Air conditioning ( Edited) PDF

Preview

Summary

Mechanical EngineeringCorrelation Courses (ME 500)Air ConditioningLecture: A classroom that normally contains 40 people is to be air- conditioned with window air- conditioning units of 5 KW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360 KJ/hr. There are 10...

Description

University of Mindanao Mechanical Engineering Correlation Courses (ME 500) Air Conditioning Lecture: 1.

2.

3.

A classroom that normally contains 40 people is to be airconditioned with window airconditioning units of 5 KW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360 KJ/hr. There are 10 light bulbs in the room, each with a rating of 100 watts. The rate of heat transfer to the classroom through the walls and the windows is estimated to be 15,00 KJ/hr. If the room air is to be maintained at a constant temperature of 21oC, determine the number of window airconditioning units required. a. 1 unit b. 2 units * c. 3 units d. 4 units A 4m x 5m x 6m room is to be heated by a baseboard resistance heater. It is desired that the resistance heater be able to raise the air temperature in the room from 7 to 23oC within 15 minutes. Assuming no heat losses from the room and an atmospheric pressure of 100 kpa, determine the required power of the resistance heater. Assume constant specific heats at room temperature. a. 2.34 KW b. 1.91 KW* c. 4.56 KW d. 6.34 KW A student in a 4m x 6m x 6m dormitory room turns on her 150 watts fan before she leaves the room on a summer day, hoping that the room will be cooler when she comes back in the evening. Assuming all the doors and windows are tightly closed and disregarding any heat transfer through the walls and the windows, determine the temperature in the room when she comes back 10 hours later. Use specific heat values at room temperature, and assume the room to be at 100 kpa and 15oC in the morning when she leaves

5.

℉

4.

℉

dry bulb and 75 ℉ wet bulb. Solar heat load is 110,000 Btu/hr and supply air at 60 ℉ , determine the amount of supply air.

7.

8.

o

a. 28.13 C b. 38.13oC c. 48.13oC d. 58.13oC* An air-conditioning system is used to maintain a house at 75OF when the temperature outside is 95OF. The house is gaining heat through the walls and windows at a rate of 1250 Btu/min, and the heat generation rate within the house from people, lights and appliances amounts to 350 Btu/min.

wet bulb temperature

when outdoor air is at 91

6.

9.

Prepared by: Engr. Edgar Kenneth Luna ME 500 Coordinator

humidity ratio of 0.015 kg/kg d.a., calculate the enthalpy of the moist air. a. 68.527 kJ/kg d.a*

Determine the minimum power input required for this airconditioning system. a. 10.06 hp b. 1.36 hp c. 1.41 hp* d. 7.94 hp An auditorium seating 1500 people is to be maintained at 80 ℉ dry bulb and 85

b.

86.527 kJ/kg d.a.

c.

65.827 kJ/kg d.a.

d.

67.528 kJ/kg d.a

10. The approach and efficiency of cooling tower are 10 oC and 65 %, respectively. If the temperature of water leaving the tower is 27 oC, determine the temperature of water entering the tower. a. 54.57 oC b.

55.47 oC

a.

93,229.17 lb/hr*

c.

45.57 oC*

b.

83,229.17 lb/hr

d.

54.75 oC

c.

73,229.17 lb/hr

d.

63,229.17 lb/hr

A room contains air at 20C and 96 kPA at a relative humidity of 75%. Determine the enthalpy of moist air. a. b.

45.919 KJ/kgda 45.515 KJ/kgda

c.

49.515 KJ/kgda*

d.

41.815 KJ/kgda

Determine the mass of water vapour contained in a 150m3 room at 100 kPa, 23C and 40% relative humidity. From steam tables: Psat @ 23C=2.810 kPA.

11.

A 10 kg/s of air enters the theatre at 16 oC. The theater is to be maintained at 27 oC DB and 20 oC WB. If the sensible heat ratio is 0.71, what is the latent heat load of the theater? a. 45.21 kW* b.

54.21 kW

c.

110.682 kW

d.

42.51 Kw

12. Re-circulated air of 10 kg/s with 53 KJ/kg dry air enthalpy and outside air of 4 kg/s with 90KJ/kg dry air enthalpy enters the conditioning unit. Determine the air conditioning capacity of air conditioning apparatus if the supply enthalpy to conditioned space is 42 kJ/kg dry air.

a.

1.6342 kg

b.

1.9342 kg

a.

154 KW

c.

1.2342 kg*

b.

302 KW*

d.

2.2342 kg

c.

204 KW

d.

