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Understanding Thermal Resistance in Systems

Thermal resistance is a measure of an object’s ability to resist heat transfer. It can be calculated using formulas comparing to fluid and electrical resistances. The ratio of the prime mover to a flow rate is vital. Thermal resistance in series explains how heat flows through each resistance sequentially, similar to current in a series circuit.

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Understanding Thermal Resistance in Systems

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  1. Resistance in Thermal Systems 4.4

  2. Thermal Resistance • a measure of an object’s ability to oppose heat transfer. • Rthermal = -ΔT/Q (Thermal resistance = temperature drop divided by heat flow rate)—Compare to fluid and electrical—Rfluid = -ΔP/V

  3. Rthermal = Δx/kA (Thermal resistance = change in thickness of slab divided by constant · Area) • ***the ratio of the prime mover to a flow rate

  4. Units—°C /cal/s or °C·s/cal—°F/Btu/s or °F·s/Btu • Temperature drop across a slab is proportional to the heat flow rate.

  5. Thermal Resistance in Series • Heat flows through each resistance, one at a time, just like current flows through each resistance in a series circuit. • Rtotal = R1 + R2

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