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Guarded Hotbox Testing in the Buildings Technology Center




ORNL maintains and operates two versatile thermal chambers for testing building components. The Rotatable Guarded Hot Box (RGHB) can test large building components at tilted orientations. Among the eight to ten commercially operated hot boxes in North America, the Large Scale Climate Simulator (LSCS) allows relatively large horizontally-mounted roof systems to be tested at extreme environmental conditions, including summer conditions. Both of the Buildings Technology Centers thermal chambers can test to the specifications in ASTM C 1363 (walls and roofs). The RGHB can meet the requirements of ASTM C 1199 (windows, doors and skylights), which the National Fenestration Rating Council uses to validate their certified U-factor ratings. We plan to modify and calibrate the RGHB for testing to the requirements of the new ISO 8990 (walls and roofs) and ISO12567 (windows, doors and skylights) test methods.

During the last decade, over two hundred wall configurations have been tested at the Buildings Technology Center. The wall assemblies were tested in the Rotatable Guarded Hot Box in accordance with ASTM C 236, the method that preceded ASTM C 1363. The test wall assemblies are installed into a specimen frame that is mounted on a moveable dolly. The specimen frame has an aperture of 4 by 3 m (13' 1" by 9' 10"). Since the wall assemblies being evaluated are all smaller than this aperture, the remaining area is filled with a thermally resistive insulation material and the thickness of the fill material is adjusted to match the thickness of the test wall assembly. The specimen frame/test wall assembly is inserted between two chambers of identical cross-section. Since the solid test wall assembly is located between the chambers, the chamber temperatures can be controlled independently. The chambers are designated as the climate (cold) and metering/guard (hot) chambers.

In the climate chamber, a full-size baffle is mounted approximately 250 mm (10 in.) from the test wall assembly. An external refrigeration system operates continuously and cooled air is circulated to and from the refrigeration system through insulated flexible ducting connected to the rear of the climate chamber behind the baffle. Five centrifugal air blowers, installed in the climate chamber behind the baffle, force the air through a bank of electrical resistance heaters and through the airspace between the baffle and test wall assembly. The air flows parallel to the wall assembly. Air temperature is controlled by controlling the temperature of the air entering the climate chamber and fine-tuning that temperature with the resistance heaters. Air velocity is controlled by adjusting the frequency of electric power input to the air blowers. Five anemometers continuously measure the wind speed in the airspace. Typical wind speeds are from 1.3 to 6.7 m/s (260 to 1300 ft/min).

In the center of the metering/guard chamber, a metering chamber is pressed against the test wall assembly. The metering chamber is approximately 2.3 m (8 ft) square by 0.4 m (1.3 ft) deep. The walls of the metering chamber are constructed from 76 mm (3 in.) thick aged extruded polystyrene foam having an approximate thermal resistance of 2.6 m2K/W (15 hr ft2oF/Btu) at 24oC (75oF). The walls of the metering chamber are reinforced by aluminum frames on the interior and exterior sides that are interconnected by fiberglass threaded rods. The edge of the metering chamber which contacts the test assembly is tapered to a thickness of 19 mm (0.75 in.) and a 13 mm (0.5in.) square, neoprene rubber gasket is affixed to this tapered edge. This gasket is very compressible and readily follows the contour of the test wall surface to minimize air leakage from the metering to the guard chamber. A baffle is mounted inside the metering chamber 150 mm (6 in.) from the exposed edge of the gasket. Behind the baffle, eight fans and four electric resistance heaters are installed. These components are situated such that air is pulled downward behind the baffle, through the resistance heaters, and upward through the airspace between the baffle and test assembly. The upper and lower rear corners of the metering box are tapered to minimize air impingement onto the metering box walls and to provide a smooth transition into the baffle space. Four anemometers continuously measure the wind speed in the air space, which varies from 0 to 0.45 m/s (0 to 88 ft/min).