4.484 KW

A 5 m x 4 m x 4 m room has an air temperature of 32 oC (psat = 4.559 kPaa, hg = 2559.9 kJ/kg) and the pressure is 101 kPaa. The mass of water vapor in air is 2.5 kg with Rv = 0.45 kJ/kg-oK. What is the relative humidity of the air? a. 94.08 %* b.

74.08 %

c.

84.08 %

d.

64.08 %

An air-vapor mixture has a dry bulb temperature of 30 oC and a

13. 30 kg of dry air enters an adiabatic drying chamber at 88 degC and with humidity ratio of 0.16 kg/kg d.a. the exhaust air leaves the drying chamber at 40 degC. What is the rate of water removal in the drying chamber? At 40C psat = 7.375 kpa; at 88, hg = 2656.9 Kj/kg a.

0.24 kg/s

b.

0.46 kg/s

c.

0.38 kg/s

University of Mindanao Mechanical Engineering Correlation Courses (ME 500) Air Conditioning d.

0.58 kg/s*

14. Compute the humidity ration of air condensing the density at 27 degC and 98 kPa is 1.32 kg/m3 a.

0.34 kgvapor/kgair

b.

0.43 kgvapor/kgair

c.

0.35 kgvapor/kgair*

d.

0.53 kgvapor/kgair

15. Compute the humidity ratio of air @ 70% relative humidity and 25˚C when the barometric pressure is 101.325 kPa from steam tables. Psat at 34˚C = 3.169 kPa a. 0.014 kg water vapor / kg dry air*

16. Wet material containing 220% moisture (dry basis) is to be dried at the rate of 1.5 kg / s in a continuous dryer to give a product containing 10%(dry basis). Find the moisture removed in kg / hr.

18. Fifty kilogram of cooling tower per second enter the condenser 15˚C and leaves at 50˚C. Find the heat carried away by water. a.

5234.45 kW

a.

3543.75 kg / hr*

b.

5233.45 kW*

b.

3513.75 kg / hr

c.

2340.52 kW

c.

3563.75 kg / hr

d.

3140.25 kW

d.

3563.75 kg / hr

17. Copra enters a dryer containing 70% moisture and leaves at 7% moisture. Find the moisture removed on each pound of solid in the final product. a.

6.258 lb

b. 0.14 kg water vapor / kg dry air

b.

1.258 lb

c. 1.4 kg water vapor / kg dry air

c.

4.258 lb

d. 0.0014 kg water vapor / kg dry air

d.

2.258 lb*

Assignment: 1.

a. b. c. d. 2.

a. b. c. d. 3.

a. b. c. d. 4.

a. b. c. d. 5.

A 10 m3 vessel contains 5 m3 of liquid water and 5 m3 of saturated water vapor at 100kpa. Calculate the internal energy of the system using the steam tables. 5 x 105 kJ 8 x 105 kJ 1 x 106 kJ 2 x 106 kJ* 3.0 lbm of air are contained at 25 psia and 100 °F. Given that Rair = 53.35 ft-lbf / lbm-°F, what is the volume of the container? 10.7 ft3 14.7 ft3 15 ft3 24.9 ft3 * Twenty grams of oxygen (O2) are compressed at a constant temperature of 30°C to 5 % of their original volume. What work is done on the system? 824 cal 924 cal 944 cal 1124 cal * A vessel with a volume of 1 cubic meter contains liquid water and water vapor equilibrium at 600 kPa. The liquid water has a mass of 1 kg. Using the steam tables, calculate the mass of the vapor. 0.99 kg 1.57 kg 3.16 kg* 2.54 kg A cylinder and a piston arrangement contains saturated

a. b. c. d. 6.

a. b. c. d. 7.

a. b. c. d. 8.

a. b. c.

Prepared by: Engr. Edgar Kenneth Luna ME 500 Coordinator

water vapor at 110 °C. The vapor is compressed in a reversible adiabatic process until the pressure is 1.6 MPa. Determine the work done by the system per kilogram of water. -637 kJ/kg -509 kJ/kg -432 kJ/kg* -330 kJ/kg Ideal Oxygen is throttled at 140degF from 10 atm to 5 atm. What is the temperature change? 0* Negative Infinity 1 Gas is enclosed in a cylinder with a weighted piston as the top boundary. The gas is heated and expands from a volume of 0.04 m3 at a constant pressure of 200 kPa. Calculate the work done by the system. 8 kJ* 10 kJ 12 kJ 14 kJ Assume the total heat of vaporization (per gram) of water can be used to supply the energy needed to tear 1 g of water molecules apart from each other. How much energy is needed per molecule for this purpose? 6.74 x 10-20 J* 1.74 x 10-20 J 3.74 x 10-20 J

