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engineering
mechanical engineering

Questions and Answers of Mechanical Engineering

The air in a room has a dry-bulb temperature of 22°C and a wet-bulb temperature of 16°C. Assuming a pressure of 100 kPa, determine (a) The specific humidity, (b) The relative humidity, and
Reconsider Prob. 14–32. Determine the required properties using EES (or other) software. What would the property values be at a pressure of 300 kPa?
The air in a room has a dry-bulb temperature of 80°F and a wet-bulb temperature of 65°F. Assuming a pressure of 14.7 psia, determine (a) The specific humidity, (b) The relative humidity,
Atmospheric air at 35°C flows steadily into an adiabatic saturation device and leaves as a saturated mixture at 25°C. Makeup water is supplied to the device at 25°C. Atmospheric pressure is 98
How is the dew-point temperature at a specified state determined on the psychrometric chart?
The air in a room is at 1 atm, 32°C, and 60 percent relative humidity. Determine (a) The specific humidity, (b) The enthalpy (in kJ/kg dry air), (c) The wet-bulb temperature, (d) The
Reconsider Prob. 14–40. Determine the required properties using EES (or other) software instead of the psychrometric chart. What would the property values be at a location at 1500 m altitude?
A room contains air at 1 atm, 26°C, and 70 percent relative humidity. Using the psychrometric chart, determine (a) The specific humidity, (b) The enthalpy (in kJ/kg dry air), (c) The wet-bulb
Reconsider Prob. 14–42. Determine the required properties using EES (or other) software instead of the psychrometric chart. What would the property values be at a location at 2000 m altitude?
A room contains air at 1 atm, 82°F, and 70 percent relative humidity. Using the psychrometric chart, determine (a) The specific humidity, (b) The enthalpy (in Btu/lbm dry air), (c) The
Reconsider Prob. 14–44E Determine the required properties using EES (or other) software instead of the psychrometric chart. What would the property values be at a location at 5000 ft altitude?
The air in a room has a pressure of 1 atm, a dry-bulb temperature of 24°C, and a wet-bulb temperature of 17°C. Using the psychrometric chart, determine (a) The specific humidity, (b) The
Reconsider Prob. 14–46. Determine the required properties using EES (or other) software instead of the psychrometric chart. What would the property values be at a location at 3000 m altitude?
What is the radiation effect? How does it affect human comfort?
How does the air motion in the vicinity of the human body affect human comfort?
Consider a tennis match in cold weather where both players and spectators wear the same clothes. Which group of people will feel colder? Why?
Why do you think little babies are more susceptible to cold?
How does humidity affect human comfort?
What is metabolism? What is the range of metabolic rate for an average man? Why are we interested in the metabolic rate of the occupants of a building when we deal with heating and air-conditioning?
Why is the metabolic rate of women, in general, lower than that of men? What is the effect of clothing on the environmental temperature that feels comfortable?
What is sensible heat? How is the sensible heat loss from a human body affected by the (a) Skin temperature, (b) Environment temperature, and (c) Air motion?
What is latent heat? How is the latent heat loss from the human body affected by the (a) Skin wettedness and (b) Relative humidity of the environment? How is the rate of evaporation from the body
An average person produces 0.25 kg of moisture while taking a shower and 0.05 kg while bathing in a tub. Consider a family of four who each shower once a day in a bathroom that is not ventilated.
An average (1.82 kg or 4.0 lbm) chicken has a basal metabolic rate of 5.47 W and an average metabolic rate of 10.2 W (3.78 W sensible and 6.42 W latent) during normal activity. If there are 100
A department store expects to have 120 customers and 15 employees at peak times in summer. Determine the contribution of people to the total cooling load of the store.
