Improved Methods of Testing Cryogenic Insulation Materials
- Created on Friday, 01 December 2000
Specimens are easy to fabricate, and thermal performance measurements are repeatable.
Two improved methods have been developed for testing continuously rolled blankets and blanketlike thermal-insulation materials typically used in cryogenic vacuum systems. Both methods, and their corresponding apparatuses, are based on the cryogen boiloff calorimeter method according to which the amount of heat that passes through an insulation specimen to a cryogenic fluid in a vessel is proportional to the rate of boiloff from that vessel. The boiloff rate is then directly related to the insulating performance of the specimen. The main challenges in the execution of this technique are to (1) eliminate (or minimize) heat leak from the ends by use of thermal guards and (2) obtain stability of the cryogen inside the measurement vessel coincident with stability of the boundary conditions in the vacuum space.
The main problem in testing high-performance materials such as multilayer insulation is the extreme care that must be exercised in their fabrication and installation. Inconsistency in wrapping techniques is the dominant source of error and poses a basic problem in the comparison of such materials. Improper treatment of the ends or seams can render a measurement several times worse than predicted. Localized compression effects, sensor installation, and outgassing are further complications. To eliminate the seam and minimize these other problems, two new methods of fabricating and testing cryogenic insulation systems have been developed.
The first method includes a cryostat test apparatus (Cryostat-1, see Figure 1), which is a liquid nitrogen boiloff calorimeter system for direct measurement of the apparent thermal conductivity at a fixed vacuum level. The cold mass is a 167-mm-diameter, 900-mm-long, vertical stainless-steel cylindrical vessel subdivided into a 10-liter measurement vessel and 2.5-liter thermal-guard vessels at both ends. Continuously rolled materials are installed around a cylindrical copper sleeve using a 1-m-wide wrapping machine. Sensors are placed between layers of the insulation to obtain temperature-thickness profiles. The sleeve is then simply slid onto the vertical cold mass of the cryostat.
During operation, all three vessels are kept filled with liquid nitrogen at near saturated condition at ambient pressure (temperature ≈77.8 K). Vacuum levels may be set at any desired pressure from 10-5 torr to 760 torr. The temperatures of the cold mass, the sleeve (cold boundary temperature), the insulation outer surface (warm boundary temperature), and the vacuum can (heated by a thermal shroud) are measured. The steady-state measurement of insulation performance is made when all temperatures and the boiloff flow rate are stable. The apparent thermal conductivity value of the insulation is directly determined from the measured boiloff rate, boundary temperature difference, latent heat of vaporization, and geometry of the test specimen. The measurable heat gain rate for Cryostat-1 is from 0.2 to 20 W.
This method offers the following advantages: (1) enables testing of continuously rolled samples for better accuracy, (2) specimens are representative of most industrial applications, (3) specimens can be easily produced to the desired specifications with an absolute minimum of handing, and (4) specimens can be fabricated off-site.
The second method includes a cryostat test apparatus (Cryostat-2, see Figure 2), which is a liquid nitrogen boiloff calorimeter system for calibrated measurement of the apparent thermal conductivity at a fixed vacuum level. The cold mass is a 132-mm-diameter, 267-mm-long stainless-steel vessel thermally guarded by a 132-mm-diameter, 127-mm-long stack of aerogel composite disks at each end. The system features a fully removable cold mass which quickly and easily mounts onto a 0.5-m-wide wrapping machine for installation of insulation material and sensors.
Cooldown and filling of the system are conveniently accomplished through a single port, using a custom ambient-pressure-regulated liquid nitrogen transfer device. Sensors and measurements are similar to those of Cryostat-1. The measurable heat gain rate for Cryostat-2 is from 0.7 to 40 W. The key benefit of this method is that it allows a high rate of testing many different samples with highly repeatable results between runs.
This work was done by James E. Fesmire, Robert A. Breakfield, Dale J. Ceballos, Philip D. Stroda, and James P. Niehoff, Jr., of Kennedy Space Center and Stan D. Augustynowicz of Dynacs Engineering Co. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.