2.74 x 10-20 J An isobaric steam generating process starts with saturated liquid at 20 psia. The change in entropy is equal to the initial entropy. What is the change in enthalpy during the process? (Hint: Not all liquid is vaporized.) a. -230.4 BTU/lbm b. 230.4 BTU/lbm* c. 0 BTU/lbm d. 196.2 BTU.lbm 10. A steam generator produces saturated steam at 100 psia from saturated liquid at 14.7 psia. If the heat source is a bath at 340F that produces 800 BTU/lbm, which of the following is true? a. The device cannot work because the pressure gradient is greater than zero. b. The device can work, but is inefficient. c. The device violates the 1st law of thermodynamics.* d. The device violates the 2nd law of thermodynamics. 11. If air is at a pressure, ρ , of 3200 lbf/ft2, and at a temperature, T , of 800°R, what is the specific volume, v? (R = 53.3 ft-lbf/lbm°R, and air can modeled as an ideal gas) a. 9.8 ft3/lbm b. 11.2 ft3/lbm c. 13.3 ft3/lbm* d. 14.2 ft3/lbm d. 9.

University of Mindanao Mechanical Engineering Correlation Courses (ME 500) Air Conditioning 12. A 5m3 vessel initially contains 50 kg of lquid water and saturated water vapor at total internal energy of 27,3000 kJ. Calculate the heat requirement to vaporize all of the liquid. a. 100,000 kJ* b. 2,000,000 kJ c. 300,000 kJ d. 400,000 kJ 13. A copper can of negligible heat capacity contains 1 kg of water just above the freezing point. A similar can contains 1 kg of water just below the boiling point. Two cans are brought to into thermal contact. Find the change in entropy of the system? a. 100 J* b. 200 J c. 300 J d. 400 J 14. The atomic weight of hydrogen is 1gram per gram-atom. What is the mass of hydrogen atom? a. 1.66 x 10-24 g/atom* b. 6.02 x 10-10 g/atom c. 1.0 x 10-23 g/atom d. 1 g/atom 15. In a gaseous mixture of 20 deg. C the partial pressures of the components are as follows: Hydrogen, 200 mmHg; Carbon Dioxide, 150 mmHg; Methane, 320 mmHg; Ethylene, 105 mmHg. What is the total pressure of the mixture? a. 755 mmHg* b. 255 mmHg c. 345 mmHg d. 800 mmHg 16. Air undergoes an isentropic compression from 14.7 psia to 180.6 psia. If the initial temperature is 68 °F and the final temperature is 621.5 °F, calculate the work done by the gas. a. -136.2 BTU/lbm b. -94.8 BTU/lbm* c. 0 BTU/lbm d. 94.8 BTU/lbm 17. Gas is enclosed in a cylinder with a weighted piston as the top boundary. The gas is heated and expands from a volume 0.04 m3 to 0.1 m3. The pressure varies such that PV = C, and the initial pressure is 200 kPa. Calculate the work done by the system. a. 6.8 kJ b. 7.33 kJ* c. 9.59 kJ d. 12.0 kJ 18. From the steam tables, determine the average constant pressure specific heat (Cp) of steam at 10 kPa and 45.8°C. a. 1.79 kJ/kg.°C* b. 10.28 kJ/kg.°C c. 30.57 kJ/kg.°C d. 100.1 kJ/kg.°C

19. For an ideal gas, what is the specific molar entropy change during an isothermal process in w/c the pressure changes from 200 kPa to 150 kPa? a. 2.00 J/moleK b. 2.39 J/moleK* c. 2.79 J/moleK d. 3.12 J/moleK 20. What is the value of the work done for a closed, reversible, isometric system? a. Zero* b. Positive c. Negative d. Positive or negative 21. If 1 lbm of steam at 14.7 psia, 63 % quality is heated isentropically, at what pressure will it reach the saturated vapor state? a. 56 psia b. 313 psia c. 1852 psia d. 2585 psia* 22. 200g of water are heated from 5C to 100C and vaporized at a constant pressure. The heat of vaporization of water at 100C is 539.2 cal/g. The heat capacity at constant pressure, Cp is 1.0 cal/gK. Determine the total change in entropy. a. 248.2 cal/K b. 298.2 cal/K c. 348.0 cal/K* d. 398.2 cal/K 23. What is true about the polytropic exponent, n , for a perfect gas undergoing an isobaric process? a. n>0 b. n < 0 c. n → infinity d. n = 0* 24. On a hot day, the temperature rises from 50F early in the morning to 99F in the afternoon. What is the ratio of the concentration (in moles/ft3 of helium in a spherical balloon in the afternoon to the concentration of helium in the balloon in the morning? a. 0.51 b. 0.69 c. 0.91* d. 1.10 25. Cool water at 9 deg. C enters hotwater from which warm water at a temperature of 80 deg. C is drawn at an average rate of 300 g/min. How much average electric power does the heater consume in order to provide hot water at this rate? a. 4.18 kW* b. 2.35 kW c. 3.31 kW d. 5.14 kW 26. Water (specific heat cv = 4.2 kJ/kgK) is being heated by a 1500W heater. What is the rate of change in temperature of 1 kg of the water? a. 0.043 K/s

