Slat heater boxes have been invented for controlling the sink temperatures of objects under test in a thermal vacuum chamber, the walls of which are cooled to the temperature of liquid nitrogen. A slat heater box (see Figure 1) includes a framework of struts that support electrically heated slats that are coated with a high-emissivity optically gray paint. The slats can be grouped together into heater zones for the purpose of maintaining an even temperature within each side.

Figure 1. A Slat Heater Box surrounds an object under test and is heated to a controlled temperature.
The sink temperature of an object under test is defined as the steady-state temperature of the object in the vacuum/ radiative environment during the absence of any internal heat source or sink. The slat heater box makes it possible to closely control the radiation environment to obtain a desired sink temperature. The slat heater box is placed inside the cold thermal vacuum chamber, and the object under test is placed inside (but not in contact with) the slat heater box.

The slat heaters occupy about a third of the field of view from any point on the surface of the object under test, the remainder of the field of view being occupied by the cold chamber wall. Thus, the radiation environment is established by the combined effects of the slat heater box and the cold chamber wall.

Given (1) the temperature of the chamber wall, (2) the fractions of the field of view occupied by the chamber wall and the slat heater box, and (3) the emissivities of the slats, chamber wall, and the surface of object under test, the slat temperature required to maintain a desired sink temperature can be calculated by solving the equations of graybody radiation for the steady-state adiabatic case (equal absorption and emission by the object under test).

Figure 2. The Sink Temperature of a test object as a function of α/ε was calculated for a representativecase of heating with a quartz infrared lamp at 10-percent power and a representative case of a slat heaterbox occupying a third of the field of view, with an emissivity of 0.865 and slat temperature of 410 K.
Slat heater boxes offer an important advantage over the infrared lamps that have been previously used to obtain desired sink temperatures: In comparison with an infrared lamp, a slat heater box provides a greater degree of sink temperature uniformity for a test-object surface that includes multiple areas with differing optical properties. This advantage can be seen by solving gray-body radiation equations for some representative cases of test objects for which the emissivity or absorptivity at wavelengths <4 μm (denoted α) differs from the emissivity or absorptivity at wavelengths >4 μm (denoted ε). [The term α is often denoted solar absorptivity because most of the power of solar radiation lies in the wavelength range below 4 μm, while the term ε is often denoted infrared or thermal emissivity because most of the power of room-temperature objects lies in the wavelength range >4 μm.] Figure 2 presents the results of one such calculation that illustrates the superiority of a slat heater box over an infrared lamp, in that the sink temperature is much less sensitive to α/ε in the case of the slat heater box.

This work was done by Eugene Ungar of Johnson Space Center.

MSC-23023


NASA Tech Briefs Magazine

This article first appeared in the October, 2003 issue of NASA Tech Briefs Magazine.

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