Basic Mechanical Engineering

Thermodynamics Viva Questions

Thermodynamics Viva Questions

Thermodynamics Viva Questions, Viva Queistions on Thermodynamics, Engineering Viva Questions, Short answer type questions on Thermodynamics, Thermodynamics Viva Questions

Short answer type Questions

Q.1. What do you understand by latent heat? Give four examples of latent heats.

Ans. For pure substance, the heat effects accompanying change in state at constant pressure (no temperature change being evident) are known as latent heats. Examples of latent heats are: heat of fusion, vaporization, sublimation, and change in crystal form.

Q.2. What is polytropic process? Under what conditions it approaches isobaric, isothermal and isometric process? In which reversible process no work is done?

Ans. A polytropic process is one that follows the equation PVn = constant (index n may have values from – ∞ to + ∞). This process approaches isobaric when n = 0, isothermal, when n = 1 and isometric when n = ∞. No work is done in isometric process.

Q.3. According to second law of thermodynamics what is the limitation for conversion of heat to work? What conclusions are drawn for efficiency of conversion of heat to work?

Ans. According to second law of thermodynamic, conversion of heat to work is limited by the temperature at which conversion occurs.

According to it, no cycle can be more efficient thou a reversible cycle operating between given temperature T1 and T2. Also the efficiently of all reversible cycles absorbing heat only at a single constant higher-temperature. T1 and rejecting heat only at a single constant lower temperature T2 must be the some.

Efficiency = \frac{W}{Q_{1}}=\frac{T_{1}-T_{2}}{T_{1}} and \frac{Q_{1}}{T_{1}}+\frac{Q_{2}}{T_{2}}=0

Q.4. What is the principle of mechanical refrigeration?

Ans. A volatile liquid will boil under the proper conditions and in so doing will absorb heat from surrounding objects.

Q.5. Why water can’t be used as refrigerant for small refrigerating equipment?

Ans. The refrigerant should be such that vapour. Volume is low so that pumping work will be low. Water vapour volume is around 4000 times. Compared to R-22 for given mass.

Q.6. Which parameter remains constant in a throttling process?

Ans. Enthalpy.

Q.7. A carnot cycle refrigerator operates between 250°K and 300°K. What is the value of COP?

Ans.

COP = \frac{T_{2}}{T_{1}-T_{2}} = \frac{250}{300-250}=5

Q.8. Out of constant volume and constant pressure line, which line o T-s diagram has higher-slope?

Ans. Constant volume.

 Q.9. A refrigerator and a heat pump are working on the reversed carrot cycle between the some temperature limits. How COP of refrigerator and heat pump related?

Ans. COP of refrigerator = COP of heat pump – 1.

Q.10. A condenser of a refrigeration system rejects heat at a rate of 60 kW. While its compressor consumes a power of 15 kW. What will be the coefficient of performance of this system?

Ans. 3.

Q.11. Define a boiler.

Ans. Boiler is a steam generator or it is a closed vessel in which water is heated and converted into steam at desired rate, pressure and temperature.

Q.12. Name water tube and fire tube boilers.

Ans. Water tube boilers are

  1. Babcox Wilcox boiler
  2. Stirling boiler

Fire tube boilers are

  1. Lancashire boiler
  2. Cornish boiler
  3. Cochran boiler
  4. Locomotive boiler.

Q.13. In which type of boilers the steam generation is more?

Ans. Rate of steam generation is more in water tube boiler.

Q.14. Does a pressure cooker used in our household work which is used to generate steam can be termed as a ‘Boiler’?

Ans. ‘No’. Any closed vessel used to generate steam cannot be termed as a ‘boiler’ until and unless it is provided with various mountings and accessories. Although accessories can be avoided but mountings are must for a closed vessel used to generate steam to be termed as a ‘Boiler’.

Q.15. What are boiler Mountings and Accessories?

Ans. Mountings are the fittings which are necessary for the safe and efficient operation of the boiler. Without mountings boiler cannot work easily. Accessories are used to improve the efficiency of the boiler plant. Without accessories boiler cannot work safely.

