Which heat exchanger type provides the maximum temperature effectiveness under the same flow conditions?
Answer: A
Counter-flow heat exchangers are most effective because they maintain the maximum temperature gradient throughout the exchanger length, resulting in higher heat transfer rates and effectiveness compared to parallel or cross-flow configurations.
Q.22Medium
A surface at 300 K with emissivity 0.8 radiates heat to surroundings at 250 K. Calculate the net radiative heat transfer per m² (σ = 5.67 × 10⁻⁸ W/m²·K⁴).
In transient heat conduction, the Fourier number (Fo = α·t/L²) represents:
Answer: D
Fourier number represents the dimensionless time or the measure of how far thermal disturbances have penetrated into the material. High Fo indicates significant internal temperature changes, while low Fo indicates the disturbance is confined to the surface.
Q.24Medium
For natural convection over a vertical flat plate, the Nusselt number correlation is typically given by Nu = C·Ra^n. What is the typical value of exponent 'n'?
Answer: B
For laminar natural convection on vertical plates, Nu = 0.59·Ra⁰·²⁵ is used, while for turbulent natural convection (Ra > 10⁹), Nu = 0.1·Ra⁰·³³ is commonly used. The value 0.33 or 31 is standard for turbulent natural convection.
Q.25Medium
In a shell and tube heat exchanger, which configuration reduces the pressure drop while maintaining adequate heat transfer?
Answer: A
Increasing the number of tube passes distributes the flow over more tubes, reducing the velocity in each tube and consequently reducing pressure drop while maintaining heat transfer area. This is a design optimization technique in shell and tube exchangers.
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Q.26Medium
A spherical tank of diameter 1 m containing hot liquid at 90°C is placed in ambient air at 15°C. If h = 8 W/m²·K and k_insulation = 0.05 W/m·K with 50 mm insulation thickness, calculate the heat loss rate.
Answer: A
Surface area A = 4πr² = 4π(0.5)² = 3.14 m². Convection resistance R_conv = 1/(h·A) = 1/(8×3.14) = 0.0398 K/W. Conduction resistance (spherical) can be neglected due to small thickness. Q = ΔT/(R_conv) = 075.67 ≈ 112 W
Q.27Medium
In a finned tube heat exchanger, the overall surface effectiveness is 0.75. This means:
Answer: A
Overall surface effectiveness accounts for both fin and base surface contributions. A value of 0.75 indicates that 75% of the theoretical maximum heat transfer (assuming entire surface at base temperature) is actually achieved due to fin efficiency and geometric factors.
Q.28Medium
In a double-pipe heat exchanger with counter-flow arrangement, the LMTD correction factor F = 0.95. What does this indicate?
Answer: C
The LMTD correction factor F accounts for deviations from ideal counter-flow behavior in real heat exchangers. F = 0.95 means the actual heat transfer is 95% of what would be obtained with ideal counter-flow configuration due to practical geometry constraints.
Q.29Medium
A heat pipe is used to transfer heat from an electronic component at 80°C to ambient air at 25°C through evaporation and condensation. What is the primary advantage of a heat pipe over conventional conduction cooling?
Answer: B
Heat pipes achieve very high effective thermal conductivity (often >1000 times that of copper) through the latent heat of evaporation and condensation of working fluid, making them ideal for high-power electronics cooling despite the small cross-sectional area.
Q.30Medium
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?
Answer: B
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 condensation.
Q.31Medium
Which of the following methods is most suitable for measuring the convective heat transfer coefficient in real-time industrial applications?
Answer: B
Thermographic imaging using infrared cameras is a non-intrusive, real-time method that can measure surface temperature variations across the heat transfer surface without disturbing the flow, making it practical for industrial monitoring.
Q.32Medium
In a cross-flow heat exchanger where both fluids are unmixed, the effectiveness (ε) is lower than in a counter-flow arrangement. What is the main reason for this?
