Heat Transfer — Part 1 MCQs with Explanations

In a multiple effect evaporator , boiling point elevation refers to the increase in the boiling point of a solution due to the presence of dissolved solids (such as salts). This phenomenon occurs because the solutes lower the vapor pressure of the solution, requiring a higher temperature for boiling to occur. As a result: Reduce the economy : The boiling point elevation increases the energy required to evaporate the solvent in each effect of the evaporator system. This increase in energy consumption reduces the overall efficiency or economy of the evaporator system. Reduce the capacity : Boiling point elevation does not directly reduce the capacity of the evaporator, as it refers to the efficiency rather than the maximum processing capacity. Increase the economy : This is incorrect because boiling point elevation actually decreases the efficiency or economy of the evaporator system. None of these : This option is incorrect because boiling point elevation does have a specific effect on the evaporator system. Therefore, the correct answer is Option B: Reduce the economy.

D-0.2 is the correct answer to “A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter 'd'. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to”. This question is standard in competitive examination syllabi for Heat Transfer.

The overall heat transfer coefficient of a tube, denoted as U , decreases when dirt is deposited on either side of the tube. This is because the dirt layer adds thermal resistance, making heat transfer less efficient. Initially, the tube has an overall heat transfer coefficient U 1 . After dirt deposition, the new coefficient is U 2 . Since the dirt layer increases thermal resistance, it reduces the overall heat transfer coefficient: U 1 > U 2 Therefore, the correct answer is Option C: U 1 > U 2 .

A: Co-current: In a co-current heat exchanger, both fluids move in the same direction. While this can be efficient in some cases, it typically results in a lower overall temperature difference between the fluids, reducing the effectiveness of heat transfer as they approach similar temperatures. This is not how automotive radiators are designed to operate. B: Cross-current: In a cross-current heat exchanger, the fluids flow perpendicular to each other. While this design can be used in some applications, it is not as efficient as counter-current systems for maximizing heat exchange. Automotive radiators do not employ this design. C: Counter-current: This is the correct answer. In a counter-current heat exchanger, one fluid flows in one direction while the other flows in the opposite direction. In a radiator, the hot coolant from the engine flows through the radiator while the cooler air flows in the opposite direction. This arrangement maintains a high temperature gradient, allowing for more effective heat transfer and cooling of the coolant. D: Direct contact: In direct contact heat exchangers, the two fluids mix directly, allowing for heat transfer through physical contact. While effective in some systems, it is not applicable for radiators where the coolant and air should remain separate to prevent contamination and maintain fluid integrity. Conclusion: Based on the explanation, the radiator of an automobile engine operates as a C: Counter-current type of heat exchanger, which is the most efficient design for transferring heat in this application.

A Heisler's chart is used in the field of heat transfer to solve problems related to transient heat conduction. Transient heat conduction refers to the transfer of heat within a material over time, where the temperature distribution changes with time. Heisler's charts provide graphical solutions for the temperature distribution and heat transfer rate in various geometries (such as slabs, cylinders, and spheres) subjected to convective boundary conditions. These charts are particularly useful because they allow engineers to quickly estimate the thermal response of a system without needing complex calculations. The other options, such as Dirhing's , Mollier's , and Cox charts, are associated with different fields and are not used for solving transient heat conduction problems.

strong>Electromagnetic spectrum for thermal radiation extends over a part of the full EM spectrum where bodies at ordinary temperatures (like furnaces, heaters, process equipment) emit significant energy. In heat transfer, thermal radiation is generally taken to be important over wavelengths from about 1 to 100 microns (μm) . This range covers: Shorter infrared waves near the visible region (around a few microns), where hot bodies at high temperature radiate strongly. Longer infrared waves (tens of microns), important for bodies at moderate temperatures that are common in chemical and process industries. Option A: 1 to 100 μm therefore represents the conventional engineering range used for radiation heat transfer calculations. Why others are wrong: Option B (0.5 to 50 μm) is too narrow; it misses part of the longer wavelength range where cooler surfaces still emit appreciable energy. Option C (10 to 100 μm) ignores the shorter wavelengths (1–10 μm), where very hot surfaces have their peak emission. Option D (100 to 1000 μm) lies mainly in the far infrared/microwave region and does not represent the main band of thermal radiation used in heat transfer. So, the important radiation range in heat transfer is 1 to 100 microns , making Option A correct.

