interview question of heat transfer.docx

HEAT EXCHANGER AND HEAT TRANSFER 1.What are the important parts of thermal power plant? - BOILER - FURNACE - TURBINE - HEAT EXCHANGER - COOLING TOWER

2.What is meant by heat transfer process? Write its 3 types. - Transfer of heat from one place to another place, Which occurs as a result of a temperature difference - Conduction - Convection - Radiation. 3. Write any 2 important differences between conduction and convection. * Conduction - Heat transfer in bodies due to fixed molecules. - This take place is solid bodies. * Convection - Heat transfer in bodies due to moving molecules. - This take place in flawed liquid of gas 4.Write Fourier law of heat conduction and explain the terms in it. - q conduction = -kA x (∆T /∆ x ) where A the cross- sectional area ∆T temperature difference ( T1-T2) between the two surfaces separated by a distance Δx

5.What is meant by natural convection heat transfer process? - is the heat transfer process where the heat transfer takes place due to the natural temperature difference or density difference and no external forces are employed for the fluid movement. 6.Write and explain the terms in the formula for calculating the radiation heat transfer process ( Name the law ) - q emitted = εσ . AT4 where A is the surface area, T is the temperature of the body, σ is a constant called Stefan-Boltzmann constant, equal to 5.67×10-8 W/m2K4, and εis a material property called emissivity. The emissivity has a value between zero and 1.

7. Write briefly about a) Laminar flow b) Turbulent flow c) Parallel flow c) Counter flow.

- Laminar flow: which the fluid particles move in regular path. - Turbulent flow: is the one in which the fluid particles move in zig-zig path - Parallel flow: is the one where the 2 fluid steams move parallel to each other. - Counter flow: the one where the fluid steams move opposite to each other.

8. Name the various types of heat exchangers. - Parallel flow heat exchanger. - Recuperation heat exchanger. - Plate heat exchanger. - Counter flow - Regenerative - Shell& tube heat exchanger. - Cross flow heat exchanger. - Direct& indirect contact - Double pipe heat exchanger

9.Write the expansion of LMTD. - Log-mean temperature difference (ΔTlm ) 10. Write the 2 limitations of Plate heat exchangers. - Plate exchangers are limited when high pressures, high temperatures, or aggressive fluids are present. - Because of this problem these type of heat exchangers have only been used in small, low pressure applications such as on oil coolers for engines. 11. Write 4 important parts of a shell and tube heat exchanger. - Shell - tubes - rear header - front header – baffles - tubes sheet

What is difference between condenser and heat exchanger? Answer: A heat exchanger is a device that transfers heat from one fluid to the others. A condenser is simply a heat exchanger. It compresses refrigerants into a hot gas to then condense them into a liquid. Majorly condenser is used to cool the required vapor and heat exchanger is used to heat the required liquid

1. What is the difference between diffusion and radiation heat transfer ?

Diffusion heat transfer is due to random molecular motion. Neighboring molecules move randomly and transfer energy between one another however there is no bulk motion. Radiation heat transfer, on the other hand, is the transport of heat energy by electromagnetic waves. All bodies emit thermal radiation. In particular, notice that unlike diffusion, radiation heat transfer does not require a medium and is thus the only mode of heat transfer in space. The time scale for radiative heat transfer is much smaller than diffusive heat transfer.

2. How is natural convection different from forced convection ?

In natural convection, the movement of the fluid is due entirely to density gradients within the fluid (e.g. hot air rises over cold air). There is no external device or phenomenon which causes fluid motion. In forced convection, the fluid is forced to flow by an external factor - e.g. wind in the atmosphere, a fan blowing air, water being pumped through a pipe. Typically heat transfer under forced convection conditions is higher than natural convection for the same fluid.

3. Define a black surface

A black surface is defined by three criteria:   

it absorbs all radiation that is incident on it it emits the maximum energy possible for a given temperature and wavelength of radiation (according to Planck's law) the radiation emitted by a blackbody is not directional (it is a diffuse emitter)

A black surface is the perfect emitter and absorber of radiation. It is an idealized concept (no surface is exactly a black surface), and the characteristics of real surfaces are compared to that of an ideal black surface.

4. What is the range of values for the emissivity of a surface ? The emissivity e ranges between 0 and 1.

21. What is a gray surface ?

A gray surface is defined as one for which the emissivity (e) and the absorptivity (a) are independent of wavelength (l).