A condenser operates with steam at 100°C condensing on a tube bank at 20°C. The saturation temperature drop is negligible. Which phase of condensation provides maximum heat transfer rate?

The correct answer is B: Dropwise condensation phase. Dropwise condensation provides heat transfer coefficients 5-10 times higher than filmwise condensation because liquid droplets continuously shed, exposing fresh surface to direct contact with steam. However, filmwise condensation is more common industrially due to stability issues with dropwise cond

Which of the following pump types operates on the principle of positive displacement with rotating screws?

The correct answer is C: Screw pump. Screw pumps (Archimedean screw or progressive cavity pumps) use rotating screws for positive displacement. They are used for high viscosity fluids.

For simultaneous heat and mass transfer in a wet bulb thermometer, the Lewis number (Le) is defined as Le = Sc/Pr. If Le = 1, what can be inferred?

The correct answer is C: Both a and b are correct. When Le = 1 (Sc/Pr = 1), both analogy relationships hold: α = D_AB and h/ρc_p = k_L, making heat and mass transfer coefficients proportionally equal.

A first-order reaction has a rate constant of 0.693 min⁻¹. What is its half-life?

The correct answer is A: 1 minute. For first-order reactions, t₁/₂ = 0.693/k = 0.693/0.693 = 1 minute

A steel rod (k = 50 W/m·K) of diameter 10 mm and length 100 mm is exposed to air at 25°C with h = 20 W/m²·K. The rod is maintained at 100°C at one end. Calculate the fin efficiency if m = √(hP/kA) = 8.37 m⁻¹.

The correct answer is B: 0.72. mL = 8.37 × 0.1 = 0.837. Fin efficiency η = tanh(mL)/(mL) = tanh(0.837)/0.837 = 0.688/0.837 ≈ 0.72

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Chemical Engineering
Heat Transfer

Process design, thermodynamics, reactions

49 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 41–49 of 49

Topics in Chemical Engineering

All Mass Transfer 100 Heat Transfer 100 Fluid Mechanics 100 Chemical Reaction Engineering 100 Thermodynamics 97

Q.41 Medium Heat Transfer

For natural convection from a vertical isothermal plate in air, which correlation applies best?

A Nu = C × (Gr × Pr)^n, where n ≈ 0.25 for laminar

B Nu = 0.023 × Re^0.8 × Pr^0.4

C Nu = constant

D Nu = 3.66 + (other terms)

Correct Answer: A. Nu = C × (Gr × Pr)^n, where n ≈ 0.25 for laminar

EXPLANATION

For natural convection: Nu = C(Ra)^n where Ra = Gr×Pr. For laminar boundary layer (Ra < 10^9), n ≈ 0.25. For turbulent (Ra > 10^9), n ≈ 0.33.

Test

Q.42 Medium Heat Transfer

For a flat plate solar collector with selective coating, which of the following is most desirable?

A High absorptivity (α) and high emissivity (ε) in solar spectrum

B High absorptivity (α) in solar spectrum and low emissivity (ε) in IR range

C Low absorptivity (α) and low emissivity (ε)

D Equal α and ε values

Correct Answer: B. High absorptivity (α) in solar spectrum and low emissivity (ε) in IR range

EXPLANATION

Selective coatings should absorb solar radiation (high α_sol) but minimize thermal radiation losses (low ε_IR), maximizing collector efficiency.

Test

Q.43 Medium Heat Transfer

In a shell-and-tube heat exchanger (1 shell pass, 2 tube passes), the effectiveness is lower than counter-current primarily because:

A Part of the flow behaves co-currently

B Fouling resistance is higher

C Pressure drop increases

D Material properties change

Correct Answer: A. Part of the flow behaves co-currently

EXPLANATION

In 1-2 arrangement, portion of shell-side flows co-currently with tube-side, reducing overall effectiveness compared to true counter-current.

Test

Q.44 Medium Heat Transfer

For turbulent flow in smooth pipes (Pr > 0.6), which correlation is most commonly used?

A Dittus-Boelert: Nu = 0.023 Re^0.8 Pr^0.4

B Sieder-Tate: Nu = 0.027 Re^0.8 Pr^(1/3)

C Gnielinski equation

D Laminar correlation only

Correct Answer: A. Dittus-Boelert: Nu = 0.023 Re^0.8 Pr^0.4

EXPLANATION

Dittus-Boelert correlation is the most widely used for turbulent flow with Pr > 0.6. Valid for Re > 10,000, smooth pipes, and fully developed flow.

Test

Q.45 Medium Heat Transfer

The log mean temperature difference (LMTD) correction factor F is required primarily for which configuration?

A Counter-current heat exchangers

B Cross-flow and more complex arrangements

C Co-current heat exchangers

D Double-pipe heat exchangers only

Correct Answer: B. Cross-flow and more complex arrangements

EXPLANATION

F-correction factor is used for non-counter-current arrangements (cross-flow, 1-2 shell-tube, etc.) where LMTD doesn't directly apply without correction.

Test

Q.46 Medium Heat Transfer

The Number of Transfer Units (NTU) is defined as which of the following?

A UA/(ṁC_p)_min

B ΔT_LMTD/ΔT_m

C Q_actual/Q_max

D hA/C_p

Correct Answer: A. UA/(ṁC_p)_min

EXPLANATION

NTU = UA/(ṁC_p)_min is the ratio of overall heat transfer capacity to minimum heat capacity rate, used in ε-NTU method.

Test

Q.47 Medium Heat Transfer

In a counter-current heat exchanger with equal heat capacities, what is the maximum possible effectiveness?

A 100% or 1.0

B 50%

C Limited by NTU

D Cannot be determined

Correct Answer: A. 100% or 1.0

EXPLANATION

For counter-current arrangement, maximum effectiveness can approach 1.0 (100%) with infinite NTU. For co-current, it's limited to (1-exp(-NTU(1+C_r)))/(1+C_r).

Test

Q.48 Medium Heat Transfer

For fully developed laminar flow in a circular pipe with constant wall heat flux, which Nusselt number (Nu) is approximately correct?

A Nu ≈ 3.66

B Nu ≈ 4.36

C Nu ≈ 5.33

D Nu ≈ 8.24

Correct Answer: B. Nu ≈ 4.36

EXPLANATION

For constant heat flux boundary condition (H1 type) in circular pipes with fully developed laminar flow, Nu ≈ 4.36. For constant wall temperature (H2), Nu ≈ 3.66.

Test

Q.49 Medium Heat Transfer

What is the Biot number (Bi) used to determine in transient heat conduction?

A Ratio of conductive to convective resistance

B Ratio of internal to external temperature difference

C Ratio of convective to conductive resistance

D Ratio of Grashof to Reynolds number

Correct Answer: A. Ratio of conductive to convective resistance

EXPLANATION

Biot number = hL_c/k determines whether lumped capacitance method is applicable. Bi << 0.1 indicates uniform internal temperature.

Test

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