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Download 4.4 Thermal Resistance Notes
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Chapter 4.4 Notes Resistance in Thermal Systems • In fluid systems, resistance opposes the flow of Fluid. • In thermal systems, resistance opposes the flow of heat. • Thermal Resistance is a measure of an object’s ability to oppose heat transfer. • Materials with high thermal resistance are used to insulate an object. • In thermal systems, the prime move is temperature difference and the flow rate is heat flow rate. • Thermal resistance = Temperature drop / Heat flow Rate • Rthermal = rT / Hfr • Thermal resistance 2 = thickness/(thermal conductivity x Area) • RThermal = x / kA • If the insulation is .5 m thick with a wall measuring 10 m by 10 m and the thermal conductivity of .00011, what is the Resistance? • Which Equation? • RThermal = x / kA • Area = 10 x 10 = 100 • RThermal = .5 / (.00011 x 100) • RThermal = .5 / .011 = 45.5 • What is our resistance if our heat flow rate is 5 with an initial temperature of 21 and final temperature of 45? • Which Equation? • Rthermal = rT / Hfr = Tf – Ti / Hfr • Rthermal = (45 – 21) / 5 • R = 24/5 = 4.8 • If you have several layers of different material in a wall, to get the total resistance, simply add the separate resistance of each layer. • Resistance total = R1 + R2 • Resistance of the sheet rock is .00032, resistance of the insulation is .031, and resistance of the brick is .00028. What is the total resistance of the wall? • Resistance total = R1 + R2 + R3 • Resistance total = .00032 + .031 + .00028 • Resistance total = .0316 Virginia – 1900’s What was used for insulation? Used by Romans – What is the insulation?