In the design of a steam generator, the pinch point is the minimum temperature approach between steam and feedwater. What is its practical significance?
Answer: C
The pinch point represents the location where the minimum temperature difference exists between hot and cold fluids. A smaller pinch point requires larger heat transfer area (higher capital cost) but improves thermal effectiveness. The pinch point design directly impacts equipment sizing and economic optimization.
Q.62Easy
In a shell-and-tube heat exchanger, what is the primary advantage of using baffles?
Answer: A
Baffles in shell-and-tube heat exchangers create turbulence by forcing the fluid to flow in a cross-flow pattern, significantly increasing the convective heat transfer coefficient on the shell side, which enhances overall heat transfer effectiveness.
Q.63Easy
The Nusselt number (Nu) relationship for laminar flow in a circular pipe is given by Nu = 3.66. What does this value represent?
Answer: B
The value of 3.66 is the fully developed Nusselt number for laminar flow in circular pipes under constant wall temperature (T_s constant) boundary condition, representing the thermal development region.
Q.64Medium
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.65Easy
In radiation heat transfer, the Stefan-Boltzmann constant σ has a value of 5.67 × 10⁻⁸ W/(m²·K⁴). A black body at 500 K radiates heat. If the temperature is doubled to 1000 K, by what factor does the radiated heat increase?
Answer: D
According to Stefan-Boltzmann law, Q = σAT⁴. When T increases from 500 K to 1000 K, the factor becomes (5001000)⁴ = 2⁴ = 16 times. Radiation is highly temperature-dependent due to the fourth-power relationship.
Advertisement
Q.66Medium
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.67Medium
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.68Medium
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.69Medium
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.70Medium
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.71Medium
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.72Easy
A flat-plate solar collector has a black absorber plate maintained at 80°C. The ambient temperature is 20°C and the heat loss coefficient (overall U value including radiation and convection) is 8 W/(m²·K). For a collector area of 4 m², what is the heat loss rate to the surroundings?
Answer: C
Q_loss = U × A × ΔT = 8 × 4 × (80-20) = 8 × 4 × 60 = 1920 W. The overall U value already accounts for both convective and radiative losses.
Q.73Medium
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.74Medium
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.75Medium
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.
Q.76Hard
A cryogenic heat exchanger operates with liquid nitrogen at 77 K on one side. The convective heat transfer coefficient on the nitrogen side is 800 W/(m²·K). The copper tubing has an inner diameter of 12 mm and outer diameter of 14 mm. Assuming the thermal conductivity of copper is 400 W/(m·K), what is the approximate overall heat transfer coefficient (considering only internal convection and conduction through copper wall)?
Answer: C
For a thin tube with copper wall, the thermal resistance is minimal. Using 1/U = 1/h_i + (r_o ln(r_o/r_i))/(k). With h_i = 800, thin wall effect: 1/U ≈ 8001 + very small value ≈ 0.00125, so U ≈ 780 W/(m²·K), closest to 775.
Q.77Hard
In the analysis of thermal stability of a convective system, the Richardson number (Ri) is used to compare natural and forced convection. Ri = Gr/Re². When Ri >> 1, what flow regime dominates?
Answer: B
The Richardson number Ri = Gr/Re² compares buoyancy effects (Grashof) to external flow effects (Reynolds). When Ri >> 1, the Grashof number is much larger, meaning buoyancy forces dominate and natural convection is the primary mechanism.
Q.78Hard
For a finned surface used in air-cooled heat exchangers, the fin efficiency is given by η_f = tanh(mL)/(mL), where m = √(hP/(kA_c)). As the fin length L increases, what happens to the fin efficiency?
Answer: B
As fin length increases, the parameter mL increases, making tanh(mL)/(mL) decrease. This is because heat must travel a longer distance through the fin material, causing temperature gradients and reducing effectiveness of the fin tip area.
Q.79Hard
In a regenerative heat exchanger (rotary wheel type), the effectiveness depends on the capacity rate ratio and heat capacity of the wheel material. If the wheel rotates slowly (high residence time), what effect does this have on effectiveness?
Answer: A
In rotary regenerators, slower rotation provides longer residence time for each sector in contact with hot and cold fluids, increasing heat transfer duration and thereby increasing the overall effectiveness of heat recovery.
Q.80Medium
The Peclet number (Pe = Re·Pr) is used to determine the relative importance of convection to conduction. For Pe << 1, which heat transfer mechanism is dominant?
Answer: B
The Peclet number Pe = Re·Pr = (velocity × characteristic length × ρ·cp)/k represents the ratio of convection to conduction. When Pe << 1, conduction dominates over convection because advective transport is very slow compared to thermal diffusion.
