Free Heat Transfer Operations MCQs with Answers

• A. Decreases
• B. Increases
• C. Is not affected
• D. May increase or decrease; depends on the feed

Explanation: Raising the liquor level in a vertical-tube evaporator increases the liquid head and generally weakens circulation, so the overall…

• A. Increases the heating area required
• B. Reduces the economy
• C. Both A. and B.
• D. Decreases the heating area required

Explanation: A colder feed requires more sensible heating before evaporation, increasing the required heating area and consuming more steam per unit of…

• A. Baffle spacing & shell diameter
• B. Tube diameter & pitch
• C. Viscosity, density & mass velocity of shell side fluid
• D. None of these

Explanation: Shell-side pressure drop depends on baffle geometry, tube layout, and fluid properties such as viscosity, density, and mass velocity.

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

Explanation: Dropwise condensation is promoted by a non-wetting surface, and an oily surface helps condensate form separate droplets rather than a…

• A. K/h0
• B. 2K/h0
• C. h0/K
• D. h0/2K

Explanation: For a sphere, balancing the insulation's added conduction resistance against the increased outer area gives the critical radius, r_c =…

• A. There is no internal heat generation
• B. Boundary surfaces are isothermal
• C. Material is anisotropic
• D. Constant temperature gradient exists

Explanation: The usual one-dimensional Fourier-conduction model assumes an isotropic material, so thermal conductivity is not direction-dependent.

• A. None of these
• B. Decreasing temperature
• C. Decreasing Reynolds number
• D. Increasing temperature

Explanation: For most liquids, increasing temperature lowers viscosity, which raises the Reynolds number and usually increases the convective…

• A. Grashoff number
• B. Eckert number
• C. Graetz number
• D. Bond number

Explanation: Natural convection is driven by buoyancy produced by density differences, and the Grashof number compares buoyancy forces with viscous…

• A. Parallel
• B. Counter
• C. Cross
• D. None of these

Explanation: The simple LMTD applies directly to parallel- and counter-flow exchangers, but cross-flow and multipass arrangements require a correction…

• A. Increased true temperature drop
• B. Increase in liquor film co-efficient
• C. Decreased effect of hydrostatic head
• D. Decreased capacity

Explanation: A higher liquid level increases the hydrostatic pressure and therefore the local boiling point, reducing the effective temperature driving…

• A. D/5 and D
• B. D/2 and 2 D
• C. D/4 and 2 D
• D. D and 2 D

Explanation: For conventional shell-and-tube exchangers, baffle spacing is normally limited to about D/5 minimum and D maximum to maintain adequate…

• A. When heat transfer area required is very high
• B. When heat transfer area required is very low, i.e. (100-200 ft2).
• C. Because it occupies less floor area
• D. Because it is less costly

Explanation: Double-pipe exchangers are simple and economical for relatively small heat-transfer areas, commonly around 100–200 ft².

• A. Zero
• B. Unity
• C. Same for all wavelengths
• D. Different for all wavelengths

Explanation: A grey body has constant monochromatic emissivity over all wavelengths, although its emissivity is less than unity.

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

Explanation: A black body absorbs and emits perfectly, so its emissivity is 1. A real surface emits less than a black body under the same conditions…

• A. None of these
• B. Sub-cooled
• C. Saturated nucleate
• D. Film

Explanation: Milk develops a protein- and fat-rich surface film during heating, which inhibits heat transfer and produces the characteristic…

• A. CP. μ/k
• B. hD/k
• C. h. CP/μ
• D. CP. μ/h

Explanation: The Nusselt number compares convective heat transfer with conduction across a fluid layer and is defined as Nu = hD/k, using the…

• A. Geometric mean temperature difference
• B. Arithmetic mean temperature difference
• C. Logarithmic mean temperature difference
• D. The difference of average bulk temperatures of hot and cold fluids

Explanation: For a heat exchanger, the temperature-driving force is represented by the logarithmic mean temperature difference, which accounts for the…

• A. mCp/kL
• B. kL/mCp
• C. mCp/kμ
• D. kμ/mCp

Explanation: The Graetz number compares convective heat transport by the flowing stream with axial conduction, and is written as mCp/(kL), where m is…

• A. Nst α f
• B. Nst α NRe
• C. NNu α f
• D. NRe α f

Explanation: Reynolds analogy relates momentum and heat transfer, giving the Stanton number as proportional to the friction factor, commonly written…

• A. Steel
• B. Nitrogen
• C. Carbon black
• D. Tar

Explanation: Gases generally conduct heat much less effectively than solids or liquids.