7/23/2019 Tercer Parcial Turbomaquinaria y Equipos Trmicos

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Tercer Examen Parcial Pgina 1de 2

Universidad de Guanajuato

Fecha:

Tercer examen parcial de Turbomquinas y Equipos Trmicos

El examen slo es en equipo. NO se permite intercambio de comunicacin entre otros

equipos, de hacer esto, los integrantes de los equipos involucrados se hacen acreedoresa reprobar el curso completo. Cada equipo tiene 24 horas para resolver el examen. Sedebe entregar el procedimiento (ecuaciones) seguido para la solucin de cada problema

en archivo de texto (Word, Latex, etc.), de igual forma, las ecuaciones deben estar escritas en el editor del archivo

de texto (no en imgenes). Para cada problema se debe entregar al menos un archivo de MATLAB.m en el cual

se resuelva cada problema. Una vez terminado el examen, los equipos deben ponerse de acuerdo de culesproblemas expondr cada uno en clase (slo se permite comunicacin entre equipos para esto).

Nota 1: todas las hojas proporcionadas fsicamente por el equipo deben llevar el nombre y firma de los

integrantes, de lo contrario, las hojas no se considerarn como parte del examen.

Duracin:24 horas.

Nombre de los Integrantes del equipo ___________________________________________________________

_________________________________________________________________________________________

_________________________________________________________________________________________

Nota: para el examen, en caso de requerir, se puede usar la siguiente ecuacin.

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1.- A small shell-and-tube exchanger with one tube pass [A= 4.64 m2and U= 280 W/m2 C] is to be used to

heat high-pressure water initially at 20 C with hot air initially at 260 C. If the exit water temperature is not to

exceed 93 C and the airflow rate is 0.45 kg/s, calculate the water-flow rate. Solve using method.Approximate solution: . /

2. - A small steam condenser is designed to condense 0.76 kg/min of steam at 83 kPa with cooling water at 10

C. The exit water temperature is not to exceed 57 C. The overall heat-transfer coefficient is 3400 W/m2 C.

Calculate the area required for a double-pipe heat exchanger. 95.6 , 2.27 10/.Suppose the inlet water temperature in the exchanger of the problem now is raised to 30 C. What percentageincrease in flow rate would be necessary to maintain the same rate of condensation?

Solve this problem using method.Approximate solution: . .

3. - A counterflow double-pipe heat exchanger is used to heat liquid ammonia from 10 to 30 C with hot water

that enters the exchanger at 60 C. The flow rate of the water is 5.0 kg/s and the overall heat-transfer coefficient

is 800 W/m2 C. The area of the heat exchanger is 30 m2. Calculate the flow rate of ammonia.

Solve this problem using method.Approximate solution: / /

4. - A counterflow double-pipe heat exchanger is employed to heat 30 kg/s of water from 20 to 40 C with a hot

oil at 200 C. The overall heat-transfer coefficient is 275 W/m2 C. Determine effectiveness and NTU for exit

oil temperatures ranging from 190 until 80 C. Plot and discuss your results.

5.- A counterflow double-pipe heat exchanger is used to heat water from 20 C to 40 C with a hot oil that enters

the exchanger at 180 C and leaves at 140 C. The flow rate of water is 3.0 kg/s and the overall heat-transfer

coefficient is 130 W/m2 C. Assume the specific heat for oil is 2100 J/kg C. Suppose the water-flow rate iscut in half. What new oil flow rate would be necessary to maintain a 40 C outlet water temperature? (The oil

flow is not cut in half.)

7/23/2019 Tercer Parcial Turbomaquinaria y Equipos Trmicos

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Tercer Examen Parcial Pgina 2de 2

6. - A light fuel oil is used in the tube side of a shell-and-tube heat exchanger with two shell passes and four tube

passes. Water is heated in the shell side from 10 C to 50 C while the oil is cooled from 90 C to 60 C. The

overall heat-transfer coefficient is 53 W/m2 C. The specific heat of the oil is 2.0 kJ/kg C. Using both the

effectiveness and LMTD methods, calculate the area of the heat exchanger for a total energy transfer of 500 kW.

