The potential temperature drop of the platen and test unit in the "Thermovac" system as a result of power failure was estimated. The temperature drop as a result of power failure cannot exceed 3.5 °C. This estimate was made using "worst case" assumptions; the true temperature is unlikely to drop at all. No additional safety procedures are required to prevent sub-cooling of the "thermovac" system in case of power failure.

The estimate assumes that the platen piping was full of liquid nitrogen when the power failed, that all of this nitrogen vaporized, and that all of the required energy for this vaporization was obtained from the platen and its internal piping. Appropriate physical property data (pipe mass and internal diameter [1], liquid nitrogen density [2], specific heats for copper and aluminum [3], latent heat of vaporization for nitrogen [4]) were obtained from Perry's Chemical Engineers' Handbook. The required energy for vaporization was 128 kJ, resulting in a temperature drop of 3.42 °C for a 100 lb aluminum platen and 650 inches of 3/8 inch copper pipe. The thermal mass of the test unit, heat transfer via the piping from the liquid nitrogen source to the platen, and the residual energy in the heaters were neglected. Including these factors would result in a smaller calculated temperature drop.

A copy of the detailed calculations is attached for your information. Estimated time on this project is approximately 1 hour in consultation with SS/L engineers to define the problem, 1 hour to gather appropriate information and complete the calculation, and 1 hour to check the answer and produce this report. Thank you for the opportunity to consider this problem.

References
[1] R.H. Perry & D. Green (eds.), Perry's Chemical Engineers' Handbook, 6th ed., McGraw-Hill, 1984, Table 6-20, p. 6-65.
[2] Ibid, Table 3-163, p. 3-113.
[3] Ibid, Table 3-182, p. 3-135.
[4] Ibid, Table 3-177, p. 3-121.

(Last modified on 2/12/98)