulated pipe. Air flow across a pipe changes the convection and thus increases heat leak into the system – a bigger factor in non-vacuum jacketed systems. The performance of foam-covered pipe also degrades rapidly as it collects moisture.

Lastly, the type of joints and valves used in a cryogenic system will add additional heat leak. Typically much more than the pipe it is connected to. Even the internal liquid flow rate can change the amount of heat leakage from a calculated value.
 
Like many specifications that are reduced to a single number, be careful around the assumptions made. For more on parameters and some typical heat leak numbers for vacuum-jacketed pipe, please see our Cryogenic Resource Library.
 

Malaria Vaccine Dewar

You may have watched a recent 60 Minutes interview with Bill Gates that talked about the Bill & Melinda Gates foundation efforts to eradicate malaria. In that interview Bill Gates visits the Intellectual Ventures Laboratory to show the device they are using to deliver vaccine in rural Africa - a ‘super thermos’ that can keep vaccines cold for 50 days only using ice. What you may not realize is that Technifab had a large role in the development of the ‘thermos’ and manufactured the vacuum-insulated container the vaccines are put into.
 
Technifab has been working with Intellectual Ventures Laboratories (IVL) for over a year helping them to make the concept a reality. It’s been an exciting project for Technifab, allowing us to apply super-insulation techniques in a non-ordinary application that can help save lives. The experience has also allowed us to develop new techniques to meet very challenging requirements, which actually have been exceeded in testing.
 
You may be interested in finding out more about our line of dewars and our custom cryogenic equipment capabilities. You can read more about the final product at the IVL website and also watch the 60 minutes interview at CBS.


Pipe Layout Sheet

Some of our customers provide us with pipe layouts for quoting almost ‘on the back of a napkin’, which is fine. We’re glad to have the opportunity to bid on any project and more than happy to help you get the layout into a more detailed form. Internally however, when we’re not using a 2D or 3D modeling program, we often do a quick sketch on a piece of paper using our isometric Pipe Layout Sheet. We’ve given them to some customers, but the layout sheet, p/n LT-31288, is also on our website. Download a copy of the PDF document if you'd like and print out a copy to use. It's handy to have if you are producing a sketch of a cryogenic liquid delivery system or any other isometric sketch.

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Heat Leak and Cryogenic Piping
Not all heat leak calculations are created the same.









































 
Technifab Manufactured Custom Dewar
Technifab Custom Dewar for Passive Vaccine Storage Device




















 
Vacuum-Jacketed (Insulated) Pipe Layout Template
Vacuum-Jacketed Pipe (VJP) Layout Sheet