What is the water-flow rate for this heat transfer?Nota: recuerde el concepto de cundo un fluido sufre el mayor cambio de temperatura. Revisar diapositivas.

Approximate solution: /

7. - Hot air for a large-scale drying operation is to be produced by routing the air over a tube bank (unmixed),

while products of combustion are routed through the tubes. The surface area of the cross-flow heat exchanger is

25, and for the proposed operating conditions, the manufacturer specifies an overall heat transfercoefficient of U = 35 W/m2 K. The air and the combustion gases may each be assumed to have a specific heat of

1040 /. Consider conditions for which combustion gases flowing at 1 kg/s enter the heat exchangerat 800 K, while air at 5 kg/s has an inlet temperature of 300 K.

What are the air and gas outlet temperatures?

Approximate solution: ,

8. - A cross-flow heat exchanger used in a cardiopulmonary bypass procedure cools blood flowing at 5 liter/minfrom a body temperature of 37 C to 25 C in order to induce body hypothermia, which reduces metabolic and

oxygen requirements. The coolant is ice water at 0 C, and its flow rate is adjusted to provide an outlet

temperature of 15 C. The heat exchanger operates with both fluids unmixed, and the overall heat transfer

coefficient is 750 W/m2 K. The density and specific heat of the blood are 1050 kg/m3 and 3740 J/kg K,respectively.

a) Determine the heat transfer rate for the exchanger.b) Calculate the water flow rate.

c) What is the surface area of the heat exchanger?

d) Calculate and plot the blood and water outlet temperatures as a function of the water flow rate for the

range 2 to 4 liter/min, assuming all other parameters remain unchanged. Comment on how the changesin the outlet temperatures are affected by changes in the water flow rate. Explain this behavior and why

it is an advantage for this application.

Approximate solution: ; . / . /, . .

9. - Saturated steam at 110 C is condensed in a shell-and-tube heat exchanger (1 shell pass; 2, 4, tube passes)

with a UA value of 2.5 kW/K. Cooling water enters at 40 C.

a)

Using MATLAB, calculate the cooling water flow rate required to maintain a heat rate of 150 kW.b) Assuming that UA is independent of flow rate, calculate and plot the water flow rate required to provide

heat rates over the range from 130 to 160 kW. Comment on the validity of your assumption.

Approximate solution: / /

10. - A single-pass, cross-flow heat exchanger with both fluids unmixed is being used to heat water ( 2 /, 4200 / ) from 20 C to 100 C with hot exhaust gases ( 1200 / entering at 320C. What mass flow rate of exhaust gases is required? Assume that UA is equal to its design value of 4700 W/K,

independent of the gas mass flow rate.

Approximate solution: / /

11.- Compressed air is used in a heat-pump system to heat water, which is subsequently used to warm a house.The house demand is 95,000 Btu/h. Air enters the exchanger at 200 F and leaves at 120 F, and water enters and

leaves the exchanger at 90 and 125 F, respectively. Choose from the following alternative units the one that is

most compact.

a) A counterflow surface with 30 BTU/h ftFand a surface-to-volume ratio of 130 ft2/ft3.b) A crossflow configuration with the water unmixed and air mixed having 40 BTU/h ftFand a

surface-to-volume ratio of 100 ft2/ft3.

c) A crossflow unit with both fluids unmixed with 50 BTU/h ftFand surface-to-volume ratio of90 ft2/ft3. Note: solve for NTU using the equation for the corresponding heat exchanger.

12. - Ethanol is vaporized at 78 C ( 846 /) in a double-pipe parallel-flow heat exchanger at a rate of0.03 kg/s by hot oil ( 2200 /) that enters at l20 C. If the heat transfer surface area and the overallheat transfer coefficients are 6.2 m2and 320 W/m2C, respectively, determine the outlet temperature and the

mass flow rate of oil using (a) the LMTD method and (b) the -NTU method.

Approximate solution: , , . / . /