Conduction, convection and radiation, then the heat exchangers that combine them.
Choose any section below to get a free practice problem. Work it with the FE Reference Handbook open, tap your answer, then check the solution.
Conduction through a plane wall is q = kAΔT/L.
A 1 m2 wall has 0.1 m of insulation (k = 0.05) and 0.2 m of concrete (k = 1.0), with 22 °C across both. What is the heat flow?
R = 0.1/0.05 + 0.2/1.0 = 2.2 K/W. q = 22/2.2 = 10 W.
Common trap: (B) uses only the concrete.
Heat TransferConductionthermal resistanceThat was 1 of 3 problems in this section. The full course has the lesson, a worked example and every problem. Try all of it free for 3 days →
Convection from a surface follows Newton’s law of cooling, q = hA(Ts − T∞).
A wall has h1 = 10 W/m2·K inside, L/k = 0.2 m2·K/W, and h2 = 25 W/m2·K outside. What is U?
1/U = 0.1 + 0.2 + 0.04 = 0.34, so U = 2.94 W/m2·K.
Common trap: (C) adds the coefficients.
ConvectionNusseltBiotoverall heat transfer coefficientThat was 1 of 3 problems in this section. The full course has the lesson, a worked example and every problem. Try all of it free for 3 days →
All surfaces radiate in proportion to the fourth power of absolute temperature: q = εσA(Ts4 − Tsur4) for a small surface inside large surroundings, with σ = 5.67 × 10−8 W/m2·K4.
If a blackbody’s absolute temperature doubles, its emissive power becomes:
E ∝ T4: 24 = 16 times.
RadiationStefan-Boltzmannemissivityview factorThat was 1 of 3 problems in this section. The full course has the lesson, a worked example and every problem. Try all of it free for 3 days →
Heat exchanger duty is q = UA·ΔTlm, where the log mean temperature difference is (ΔT1 − ΔT2)/ln(ΔT1/ΔT2), using the end temperature differences.
A counterflow exchanger cools hot fluid from 150 to 90 °C while cold fluid warms from 30 to 70 °C. With U = 500 W/m2·K and A = 10 m2, what is its duty?
Ends: ΔT1 = 150 − 70 = 80, ΔT2 = 90 − 30 = 60. LMTD = 20/ln(80/60) = 69.5 °C. q = 500(10)(69.5) = 348 kW.
Common trap: (B) uses ΔT2 = 60 instead of the LMTD.
Heat ExchangersLMTDeffectivenessTransient ConductionThat was 1 of 3 problems in this section. The full course has the lesson, a worked example and every problem. Try all of it free for 3 days →
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