# humidity flow ```FACULTY OF ENGINEERING AND COMPUTER SCIENCE
DEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING
COURSE
NUMBER
Thermodynamics II
EXAMINATION
Final
SECTION
MECH 351
DATE
TIME &amp; PLACE
April 24, 2012
9:00 – 12:00
PROFESSORS
Room:
All Sections
# OF PAGES
18
LAB INSTRUCTOR
MATERIALS ALLOWED

X NO
CALCULATORS ALLOWED
NO
x YES (non programmable)

SPECIAL INSTRUCTIONS:
State clearly any assumptions you make.
Draw a clear sketch of the problem.
Return the Exam paper with the answers’ book.
Name: _______________________________
I.D.: ______________________
Surname, given names
1
Question no. 1 (30 Points)
Consider a cogeneration power plant modified with regeneration. Steam enters the turbine at 6 MPa
and 450&deg;C and expands to a pressure of 0.4 MPa. At this pressure, 60 percent of the steam is
extracted from the turbine, and the remainder expands to 10 kPa. Part of the extracted steam is used
to heat the feedwater in a feedwater heater. The rest of the extracted steam is used for process heating
and leaves the process heater as a saturated liquid at 0.4 MPa. It is subsequently mixed with
feedwater heater, and the mixture is pumped to the boiler pressure.
Assuming the turbine and the pumps to be isentropic,
a- Show the cycle on a T-s diagram with respect to saturation lines.
b- Determine the mass flow rate of steam through the boiler for a net power output of 15 MW.
c- Compute the utilization factor.
Question no. 2 (25Marks):
A stationary gas-turbine power plant, as shown in figure, operates on an ideal regenerative Brayton
q regen,act
cycle ( = 75 percent,  
) with air as the working fluid. Air enters the compressor (state
q regen,max
1) at 100 kPa and 290 K and the turbine (state 3) at 610 kPa and 1100 K. The compressor and
turbine isentropic efficiencies are 84 and 100%, respectively.
(a) Plot the T-s diagram of the cycle.
(b) Determine the thermal efficiency of the cycle.
(c) Determine the net work delivered by this plant.
Assume constant specific heats with temperature, Cv = 0.718 kJ/kg K and Cp= 1.005 kJ/kg
K.
2
Question no. 3 (20 Marks)
Atmospheric air enters an air-conditioning system at 30oC and 70 percent relative humidity
with a volume flow rate of 4 m3/min and is cooled to 20oC at a pressure of 1 atm, as shown in
the figure. The system uses refrigerant-134a as the cooling fluid that enters the cooling section
at 350kPa with a quality of 20 percent and leaves as a saturated vapor.
a- Sketch the process on the Psychrometric chart.
b- What is the heat transfer from the air to the cooling coils, in kW?
c- If any water is condensed from the air, how much water will be condensed from the
atmospheric air per min?
d- Determine the mass flow ate of the refrigerant, in kg/min.
3
Question no. 4 (25 Marks)
Benzene gas (C6H6) at 25&deg;C is burned during a steady-flow combustion process with 95 percent
theoretical air that enters the combustion chamber at 25&deg;C and 1 atm. All the hydrogen in the fuel burns
to H2O, but part of the carbon burns to CO. If the products leave at 1000 K, determine,
a- The mole fraction of the CO in the products.
b- The heat transfer from the combustion chamber during this process.
4
s
THERMODYNAMICS DATA SHEET
R = 8.315 kJ/kmole&deg;K
1 bar = 100kPa
1 gm/cm3 = 103 kg/m3
Molar masses and specific heats:
Air:
M = 29 kg/kmol
Cp= 1.005kJ/kg K
Enthalpy of Formation:
5
Note: Cp = Cv + R
6
7
8
9
10
11
12
13
14
15
.
16
Formula Sheet – Thermodynamics II
 T2   V1 
  
 T1   V2 
K 1
T   P 
; 2    2 
 T1   P1 
K 1
K
MEP=
K
 P  V 
W
W
C
;  2    1  ; where K= P ;  P  s ;T  a ;
Wa
Ws
CV
 P1   V2 
W
Vmax  Vmin
;
 cycle 
Wnet
Qin
Ni
m
; Average molar mass M m = m =  yi M i ; gas constant:
Nm
Nm
P
V
N
R
Rm  u  Ru  8.314 KJ /( Kmol.K )  ; PV  ZNRuT ; (Zm   yi Zi ) ; i  i  i  yi ;
Mm
Pm Vm Nm
Mole number: Ni

mi
; Mole fraction: yi
Mi

Dalton’s law: Pm   Pi (Tm .Vm ) ; Amagat’s law: Vm   Vi (Tm .Pm ) ;
U m =  mfiU i , U m =  yi U i (same for h and s); Cv,m   mfiCv,i ; C v,m   yi C v,i , same for Cpm
Real gasses: h  (h2
 h1 ) ideal  Ru Tcr ( Z h 2  Z h1 ) ; u  (h2  h1 )  Ru (Z 2T2  Z1T1 ) ;
s  (s2  s1 )ideal  R(Zs2  Zs1 ) ;
.
Air conditioning energy balance:
Q
Air fuel ratio: AF 
.
in
.
.
.
.
.
; m=NM ;
m fuel
.
out

.
m air
.
 W in   mi hi  Q  W out   me h e , m  m a 
Pvi
Ptotal

mair,act
N
Ni

; Percentage of theoretical air: 
or air ,act
Ntotal
mair,th
N air,th
Enthalpy of combustion : hc=H prod  H react ;
HHV  LHV  (mhfg )H2O ;
First law analysis of reacting systems:
Q  W   N p (h f   h  h) p  Nr (h f   h  h)r
Entropy generation of reacting systems:
S gen  N p s p  N r s r

yP
 N p  s o i T , Po   Ru ln i m
Po



yP

 N r  s o i T , Po   Ru ln i m
Po
 Pr od

17


 react
Question No 1:
Required
(1) Mass flow rate
(2) Utilization factor
Question No 2:
Required
Thermal efficiency
Work in kW
Question No 3:
Required