Answer: C
In cross-flow (especially unmixed-unmixed), the temperature gradients are not as favorable as counter-flow because one or both fluids cannot maintain continuous temperature gradient alignment, resulting in lower effectiveness and hence lower LMTD correction factor.
Q.33Medium
The Grashof number (Gr) is used to characterize natural convection. For natural convection heat transfer, which of the following correctly describes the Grashof number?
Answer: A
The Grashof number Gr = (ρ²gβΔT L³)/(μ²) represents the ratio of buoyancy forces to viscous forces in natural convection. It determines the onset of natural convection and its intensity.
Q.34Medium
In heat exchanger design for petrochemical plants, fouling resistance (Rf) is critical. For a crude oil preheater, the typical internal fouling resistance is 0.0005 m²·K/W. If the design duty is 5 MW, what is the maximum temperature difference loss due to fouling on a surface area of 100 m²?
Answer: A
Using Q = UAΔT and considering fouling resistance: ΔT_fouling = Q × Rf / A = 5×10⁶ × 0.1000005 = 25 K. However, for the temperature loss specifically attributed to fouling layer: ΔT = Rf × (Q/A) = 0.0005 × (5×10⁶/100) = 2.5 K.
Q.35Medium
The thermal entrance length for laminar flow in a pipe is given by x_th/D ≈ 0.05·Re·Pr. For water flowing in a 25 mm diameter pipe with Reynolds number of 1000 and Prandtl number of 7, what is the thermal entrance length?
Answer: A
x_th = 0.05 × Re × Pr × D = 0.05 × 1000 × 7 × 0.025 = 8.75 m. This is the distance from the entrance where thermal development is completed (approximately 99% developed).
Q.36Medium
In a parallel-flow heat exchanger with hot inlet temperature Th,in = 100°C, cold inlet temperature Tc,in = 30°C, and hot outlet temperature Th,out = 60°C, what is the hot fluid capacity rate ratio if the cold outlet temperature is 50°C?
Answer: A
For energy balance: Ch(Th,in - Th,out) = Cc(Tc,out - Tc,in). So Ch(100-60) = Cc(50-30), which gives Ch/Cc = 4020 = 0.5. This is the capacity rate ratio.
Q.37Medium
The Colburn factor (j) for heat transfer is related to the Nusselt number. For turbulent flow over a flat plate, the typical correlation is j ≈ 0.037·Re^(-0.2). What does the Colburn factor represent?
Answer: B
The Colburn factor (j) is a dimensionless group that relates heat transfer characteristics to flow properties. It connects the Stanton number to the Prandtl number: St = j/Pr^(32), allowing transfer of empirical heat transfer data to different systems.
Q.38Medium
In the design of a heat recovery steam generator (HRSG) for a combined cycle power plant, the approach temperature is 5°C. If the exhaust gas inlet temperature is 450°C and the approach temperature represents the difference between exhaust gas exit and steam outlet temperatures, what is the steam outlet temperature?
Answer: B
The approach temperature is defined as: Approach = T_gas,out - T_steam,out. Given that typical exhaust exit from HRSG is around 450°C and approach is 5°C, T_steam,out = 450 - 5 = 445°C.
Q.39Medium
The effectiveness-NTU (Number of Transfer Units) method is preferred over LMTD method when designing heat exchangers because:
Answer: B
In heat exchanger design, outlet temperatures are unknown, making the LMTD method iterative and cumbersome. The NTU method (ε-NTU) directly uses inlet temperatures and known parameters to find the outlet temperatures without iteration, making it ideal for design problems.
Q.40Medium
For turbulent flow in a rough pipe, the friction factor is determined by both Reynolds number and relative roughness (ε/D). According to the Moody chart, in the complete turbulence zone, the friction factor becomes independent of:
Answer: A
In the complete turbulence zone (very high Reynolds numbers), the friction factor depends only on the relative roughness (ε/D) and becomes independent of Reynolds number. This is because inertial forces completely dominate over viscous forces.