The Biot number (Bi) is a dimensionless quantity used in heat transfer. It represents the ratio of the internal thermal resistance within a solid to the external thermal resistance between the solid and its surrounding fluid. In the Biot number calculation, the characteristic length is defined as the ratio of the volume of the solid to its surface area . This characteristic length helps determine whether temperature gradients within the solid are significant compared to those at the surface.

In a shell and tube heat exchanger, the arrangement or spacing of the tubes within the shell can have different effects on various parameters, including pressure drop and heat transfer efficiency. Let's examine each option: Option A: Gives a higher shell side pressure drop This statement is generally correct. In a shell and tube heat exchanger with a square pitch arrangement, the tubes are spaced more closely together compared to a triangular pitch arrangement. The closer spacing can lead to a higher pressure drop on the shell side due to increased resistance to the flow of the fluid around the tubes. Option B: Gives a lower shell side pressure drop This statement is generally not correct. As mentioned earlier, the square pitch arrangement typically results in a higher pressure drop on the shell side due to the closer tube spacing. Option C: Can pack more surface area into a shell of given diameter This statement is generally correct. The square pitch arrangement allows for a higher density of tubes within the same shell diameter, which means more heat transfer surface area can be packed into the same space compared to a triangular pitch arrangement. Option D: Both (A) and (B) This option is not accurate because it suggests contradictory effects on pressure drop. Option E: Both (A) and (C) This option is accurate. Square pitch compared to triangular pitch can indeed result in both a higher shell side pressure drop ( Option A ) and the ability to pack more surface area into a shell of given diameter ( Option C ). So, the correct answer is Option E: Both (A) and (C) in the context of square pitch compared to triangular pitch in a shell and tube heat exchanger.

0.1 is the correct answer to “200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, N c = 0.5 × 10 -3 kg/m 2 .s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m 2 /1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)”. This question is standard in competitive examination syllabi for Heat Transfer.

3.06 × 105 is the correct answer to “It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/(kgK). The rate at which the heat has to be supplied in (kJ/min) to the evaporator is”. This question is standard in competitive examination syllabi for Heat Transfer.

A higher capacity is the correct answer to “Fqra cold viscous feed, backward feed gives __________ than forward feed.”. This question is standard in competitive examination syllabi for Heat Transfer.

Four times is the correct answer to “In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.”. This question is standard in competitive examination syllabi for Heat Transfer.

Approach is the correct answer to “Terminal point temperature differences between fluids in case of a heat exchanger is termed as”. This question is standard in competitive examination syllabi for Heat Transfer.

1 is the correct answer to “For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is”. This question is standard in competitive examination syllabi for Heat Transfer.

1-1.3 is the correct answer to “Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.”. This question is standard in competitive examination syllabi for Heat Transfer.

$$\frac{1}{{{{\text{t}}^4}}}$$ is the correct answer to “Stefan-Boltzman law which describes the radiation heat transfer states that, it is proportional to (where, t = temperature in °C T = absolute temperature in °K)”. This question is standard in competitive examination syllabi for Heat Transfer.

20.8 is the correct answer to “A single pass air heater is connected to a two pass unit. For the air flow rate and other conditions remaining the same, the film heat transfer co-efficient for air will vary in the ratio of”. This question is standard in competitive examination syllabi for Heat Transfer.

Increasing temperature of the vapour is the correct answer to “The film co-efficient between condensing vapour and metal wall increases with”. This question is standard in competitive examination syllabi for Heat Transfer.

