21. The following figure was generated from experimental data relating spectral black body emissive power to wave length at the three temperatures T₁, T₂ and T₃ (T₁ > T₂ >T₃).
    
    The conclusion in that the measurements are

1. 1. correct because the maxima in E_bλ show the correct trend
      

   
   
   

   2. correct because planck's law is satisfied
      

   
   
   

   3. wrong because the Stefan Boltzmann law in not satisfied
      

   
   
   

   4. wrong because Wien's displacement law is not satisfied
      

   
   
   

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1. View Hint View Answer Discuss in Forum

   Wien's displacement law is not satisfied, i.e., λ_max T = C
   which tells lower λ_max for higher temperature.

   Correct Option: D

   Wien's displacement law is not satisfied, i.e., λ_max T = C
   which tells lower λ_max for higher temperature.

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22. For an opaque surface, the absorptivity (α), transmissivity (τ) and reflectivity ρ are related by the equation
    

1. 1. α + ρ = τ
      

   
   
   

   2. ρ + α + τ = 0
      

   
   
   

   3. α + ρ = 1
      

   
   
   

   4. α + ρ = 0

   
   
   

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1. View Hint View Answer Discuss in Forum

   α + τ + ρ = 1
   for opaque surface, τ = 0
   ∴ α + ρ = 1

   Correct Option: C

   α + τ + ρ = 1
   for opaque surface, τ = 0
   ∴ α + ρ = 1

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23. The radiative heat transfer rate per unit area (W/m²) between two plane parallel gray surfaces (emissivity = 0.9) maintained at 400 K and 300 K is (σ = Stefan Boltzmann constant = 5.67 × 10^–8 W/m²K⁴)
    

1. 1. 992
      

   
   
   

   2. 812
      

   
   
   

   3. 464
      

   
   
   

   4. 567

   
   
   

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1. View Hint View Answer Discuss in Forum

   For plane surfaces
   

   Q	=	σ(T14 - T24)
   A		1	+	1	- 1
   		ε1		ε2

   
   Here , ε₁ = ε₂ = 0.9
   

   Q	=	5.67 × 10-8(4004 - 3004)
   A		1	+	1	- 1
   		0.9		0.9

   
   

   = 8 ∥ .84	w	≈ 812	w
   	m2		m2

   Correct Option: B

   For plane surfaces
   

   Q	=	σ(T14 - T24)
   A		1	+	1	- 1
   		ε1		ε2

   
   Here , ε₁ = ε₂ = 0.9
   

   Q	=	5.67 × 10-8(4004 - 3004)
   A		1	+	1	- 1
   		0.9		0.9

   
   

   = 8 ∥ .84	w	≈ 812	w
   	m2		m2

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24. In shell and tube heat exchanger, baffles are mainly used to
    

1. 1. increase the mixing of fluid
      

   
   
   

   2. increase the heat transfer area
      

   
   
   

   3. deflect the flow in desired direction
      

   
   
   

   4. reduce fouling of the tube surface

   
   
   

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1. View Hint View Answer Discuss in Forum

   Baffles are used to deflect the flow.

   Correct Option: C

   Baffles are used to deflect the flow.

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25. The practice to use steam on the shell side and water on the tube side in condensers of steam power plant is because
    

1. 1. to increase overall HT coefficient, water side velocity can be increased if water is on the tube side
      

   
   
   

   2. condenser can act as a Storage unit for condensed steam
      

   
   
   

   3. rate of condensation of steam is invariably smaller than the mass flow rate of cooling water
      

   
   
   

   4. it is easier to maintain vacuum on the shell side than on the tube side

   
   
   

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1. View Hint View Answer Discuss in Forum

   Air leaked into the condensor can be easily extracted.

   Correct Option: D

   Air leaked into the condensor can be easily extracted.

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