1. To improve heat exchange between a gas & a liquid stream in a heat exchanger, it is decided to use fins. Correct the suitable option.

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1 1. To improve heat exchange between a gas & a liquid stream in a heat exchanger, it is decided to use fins. Correct the suitable option. a) Fins are generally attached on gas side. b) Fins are generally attached on liquid side. c) Fins are generally attached on both sides. d) To improve the rate of heat transfer, addition of fins is not a good decision. Ans: (a) Fins are generally attached on gas side as heat transfer coefficient on gas side is small 2. In a counter-flow heat exchanger, the hot fluid is cooled from 110 C to 80 C by a cold fluid which gets heated from 30 C to 60 C. Mean temperature difference for the heat exchanger is a) 20 C b) 30 C c) 50 C d) 70 C Ans: c As the temperature differences at two ends of the counter-flow heat exchanger are same, temperature difference should be same throughout the length of the heat exchanger. It can be also mathematically shown. In case of counter-flow heat exchanger, mean temperature difference is same as log mean temperature difference. T lm T1 T2 ln T / T 1 2 Here, a special case of counter-flow heat exchanger is arising as T1 T2 T. T T 0 Tlm ln T / T 0 (indeterminate value)

2 For such a case, T lm can be found by applying L Hospital s rule: lim T T T T 1 T 2 2 lim ln / ln / T T / 1 T T T1 T2 T1 T Let, T 1 T2 R. Hence, the above expression can be expressed as: T R1 lim R1 ln R Now, differentiating the numerator and denominator with respect to R and taking limits, lim R1 1/ T T R 3. In a counter-flow heat exchanger, the product of specific heat and mass flow rate is same for the hot and cold fluids. If NTU is equal to 0.5, then the effectiveness of the heat exchanger is a) 1.0 b) 0.5 c) 0.33 d) 0.2 Ans: c As the product of specific heat and mass flow rate is same for the hot and cold fluids the effectiveness of the heat exchanger is NTU 1 NTU

3 4. Match the following. a) LMTD i) Effectiveness. b) NTU ii) Shell & tube exchanger. c) Baffel iii) Correction factor. d) Extended surface iv) Plate fin heat exchanger A) a-(iii), b-(i), c-(ii), d-(iv) B) a-(i), b-(iii), c-(iv), d-(ii) C) a-(iv), b-(ii), c-(iii), d-(i) D) a-(iii), b-(iv), c-(i), d-(ii) Ans: A) a-(iii), b-(i), c-(ii), d-(iv) 5. Identify the correct expression of NTU (Net Heat Transfer Unit) in a heat exchanger. (All symbols have usual meaning). a) b) NTU NTU UA C min UA C max C NTU C c) min max d) NTU AU C Ans: a 6. Select the cycles which could be used as bottoming cycles

4 a) Otto cycle. b) Joule cycle. c) Kalina cycle. d) Trilateral flash cycle. Ans: c, d 7. Identify the correct statements for Kalina cycle a) Evaporation takes place at a constant temperature. b) Condensation tales place over a temperature range. c) Pure fluid is used as the working medium. d) Different variations of the plant configuration are possible. Ans: b, d. 8. Identify the correct statements for a Trilateral flash cycle. a) Expander handles a mixture of liquid and vapor. b) Expander handles dry vapor. c) Condensation takes place over a temperature range. d) Substantial amount of heat is transferred to the working fluid in subcooled condition. Ans: a, d. 9. Identify the correct statements for Kalina cycles. a) Phase change processes are modified to reduce irreversibility. b) NH3-H2O mixture is a suitable working fluid. c) It requires only four equipment, namely vapor generator, turbine, condenser and pump.

5 d) Throughout the cycle the composition of NH3-H2O mixture is uniform. Ans: a, b 10. Identify the correct statements for the two figures given below. (i) The process 1-2 in figures (a) and (b) show wet and dry expansion, respectively, in the turbine of Organic Rankine cycle. (ii) The process 1-2 in figures (a) and (b) show dry and wet expansion, respectively, in the turbine of Organic Rankine cycle. (iii) Superheating is required for the fluid in figure (a) for dry expansion in the turbine. (iv) Superheating is required for the fluid in figure (b) for dry expansion in the turbine.

6 Ans: i, iii. 11. A counterflow, concentric tube heat exchanger is used to cool the lubricating oil for a large industrial gas turbine engine. The flow rate of cooling water through the inner tube is 0.2 kg/s, while the flow rate of oil through the outer annulus is 0.1 kg/s. The oil and water enter at temperature of 100 C and 30 C, respectively. What will be outlet temperature of water if outlet temperature of the oil is to be 60 C? Specific heat of oil and water are 2131 J/kg K and 4178 J/kg K, respectively. (a) 40.2 C, (b) 52.5 C, (c) 33.9 C, (d) 25.5 C Ans: a. q m c ( T T ) W h p, h h, i h,o q 8524 Tc, o T. c, i 30 C 40.2 C mc c p, c 12. In the above problem (Question 11), what is LMTD of this heat exchanger? (a) 38.2 C, (b) 43.2 C, (c) 33.9 C, (d) 50.5 C Ans: b ( T T ) ( T T ) h, i c, o h, o c, i Tlm ln[( Th, i Tc, o) / ( Th, o Tc, i) ln (59.8 / 30) 43.2 C 13. In a Cogeneration plant with a pass-out turbine, the rate of steam extraction for process heating is kg/h The T-s diagram of the plant is shown below. The enthalpy values at different locations corresponding to the diagram is given. Find out the boiler capacity (kg/h).

7 h 5 = kj/kg h 7 = kj/kg h 8 = 275 kj/kg (a) , (b) , (c) , (d) Ans: (c) (w s ) 153.8= w s 275 w s = kg/h 14. Hot exhaust gases, which enters a finned-tube, cross-flow heat exchanger at 300 C and leave at 100 C, are used to heat pressurized water at a flow rate of 1 kg/s from 35 C to 125 C. The overall heat transfer coefficient based on the gas-side surface is U h =100 W/m 2. K. Assume, specific heat of cold fluid, cp, c = 4197 J/kg. K. Find out maximum possible heat transfer. (a) 250 kw, (b) 425 kw, (c) 500 kw, (d) 850 kw Ans: c

8 . Cc mc cp, c J Kg. K T T. c, o c, i Ch mh cp, h Cc T h, i T h, o = 4197* = 1889 W/K= C min q C ( T T ) = 1889*(300-35) = 500 kw max min h, i c, i 15. For the above problem (question 14), determine the effectiveness of heat exchanger. (a) 0.955, (b) 0.755, (c) 0.455, (d) 1.25 Ans: b Actual Heat transfer: q C ( T, T, ) = 4197(125-35) = 378 kw c c o c i = q q max = 0.755