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MCQs

Total Questions : 448 | Page 28 of 45 pages
Question 271. The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
  1.    Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)
  2.    Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
  3.    Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
  4.    Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
 Discuss Question
Answer: Option C. -> Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
Question 272. Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ρ = Density of substance, S = Specific heat, and k = Thermal conductivity)
  1.    h = k/ ρS
  2.    h = kρ/S
  3.    h = S/ρk
  4.    h = ρS/k
 Discuss Question
Answer: Option A. -> h = k/ ρS
Question 273. In heat exchangers, degree of approach is defined as the difference between temperatures of
  1.    Hot medium inlet and outlet
  2.    Hot medium outlet and cold water outlet
  3.    Cold water inlet and outlet
  4.    Hot medium outlet and cold water inlet
 Discuss Question
Answer: Option B. -> Hot medium outlet and cold water outlet
Question 274. Free convection flow depends on
  1.    Coefficient of viscosity
  2.    Density
  3.    All of these
  4.    Gravitational force
 Discuss Question
Answer: Option C. -> All of these
Question 275. Fouling factor is used
  1.    None of these
  2.    When a liquid exchanges heat with a gas
  3.    In case of Newtonian fluids
  4.    In heat exchanger design as a safety factor
 Discuss Question
Answer: Option D. -> In heat exchanger design as a safety factor
Question 276. Unit of thermal conductivity in M.K.S. units is
  1.    K cal/hr m² °C
  2.    K cal/kg m² °C
  3.    K calm/hr °C
  4.    K cal m/hr m² °C
 Discuss Question
Answer: Option D. -> K cal m/hr m² °C
Question 277. The critical thickness of insulation for a sphere is
  1.    h₀/2k
  2.    2k/h₀
  3.    h₀/k
  4.    k/h₀
 Discuss Question
Answer: Option B. -> 2k/h₀
Question 278. The logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)
  1.    tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)
  2.    tm = loge (Δt1 - Δt2)/ Δt1/Δt2
  3.    tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
  4.    tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
 Discuss Question
Answer: Option C. -> tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
Question 279. An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be
  1.    800 mm
  2.    40 mm
  3.    160 mm
  4.    25 mm
 Discuss Question
Answer: Option D. -> 25 mm
Question 280. The natural convection air cooled condensers are used in
  1.    Room air conditioners
  2.    Domestic refrigerators
  3.    Water coolers
  4.    All of these
 Discuss Question
Answer: Option D. -> All of these

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