8-33
8-48 Chickens are to be cooled by chilled water in an immersion chiller that is also gaining heat from the surroundings. The
rate of heat removal from the chicken and the rate of exergy destruction during this process are to be determined.
Assumptions 1 Steady operating conditions exist. 2 Thermal properties of chickens and water are constant. 3 The
temperature of the surrounding medium is 25C.
Properties The specific heat of chicken is given to be 3.54 kJ/kg.°C. The specific heat of water at room temperature is 4.18
kJ/kg.C (Table A-3).
Analysis (a) Chickens are dropped into the chiller at a rate of 700 per hour. Therefore, chickens can be considered to flow
steadily through the chiller at a mass flow rate of
m chicken  (700 chicken/h)(1.6 kg/chicken)  1120 kg/h = 0.3111 kg/s
Taking the chicken flow stream in the chiller as the system, the energy balance for steadily flowing chickens can be
expressed in the rate form as
E  E

E system 0 (steady)
 0  E in  E out
inout



Rate of net energy transfer
by heat, work, and mass
Rate of change in internal, kinetic,
potential, etc. energies
m h1  Q out  m h2 (since ke  pe  0)
Q out  Q chicken  m chicken c p (T1  T2 )
Then the rate of heat removal from the chickens as they are cooled from 15C to 3ºC becomes
Q
(m c T )
 (0.3111 kg/s)(3.54 kJ/kg.º C)(15  3)º C  13.22 kW
chicken
p
chicken
The chiller gains heat from the surroundings as a rate of 200 kJ/h = 0.0556 kJ/s. Then the total rate of heat gain by the
water is
Q
 Q
 Q
 13.22 kW  (400 / 3600) kW  13.33 kW
water
chicken
heat gain
Noting that the temperature rise of water is not to exceed 2ºC as it flows through the chiller, the mass flow rate of water
must be at least
Q water
13.33 kW
m water 

 1.594 kg/s
(c p T ) water (4.18 kJ/kg.º C)(2º C)
(b) The exergy destruction can be determined from its definition Xdestroyed = T0Sgen. The rate of entropy generation during
this chilling process is determined by applying the rate form of the entropy balance on an extended system that includes the
chiller and the immediate surroundings so that the boundary temperature is the surroundings temperature:
S  S out
 S gen
 S system 0 (steady)
in



Rate of net entropy transfer
by heat and mass
Rate of entropy
generation
m 1 s1  m 3 s 3  m 2 s 2  m 3 s 4 
Qin
 S gen  0
Tsurr
m chicken s1  m water s 3  m chicken s 2  m water s 4 
Qin
 S gen  0
Tsurr
Rate of change
of entropy
Q
S gen  m chicken ( s 2  s1 )  m water ( s 4  s 3 )  in
Tsurr
Noting that both streams are incompressible substances, the rate of entropy generation is determined to be
Q
T
T
S gen  m chicken c p ln 2  m water c p ln 4  in
T1
T3 Tsurr
 (0.3111 kg/s)(3.54 kJ/kg.K) ln
276
275.5 (400 / 3600) kW
 (1.594 kg/s)(4.18 kJ/kg.K) ln

288
273.5
298 K
 0.001306 kW/K
Finally,
X destroyed  T0 S gen  (298 K)(0.001306 kW/K) = 0.389 kW
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