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HEAT EXCHANGERS TEST 1
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Question 1 of 20
1. Question
1 pointsA long thinwalled doublepipe heat exchanger with tube and shell diameters of 1.0 cm and 2.5 cm, respectively, is to condense refrigerant 134a by water at 20°C. The refrigerant flows through the tube, with a convection heat transfer coefficient of hi = 5000 W/m² · °C. Water flows through the shell at a rate of 0.3 kg/s. Determine the overall heat transfer coefficient of this heat exchanger.
Correct
2020 W/m² · °C
Incorrect
2020 W/m² · °C

Question 2 of 20
2. Question
1 pointsSteam in the condenser of a steam power plant is to be condensed at a temperature of 50°C (hfg = 2305 kJ/kg) with cooling water (Cp = 4180 J/kg · °C) from a nearby lake, which enters the tubes of the condenser at 18°C and leaves at 27°C. The surface area of the tubes is 58 m2 , and the overall heat transfer coefficient is 2400 W/m² · °C. Determine the mass flow rate of the cooling water needed
Correct
101 kg/s
Incorrect
101 kg/s

Question 3 of 20
3. Question
1 pointsSteam in the condenser of a steam power plant is to be condensed at a temperature of 50°C (hfg = 2305 kJ/kg) with cooling water (Cp = 4180 J/kg · °C) from a nearby lake, which enters the tubes of the condenser at 18°C and leaves at 27°C. The surface area of the tubes is 58 m2 , and the overall heat transfer coefficient is 2400 W/m² · °C. Determine the rate of condensation of
the steam in the condenser. (answer upto two decimal)Correct
1.65 kg/s
Incorrect
1.65 kg/s

Question 4 of 20
4. Question
1 pointsA test is conducted to determine the overall heat transfer coefficient in a shellandtube oiltowater heat exchanger that has 24 tubes of internal diameter 1.2 cm and length 2 m in a single shell. Cold water (Cp= 4180 J/kg · °C) enters the tubes at 20°C at a rate of 5 kg/s and leaves at 55°C. Oil (Cp = 2150 J/kg · °C) flows through the shell and is cooled from 120°C to 45°C. Determine the overall heat transfer coefficient Ui of this heat exchanger based on the inner surface area of the tubes(answer upto one decimal)
Correct
13.9 kW/m² · °C
Incorrect
13.9 kW/m² · °C

Question 5 of 20
5. Question
1 pointsEngine oil (Cp = 2100 J/kg · °C) is to be heated from 20°C to 60°C at a rate of 0.3 kg/s in a 2cmdiameter thinwalled copper tube by condensing steam outside at a temperature of 130°C (hfg = 2174 kJ/kg). For an overall heat transfer coefficient of 650 W/m² · °C, determine the rate of heat transfer(answer in one decimal)
Correct
25.2 kW
Incorrect
25.2 kW

Question 6 of 20
6. Question
1 pointsEngine oil (Cp = 2100 J/kg · °C) is to be heated from 20°C to 60°C at a rate of 0.3 kg/s in a 2cmdiameter thinwalled copper tube by condensing steam outside at a temperature of 130°C (hfg = 2174 kJ/kg). For an overall heat transfer coefficient of 650 W/m² · °C, determine the the length of the tube required to achieve it
Correct
7.0 m
Incorrect
7.0 m

Question 7 of 20
7. Question
1 pointsA shellandtube heat exchanger with 2shell passes and 12tube passes is used to heat water (Cp = 4180 J/kg · °C) in the tubes from 20°C to 70°C at a rate of 4.5 kg/s. Heat is supplied by hot oil (Cp = 2300 J/kg · °C) that enters the shell side at 170°C at a rate of 10 kg/s. For a tubeside overall heat transfer coefficient of 600 W/m² · °C, determine the heat transfer surface area on the tube side
Correct
15 m²
Incorrect
15 m²

Question 8 of 20
8. Question
1 pointsHot oil (Cp = 2200 J/kg · °C) is to be cooled by water (Cp = 4180 J/kg · °C) in a 2shellpass and 12tubepass heat exchanger. The tubes are thinwalled and are made of copper with a diameter of 1.8 cm. The length of each tube pass in the heat exchanger is 3 m, and the overall heat transfer coefficient is 340 W/m² · °C. Water flows through the tubes at a total rate of 0.1 kg/s, and the oil through the shell at a rate of 0.2 kg/s. The water and the oil enter at temperatures 18°C and 160°C, respectively. Determine the rate of heat transfer in the heat exchanger ( answer upto one decimal)
Correct
36.2 kW
Incorrect
36.2 kW