In a movie theater in winter, 500 people, each generating sensible heat at a rate of 70 W, are watching a movie. The heat losses through the walls, windows, and the roof are estimated to be 130,000
For an infiltration rate of 1.2 air changes per hour (ACH), determine sensible, latent, and total infiltration heat load of a building at sea level, in kW, that is 20 m long, 13 m wide, and 3 m high
Repeat Prob. 14–64 for an infiltration rate of 1.8 ACH
How do relative and specific humidities change during a simple heating process? Answer the same question for a simple cooling process.
Why does a simple heating or cooling process appear as a horizontal line on the psychrometric chart?
Air enters a heating section at 95 kPa, 12°C, and 30 percent relative humidity at a rate of 6 m3/min and it leaves at 25°C. Determine (a) The rate of heat transfer in the heating section and (b)
A heating section consists of a 15-in.-diameter duct that houses a 4-kW electric resistance heater. Air enters the heating section at 14.7 psia, 50°F, and 40 percent relative humidity at a velocity
Air enters a 40-cm-diameter cooling section at 1 atm, 32°C, and 30 percent relative humidity at 18 m/s. Heat is removed from the air at a rate of 1200 kJ/min. Determine (a) the exit temperature,
Repeat Prob. 14–70 for a heat removal rate of 800 kJ/min.
Why is heated air sometimes humidified?
Air at 1 atm, 15°C, and 60 percent relative humidity is first heated to 20°C in a heating section and then humidified by introducing water vapor. The air leaves the humidifying section at 25°C and
Air at 14.7 psia, 50°F, and 60 percent relative humidity is first heated to 72°F in a heating section and then humidified by introducing water vapor. The air leaves the humidifying section at 75°F
An air-conditioning system operates at a total pressure of 1 atm and consists of a heating section and a humidifier that supplies wet steam (saturated water vapor) at 100°C. Air enters the
Repeat Prob. 14–75 for a total pressure of 95 kPa for the air stream.
Air enters a window air conditioner at 1 atm, 32°C, and 70 percent relative humidity at a rate of 2 m3/min, and it leaves as saturated air at 15°C. Part of the moisture in the air that condenses
An air-conditioning system is to take in air at 1 atm, 34°C, and 70 percent relative humidity and deliver it at 22°C and 50 percent relative humidity. The air flows first over the cooling coils,
Air enters a 30-cm-diameter cooling section at 1 atm, 35°C, and 60 percent relative humidity at 120 m/min. The air is cooled by passing it over a cooling coil through which cold water flows. The
Reconsider Prob. 14–80. Using EES (or other) software, develop a general solution of the problem in which the input variables may be supplied and parametric studies performed. For each set of input
Repeat Prob. 14–80 for a total pressure of 95 kPa for air.
Two unsaturated airstreams are mixed adiabatically. It is observed that some moisture condenses during the mixing process. Under what conditions will this be the case?
Two airstreams are mixed steadily and adiabatically. The first stream enters at 32°C and 40 percent relative humidity at a rate of 20 m3/min, while the second stream enters at 12°C and 90
Repeat Prob. 14–102 for a total mixing-chamber pressure of 90 kPa.
During an air-conditioning process, 900 ft3/min of conditioned air at 65°F and 30 percent relative humidity is mixed adiabatically with 300 ft3/min of outside air at 80°F and 90 percent relative
Reconsider Prob. 14–104E Using EES (or other) software, develop a general solution of the problem in which the input variables may be supplied and parametric studies performed. For each set of
A stream of warm air with a dry-bulb temperature of 40°C and a wet-bulb temperature of 32°C is mixed adiabatically with a stream of saturated cool air at 18°C. The dry air mass flow rates of the
Reconsider Prob. 14–106. Using EES (or other) software, determine the effect of the mass flow rate of saturated cool air stream on the mixture temperature, specific humidity, and relative humidity.
How does a natural-draft wet cooling tower work?
What is a spray pond? How does its performance compare to the performance of a wet cooling tower?
