Chemical Engineering - MCQ Practice Questions

In turbulent flow heat transfer for gases (Pr ≈ 0.7), the viscous sublayer thickness δ_v relates to thermal boundary layer thickness δ_t as:

In a once-through steam generator (OTSG) for heat recovery, the effectiveness-NTU relation for counterflow is ε = 1 - exp(-NTU(1-C_r))/(1-C_r·exp(-NTU(1-C_r))) where C_r = C_min/C_max. When C_r = 1, this simplifies to:

For condensation of saturated steam on a vertical cold surface, the local heat transfer coefficient h_x at height x is given by Nusselt equation: h_x = 0.943[ρ_l(ρ_l-ρ_v)gk_l³h_fg/(μ_l·ΔT·x)]^($\frac{1}{4}$). When condensate film thickness increases:

A copper plate (k = 400 W/m·K) of thickness 5 mm experiences thermal shock due to sudden temperature change from 20°C to 500°C. Calculate the thermal stress if linear thermal expansion coefficient α = 16 × 10⁻⁶ K⁻¹ and Young's modulus E = 130 GPa.

Which of the following statements about the thermal boundary layer in forced convection is incorrect?

In a gas turbine blade cooling system, film cooling effectiveness is defined as η = (T_g - T_surface)/(T_g - T_coolant). If T_g = 1200 K, T_coolant = 600 K, and measured T_surface = 900 K, calculate the cooling effectiveness.

In turbulent forced convection over a flat plate, the local Nusselt number varies as Nu_x ∝ x^n. What is the typical exponent 'n'?

In the design of a steam generator, the pinch point is the minimum temperature approach between steam and feedwater. What is its practical significance?

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)?

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?

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?

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?

In pipeline heat loss calculations for process industries, the critical radius of insulation is the thickness at which the outer surface area increase due to added insulation equals the reduction in heat transfer coefficient. For a pipe with outer radius r_o = 50 mm and insulation thermal conductivity k = 0.05 W/(m·K), what is the approximate critical radius?

In the 2024-2025 update to heat exchanger design standards (TEMA), which of the following represents the most significant advancement in fouling prediction for industrial heat exchangers?

A double-pipe heat exchanger (counterflow) is used to cool 5 kg/s of oil (Cp = 2.0 kJ/kg·K) from 80°C to 50°C using water at 20°C entering with a temperature rise of 15°C. What is the water flow rate required?

A 10 cm diameter steel pipe (k = 50 W/m·K) with inner diameter 9 cm carries hot water at 90°C. Outer surface is at 70°C. Calculate heat transfer rate per meter length.

In cross-flow heat exchangers (unmixed-unmixed configuration), the effectiveness is lower than parallel/counterflow because:

For turbulent flow over a flat plate, the local Nusselt number varies with distance x according to which relationship?

For a given pipe and fluid, if the flow velocity doubles, the pressure drop in turbulent flow will approximately:

For compressible flow through a converging-diverging nozzle (de Laval nozzle), the throat conditions occur where: