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11-155
11-175 The inlet and exit temperatures and the volume flow rates of hot and cold fluids in a heat exchanger are given. The
rate of heat transfer to the cold water, the overall heat transfer coefficient, the fraction of heat loss, the heat transfer
994.8 and 998.6 kg/m3, respectively. The specific heat at the average
temperature is 4178 J/kg.C for hot water and 4184 J/kg.C for cold
The rates of heat transfer from the hot water and to the cold water are
19.8C
27.0C
11-159
11-182 Cold water (cp = 4.18 kJ/kgºC) enters a counter-flow heat exchanger at 18ºC at a rate of 0.7 kg/s where it is heated by
hot air (cp = 1.0 kJ/kgºC) that enters the heat exchanger at 50ºC at a rate of 1.6 kg/s and leaves at 25ºC. The maximum
possible outlet temperature of the cold water is
(a) 25.0ºC (b) 32.0ºC (c) 35.5ºC (d) 39.7ºC (e) 50.0ºC
Answer (c) 35.5ºC
11-183 A heat exchanger is used to condense steam coming off the turbine of a steam power plant by cold water from a
nearby lake. The cold water (cp = 4.18 kJ/kgºC) enters the condenser at 16ºC at a rate of 42 kg/s and leaves at 25ºC
while the steam condenses at 45ºC. The condenser is not insulated and it is estimated that heat at a rate of 8 kW is lost
from the condenser to the surrounding air. The rate at which the steam condenses is
(a) 0.228 kg/s (b) 0.318 kg/s (c) 0.426 kg/s (d) 0.525 kg/s (e) 0.663 kg/s
11-161
11-186 In a parallel-flow, liquid-to-liquid heat exchanger, the inlet and outlet temperatures of the hot fluid are 150ºC and
90ºC while that of the cold fluid are 30ºC and 70ºC, respectively. For the same overall heat transfer coefficient, the
percentage decrease in the surface area of the heat exchanger if counter-flow arrangement is used is
(a) 3.9% (b) 9.7% (c) 14.5% (d) 19.7% (e) 24.6%
Answer (e) 24.6%
11-187 A counter-flow heat exchanger is used to cool oil (cp = 2.20 kJ/kgºC) from 110ºC to 85ºC at a rate of 0.75 kg/s by
cold water (cp = 4.18 kJ/kgºC) that enters the heat exchanger at 20ºC at a rate of 0.6 kg/s. If the overall heat transfer
coefficient is 800 W/m2ºC, the heat transfer area of the heat exchanger is
(a) 0.745 m2 (b) 0.760 m2 (c) 0.775 m2 (d) 0.790 m2 (e) 0.805 m2
U=0.800 [kW/m^2-C]
Q_dot=m_dot_h*c_p_h*(T_h_in-T_h_out)
11-163
11-190 In a parallel-flow heat exchanger, the NTU is calculated to be 2.5. The lowest possible effectiveness for this heat
exchanger is
NTU=2.5
c=1 "The effectiveness is lowest when c = 1"
11-191 Cold water (cp = 4.18 kJ/kgºC) enters a counter-flow heat exchanger at 10ºC at a rate of 0.35 kg/s where it is heated
by hot air (cp = 1.0 kJ/kgºC) that enters the heat exchanger at 50ºC at a rate of 1.9 kg/s and leaves at 25ºC. The effectiveness
of this heat exchanger is
(a) 0.50 (b) 0.63 (c) 0.72 (d) 0.81 (e) 0.89
Answer (d) 0.81
11-166
11-201 A counter flow double-pipe heat exchanger is used for cooling a liquid stream by a coolant. The rate of heat transfer
and the outlet temperatures of both fluids are to be determined. Also, a replacement proposal is to be analyzed.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings
