First Law of Thermodynamics

The centrifugal air compressor of a gas turbine receives air from the ambient where the pressure is 1 bar and the temperature is 27°C. At the compressor discharge, the pressure is 4 bars and the temperature is 200°C. For a mass flow rate of 100 kg/min, the exit velocity is determined to be 100 m/s. Compute the power required to drive the compressor.

• 385 kW

• 298 kW

• 490 kW

• None of the above

The enthalpy of air is increased by 139.586 kJ/kg in a compressor. The rate of air flow is 16.42 kg/min. The power input is 48.2 kW. How much there is heat transfer from the compressor in kW?

• 21 kW

• 20 kW

• 10 kW

• 5.8 kW

In large gas-turbine power plants, air is preheated by the exhaust gases in a heat exchanger called the regenerator before it enters the combustion chamber. Air enters the regenerator at 1 MPa and 550 K at a mass flow rate of 800 kg/min. Heat is transferred to the air at a rate of 3200 kJ/s. Exhaust gases enter the regenerator at 140 kPa and 800 K and leave at 130 kPa and 600 K. Treating the exhaust gases as air, determine the exit temperature of the air and the mass flow rate of exhaust gases.

• 754 K & 10 kg/s

• 780 K & 15 kg/s

• 780 K & 13.85 kg/s

Refrigerant-134a is throttled from the saturated liquid state at 700 kPa to a pressure of 160 kPa. Determine the temperature drop during this process.

• 42.3°C

• 38.5°C

• 49.0°C

• -15.6°C

• 26.7°C

Steam flows steadily through an adiabatic turbine. The inlet conditions of the steam are 4 MPa, 500°C, and 80 m/s, and the exit conditions are 30 kPa, 92 percent quality, and 50 m/s. The turbine power output is 38.5 MW. Determine the mass flow rate of the steam through the turbine.

@ 4 MPa ann 500°C

 $h_1=3445\ \frac{kJ}{kg}$ 

@ 30 kPa and 92% quality

 $h_2=2437.4\ \frac{kJ}{kg}$  

• 43.16 kg/s
  

• 38.14 kg/s
  

• 51.17 kg/s
  

• 42.51 kg/s