One fine way to test the thermal performance of insulation systems is by the method of boiloff calorimetry using liquid nitrogen. The liquid nitrogen works as an energy meter. The boiloff (or evaporation) of the liquid is directly related to the energy going through the insulation system from the outside (warm boundary) to the inside (cold boundary). An article on cryogenic insulation testing, including both cylindrical and flat plate type instruments is found in the Cold Facts magazine series on Boiloff Calorimetry
The enthalpy of vaporization (or heat of vaporization) for liquid nitrogen is a constant at any given pressure. We can therefore measure the boiloff flow rate and directly calculate the heat going (Q). That is it: the flow rate is directly proportional to the heat energy. For example, a boiloff flowrate of 1,000 sccm (or one liter per minute) equates to 4,138 W (see Table and Chart below). Knowing the boundary temperatures and geometry of the insulation test specimen, we can then calculate thermal properties such as heat flux (q) and effective thermal conductivity (Ke). The heat flux depends on the effective heat transfer area which is a small test specimen of 146-mm diameter in this example.
| t = 25.4 mm, 293 K/78 K, Hfg = 198.6 J/g |
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| flow |
Q |
q |
| sccm |
W |
W/m2 |
| 1 |
0.004 |
0.247 |
| 10 |
0.041 |
2.47 |
| 100 |
0.414 |
24.7 |
| 1000 |
4138 |
247 |
| 10000 |
41.38 |
2470 |
| 100000 |
413.8 |
24698 |
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| Heat Flow Rate (Q) as a function of LN2 Boiloff Flow Rate |
The basic apparatuses for both flat-plate (round disk) and cylindrical type instruments for cryogenic boiloff calorimetry testing of thermal insulation systems are shown in the Figures below. For more information, see ASTM C1774, Standard Guide for Thermal Performance Testing of Cryogenic Insulation Systems. See also the recent presentations from the 2015 Cryogenic Engineering Conference in Tucson, Arizona: Cylindrical Cryostats and Flat Plate Cryostats.
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| Basic Flat Plate Apparatus for Cryogenic Boiloff Testing of Thermal Insulation Systems |
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| Basic Cylindrical Apparatus for Cryogenic Boiloff Testing of Thermal Insulation Systems |