Q.16. What is draught?

Ans. Draught is the pressure difference which is necessary to supply required quantity of air for combustion and to remove the burnt gases out of the system.

Q.17. What are the types of draught?

Ans. The various types of draught are:

  1. Natural or chimney draught
  2. Artificial draught

(a) Forced draught

(b) Induced draught

(c) Balanced draught.

Q.18. Why the feed water is treated?

Ans. Water, before supplying to the boiler drum, is chemically treated so as to reduce scale formation on the inner surface of boiler drum.

Q.19. What type of draught is used in case of locomotive boiler?

Ans. Induced type steam jet draught.

Q.20. What is the function of red cheek valve?

Ans. Through the feed check valve water enters the drum. It allows the water to flow into the drum. It will not allow the water to flow out of the drum so it is called as feed check valve.

Q.21. What is the function of blow-off valve?

Ans. Its function is to blow out the impurities at regular intervals.

Q.22. Why is manhole elliptical in shape?  

Ans. The shape is so made to enable the boiler attendant to enter easily in the boiler.

Q.23. What is meant by high pressure and low pressure boiler?

Ans. When boilers develop pressure above 80 bar they are called as high pressure boilers.

When boilers develop pressure below 80 bar they are called as low pressure boilers.

Q.24. Where do you use a dead weight safety valve in everyday life?

Ans. It is used in pressure cooker.

Q.25. Name the fusible materials used in fusible plug.

Ans. Lead and tin.

Q.26. Why is the plug of a fusible plug made of copper?

Ans. For better heat transfer.

Q.27. Whether the pressure of wager in the economiser is more or less than the boiler pressure?

Ans. It is always more.

Q.28. Define sensible heat.

Ans. The heat supplied to a substance if sensed (measured) by a thermometer then it is known as sensible heat. There is no phase change during sensible heating, only temperature rises.

Q.29. What is latent heat?

Ans. The heat supplied to a substance if it cannot be sensed by a thermometer then it is known as latent heat. Here phase change occurs but temperature remains constant during latent heating.

Q.30. What do you understand by superheat?

Ans. Heating of dry steam is known as super heating. This steam is known as superheated steam.

Q.31. What is degree of superheat?

Ans. The difference between superheated and saturated temperature of steam is called as ‘Degree of superheat’.

Q.32. What is Internal Energy?

Ans. Internal Energy may be defined as the difference in the Enthalpy of steam and the external work of evaporation i.e.,

Internal energy of steam = Enthalpy of steam – external work of evaporation.

Q.33. What do you mean by specific volume?

Ans. The volume per unit mass of the steam is called the specific volume of the steam.

Q.34. What is enthalpy of steam?

Ans. The amount of heat required to convert one kg of water of its freezing point to steam of required quality is enthalpy.

In other words, it is the sum of sensible heat of the water, latent heat of vaporization and enthalpy of superheating.

Q.35. What is internal energy of steam?

Ans. The internal energy of steam is the heat energy above freezing point of water, stored in steam.

Q.36. What is entropy of steam?

Ans. Entropy is a property of system which increases with the addition of heat and decreases with the removal of heat. The entropy is measured in kJ/kg K.

Q.37. What is dryness fraction?

Ans. This gives the indication regarding the dryness of wet steam. If in 1 kg of liquid vapour mixture, x kg is the mass of vapour and (1 – x) kg is the mass of liquid, then x is known as the quality or dryness fraction of the liquid-vapour mixture.

Q.38. For which purpose Barrel or Tank calorimeter is used?

Ans. Barrel calorimeter is used to determine dryness fraction of steam.

Q.39. What are the various forms in which the steam exists?

Ans. The various forms in which steam can exist are saturated form, supersaturated form, dry saturated in mixed state etc.

Q.40. What is ‘steam’?

Ans. Steam is a vapour of water and invisible when pure and dry. Steam is a pure substance because its chemical composition does not change with the Phase change i.e., if ice melts to water or water evaporates to vapours or steam condenses to water, the system remains in constant chemical composition.

Viva Questions of Materials in Basic Mechanical

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