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 = T / 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 = T / Hfr = Tf – Ti / Hfr

• Rthermal = (45 – 21) / 5

• Rthermal = 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?