Question 9 of 20
9. Question
1 pointsHot oil (Cp=2200 J/kg · °C) is to be cooled by water (Cp = 4180 J/kg · °C) in a 2shellpass and 12tubepass heat exchanger. The tubes are thinwalled and are made of copper with a diameter of 1.8 cm. The length of each tube pass in the heat exchanger is 3 m, and the overall heat transfer coefficient is 340 W/m² · °C. Water flows through the tubes at a total rate of 0.1 kg/s, and the oil through the shell at a rate of 0.2 kg/s. The water and the oil enter at temperatures 18°C and 160°C, respectively.Determine the outlet temperatures of the water ( answer upto one decimal)
Correct
104.6°C
Incorrect
104.6°C

Question 10 of 20
10. Question
1 pointsHot oil (Cp=2200 J/kg · °C) is to be cooled by water (Cp = 4180 J/kg · °C) in a 2shellpass and 12tubepass heat exchanger. The tubes are thinwalled and are made of copper with a diameter of 1.8 cm. The length of each tube pass in the heat exchanger is 3 m, and the overall heat transfer coefficient is 340 W/m² · °C. Water flows through the tubes at a total rate of 0.1 kg/s, and the oil through the shell at a rate of 0.2 kg/s. The water and the oil enter at temperatures 18°C and 160°C, respectively.Determine the outlet temperatures of the oil ( answer upto one decimal)
Correct
77.7°C
Incorrect
77.7°C

Question 11 of 20
11. Question
1 pointsA crossflow airtowater heat exchanger with an effectiveness of 0.65 is used to heat water (Cp = 4180 J/kg · °C) with hot air (Cp = 1010 J/kg · °C). Water enters the heat exchanger at 20°C at a rate of 4 kg/s, while air enters at 100°C at a rate of 9 kg/s. If the overall heat transfer coefficient based on the water side is 260 W/m2 · °C, determine the heat transfer surface area of the heat exchanger on the water side. Assume both fluids are unmixed.(answer upto one decimal)
Correct
52.4 m²
Incorrect
52.4 m²

Question 12 of 20
12. Question
1 pointsCold water (Cp = 4180 J/kg · °C) leading to a shower enters a thinwalled doublepipe counterflow heat exchanger at 15°C at a rate of 0.25 kg/s and is heated to 45°C by hot water (Cp = 4190 J/kg · °C) that enters at 100°C at a rate of 3 kg/s. If the overall heat transfer coefficient is 950 W/m² · °C, determine the rate of heat transfer(answer upto two decimal)
Correct
31.35 kW
Incorrect
31.35 kW

Question 13 of 20
13. Question
1 pointsCold water (Cp = 4180 J/kg · °C) leading to a shower enters a thinwalled doublepipe counterflow heat exchanger at 15°C at a rate of 0.25 kg/s and is heated to 45°C by hot water (Cp = 4190 J/kg · °C) that enters at 100°C at a rate of 3 kg/s. If the overall heat transfer coefficient is 950 W/m² · °C, determine the heat transfer surface area of the heat exchanger using the ε–NTU method.(answer upto three decimal)
Correct
0.482 m²
Incorrect
0.482 m²

Question 14 of 20
14. Question
1 pointsA shellandtube heat exchanger with 2shell passes and 8tube passes is used to heat ethyl alcohol (Cp = 2670 J/kg · °C) in the tubes from 25°C to 70°C at a rate of 2.1 kg/s. The heating is to be done by water (Cp = 4190 J/kg · °C) that enters the shell at 95°C and leaves at 60°C. If the overall heat transfer coefficient is 800 W/m² · °C, determine the heat transfer surface area of the heat exchanger using the LMTD method (answer in one decimal)
Correct
11.4 m²
Incorrect
11.4 m²

Question 15 of 20
15. Question
1 pointsThe condenser of a large power plant is to remove 500 MW of heat from steam condensing at 30°C (hfg = 2430 kJ/kg). The cooling is to be accomplished by cooling water (Cp = 4180 J/kg · °C) from a nearby river, which enters the tubes at 18°C and leaves at 26°C. The tubes of the heat exchanger have an internal diameter of 2 cm, and the overall heat transfer coefficient is 3500 W/m² · °C. Determine the total length of the tubes required in the condenser. What type of heat exchanger is suitable for this task?(answer upto one decimal)
Correct
312.3 km
Incorrect
312.3 km

