Environmental Enclosure

The EE is the temperature-controlled housing for the water flow system and the CC. Below is a photograph of the workspace of the EE. On the left is the flowmeter and water reservoir (gallon jug). On the far left at the bottom is the positive-displacement pump. On the right is the CC. In the foreground at the bottom are the terminal strips that provide all electrical connections to the interior of the CC.

The EE is a 19 mm thick plywood shell lined with two layers of 19 mm thick rigid polystyrene foam insulation[1]that has a thermal conductivity of 0.036 W/m/K. Estimated heat leakage from the laboratory room is 22 W for a 3 °C temperature difference between the room and the enclosure interior. Exterior dimensions of the enclosure are 1.5 m wide by 1.15 m high by 0.77 m deep.

The EE has two plenums, an air conditioner, and a main workspace. The workspace is 1 m wide by 0.66 m high by 0.66 m deep. Grilles form the ceiling and floor. The grilles are 6.35 mm aluminum plates perforated with 6.35 mm diameter holes on a staggered 12.7 mm pattern. The holes permit airflow through the workspace and provide a means of mounting equipment. Three of the walls of the workspace are solid and the fourth wall has a removable insulated door. A triple-glazed window in the door permits viewing the interior. The two plenums are located above and below the workspace. The air conditioner forms the left-hand portion of the EE. The plenum above the ceiling grille collects return air and is connected to the inlet of the air conditioner. The outlet of the air conditioner connects to the plenum below the floor grille. Four 2.94 m3/min fans move the air from the return air plenum into the air conditioner. The fans operate from regulated DC power to minimize speed variations. Each fan dissipates approximately 5 W. Removable insulated panels cover the air conditioner and plenums.

The temperature of the EE is controlled by a two stage Peltier air conditioner. The first stage contains two dual-element Peltier assemblies while the second stage has a single dual-element assembly. Each assembly consists of two commercial Peltier devices[2] of nominal 50-watt capacity sandwiched between a substantial, finned aluminum heat sink and a water-cooled copper block. Each fan on the Peltier assemblies dissipates approximately 5 W. Baffles in the air conditioner improve the air mixing by adding turbulence. Temperature sensors between stages and at the outlet provide signals for regulation of the air temperature. Additional sensors monitor the temperature of the EE air at the ceiling and at the inlet to the air conditioner.

The water circuit for these Peltiers consists of a 114-liter vessel, a small circulating pump, and a fan and radiator assembly. The large external vessel serves as a thermal capacitor, reducing the effect of sudden changes in room temperature.

The capacity of the EE air conditioner is adequate for CC dissipation of about 20 W plus the other heat sources in the EE. The additional loading of the air conditioner comes from the flowmeter equipment, circulating fans, ohmic losses in the electrical circuits, Active Insulation (AI) heaters, EE light, and heat ingress from the laboratory room. The air conditioner maintains the EE at 24.5 °C for most experiments. The control system permits a smooth transition between cooling and heating, minimizing temperature lag and overshoot. The Peltier assemblies also provide enclosure heating when needed. The relative humidity in the EE is about 45% at 25 °C. Under these conditions, the dew point is approximately 15 °C. We have not observed condensation on the air conditioning cooling assemblies. Condensate drains are not used.

[1] Dow Chemical Company Styrofoamâ; Extruded polystyrene foam insulation board; R = 3.75 ft2h°F/Btu; (www.dow.com/styrofoam)

[2] Melcor Corporation; Type CP 1.4-127-06L; 6 Ampere, 51.4 watts at DT = 0; (www.melcor.com)


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