Prepared by: Engr. Edgar Kenneth Luna ME 500 Coordinator

b. c. d. 27.

a. b. c. d. 28.

a. b. c. d. 29.

a. b. c. d. 30.

a. b. c. d. 31.

a. b. c. d. 32.

a. b. c. d. 33.

a. b. c. d. 34.

0.719 K/s 0.357 K/s* 1.50 K/s Calculate the change in enthalpy as 1 kg of nitrogen is heated from 1000 K to 1500 K, assuming the nitrogen is an ideal gas at a constant pressure. The temperature-dependent specific heat of nitrogen is: cp = 39.06 - 512.79T-1.5 + 1072.7T-2 - 820.4T-3 cp is in kJ/Kmole-K and T is in K. 600 kJ 700 kJ* 800 kJ 900 kJ Helium is compressed isothermally from 14.7 psia and 60 °F. The compression ratio is 4. Calculate the change in entropy of the gas given that RHe = 0.4961 BTU/lbm-°R. -2.76 BTU/lbm-°R -0.689 BTU/lbm-°R* 0 BTU/lbm-°R 0.689 BTU/lbm-°R One kilogram of water (cv = 4.2 kJ/kg-K) is heated by 300 BTU of energy. What is the change in temperature, in K? 17.9 K 71.4 K 73.8 K 75.4 K* In an isentropic compression, p1 = 200 psia, p2 = 300 psia, V1 = 10 in3, and k= 1.4. Find V2. 3.509 in3 4.500 in3 5.000 in3 6.095 in3* 200 mL of oxygen are collected over water at 23C and a pressure of 1 atmosphere. What volume would the oxygen occupy dry at 273K and 1 atmosphere? 179.3 ml* 184.4 ml 190.0 ml 194.5 ml A gas has a density of 0.094 lb/ft3 at 100degF and 2 atm. What pressure is needed to change the density to 0.270 lb/ft3 at 250degF? 7.28 atm* 6.32 atm 3.45 atm 5.25 atm If atmospheric air 14.7 psia and 60degF at sea level, what is the pressure at 12000 ft altitude if air is incompressible. Note: @ 60degF; the density of air is 0.0763 lbm/ft3; P1 = 14.7 psia 8.342 psia* 5.34 psia 6.72 psia 9.32 psia A cold tire contains 1000 in.3 of air at 24 psia and 32degF. What pressure in the tire is needed if the

University of Mindanao Mechanical Engineering Correlation Courses (ME 500) Air Conditioning

a. b. c. d. 35.

a. b. c. d. 36.

a. b. c. d. 37.

a. b. c. d. 38.

a. b. c. d. 39.

a. b. c. d.

temperature and volume are increased to 35degF and 1020 in.3 respectively? 23.57 psia* 32.45 psia 43.54 psia 34.57 psia Determine the specific gravity of carbon dioxide gas (molecular weight = 44) at 150degF (66degC) and 20 psia (138 kPa). 1.67* 1.45 1.87 1.30 An empty polyethylene telemetry balloon and payload have a mass of 500 lbm. The balloon is charged with helium when the atmospheric conditions are 60degF and 14.8 psia. The specific gas constant of helium is 386.3 ft-lbf/lbm-R. What is the volume of helium is required? 7544 ft3 * 6754 ft3 7455 ft3 6456 ft3 What is the change in entropy of 2 kg water molecules when transformed at constant pressure of 1 atmosphere from water at 100degC to steam at the same temperature? 12.12 kJ/K* 10.43 kJ/K 9.45 kJ/K 10.45 kJ/K In a gaseous mixture of 20 deg. C the partial pressures of the components are as follows: Hydrogen, 200 mmHg; Carbon Dioxide, 150 mmHg; Methane, 320 mmHg; Ethylene, 105 mmHg. What is the mass fraction of Hydrogen ( m(H2) = 2, m(CO2) = 44, m(meth) = 16 and m(eth) = 30 kg/mol) ? 0.026 * 0.056 0.076 0.016 Assume the total heat of vaporization (per gram) of water can be used to supply the energy needed to tear 1 g of water molecules apart from each other. What is the ratio of energy needed per molecule to kT where k = 1.38 x 10-23 J/K? 11.2 12.5 13.1 * 12.3