Question 16 of 20
16. Question
1 pointsCold water (Cp = 4180 J/kg · °C) enters the tubes of a heat exchanger with 2shell passes and 20–tube passes at 20°C at a rate of 3 kg/s, while hot oil (Cp = 2200 J/kg · °C) enters the shell at 130°C at the same mass flow rate and leaves at 60°C. If the overall heat transfer coefficient based on the outer surface of the tube is 300 W/m² · °C, determine the rate of heat transfer
Correct
462 kW
Incorrect
462 kW

Question 17 of 20
17. Question
1 pointsCold water (Cp = 4180 J/kg · °C) enters the tubes of a heat exchanger with 2shell passes and 20–tube passes at 20°C at a rate of 3 kg/s, while hot oil (Cp = 2200 J/kg · °C) enters the shell at 130°C at the same mass flow rate and leaves at 60°C. If the overall heat transfer coefficient based on the outer surface of the tube is 300 W/m² · °C, determine the heat transfer surface area on the outer side of the tube.(answer upto one decimal)
Correct
29.2 m²
Incorrect
29.2 m²

Question 18 of 20
18. Question
1 pointsThe condenser of a room air conditioner is designed to reject heat at a rate of 15,000 kJ/h from Refrigerant134a as the refrigerant is condensed at a temperature of 40°C. Air (Cp = 1005 J/kg · °C) flows across the finned condenser coils, entering at 25°C and leaving at 35°C. If the overall heat transfer coefficient based on the refrigerant side is 150 W/m² · °C, determine the heat transfer area on the refrigerant side(answer upto two decimal)
Correct
3.05 m²
Incorrect
3.05 m²