The cooling water from the condenser of a power plant enters a wet cooling tower at 40°C at a rate of 90 kg/s. The water is cooled to 25°C in the cooling tower by air that enters the tower at 1
The cooling water from the condenser of a power plant enters a wet cooling tower at 110°F at a rate of 100 lbm/s. Water is cooled to 80°F in the cooling tower by air that enters the tower at 1 atm,
A wet cooling tower is to cool 60 kg/s of water from 40 to 26°C. Atmospheric air enters the tower at 1 atm with dry- and wet-bulb temperatures of 22 and 16°C, respectively, and leaves at
A wet cooling tower is to cool 25 kg/s of cooling water from 40 to 30°C at a location where the atmospheric pressure is 96 kPa. Atmospheric air enters the tower at 20°C and 70 percent relative
A natural-draft cooling tower is to remove waste heat from the cooling water flowing through the condenser of a steam power plant. The turbine in the steam power plant receives 42 kg/s of steam from
The condensation of the water vapor in compressed air lines is a major concern in industrial facilities, and the compressed air is often dehumidified to avoid the problems associated with
The relative humidity of air at 80°F and 14.7 psia is increased from 25 to 75 percent during a humidification process at constant temperature and pressure. Determine the percent error involved in
Dry air whose molar analysis is 78.1 percent N2, 20.9 percent O2, and 1 percent Ar flows over a water body until it is saturated. If the pressure and temperature of air remain constant at 1 atm and
Determine the mole fraction of the water vapor at the surface of a lake whose surface temperature is 60°F, and compare it to the mole fraction of water in the lake, which is very nearly 1.0. The air
Determine the mole fraction of dry air at the surface of a lake whose temperature is 18°C. The air at the lake surface is saturated, and the atmospheric pressure at lake level can be taken to be 100
Consider a room that is cooled adequately by an air conditioner whose cooling capacity is 7500 Btu/h. If the room is to be cooled by an evaporative cooler that removes heat at the same rate by
The capacity of evaporative coolers is usually expressed in terms of the flow rate of air in ft3/min (or cfm), and a practical way of determining the required size of an evaporative cooler for an
A cooling tower with a cooling capacity of 100 tons (440 kW) is claimed to evaporate 15,800 kg of water per day. Is this a reasonable claim?
The U.S. Department of Energy estimates that 190,000 barrels of oil would be saved per day if every household in the United States raised the thermostat setting in summer by 6°F (3.3°C). Assuming
The thermostat setting of a house can be lowered by 2°F by wearing a light long-sleeved sweater, or by 4°F by wearing a heavy long-sleeved sweater for the same level of comfort. If each °F
The air-conditioning costs of a house can be reduced by up to 10 percent by installing the outdoor unit (the condenser) of the air conditioner at a location shaded by trees and shrubs. If the
A 3-m3 tank contains saturated air at 25°C and 97 kPa. Determine (a) The mass of the dry air, (b) The specific humidity, and (c) The enthalpy of the air per unit mass of the dry air.
Reconsider Prob. 14–126. Using EES (or other) software, determine the properties of the air at the initial state. Study the effect of heating the air at constant volume until the pressure is 110
Air at 15 psia, 60°F, and 50 percent relative humidity flows in an 8-in.-diameter duct at a velocity of 50 ft/s. Determine (a) The dew-point temperature, (b) The volume flow rate of air, and
Air enters a cooling section at 97 kPa, 35°C, and 30 percent relative humidity at a rate of 6 m3/min where it is cooled until the moisture in the air starts condensing. Determine (a) The
Outdoor air enters an air-conditioning system at 10°C and 40 percent relative humidity at a steady rate of 22 m3/min and it leaves at 25°C and 55 percent relative humidity. The outdoor air is first
Air enters an air-conditioning system that uses refrigerant-134a at 30°C and 70 percent relative humidity at a rate of 4 m3/min. The refrigerant enters the cooling section at 700 kPa with a quality
Repeat Prob. 14–131 for a total pressure of 95 kPa for air.