Click Here To See Detailed Hot Box R-value Test Results

Rotatable Guarded Hot Box (RGHB) and Sample of Tested Wall Technologies
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  Hot Box Testing of Full-Scale Attic and Roof Assemblies

Since construction of the Large Scale Climate Simulator (LSCS) in the late 1980's, many tests of commercial low-slope roof and residential attic/roof assemblies have been accomplished. Similar to the situation in the Rotatable Guarded Hot Box, many of these tests have been at steady-state in accordance with ASTM C 1363 and its predecessor. Some tests have also been done with diurnal conditions. Roof assemblies are constructed in a test platform that has a 3.8 m (12.5 ft) square opening. The assembly can weigh as much as 10 Tons and can be 6 ft high. The fully assembled and instrumented test section is moved by a crane into the LSCS. The test platform rests on an inflatable gasket that seals the platform between the climate chamber above it and the metering/guard chambers below it.

Air temperatures in the climate chamber can simulate any outdoor condition of interest. Steady-state temperatures from 66oC to -40oC (150oF to -40oF) can be achieved. Infrared lamps can preferentially heat roof surface temperatures to 93oC (200oF). An external refrigeration system supplies chilled refrigerant through a control valve to direct expansion evaporator coils in the air handler of the climate chamber. This allows extremely cold air temperatures in the climate chamber. After passing over the coils, the air can be reheated if necessary by electric resistance heaters for moderate air temperatures in the climate chamber. If no refrigerant is flowing through the evaporator, the air can be heated by the electric resistance heaters to extremely hot temperatures in the climate chamber. A wide range of relative humidities are also available in the climate chamber through control of dew point temperature from 3oC to 50o C (37oF to 122oF). Additional refrigeration coils and direct steam humidification are used for control of dew point temperature. The air in the climate chamber is pulled over the coils and heaters by centrifugal fans which discharge the conditioned air at the top of the climate chamber. The air then flows along the ceiling and back over the test section before being drawn into the bottom of the air handler. Flow velocity over a low-slope roof test section is 0.5 to 0.8 m/s (95 to 160 ft/min).

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Large Scale Climate Simulator (LSCS)

For guarded hot box tests in the LSCS, a 1.5 m (5 ft) high metering box with 2.4 m (8 ft) inside cross-sectional area is raised until it is sealed against the bottom of the test section. The metering box is surrounded by a guard chamber. The walls of the metering box are made from 10.2 cm (4 in.) thick polyisocyanurate foam insulation covered by hard plastic. R-value of the walls is 4.4 m2 K/W (25.2 hr ft2 oF/Btu) at 24oC (75oF). Thermocouples are placed across the walls of the metering box in order to determine the heat flow between the metering and guard chambers. The air temperatures in the guard and metering chambers are kept constant by automatic controls. Set point for the guard chamber air temperature is determined from the thermocouples on the walls of the metering chamber, in order to minimize the heat flow between the metering and guard chambers. The top of the metering chamber has a thick gasket to ensure an air-tight seal with the test section and corrections are made for thick test sections that account for two-dimensional heat flow around the perimeter of the metered area. The LSCS is a particularly versatile guarded hot box because it has the capability for cooling in both the metering and guard chambers. Chilled water from a common chiller loop serves a cooling coil in each chamber. This allows tests wherein summer conditions can be imposed in the climate chamber besides the usual winter conditions for most hot box tests. Using electrical resistance heaters alone in the metering chamber, or with electrical resistance heaters to offset the effects of cooling, temperatures from 40oF to 150oF can be held below the metered area. In the metering chamber, a baffle is located about 23 cm (9 in) below its top. Ten axial flow fans pull air from the bottom of the metering chamber, over the cooling coil and electric resistance heaters, and deliver it parallel to the bottom of the test section. At full flow through the fans, air flow velocity over the test section varies from 0.7 to 0.9 m/s (150 to 175 ft/min).



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