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| | Description of Thermal System | Warm a test instrument in an avionics system from as cold as -45°C to 70°C within two minutes with ±2°C accuracy. The instrument is a cylinder 1.25" (32 mm) diameter and 3.5" (89 mm) tall, providing a heating area of 3.9 x 3.5" (100 x 89 mm). The available voltage on the aircraft is 28 VDC. | Maintain 96 sample vials, each containing 10 ml of human blood, at 37°C. The vials are positioned in drilled blind holes in an aluminum block measuring 4.0" x 6.0" x 1.5" (102 x 152 x 38 mm) with a total mass of 500 g. The sample temperature must never exceed 40°C at 24 VDC. | A 300 mm silicon wafer placed on a 325 mm diameter aluminum chuck must be heated from 40°C to 220°C during processing. Input voltage is 208 VAC. | |
| | Wattage requirements | From Thermal Calc*, we need 60 watts warmup power and 25 watts maintenance power. | From Thermal Calc*, we need 60 watts for warmup and maintenance. | From Thermal Calc*, we need 800 watts to reach the required temperature within the time limit. | |
| | Electrical parameters | R = E7W = 28760 = 13.1 Q | R = E2/W = 242/60 = 9.6 Q | R = E2/W = 2082/800 = 54.1 Q | |
| | Heater selection | Commercial and military avionics systems typically specify Polyimide insulated heaters. Model HK5482R12.1L12A is selected. | Specifying polyimide because it is resistant to most chemicals and does not outgas, the best choice is HK5491R9.4L12B | The required temperature exceeds the limit for polyimide heaters, and the vacuum process does not allow silicone rubber. An All-Polyimide heater, factory mounted to the chuck, is required. | |
| | Actual wattage | Wattage is 28712.1 = 65 W | Wattage is 242/9.4 = 61 W | Wattage is 2082/54.1 = 800 W | |
| | Watt density | Watt density = W/effective area = 65/9.8 in2 = 6.6 W/in2 (1.0 W/cm2) | Watt density = W/effective area = 61/21.54 in2 = 2.8 W/in2 (0.4 W/crrf) | Watt density = W/effective area = 800/109.9 in2 = 7.3 W/in2 (1.1 W/cnf) | |
| | Installation | For this cylindrical shape heat sink, a BM3 Shrink Band is selected. | Any type of heater mounting will handle the watt density. We recommend acrylic PSA for fast availability of prototypes. | Factory lamination of AP heaters provides optimum heat transfer and allows operating temperatures higher than other adhesives. | |
| | Leadwire current | AWG 26 current rating is 5.0 A. Actual current is: I = 28/12.1 = 2.3 A (OK). | AWG 24 leadwire current rating is 7.5 A. Actual current is: I = 24/9.4 = 2.6 A (OK). | AWG 20 leadwire current rating is 13.5 A. Actual current is: I = 208/54.1 = 3.8 A (OK). | |
| | Control | The CT325 controller will be used to control the heater. | A custom control circuit integrated into the system electronics will control the heater. The controller is designed for a 1000 Q platinum RTD element input. | All electrical and motion control of the wafer processing system is centrally controlled by a computer. Thermal control is integrated into the system. | |
| | Sensor | An S665 Thermal Tab RTD provides easy installation in the prototype test system. | A 1000 Q platinum Thermal-Tab™ RTD sensor is used. The customer tests the sensor in several positions around the aluminum block to determine the optimum location. | An S247 thin film RTD element with high-temperature extension leads will be cemented into a hole in the platen. | |
| | Custom options | Experiments confirm the power requirements, but also show that the sensor measures only one point rather than the average temperature of the cylinder. In the final custom design an integrated Thermal-Ribbon strip sensor wraps around the circumference of the cylinder to measure the average temperature. | Testing showed that edge losses required 20% higher watt density around the periphery of the heater to equalize temperature within the block. A custom design with profiled power output, integrated sensor, and 40°C thermal fuse provides a complete thermal system in one package. | The leads exit is located at the center of the heater to fit with the design requirements of the machine. | |
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