Thermodynamics Flashcards
(31 cards)
In a non-flow process there is heat transfer loss of 1055 kJ and an internal energy increase of 210 kJ. Determine the work transfer and state whether the process is an expansion or
compression.
-1265 kJ, compression
Solution:
Q + W = dU
1055 + W = - 210 (will make sense kung ganto)
W = - 1265 kJ
In a non-flow process carried out on 5.4 kg of a substance, there was a specific internal
energy decrease of 50 kJ/kg and a work transfer from the substance of 85 kJ/kg. Determine
the heat transfer and state whether it is gain or loss.
189 kJ, gain
For closed systems, Q - W = dU
Q = 5.4 (- 50) + 5.4(85) = - 270 + 459
Q = 189 kJ
During the working stroke of an engine the heat transferred out of the system was 150
kJ/kg of the working substance. If the work done by the engine is 250 kJ/kg, determine the
change in internal energy and state whether it is decrease or increase.
-400 kj/kg, decrease
Steam enters a cylinder fitted with a piston at a pressure of 20 MN/m2 and a temperature of
500 deg C. The steam expands to a pressure of 200 kN/m2 and a temperature of 200 deg C.
During the expansion there is a net heat loss from the steam through the walls of the
cylinder and piston of 120 kJ/kg. Determine the displacement work done by one kg of
steam during this expansion.
168.6 kj/kg
State 1: at 20 MPa, 500 C: u = 2942.9 kJ/kg
State 2: at 200 kPa, 200C: u = 2654.4 kJ/kg
Closed system for which the first law of Termodynamics applies,
Q - W = dU
Rearranging to determine the work done:
W = Q -
dU
U = (-120) – (2654.4 -2942.9) = 168.5 kJ/kg
A closed rigid system has a volume of 85 litres contains steam at 2 bar and dryness fraction of 0.9. Calculate the quantity of heat which must be removed from the system in order to reduce the pressure to 1.6 bar. Also determine the change in enthalpy and entropy per unit
mass of the system.
-38 kJ
2 kg of air is heated at constant pressure of 2 bar to 500 degC. Determine the change in its entropy if the initial volume is 0.8 m3.
2.04 kJ/K
T1 = (2x10^5)(0.8)/(2)(287) = 278.746 K
dS = (2x10^5)ln(500+273)/278.7 = 2.05 kJ/K
A boiler is designed to work at 14 bar and evaporate 8 kg/s of water. The inlet water to the
boiler has a temperature of 40 deg C and at exit the steam is 0.95 dry. The flow velocity at
inlet is 10 m/s and at exit 5 m/s and the exit is 5 m above the elevation at entrance.
Determine the quantity of heat required. What is the significance of changes in kinetic and
potential energy on the result?
20.186 MW
Steam flows along a horizontal duct. At one point in the duct the pressure of the steam is 1 bar and the temperature is 400°C. At a second point, some distance from the first, the pressure is 1.5 bar and the temperature is 500°C. Assuming the flow to be frictionless and
adiabatic, determine whether the flow is accelerating or decelerating.
Decelerating
Steam is expanded isentropically in a turbine from 30 bar and 400°C to 4 bar. Calculate the work done per unit mass flow of steam. Neglect changes in Kinetic and Potential energies.
476 kJ/kg
A compressor takes in air at 1 bar and 20°C and discharges into a line. The average air velocity in the line at a point close to the discharge is 7 m/s and the discharge pressure is 3.5 bar. Assuming that the compression occurs isentropically, calculate the work input to the compressor. Assume that the air inlet velocity is very small.
-126.6 kW/kg
Air is expanded isentropically in a nozzle from 13.8 bar and 150°C to a pressure of 6.9 bar.
The inlet velocity to the nozzle is very small and the process occurs under steady-flow,
steady-state conditions. Calculate the exit velocity from the nozzle knowing that the nozzle
is laid in a horizontal plane and that the inlet velocity is 10 m/s.
