Thermo Actual, Reheat, and Regen Review - Multiple Choice

A simple Rankine cycle is operating between 4MPA and 20 kPa using water as the working fluid. What is the pump's isentropic efficiency if the actual enthalpy leaving the pump is 256.18 kJ/kg.

Reminder: η_pump = (h_2_isentropic - h_1)/(h_2_actual - h_1)

• .8

• .85

• .9

• .95

• 1

Tags

NGSS.HS-PS3-1

A simple Rankine cycle is operating between 4MPA and 20 kPa using water as the working fluid. If steam enters the turbine at 700C and leaves the turbine with an enthalpy of 2653 kJ/kg, what is the isentropic efficiency of the turbine?

Reminders: η_turb = (h_3 - h_4_actual)/(h_3 - h_4_ideal)

h_4_ideal has the same entropy as the steam entering the turbine.

• .8

• .85

• .9

• .95

• 1

A simple Rankine cycle is operating between 4MPA and 20 kPa using water as the working fluid. If steam leaves the turbine with an enthalpy of 2513.7 kJ/kg and water leaves the condenser with 10C of subcooling, how much heat is rejected by the condenser?

Reminder: The enthalpy of a subcooled liquid ≅ the enthalpy of a saturated liquid at that temperature

• 255 kJ/kg

• 2262 kJ/kg

• 2304 kJ/kg

• 2565 kJ/kg

• 3906 kJ/kg

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-4

An ideal Reheat Rankine cycle operates its boiler at 17.5 MPa, its reheater at 1 MPa, and its condenser at 30 kPa using water as the working fluid. The steam inlet to the high pressure turbine is at 550C and the low pressure at 500C. What is the work contribution of the high and low pressure turbines, respectively?

Reminder: The steam can leave the high pressure turbine as a mixture.

The reheat states (4 and 5) are at the same pressure.

w_turb = h_inlet - h_outlet

• w_H = 692 kJ/kg
  w_L = 843 kJ/kg

• w_H = 719 kJ/kg
  w_L = 856 kJ/kg

• w_H = 892 kJ/kg
  w_L = 642 kJ/kg

• w_H = 642 kJ/kg
  w_L = 892 kJ/kg

• w_H = 582 kJ/kg
  w_L = 941 kJ/kg

Tags

NGSS.HS-PS3-2

NGSS.HS-PS3-4

An ideal Reheat Rankine cycle operates its boiler at 17.5 MPa, its reheater at 2 MPa, and its condenser at 50 kPa using water as the working fluid. The steam enters both turbines at 550C. If the high and low pressure turbines have 582 kJ/kg and 941 kJ/kg of work output respectively, how much additional turbine work is provided by reheat than if there was only 1 turbine?

Reminder: With only one turbine stage, the entropy leaving the turbine would have the same entropy as the steam entering at state 3.

w_turb = h_inlet - h_outlet

• 723 kJ/kg

• 2232 kJ/kg

• 0 kJ/kg

• 1192 kJ/kg

• 331 kJ/kg

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-4

Reheat improves the efficiency of a Rankine Cycle by reducing the pump work required

• True

• False

Tags

NGSS.HS-PS3-4

Reheat helps reduce the risk of having low quality at the end of the expansion process in a Rankine Cycle.

• True

• False

Tags

NGSS.HS-PS3-4

Open and Closed Feed Water heaters are a way to add regeneration to a Rankine Cycle.

• True

• False

Tags

NGSS.HS-PS3-4

Open and Closed Feed Water heaters improve cycle efficiency in a Rankine cycle by making the boiler heat transfer process more irreversible.

• True

• False

Answer explanation

Open and Closed Feed Water heaters make the heat transfer process in the boiler MORE reversible.

Tags

NGSS.HS-PS3-4

An ideal Rankine cycle using an Open Feedwater Heater operates between 6 MPa and 20 kPa using water as the working fluid. If the enthalpy of the mixed state (state 3) is 655.77 kJ/kg, what pressure is steam being extracted from the turbine?

Reminder: The pressure at state 3 is the same as the pressure at state 6 (where steam is extracted from the turbine). Use a table.

• 0.3 MPa

• 0.35 MPa

• 0.4 MPa

• 0.55 MPa

• .065 MPa

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-2

NGSS.HS-PS3-4