Jabatan Kejuruteraan Loji dan AutomotifFakulti Kejuruteraan Mekanikal dan PembuatanUniversiti Tun Hussein Onn Malaysia

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Tutorial 6Mixtures

BDA 3043 – Thermodynamics IIWinardi Sani

¶ A vessel of volume 0.4 m3 contains 0.45 kg CO and 1 kg of air, at 15 °C.Calculate:

(a) The partial pressure of each component(b) The total pressure in the vessel

The gravimetric analysis of air is to be taken as 23.3 % O2 and 76.7 % of N2. Takethe molar mass of CO, O2, and N2 as 28, 32, and 28 kg/kmol, respectively.

· A mixture of 1 kmol CO2 and 3.5 kmol of air is contained in a vessel at 1 barand 15 °C. The volumetric analysis of air can be taken as 21 % O2 and 79% N2.Calculate for the mixture:

(a) The masses of CO2, O2, N2, and the total mass(b) The percentage carbon content by mass(c) The molar mass and the specific gas constant for the mixture(d) The specific volume of the mixture

(MC = 12, MO2 = 32 , MN2 = 28 kg/kmol, R = 8.3145 kJ/kmol K)

¸ Moist air enters a duct at 10 °C, 80 % RH, and a volumetric flow rate of 150m3/min. The mixture is heated as it flows through the duct and exits at 30 °C. Nomoisture is added or removed, and the mixture pressure remains approximatelyconstant at 1 bar. For steady-state operation,

(a) Sketch on T – s diagram the heating process, and determine(b) The rate of heat transfer, in kJ/min(c) The relative humidity at the exit.(d) Compare your result with the psychrometric chart analysis

Q. 2

1

.3V = 150 m /min

T = 10 Co

p = 1 barT = 30 Com

ois

t a

ir

control volume

RH = 80%

¹ Moist air at 30 °Cand 50% RH enters a dehumidifier operating at steady statewith a volumetric flow rate of 280 m3/min. The moist air passes over a coolingcoil and water vapor condenses. Condensate exits the humidifier saturated at10 °C. Saturated moist air exits in a separate stream at the same temperature.There is no a significant loss of energy by heat transfer to the surroundings andpressure remains constant at 1.013 bar. Determine:

2Jabatan Kejuruteraan Loji dan AutomotifFakulti Kejuruteraan Mekanikal dan PembuatanUniversiti Tun Hussein Onn Malaysia

(a) The mass flow rate of the dry air, in kg/min

(b) The rate at which water is condensed, in kg per kg of dry air flowing throughthe control volume

(c) The required refrigerating capacity, in tons.

(d) Compare your result with the psychrometric chart analysis

º Cooling water leaves the condenser of a power plant and enters a wet coolingtower at 35 °C at a rate of 100 kg/s. Water is cooled to 22 °C in the coolingtower by air that enters the tower at 1 atm, 20 °C, and 60 % RH, and leavessaturated at 30 °C. Neglecting the power input to the fan, determine:

(a) The volume flow rate of air into the cooling tower

(b) The mass flow rate of the required makeup water

» Saturated air leaving the cooling section of an air-conditioning system at 14°C at rate of 50 m3/min is mixed adiabatically with the outside air at 32 °C and60 % RH at a rate of 20 m3/min. Assuming that the mixture process occurs at apressure of 1 atm, determine:

(a) The specific humidity

(b) The relative humidity

(c) The DBT

(d) The volume flow rate of the mixture.

(Answers: 0.0122 kg/kg d.a; 89 %; 19.0 °C; 70.1 m3/min)

¼ A small size cooling tower is designed to cool 5.5 litres of water per second , theinlet temperature is 44 °C. The motor-driven fan induces 9 m3/s of air through thetower and the power absorbed is 4.75 kW. The air entering the tower is at 18 °C,and has a relative humidity of 60 %. The air leaving the tower can be assumed tobe saturated and its temperature is 26 °C. Assuming that the pressure throughoutthe tower is contant at 1.013 bar, and makeup water is added outside the tower,calculate:

(a) The mass flow rate of makeup water required

(b) The final temperature of the water leaving the tower.

½ Two air streams are mixed steadily and adiabatically. The first streams enters at32 °C and 40 % RH at a rate of 20 m3/min, while the second stream enters at 12°C and 90% RH at a rate of 25 m3/min. Assuming that the mixing process occursat a pressure of 1 atm, determine:

(a) The specific humidity

(b) The relative humidity

(c) The dry-bulb temperature

(d) The volume flow rate of the mixture

(Answers: 0.0096 H2O/kg dry air; 63.4 %; 20.6 °C; 45.0 m3/min.)

Jabatan Kejuruteraan Loji dan AutomotifFakulti Kejuruteraan Mekanikal dan PembuatanUniversiti Tun Hussein Onn Malaysia

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(M1) 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) 2.04 kg (E) 3.17 kg

(M2) A room contains 50 kg of dry air and 0.6 kg of water vapor at 25 °C and 95 kPatotal pressure. The relative humidity of air in the room is, in %:(A) 1.2 (B) 18.4(C) 56.7 (D) 65.2 (E) 78.0

(M3) A room contains air at 30 °C and a total pressure of 96.0 kPa with 75 % RH. Thepartial pressure of dry air is, in kPa:(A) 82.0 (B) 85.8(C) 72.0 (D) 92.8 (E) 90.6

(M4) The air in a house is at 20 °C and 50 % RH. Now the air is cooled at a constantpressure. The temperature at which the moisture in the air will start condensingis, in °C:(A) 8.7 (B) 11.3(C) 13.8 (D) 9.3 (E) 10.0

(M5) Air is cooled and dehumidified as it flows over the coils of a refrigeration systemat 85 kPa from 30 °C and a humidity ratio of 0.023 kg/kg dry air to 15 °C and ahumidity ratio of 0.015 kg/kg dry air. If the mass flow rate of dry air is 0.7 kg/s, therate of heat removal from the air is, in kJ/s:(A) 5 (B) 10(C) 15 (D) 20 (E) 25