ChE Thermo Problems Flashcards
(40 cards)
Determine for a unit mass of air, the change in enthalpy when heated from zero°C to 100°C if:
i. Cp = 1 kJ/kg-K constant
H= 100 kJ/kg
ii. Cp = 0.95 + 0.00002T – 0.03x10-6 T2
H= 96.09 kJ/kg
100 kJ/kg; 96.09 kJ/kg
An insulated, constant-volume system containing
1.36 kg of air receives 53 kJ of paddle work. The
initial temperature is 27°C. Determine
a) The change of internal energy.
U= + 53 kJ
b) The final temperature
Assume a mean value Cv = 0.718 kJ/kg-K
T= 81.3°C
+ 53 kJ; 81.3°C
Determine using Steam Tables, the volume occupied by 2 kg of steam at 500 kPa, under the
following conditions and specify the state of steam.
a) Pure liquid state
V= 0.00218 m3
b) When it is in a pure vapour state
V= 0.7498 m3
0.00218 m3; 0.7498 m3
A nuclear reactor generates 3000 MW of heat. The
heat is transferred in a heat exchanger of energy
transfer efficiency 75% into steam which is expanded in a turbine in order to produce a power output. The steam is condensed in a condenser,
releasing 1800 MW of heat, and pumped back through the heat exchanger by a feed pump which requires 3% of the power output from the turbine.
Determine:
a) The net power output from the plant.
P= 463.9 MW
b) The power output from the turbine.
P= 450 MW
463.9 MW; 450 MW
A boiler receives feed water at 40°C and delivers steam at 2 MPa and 500°C. if the furnace is oil fired, the calorific value of oil being 42000 kJ/kg and
4000 kg oil are burned while 45000 kg of steam are produced, determine:
a) The heat supplied in the boiler.
Q= 1.484x10^8 kJ
b) The efficiency of the boiler.
e= 88%
Assume the values of enthalpies at the two state points as:
h1 = hf@40°C = 169.33 kJ/kg at 2 MPa, 500°C, h2 = 3467.6 kJ/kg
1.484x10^8 kJ; 88%
A motorist equips his automobile tyres with a relieftype valve so that the pressure inside the tyre will never exceed 220 kPa (gauge). He starts the trip with a pressure of 200 kPa (gauge) and a temperature of 23°C in the tyres. During the long drive the temperature of the air in the tyres reaches 83°C. each tyre contains 0.11 kg of air. Determine the mass of air escaping each tyre
m= 0.0094 kg
0.0094 kg
Determine the pressure of water vapour at 300°C, if its specific volume is 0.2579 m3/kg, using the following method:
a) Ideal gas equation
P= 1.025 MPa
b) Van-der-Waals equation
P= 1.006 MPa
1.025 MPa; 1.006 MPa
Oxygen has a molecular weight **of 32 and a specific heat at constant pressure of 0.91 kJ/kgK.
a) Determine the ratio of the specific heats.
n= 1.4
b) Calculate the change in internal energy and
enthalpy if the gas is heated from 300 to 400 K
U= 65 kJ/kg, H= 91 kJ/kg
1.4, 65 kJ/kg, 91 kJ/kg
Calculate the density of Ethane at 171 bar and 458K; assume for Ethane:
TC = 305 K
PC = 48.80 bar
R = 319.3 J/kg-K
a) Assume it behaves as a perfect gas.
density= 117 kg/m3
b) Using the compressibility chart.
Z= 146 kg/m3
117 kg/m3, 146 kg/m3
You have a 200 gram cup of coffee at 100°C, too
hot to drink. How much will you cool it by adding 50 gm
of water at 0°C?
80°C
An ideal gas occupies a volume of 0.5 m3 at a temperature of 340 K and a given pressure. The gas undergoes a constant pressure process until the
temperature decreases to 290 K. Determine
a) The final volume.
V= 0.426 m3
b) The work if the pressure is 120 kPa.
W= -8.88 kJ
0.426 m3; -8.88 kJ
A reciprocating internal combustion engine has a
clearance volume of 0.0001 m3 and a compression ratio (volume ratio) of 10. The pressure and temperature of the combustion gases when the piston is at top dead centre are 4000 kN/m2 and 1800°C respectively. Assuming that the expansion process follows PV^1.3 = constant, calculate:
a) The work transfer in this process; and
W= 66.7 J
b) The temperature of the gases at the end of the
process
T= 1039 K
66.7 J; 1039 K
An ideal centrifugal air compressor takes in air at 1
bar, 15°C and compresses it reversible and adiabatically to a pressure of 4 bar.
a) Calculate the delivery temperature of the gas.
T= 428 K
b) If kinetic energy and potential energy changes
are negligible calculate the specific work transfer in the compression process.
W= -140.7 kJ/kg
428 K; -140.7 kJ/kg
Milk initially at 30°C is to be kept in a chilled tank at 5°C. If the total volume of milk is 100 litres, its density is 1100 kg/m3 and the specific heat capacity
of 4.2 kJ/kg-K. Determine the heat extraction rate assuming the chiller to be perfectly insulated.
Q= 11.55 kW
11.55 kW
Self-ignition would occur in the engine using certain brand of petrol if the temperature due to compression reached 350°C. Calculate the highest ratio of compression that may be used to avoid preignition if the law of compression is PV^1.3 = c .
r= 11.36
11.36
A boiler receives feed water at 20 kPa as saturated liquid and delivers steam at 2 MPa and 500oC. If the furnace of this boiler is oil fired, the calorific value of oil being 42000 kJ/kg determine the efficiency of
the combustion when 4.2 tonnes of oil was required
to process 42000 kg of steam.
96%
96%
2 kg of air is heated at constant pressure of 2 bar to
500oC. Determine the change in its entropy if the
initial volume is 0.8 m3.
S= 2.04 kJ/K
2.04 kJ/K
Find the specific volume for H2O at 10 MPa and 500°C using:
a) Compressibility chart;
V= 0.032 m3/kg
b) Steam tables
V= 0.03279 m3/kg
0.032 m3/kg; 0.03279 m3/kg
10 kg/s steam at 6 MPa and 500oC, expands isentropically in a turbine to a pressure of 100 kPa. If the heat transfer from the casing to surroundings represents 1 per cent of the overall change of
enthalpy of the steam, calculate the power output of
the turbine. Assume exit is 5 m above entry and that initial velocity of steam is 100 m/s whereas exit velocity is 10 m/s.
96%
96%
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
V= 0.2833 m3/kg
0.2833 m3/kg
Self-ignition would occur in an engine using certain brand of petrol if the temperature due to compression reaches 350°C; when the inlet condition is 1 bar, 27°C. Calculate the highest compression ratio possible in
order to avoid self-ignition, if the compression is according to
a) Adiabatic, with index of 1.4; n= 6.2
b) Polytropic, with index of 1.3
n= 11.4
6.2; 11.4
Air at 27°C receives heat at constant volume until its temperature reaches 927°C. Determine the heat added per kilogram? Assume for air Cv = 0.718 kJ/kg-K
Q= 646.2 kJ/kg
646.2 kJ/kg
** 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.
W= -1265 kJ, compression
-1265 kJ, compression
**An unknown gas has a mass of 1.5 kg contained in a bottle of volume 1.17 m3 while at a temperature of 300K, and a pressure of 200 kPa.
Determine the ideal gas constant and deduce the gas?
R = 520 J/kg-K;
M = Ro/R = 15.99]
R = 520 J/kg-K; M = Ro/R = 15.99]