An air-conditioning system operates at a total pressure of 1 atm and consists of a heating section and an evaporative cooler. Air enters the heating section at 10°C and 70 percent relative humidity
Reconsider Prob. 14–133. Using EES (or other) software, study the effect of total pressure in the range 94 to 100 kPa on the results required in the problem. Plot the results as functions of total
Repeat Prob. 14–133 for a total pressure of 96 kPa.
Conditioned air at 13°C and 90 percent relative humidity is to be mixed with outside air at 34°C and 40 percent relative humidity at 1 atm. If it is desired that the mixture have a relative
Reconsider Prob. 14–136. Determine the desired quantities using EES (or other) software instead of the psychrometric chart. What would the answers be at a location at an atmospheric pressure of 80
A natural-draft cooling tower is to remove 50 MW of waste heat from the cooling water that enters the tower at 42°C and leaves at 27°C. Atmospheric air enters the tower at 1 atm with dry- and
Reconsider Prob. 14–138. Using EES (or other) software, investigate the effect of air inlet wet-bulb temperature on the required air volume flow rate and the makeup water flow rate when the other
Atmospheric air enters an air-conditioning system at 30°C and 70 percent relative humidity with a volume flow rate of 4 m3/min and is cooled to 20°C and 20 percent relative humidity at a
An uninsulated tank having a volume of 0.5 m3 contains air at 35°C, 130 kPa, and 20 percent relative humidity. The tank is connected to a water supply line in which water flows at 50°C. Water is
Air flows steadily through an isentropic nozzle. The air enters the nozzle at 35°C, 200 kPa and 50 percent relative humidity. If no condensation is to occur during the expansion process, determine
A room is filled with saturated moist air at 25°C and a total pressure of 100 kPa. If the mass of dry air in the room is 100 kg, the mass of water vapor is (a) 0.52 kg (b) 1.97 kg (c) 2.96 kg (d)
A room contains 50 kg of dry air and 0.6 kg of water vapor at 25°C and 95 kPa total pressures. The relative humidity of air in the room is (a) 1.2% (b) 18.4% (c) 56.7% (d) 65.2% (e) 78.0%
A 40-m3 room contains air at 30°C and a total pressure of 90 kPa with a relative humidity of 75 percent. The mass of dry air in the room is (a) 24.7 kg (b) 29.9 kg (c) 39.9 kg (d) 41.4 kg (e) 52.3
A room contains air at 30°C and a total pressure of 96.0 kPa with a relative humidity of 75 percent. The partial pressure of dry air is (a) 82.0 kPa (b) 85.8 kPa (c) 92.8 kPa (d) 90.6 kPa (e) 72.0
The air in a house is at 20°C and 50 percent relative humidity. Now the air is cooled at constant pressure. The temperature at which the moisture in the air will start condensing is (a) 8.7°C (b)
On the psychrometric chart, a cooling and dehumidification process appears as a line that is (a) Horizontal to the left (b) Vertical downward (c) Diagonal upwards to the right (NE direction) (d)
On the psychrometric chart, a heating and humidification process appears as a line that is (a) Horizontal to the right (b) Vertical upward (c) Diagonal upwards to the right (NE direction) (d)
An air stream at a specified temperature and relative humidity undergoes evaporative cooling by spraying water into it at about the same temperature. The lowest temperature the air stream can be
Air is cooled and dehumidified as it flows over the coils of a refrigeration system at 85 kPa from 30°C and a humidity ratio of 0.023 kg/kg dry air to 15°C and a humidity ratio of 0.015 kg/kg dry
Air at a total pressure of 90 kPa, 15°C, and 75 percent relative humidity is heated and humidified to 25°C and 75 percent relative humidity by introducing water vapor. If the mass flow rate of dry
How does the presence of N2 in air affect the outcome of a combustion process?
How does the presence of moisture in air affect the outcome of a combustion process?
What does the dew-point temperature of the product gases represent? How is it determined?
Is the number of atoms of each element conserved during a chemical reaction? How about the total number of moles?

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