40. Steam at 1000 lbf/ft2 pressure and 300°R has a specific volume of 6.5 ft3/lbm and a specific enthalpy of 9800 lbf-ft/lbm. Find the internal energy per pound mass of steam. a. 2500 lbf-ft/lbm b. 3300 lbf-ft/lbm*

c. 5400 lbf-ft/lbm b. d. 6900 lbf-ft/lbm 41. Calculate the entropy of steam at c. 60 psia with a quality of 0.6. a. 0.4274 BTU/lbm-°R d. b. 0.7303 BTU/lbm-°R c. 1.1577 BTU/lbm-°R* 50. d. 1.2172 BTU/lbm-°R 42. Find the change in internal energy a. of 5 lbm of oxygen gas when the b. temperature changes from 100°F c. to 120°F. cv = 0.157 BTU/lbm-°R. a. 14.7 BTU d. b. 15.7 BTU* 51. c. 16.8 BTU d. 147 BTU 43. Determine the change in enthalpy per lbm on nitrogen gas as its a. temperature changes from 500°F b. to 200°F. cp = 0.2483 BTU/lbmc. °R. d. a. -74.49 BTU/lbm * 52. b. -72.66 BTU/lbm c. -68.47 BTU/lbm d. 63.78 BTU/lbm 44. What is the resulting pressure when one pound of air at 15 psia and 200°F is heated at constant volume to 800°F? a. a. 15 psia b. c. b. 28.6 psia* d. c. 36.4 psia 53. d. 52.1 psia 45. What horsepower is required to 3 isothermally compress 800 ft of air per minute from 14.7 psia to 120 psia? a. 28 Hp b. 108 Hp* c. 256 Hp d. 13,900 Hp 46. Calculate the work done by a system in which 1 kg mole of water completely evaporates at 100°C and 1 atmosphere pressure. a. 1000 kJ power. b. 2130 kJ a. c. 2490 kJ b. d. 3050 kJ* c. 47. What is the equation for a work d. done by a constant temperature 54. system? a. W = mRTln(V2 – V1) b. W = mR(T2 – T1)ln V2/V1 c. W = mRTln V2/V1 * d. W = RTln V2/V1 48. A piston-cylinder system contains a gas which expands under a constant pressure of 1200 lbf/ft2. If the piston is displaced during the work. process, and the piston diameter is 24”, what is the work done by the gas on the piston? a. 1768 ft-lbf a. b. 1890 ft-lbf b. c. 2387 ft-lbf c. d. 3768 ft-lbf* d. 49. How does an adiabatic process 55. compare to an isentropic process? a. Adiabatic: heat transfer = 0; Isentropic: heat transfer ≠ 0.

Prepared by: Engr. Edgar Kenneth Luna ME 500 Coordinator

Adiabatic: heat transfer ≠ 0; Isentropic: heat transfer = 0. Adiabatic: reversible; Isentropic: not reversible. Both: heat transfer = 0; Isentropic: reversible.* In an adiabatic, isentropic process, p1 = 200 psia, p2 = 300 psia, and T1 = 700 °R. Find T2, using γ= 1.4. 576 °R 590 °R 680 °R 786 °R* Nitrogen is expanded isentropically. Its temperature changes from 620 °F to 60 °F. Find the pressure Ratio (p1/p2). 0.08 12.9* 23.2 3547 Nitrogen is expanded isentropically. Its temperature changes from 620 °F to 60 °F. The volumetric ratio is V2/V1 = 6.22 and the value of R for nitrogen is 0.0787 BTU/lbm-°R. What is the work done by the gas? -1112.7 BTU/lbm -99.22 BTU/lbm 1112.7 BTU/lbm 99.22 BTU/lbm* A device that is meant to extract power from waste process steam starts w/ steam of 75% quality at 100 psia. The exit conditions of the steam are 70% quality at 14.7 psia. Which of the following statements are true? I. The device violates the 1st law of thermodynamics. II. The device violates the 2nd law of thermodynamics. III. The device generates positive net power IV....