Free Heat Transfer Operations MCQs with Answers

• A. Shape and porosity of the body
• B. Surface temperature of the body
• C. Nature of the surface
• D. Wavelength of radiation

Explanation: Radiation emission depends on wavelength, surface temperature and surface nature or emissivity.

• A. Constant overall heat transfer co-efficient.
• B. Constant rate of fluid flow
• C. Constant specific heat
• D. No partial phase change in the system

Explanation: The LMTD derivation assumes a constant flow rate, but density and therefore volumetric flow can change substantially as a fluid is heated…

• A. (r1 - r2)/-ln(r1/r2)
• B. (r1 - r2)/ln(r2/r1)
• C. (r2 - r1)/ln(r1/r2)
• D. (r1 - r2)/ln(r1/r2)

Explanation: The logarithmic mean is obtained by dividing the difference of the two values by the logarithm of their ratio: (r1 − r2)/ln(r1/r2).

• A. 4.36
• B. 2.36
• C. 120.36
• D. Dependent on NRe only

Explanation: For fully developed laminar flow in a circular tube with constant wall heat flux, the standard Nusselt number is Nu = 4.36.

• A. Average
• B. Sum
• C. Product
• D. Geometric mean

Explanation: In series, heat passes successively through each resistance, so the temperature drops add.

• A. n = 0.4 is used for heating
• B. n = 0.3 is used for cooling
• C. Reynolds number for the flow involved is < 2100
• D. Reynolds number for the flow involved is > 10000

Explanation: The Dittus-B​oelter correlation is intended for turbulent flow, typically with Reynolds number above about 10,000.

• A. Pe = Nu.Re
• B. Pe = Re/Pr
• C. Pe = Pr/Re
• D. Pe = Re.Pr

Explanation: For heat transfer, the Peclet number compares convective transport with thermal diffusion and is Pe = Re × Pr.

• A. Thermal conductivity
• B. Thermal diffusivity
• C. Heat transfer co-efficient
• D. Stefan-Boltzmann constant

Explanation: Fourier’s law is q = −kA(dT/dx), where k is the thermal conductivity. Thermal diffusivity instead measures the rate at which temperature…

• A. Free convection
• B. Mixed convection (both free and forced)
• C. Entry length problem in laminar forced conduction (developing thermal boundary layer)
• D. Forced convection

Explanation: The ratio Gr/Re² compares buoyancy effects with forced-flow effects. When it is approximately one, both mechanisms are important, so the…

• A. J.m-2.K-1
• B. J.m-1.K-1
• C. W.m-2.K-1
• D. W-1m2K

Explanation: Thermal resistance is defined as temperature difference divided by heat-transfer rate, giving units K/W, equivalent to W⁻¹m²K for an…

• A. None of these
• B. Decreased
• C. No effect on
• D. Increased

Explanation: With hot feed in a multiple-effect evaporator, forward feeding generally improves economy because the feed enters at a relatively high…

• A. Temperature of the heating surface is more than the boiling point of the liquid
• B. Temperature of the heating surface is less than the boiling point of the liquid
• C. Bubbles from heating surface are absorbed by the mass of the liquid
• D. Very large vapour space is necessary

Explanation: In sub-cooled boiling, the bulk liquid is below its saturation temperature, while the heating surface must be above the local boiling…

• A. Between two bodies in physical contact with each other
• B. Between two bodies not in physical contact with each other
• C. From one part of a body to the another part of the same body
• D. Both B & C

Explanation: Conduction requires molecular or electronic interaction across a material interface, so two bodies must be in physical contact.

• A. Condensation rate
• B. Surface configuration
• C. All A., B. and C.
• D. Liquid flow rate from the surface

Explanation: The condensate-film thickness is governed by the condensate flow produced by condensation and by the surface geometry, such as a vertical…

• A. Greater than
• B. Half
• C. Is same as
• D. Lower than

Explanation: Dropwise condensation exposes much more of the cold surface directly to fresh condensate-free regions, giving a higher heat-transfer…

• A. 1
• B. > 1
• C. < 1
• D. Between 0 and 1

Explanation: Steam economy is the kilograms of water evaporated per kilogram of steam supplied.

• A. Viscosity of liquid is highest in first effect
• B. Transfer from effect to effect is done by pumps
• C. None of these
• D. No pump is required to withdraw the product from the last effect

Explanation: In forward feed, liquid flows from one effect to the next because of the pressure difference, without inter-effect pumps.

• A. Glazed
• B. Smooth
• C. Coated
• D. Oily

Explanation: Dropwise condensation requires a non-wetting surface so that condensate forms separate droplets rather than a continuous film.

• A. Density
• B. Pressure
• C. Viscosity
• D. Corrosiveness & fouling characteristics

Explanation: Fluid routing depends strongly on corrosiveness, fouling tendency, pressure and viscosity because these affect material selection…

• A. 6.12 x 10^5
• B. 3.06 x 10^5
• C. 7.24 x 10^5
• D. 9.08 x 10^5

Explanation: Salt in the feed is 300(0.20) = 60 kg/min, so the concentrated product is 60/0.30 = 200 kg/min and water evaporated is 100 kg/min.