EXAM ∑
1. Benzene gas (C6H6) is burned with 95% theoretical air during a steady-flow combustion process. The mole fraction of the CO in the products is: (a) 8.3% (b) 4.7% (c) 2.1% (d) 1.9% (e) 14.3%
2. Air at a total pressure of 90 kPa, 15ºC and ø= 75% is heated and humidified to 25ºC and ø= 75% by introducing water vapor. If the mass flow rate of dry air is 4 kg/s, the rate at which steam is added to the air is: (a) 32 g/s (b) 13 g/s (c) 19 g/s (d) 7.9 g/s (e) 0 g/s
3. A refrigerator removes heat from a refrigerated space at 0ªC at a rate of 2.2 kJ/s and rejects it to an environment at 20ºC. The minimum required power input is: (a) 89 W (b) 150 W (c) 161 W (d) 557 W (e) 2,200 W
4. Consider a simple ideal Rankine cycle. If the condenser pressure is lowered while keeping turbine inlet state the same,
(a) the turbine work output will decrease (b) the amount of heat rejected will decrease (c) the cycle efficiency will decrease
(d) the moisture content at turbine exit will decrease (e) the pump work input will decrease.
5. In a shower, cold water at 10ºC flowing at a rate of 5 kg/min is mixed with hot water at 60ºC flowing at a rate of 2 kg/min. The exit temperature of the mixture is [in ºC]: (a) 24.3 (b) 35.0 (c) 40.0 (d) 44.3 (e) 55.2
EXAM Ω
6. One kmol of methane CH4 is burned with an unknown amount of air during combustion process. If the combustion is complete and there are 1 kmol of free O2 in the products, the air-fuel mass ratio is: (a) 34.6 (b) 25.7 (c) 17.2
(d) 14.3 (e) 11.9
7. Air is cooled and dehumidified as it flows over the coils of a refrigeration system at 85 kPa from 35ºC and w= 23 g/kg to 15ºC and w= 15 g/kg. If the mass flow rate of dry air is 0.4 kg/s, the rate of heat removal from the air is:
(a) 4 kJ/s (b) 8 kJ/s (c) 12 kJ/s (d) 16 kJ/s (e) 20 kJ/s
8. Consider a refrigerator that operates on the vapor compression refrigeration cycle with R-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 160 kPa (h=241.1 kJ/kg), and exits at 800 kPa and 50ºC (h=286.7 kJ/kg), and leaves the condenser as saturated liquid at the same pressure (h=95.47 kJ/kg). The coefficient of performance of this refrigerator is:
(a) 2.6 (b) 1.0 (c) 4.2 (d) 3.2 (e) 4.4
9. Consider a simple ideal Rankine cycle with fixed boiler and condenser pressures. If the cycle is modified with regeneration that involves one open feed water heater (select the correct statement per unit mass of steam flowing through the boiler),
(a) the turbine work output will decrease (b) the amount of heat rejected will increase (c) the cycle thermal efficiency will decrease (d) the quality of steam at turbine exit will decrease (e) the amount of heat input will increase.
10. In a heating system, cold outdoor air at 7ºC flowing at a rate of 4 kg/min is mixed adiabatically with heated air at 70ºC flowing at a rate of 3 kg/min. The exit temperature of the mixture is [in ºC]: (a) 34 (b) 39 (c) 45 (d) 63 (e) 77