Liquid Nitrogen Boiloff Calorimeter

A liquid nitrogen boiloff calorimeter was fabricated at KSC for testing cryogenic insulation materials. The unit was previously used at Aspen Systems, Inc., of Marlborough, Massachusetts, for measuring the thermal performance of new aerogel-based superinsulations. The evaluation of commercially available cryogenic insulation as well as further testing of the aerogel composites is scheduled. The calorimeter features a 24-inch-long, 6-inch-diameter test chamber (volume is approximately 10 liters), which is oriented vertically with a thermal guard chamber on each end. The test chamber is enclosed by a 14-inch-diameter by 48-inch-long vacuum can. An overall view of the system is shown in the photograph and the schematic diagram. The insulation test article is installed around the stack of chambers in the desired thickness (up to 2 inches). Temperature sensors are installed on the surface of each chamber and on the outer surface of the insulation. In operation, all three chambers are filled to capacity and maintained full until a steady-state thermal equilibrium is obtained (heavy-wall stainless steel construction provides added thermal stability). The test chamber is then isolated while replenish flows of liquid nitrogen are continuously supplied to the two guard chambers. The upper chamber is maintained at a slightly higher pressure [approximately 0.5 pound per square inch gage (psig)] to prevent the test chamber's boiloff gas from liquefying. The boiloff gas is routed to a flowmeter where it can be measured and monitored as required. This flow of nitrogen gas is directly proportional to the radial conduction of heat through the insulation. The basic relation is as follows:




Formula 



where,

hfg = heat of vaporization (joule per gram)
m = boiloff rate (gram per second)
L = length of chamber (meter)
k = thermal conductivity (watts per meter-kelvin)
To = insulation surface temperature (kelvin)
Ti= chamber surface temperature (kelvin)
Do = insulation outer diameter (meter)
Di = chamber outer diameter (meter)

Thus, the apparent thermal conductivity of the insulation can be computed by knowing the boiloff rate, the geometry of the chamber, and the temperature difference across the insulation thickness. The boiloff calorimeter is designed to operate at a test pressure of below 10-4 torr or up to 15 psig. The liquid nitrogen supply is provided through a 0.5-inch line at approximately 20 psig.. The system design incorporates a removable vacuum enclosure that allows quick reconfiguration of the insulation test article. Monitoring and data acquisition are accomplished using a LabView computer interface.

Contact:
James Fesmire
DM-MGD, (407) 867-7969

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Author: James Fesmire <james.fesmire@ksc.nasa.gov> / NASA
Curator: David Adcock <david.adcock@ksc.nasa.gov> / NASA
Last Revised: May 1, 1997

A service of NASA / Kennedy Space Center Engineering Development Directorate, MECHANICAL AND ELECTRICAL GROUND SUPPORT SYSTEMS OFFICE, Roger Hall, Chief (roger.hall-1@ksc.nasa.gov)