3-1 Calculate the specific volume of an air-vapor mixture in cubic meters per kilogram of dry air when the following conditions prevail: t=30°C, W = 0.015 kg/kg, and p₁ = 90 kPa. Ans. 0.99 m³/kg. 3-2 A sample of air has a dry-bulb temperature of 30°C and a wet-bulb temperature PSYCHROMETRY AND WETTED-SURFACE HEAT TRANSFER 57 of 25°C. The barometric pressure is 101 kPa. Using steam tables and Eqs. (3-2), (3-3), and (3-5), calculate (a) the humidity ratio if this air is adiabatically saturated, (b) the enthalpy of the air if it is adiabatically saturated, (c) the humidity ratio of the sample using Eq. (3-5), (d) the partial pressure of water vapor in the sample, and (e) the rela- tive humidity. Ans. (a) 0.0201 kg/kg, (b) 76.2 kJ/kg, (c) 0.0180 kg/kg, (d) 2840 Pa, (e) 67%. 3-3 Using humidity ratios from the psychrometric chart, calculate the error in con- sidering the wet-bulb line to be the line of constant enthalpy at the point of 35°C dry-bulb temperature and 50 percent relative humidity. 3-4 An air-vapor mixture has a dry-bulb temperature of 30°C and a humidity ratio of 0.015. Calculate at two different barometric pressures, 85 and 101 kPa, (a) the enthalpy and (b) the dew-point temperature. Ans. (a) 68.3 and 68.3 kJ/kg, (b) 17.5 and 20.3°C. 3-7 A stream of outdoor air is mixed with a stream of return air in an air-conditioning system that operates at 101 kPa pressure. The flow rate of outdoor air is 2 kg/s, and its condition is 35°C dry-bulb temperature and 25°C wet-bulb temperature. The flow rate of return air is 3 kg/s, and its condition is 24°C and 50 percent relative humidity. Determine (a) the enthalpy of the mixture, (b) the humidity ratio of the mixture, (c) the dry-bulb temperature of the mixture from the properties determined in parts (a) and (b), and (d) the dry-bulb temperature by weighted average of the dry-bulb temperatures of the entering streams. Ans. (a) 59.1 kJ/kg, (b) 0.01198 kg/kg, (c) 28.6°C, (d) 28.4°C. 3-8 The air conditions at the intake of an air compressor are 28°C, 50 percent relative humidity, and 101 kPa. The air is compressed to 400 kPa, then sent to an intercooler. If condensation of water vapor from the compressed air is to be prevented, what is the minimum temperature to which the air can be cooled in the intercooler? Ans. 40.3°C. 3-9 A winter air-conditioning system adds for humidification 0.0025 kg/s of saturated steam at 101 kPa pressure to an airflow of 0.36 kg/s. The air is initially at a tempera- ture of 15°C with a relative humidity of 20 percent. What are the dry- and wet-bulb temperatures of the air leaving the humidifier? Ans. 16.0 and 13.8°C. 3-10 Determine for the three cases listed below the magnitude in watts and the direction of transfer of sensible heat [using Eq. (3-8)], latent heat [using Eq. (3-9)], and total heat [using Eq. (3-14)]. The area is 0.15 m² and he 30 W/m2K. Air at 30°C and 50 percent relative humidity is in contact with water that is at a tempera- ture of (a) 13°C, (b) 20°C, and (c) 28°C. Ans. (a) -76.5, -42.3, -120.4 W; (b) -45.0, 15.1,-29.6 W; (c) -9.0, 116.5, 113.8 W. 3-5 A cooling tower is a device that cools a spray of water by passing it through a stream of air. If 15 m³/s of air at 35°C dry-bulb and 24°C wet-bulb temperature and an atmospheric pressure of 101 kPa enters the tower and the air leaves saturated at 31°C, (a) to what temperature can this airstream cool a spray of water entering at 38°C with a flow rate of 20 kg/s and (b) how many kilograms per second of makeup water must be added to compensate for the water that is evaporated? Ans. (a) 31.3°C, (b) 0.245 kg/s. 3-6 In an air-conditioning unit 3.5 m³/s of air at 27°C dry-bulb temperature, 50 per- cent relative humidity, and standard atmospheric pressure enters the unit. The leaving condition of the air is 13°C dry-bulb temperature and 90 percent relative humidity. Using properties from the psychrometric chart, (a) calculate the refrigerating capacity in kilowatts and (b) determine the rate of water removal from the air. Ans. (a) 88 kW, (b) 0.0113 kg/s. 3-1 Calculate the specific volume of an air-vapor mixture in cubic meters per kilogram of dry air when the following conditions prevail: r = 30°C, W = 0.015 kg/kg, and p₁ = 90 kPa. Ans. 0.99 m³/kg. 3-2 A sample of air has a dry-bulb temperature of 30°C and a wet-bulb temperature PSYCHROMETRY AND WETTED-SURFACE HEAT TRANSFER 57 of 25°C. The barometric pressure is 101 kPa. Using steam tables and Eqs. (3-2), (3-3), and (3-5), calculate (a) the humidity ratio if this air is adiabatically saturated, (b) the enthalpy of the air if it is adiabatically saturated, (c) the humidity ratio of the sample using Eq. (3-5), (d) the partial pressure of water vapor in the sample, and (e) the rela- tive humidity. Ans. (a) 0.0201 kg/kg, (b) 76.2 kJ/kg, (c) 0.0180 kg/kg, (d) 2840 Pa, (e) 67%.