- 9 m/s
A rotary air compressor takes in air (which may be treated as a perfect gas) at a pressure of
1 bar and a temperature of 20°C and compress it adiabatically to a pressure of 6 bar. The isentropic efficiency of the processes is 0.85 and changes in kinetic and potential energy may be neglected. Calculate the specific entropy change of the air. Take R = 0.287 kJ/kg. K and Cp = 1.006 kJ/kg K.
0.07 kJ/kg. K
An air receiver has a capacity of 0.86m3 and contains air at a temperature of 15°C and a
pressure of 275 kN/m2
An additional mass of 1.7 kg is pumped into the receiver. It is then left until the temperature becomes 15°C once again. Determine:
a) the new pressure of the air in the receiver, and
b) the specific enthalpy of the air at 15°C if it is assumed that the specific enthalpy of the
air is zero at 0°C.
Take Cp = 1.005 kJ/kg, Cv = 0.715 kJ/kg K
442 kN/m2
15.075 kJ/kg
Oxygen has a molecular weight of 32 and a specific heat at constant pressure of 0.91 kJ/kg
K.
a) Determine the ratio of the specific heats.
b) Calculate the change in internal energy and enthalpy if the gas is heated from 300 to
400 K.
1.4, 65 kJ/kg
91 kJ/kg
A steam turbine inlet state is given by 6 MPa and 500°C. The outlet pressure is 10 kPa.
Determine the work output per unit mass if the process:-
a) is reversible and adiabatic (ie 100% isentropic),
b) such that the outlet condition is just dry saturated
C) such that the outlet condition is 90% dry.
1242.7 kJ/kg
837.5 kJ/kg
1076.8 kJ/kg
Determine the volume for carbon dioxide contained inside a cylinder at 0.2 MPa, 27°C:-
a) assuming it behaves as an ideal gas
b) taking into account the pressure and volume associated with its molecules
0.2833 m3/kg
A cylindrical storage tank having an internal volume of 0.465 m3 contains methane at 20°C
with a pressure of 137 bar. If the tank outlet valve is opened until the pressure in the
cylinder is halved, determine the mass of gas which escapes from the tank assuming the tank temperature remains constant.
20.972 kg
Find the specific volume for H20 at 1000 kN/m2 and 300°C by using:-
a) the ideal gas equation assuming R = 461.5 J/kg K
b) steam tables
0.264m3/kg
0.258 m3/kg
Determine the specific volume of steam at 6 MPa using the steam tables for the following
conditions:-
a) dryness fraction x = 0
b) dryness fraction x = 0.5
c) dryness fraction x = 1
d) its temperature is 600 degC
0.001319
0.01688
0.03244
0.06525 m3
/kg
Steam at 4 MPa, 400 degC expands at constant entropy till its pressure is 0.1 MPa.
Determine:
a) the energy liberated per kg of steam
b) repeat if the process is 80% isentropic
758 kJ/kg
606 kJ/kg
Steam (1 kg) at 1.4 MPa is contained in a rigid vessel of volume 0.16350 m3. Determine its temperature.
a) If the vessel is cooled, at what temperature will the steam be just dry saturated?
b) If cooling is continued until the pressure in the vessel is 0.8 MPa; calculate the final
dryness fraction of the steam, and the heat rejected between the initial and the final
states.
250 degC,
188 degC,
0.678; 8181 kJ]
Steam (0.05 kg) initially saturated liquid, is heated at constant pressure of 0.2 MPa until its volume becomes 0.0658 m3. Calculate the heat supplied during the process.
128.355 kJ
Steam at 0.6 MPa and dryness fraction of 0.9 expands in a cylinder behind a piston isentropically to a pressure of 0.1 MPa. Calculate the changes in volume, enthalpy and temperature during the process.
1.1075 m3
-276 kJ/kg,
-59 drgC
The pressure in a steam main pipe is 1.2 MPa; a sample is drawn off and throttled where its pressure and temperature become 0.1 MPa, 140 defC respectively. Determine the dryness fraction of the steam in the main stating reasonable assumptions made!
0.986 assuming constant enthalpy
