Download 4.4 Thermal Resistance Notes

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?