Enhance turbulence is the correct answer to “The main function of baffles provided in a shell and tube heat exchanger is to”. This question is standard in competitive examination syllabi for Heat Transfer.

Saturated is the correct answer to “The maximum heat transfer co-efficient from steam heating will be attained when the steam is”. This question is standard in competitive examination syllabi for Heat Transfer.

Shape and porosity of the body is the correct answer to “The rate of emission of radiation by a body does not depend upon the”. This question is standard in competitive examination syllabi for Heat Transfer.

Lewis relationship is the correct answer to “Which of the following is concerned with both heat and mass transfer”. This question is standard in competitive examination syllabi for Heat Transfer.

Prandtl is the correct answer to “The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.”. This question is standard in competitive examination syllabi for Heat Transfer.

Reyleigh is the correct answer to “(N Gr × N Pr ) is called the __________ number.”. This question is standard in competitive examination syllabi for Heat Transfer.

Nusselt is the correct answer to “h.D/K is called the __________ number.”. This question is standard in competitive examination syllabi for Heat Transfer.

1000 is the correct answer to “The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m 2 . The approximate surface temperature (K) is (Stefan-Boltzman constant = 5.67 × 10 -8 W/m 2 .K 4 )”. This question is standard in competitive examination syllabi for Heat Transfer.

All of the above is the correct answer to “Crystal size in a continuous crystalliser depends upon the”. This question is standard in competitive examination syllabi for Heat Transfer.

Logarithmic is the correct answer to “For heat flow through very thick walled cylinder, use____mean radius.”. This question is standard in competitive examination syllabi for Heat Transfer.

0.75 D is the correct answer to “In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)”. This question is standard in competitive examination syllabi for Heat Transfer.

Sum is the correct answer to “Tube pitch is the __________ of tube diameters and the clearances.”. This question is standard in competitive examination syllabi for Heat Transfer.

$${\left( {\Delta {\text{T}}} \right)^{\frac{1}{4}}}$$ is the correct answer to “The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, $$\Delta {\text{T}}$$ = Temperature drop across condensate film)”. This question is standard in competitive examination syllabi for Heat Transfer.

Increase the steam economy & the capacity is the correct answer to “Multiple effect evaporators ar used to”. This question is standard in competitive examination syllabi for Heat Transfer.

Radiation becomes important is the correct answer to “Heat flux increases with temperature drop beyond the Leiden frost point in the plot of heat flux vs. temperature drop for a boiling liquid, because”. This question is standard in competitive examination syllabi for Heat Transfer.

0.794 h1 is the correct answer to “For a laminar flow of fluid in a circular tube, 'h 1 ' is the convective heat transfer co-efficient at velocity 'V 1 '. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is”. This question is standard in competitive examination syllabi for Heat Transfer.

Always between h₁ and h₂ is the correct answer to “If h₁ = inner film co-efficient and /h₂ = outer film co-efficient, then the overall heat transfer co-efficient is”. This question is standard in competitive examination syllabi for Heat Transfer.

BTU/hr.ft2°F is the correct answer to “The unit of heat transfer co-efficient is”. This question is standard in competitive examination syllabi for Heat Transfer.

$${{\text{h}}_{\text{a}}} = 2{{\text{h}}_l}$$ is the correct answer to “If average heat transfer co-efficient is $${{\text{h}}_{\text{a}}}$$ and the local coefficient at the end of the plate is $${{\text{h}}_l}$$ then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length”. This question is standard in competitive examination syllabi for Heat Transfer.

Viscous sub-layer is the correct answer to “At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the”. This question is standard in competitive examination syllabi for Heat Transfer.

Nucleate boiling is the correct answer to “When vaporisation takes place directly at the heating surface, it is called”. This question is standard in competitive examination syllabi for Heat Transfer.

Open pan evaporation is the correct answer to “Which is the most suitable for the con-certration of highly concentrated solution”. This question is standard in competitive examination syllabi for Heat Transfer.