191. A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid stream with a convective heat transfer coefficient of 100 W/m².K. The Biot number is
5.6
0.025
3.5
0.0035
✅ The correct answer is option B.
Heat Transfer MCQs
192. Value of Nusselt number [Nu = (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is
0.5
2
10
100
✅ The correct answer is option B.
193. Which of the following has the minimum absorptivity?
Aluminium foil
Coal dust
Refractory bricks
Iron plates
✅ The correct answer is option A.
194. If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
Remains unchanged
Increases
Increases or decreases depending on number of shell passes
Decreases
✅ The correct answer is option D.
195. A diathermaneous substance __________ the thermal radiation completely.
Absorbs
Reflects
Transmits
None of these
✅ The correct answer is option C.
196. Multipass heat exchangers are used
Because of simplicity of fabrication
For low heat load
To obtain higher heat transfer co-efficient and shorter tube
To reduce the pressure drop
✅ The correct answer is option C.
197. Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
Square pitch
Triangular pitch
Diagonal square pitch
None of these
✅ The correct answer is option B.
198. Multiple effect evaporation is generally recommended, when the
Large scale evaporation of liquor is needed
Corrosive liquids are to be concentrated
Fuel is cheaply available
Evaporation on small scale is to be done
✅ The correct answer is option A.
199. Prandtl number is the reciprocal of
Thermal diffusivity/Momentum diffusivity
Thermal diffusivity x Momentum
Thermal diffusivity x Mass diffusivity
Mass diffusivity x Momentum diffusivity
✅ The correct answer is option A.
200. Hot water (0.01 m³ /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m³/min) of density 800 kg/m³ and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately
32
37
45
50
✅ The correct answer is option A.