Free Heat Transfer Operations MCQs with Answers

• A. May increase or decrease, depends on the vacuum
• B. Decreases
• C. Remain constant
• D. Increases

Explanation: Creating a vacuum lowers the boiling temperature, usually reducing the sensible heat needed to bring the feed to its boiling point…

• A. Laminar
• B. Turbulent
• C. Creeping
• D. Transition region

Explanation: Turbulent flow produces strong mixing and a thinner thermal boundary layer, which raises the convective heat-transfer coefficient.

• A. High heat transfer co-efficient
• B. Large temperature differentials
• C. Low pressure drop
• D. Less corrosion of tubes

Explanation: A floating head allows one tube sheet to move relative to the shell, accommodating differential thermal expansion.

• A. For fluids in laminar flow
• B. When Grashoff number is very important
• C. For fluids in turbulent flow
• D. For liquid metals

Explanation: The Dittus-Boelter correlation is an empirical relation for forced convection in fully developed turbulent flow, commonly for smooth…

• A. Decreases linearly
• B. Decreases
• C. Remain same
• D. Increases

Explanation: For metallic conductors, increasing temperature generally increases the total emissivity because electrical resistivity and the associated…

• A. Higher steam economy
• B. Lower steam economy
• C. Lower heat transfer area
• D. Higher solute concentration in the product

Explanation: In a multiple-effect evaporator, vapour from one effect supplies heat to the next, allowing the same steam to evaporate more solvent.

• A. Square pitch
• B. Triangular pitch
• C. Diagonal square pitch
• D. None of these

Explanation: A triangular pitch fits more tubes into a given shell area and gives a higher shell-side heat-transfer coefficient.

• A. Wien's displacement
• B. Planck's
• C. Kirchoff's
• D. Stefan-Boltzmann

Explanation: Kirchhoff's law states that at thermal equilibrium, emissive power divided by absorptivity is the same for bodies at the same temperature.

• A. Lowest in last effect
• B. Highest in last effect
• C. Same in all effects
• D. Dependent on the number of effects

Explanation: In forward feed, the liquid flows from the first effect to successive effects while the pressure and boiling temperature are progressively…

• A. Increases
• B. Decreases
• C. Remain same
• D. First increases upto a certain temperature and then becomes constant

Explanation: The thermal conductivity of most common liquids decreases as temperature rises, although a few liquids show exceptional behaviour over…

• A. Once
• B. Thrice
• C. Twice
• D. Four times

Explanation: A longitudinal baffle divides the shell into two parallel flow passages, so the shell-side fluid travels through the exchanger twice…

• A. x/KA
• B. A/Kx
• C. K/Ax
• D. KA/x

Explanation: From the conduction relation Q = KAΔT/x, conductance is Q/ΔT = KA/x. Therefore, increasing area or conductivity increases conductance…

• A. All A. B. and C.
• B. Baffled heat exchanger
• C. Condensation of mixed vapour in a condenser
• D. Multipass heat exchanger

Explanation: The basic LMTD must be multiplied by a correction factor when the exchanger arrangement causes temperature crossflow or multiple passes…

• A. 1.5
• B. 1
• C. > 1
• D. < 1

Explanation: The Reynolds analogy gives St = f/2 when the momentum and thermal diffusivities are equal, which corresponds to Pr = 1.

• A. None of these
• B. Directly proportional to LMTD and inversely proportional to the area
• C. Varies as square of the area
• D. Varies directly as the area and the LMTD

Explanation: For a heat exchanger, the heat-transfer rate is Q = UAΔTlm, so it increases directly with both heat-transfer area and the log mean…

• A. gD3.β.Δtρ2/μ2
• B. gD2βΔtρ/μ2
• C. gD2βΔtP2μ
• D. gD3βΔtP2/μ

Explanation: The Grashof number is Gr = gβΔTD³/ν². Since ν = μ/ρ, this becomes gD³βΔTρ²/μ², matching option a.

• A. Tar dolomite bricks followed by asbestos
• B. 85% magnesia cement and glass wool
• C. Cotton followed by aluminium foil
• D. Fireclay refractory followed by aluminium sheet

Explanation: Steam pipelines commonly use magnesia-based insulation, often with glass wool, because it has low thermal conductivity and can withstand…

• A. Horizontal
• B. Inclined
• C. Vertical
• D. Random

Explanation: Evaporator tubes are generally vertical because vapor and concentrated liquor can rise through them, promoting circulation and reducing…

• A. Less than
• B. More than
• C. Equal to
• D. Either more or less than (depending upon the fluid)

Explanation: In parallel flow, the hot and cold streams move in the same direction, and the hot-fluid outlet cannot practically fall below the…

• A. Grashoff
• B. Reynolds
• C. Both 'a' & 'b'
• D. Prandtl & Grashoff

Explanation: Natural-convection transition is governed by the Rayleigh number, Ra = Gr·Pr, so both the Grashof and Prandtl numbers are involved.