Question 19 of 20
19. Question
1 pointsIn a dairy plant, milk is pasteurized by hot water supplied by a natural gas furnace. The hot water is then discharged to an open floor drain at 80°C at a rate of 15 kg/min. The plant operates 24 h a day and 365 days a year. The furnace has an efficiency of 80 percent, and the cost of the natural gas is $0.40 per therm (1 therm = 105,500 kJ). The average temperature of the cold water entering the furnace throughout the year is 15°C. The drained hot water cannot be returned to the furnace and recirculated, because it is contaminated during the process. In order to save energy, installation of a watertowater heat exchanger to preheat the incoming cold water by the drained hot water is proposed. Assuming that the heat exchanger will recover 75 percent of the available heat in the hot water, determine the heat transfer rating of the heat exchanger that needs to be purchased and suggest a suitable type. Also, determine the amount of money this heat exchanger will save the company per year from natural gas savings.
Assumptions 1 Steady operating conditions exist. 2 The effectiveness of the heat exchanger remains constant.
Correct
SOLUTION A watertowater heat exchanger is to be installed to transfer energy from drained hot water to the incoming cold water to preheat it. The rate of heat
transfer in the heat exchanger and the amount of energy and money saved per year are to be determinedAnalysis A schematic of the prospective heat exchanger is given in Figure The heat recovery from the hot water will be a maximum when it leaves the heat exchanger at the inlet temperature of the cold water. Therefore,
Qmax=mhCp(Th, in – Tc, in)
=( 15/60 kg/s)( 4.18 kj/kg . °C ) (80 15)°C
=67.9 kJ/s
That is, the existing hot water stream has the potential to supply heat at a rate of 67.9 kJ/s to the incoming cold water. This value would be approached in a counterflow heat exchanger with a very large heat transfer surface area. A heat exchanger of reasonable size and cost can capture 75 percent of this heat transfer potential. Thus, the heat transfer rating of the prospective heat exchanger must be
Q= εQmax=(0.75)(67.9 kJ/s) =50.9 kJ/s
That is, the heat exchanger should be able to deliver heat at a rate of 50.9 kJ/s from the hot to the cold water. An ordinary plate or shellandtube heat exchanger should be adequate for this purpose, since both sides of the heat exchanger involve the same fluid at comparable flow rates and thus comparable heat transfer coefficients. (Note that if we were heating air with hot water, we would have to specify a heat exchanger that has a large surface area on the air side.) The heat exchanger will operate 24 h a day and 365 days a year. Therefore, the annual operating hours are
Operating hours=(24 h/day)(365 days/year)=8760 h/year
Noting that this heat exchanger saves 50.9 kJ of energy per second, the energy
saved during an entire year will beEnergy saved=(Heat transfer rate)(Operation time)=(50.9 kJ/s)(8760 h/year)(3600 s/h)=1.605 x 10^9 kJ/year
The furnace is said to be 80 percent efficient. That is, for each 80 units of heat supplied by the furnace, natural gas with an energy content of 100 units must be supplied to the furnace. Therefore, the energy savings determined above result in fuel savings in the amount of
Fuel saved=Energy saved/Furnace efficiency=1.605 x 10^9 kJ/years/0.80(1 therm/105,500 kJ)
Noting that the price of natural gas is $0.40 per therm, the amount of money saved becomes
Money saved=(Fuel saved) x (Price of fuel)
=(19,020 therms/year)($0.40/therm)=$7607/ year
Therefore, the installation of the proposed heat exchanger will save the company $7607 a year, and the installation cost of the heat exchanger will probably be paid from the fuel savings in a short time.
Incorrect
SOLUTION A watertowater heat exchanger is to be installed to transfer energy from drained hot water to the incoming cold water to preheat it. The rate of heat
transfer in the heat exchanger and the amount of energy and money saved per year are to be determinedAnalysis A schematic of the prospective heat exchanger is given in Figure The heat recovery from the hot water will be a maximum when it leaves the heat exchanger at the inlet temperature of the cold water. Therefore,
Qmax=mhCp(Th, in – Tc, in)
=( 15/60 kg/s)( 4.18 kj/kg . °C ) (80 15)°C
=67.9 kJ/s
That is, the existing hot water stream has the potential to supply heat at a rate of 67.9 kJ/s to the incoming cold water. This value would be approached in a counterflow heat exchanger with a very large heat transfer surface area. A heat exchanger of reasonable size and cost can capture 75 percent of this heat transfer potential. Thus, the heat transfer rating of the prospective heat exchanger must be
Q= εQmax=(0.75)(67.9 kJ/s) =50.9 kJ/s
That is, the heat exchanger should be able to deliver heat at a rate of 50.9 kJ/s from the hot to the cold water. An ordinary plate or shellandtube heat exchanger should be adequate for this purpose, since both sides of the heat exchanger involve the same fluid at comparable flow rates and thus comparable heat transfer coefficients. (Note that if we were heating air with hot water, we would have to specify a heat exchanger that has a large surface area on the air side.) The heat exchanger will operate 24 h a day and 365 days a year. Therefore, the annual operating hours are
Operating hours=(24 h/day)(365 days/year)=8760 h/year
Noting that this heat exchanger saves 50.9 kJ of energy per second, the energy
saved during an entire year will beEnergy saved=(Heat transfer rate)(Operation time)=(50.9 kJ/s)(8760 h/year)(3600 s/h)=1.605 x 10^9 kJ/year
The furnace is said to be 80 percent efficient. That is, for each 80 units of heat supplied by the furnace, natural gas with an energy content of 100 units must be supplied to the furnace. Therefore, the energy savings determined above result in fuel savings in the amount of
Fuel saved=Energy saved/Furnace efficiency=1.605 x 10^9 kJ/years/0.80(1 therm/105,500 kJ)
Noting that the price of natural gas is $0.40 per therm, the amount of money saved becomes
Money saved=(Fuel saved) x (Price of fuel)
=(19,020 therms/year)($0.40/therm)=$7607/ year
Therefore, the installation of the proposed heat exchanger will save the company $7607 a year, and the installation cost of the heat exchanger will probably be paid from the fuel savings in a short time.

Question 20 of 20
20. Question
1 pointsA counterflow doublepipe heat exchanger is to heat water from 20°C to 80°C at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal water available at 160°C at a mass flow rate of 2 kg/s. The inner tube is thinwalled and has a diameter of 1.5 cm. If the overall heat transfer coefficient of the heat exchanger is 640 W/m² · °C, determine the length of the heat exchanger required to achieve the desired heating.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well insulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 There is no fouling. 5 Fluid properties are constant.
Properties We take the specific heats of water and geothermal fluid to be 4.18 and 4.31 kJ/kg · °C, respectively.
Correct
SOLUTION Water is heated in a counterflow doublepipe heat exchanger by geothermal water. The required length of the heat exchanger is to be determined.
Analysis The schematic of the heat exchanger is given in Figure below . The rate of heat transfer in the heat exchanger can be determined from
To provide this much heat transfer surface area, the length of the tube must be
As=πDL→ L=As/πD=5.11m²/π(0.015m)=108m
Incorrect
SOLUTION Water is heated in a counterflow doublepipe heat exchanger by geothermal water. The required length of the heat exchanger is to be determined.
Analysis The schematic of the heat exchanger is given in Figure below . The rate of heat transfer in the heat exchanger can be determined from
To provide this much heat transfer surface area, the length of the tube must be
As=πDL→ L=As/πD=5.11m²/π(0.015m)=108m