ICAM2
– Heat Exchangers
ICAM (LILLE) - HEAT EXCHANGER COURSE
ICAM 2 - AUTUMN 2003
Heat Exchanger Examples - Sheet 2
1. Consider a double-pipe heat exchanger:
Cold fluid, water
mass flow rate – 4,540 kg/h
heat capacity – 4,180 J/kg K
inlet temperature - 16 C
Hot fluid, oil
mass flow rate – 8,050 kg/h
heat capacity – 2,090 J/kg K
inlet temperature - 93 C
outlet temperature - 66 C
If the overall coefficient of heat exchange U = 283.3 W/m2 K, calculate the surface area of
the heat exchanger. Consider the possibility of using either counter-flow or parallel flow.
[Ans: 8.65 m2, 9.47 m2]
2. Calculate the heat exchange surface area required in a heat exchanger comprised of tubes
with an external diameter of 25 mm in order to cool a solution of ethyl alcohol (mass flow
rate: 24,948 kg/h, heat capacity: 3,804 J/kg K) from 66C to 40C using a water flow (mass
flow rate: 22,680 kg/h, heat capacity: 4180 J/kg K) entering at 10C. The overall heat
transfer coefficient related to the outside tube surface is 566 W/m2 K. Consider the following
cases:
a) Parallel flow in a double-pipe exchanger
b) Counter-flow in a double-pipe exchanger
c) Shell and tube heat exchanger with 2 shell-passes and 72 tube-passes. The alcohol
flows in the shell and the water in the tubes. Calculate also the exchanger length.
d) Cross-flow heat exchanger with both fluids making one pass. Consider the shell fluid to
be "mixed".
[Ans: a) 61.5 m2; b) 40.4 m2; c) 46.4 m2; d) 45.9 m2]
3. It is required to cool to 30C, in a counter-flow heat exchanger, an oil whose initial
temperature, flow rate and heat capacity are respectively 110C, 5,000 kg/h and 2,100 J/kg
K. The other fluid is water entering the exchanger at 12C with a flow rate of 12,000 kg/h. If
the overall heat transfer coefficient of the exchanger is 300 W/m2 K, calculate the outlet
temperature of the water and the required heat exchanger surface area.
[Ans: 28.75°C, 18.5 m2]
4. Consider a one tube-pass, one shell-pass, shell and tube heat exchanger operating in
counter-flow.
Cold fluid, water
mass flow rate – 1,000 kg/h
heat capacity – 4,180 J/kg K
inlet temperature - 10 C
Hot fluid, lubricating oil
mass flow rate – 2,500 kg/h
heat capacity – 2,131 J/kg K
inlet temperature - 120 C
Given that the surface area of the heat exchanger is 300 W/m2 K and the corresponding
overall heat transfer coefficient is 5 m2, find:
a) The heat transfer rate
b) The outlet temperatures of hot and cold fluids.
[Ans: a) 76.39 kW; b) 75.8°C, 68.4°C]
Mike Patrick
Page 1
18/02/2016
ICAM2
– Heat Exchangers
5. A preliminary design study is to be carried out on an oil-oil heat exchanger with the following
specification:
Cold fluid, oil
mass flow rate - 64.0 t/h
heat capacity – 2,612 J/kg K
inlet temperature - 171.5C
outlet temperature - 239C
Hot fluid, oil
mass flow rate - 28.8 t/h
heat capacity – 2,612 J/kg K
inlet temperature - 340C
If the overall coefficient of heat exchange U = 502 W/m2 K,
a) Calculate the heat exchange surface area required for:
 Simple counter-flow
 One shell-pass, two tube-pass
 Two shell-pass, four tube-pass
 Three shell-pass, six tube-pass
b) Given that there is already available a one shell-pass, two tube-pass exchanger of
surface area 210 m2, specify what additional arrangement of heat exchangers in series
would be necessary to deliver the required performance.
[Ans: a) 461 m2, no solution, 634 m2, 530 m2; b) 2 exchangers in series each of 149 m2]
6. A number of possible heat exchange arrangements are to be considered based on the
availability of a pair of water-water cross flow heat exchangers of heat transfer surface area
170 m2 operated with the hot fluid mixed. The inlet temperatures for hot and cold fluid are
90C and 10C respectively while the overall heat exchange coefficient is given by the
relationship:
U = 52 (mc + 0.8 mh)0.5 + 65 W/m2K
where mh and mc are the mass flow rates of the hot and cold streams respectively
expressed in kg/s.
a) Determine the effectiveness of a single exchanger for the following flow rates:
 mc = 9.6 kg/s, mh = 8.2 kg/s
 mc = 4.8 kg/s, mh = 8.2 kg/s
 mc = 9.6 kg/s, mh = 4.1 kg/s
b) Determine the overall effectiveness and the outlet temperatures when the two
exchangers are mounted in series for the first set of flow rates.
c) Determine the overall effectiveness and the outlet temperatures when the two
exchangers are operated with the hot water flowing in series and the cold water flowing
in parallel again for the first set of flow rates.
[Ans: a) 0.552, 0.681, 0.785; b) 0.727, 31.8C, 59.7C; c) 0.638, 38.9C, 53.6C]
7. Cold water flows inside the tubes of a condenser with a flow rate of 400 kg/h and an inlet
temperature of 18C. The water is heated by the condensation of steam at a temperature of
104C. The following data is provided: Internal diameter of tubes, 12.5 mm, External
diameter of tubes 16 mm, tube length 2.4 m, thermal conductivity of material of the pipe
wall, 46 W/m K. For the water, the kinematic viscosity is 0.7 x 10-6 m2/s and the Prandtl
number is 5.5.
a) Calculate the heat transfer coefficient on the inside of the tube
b) If the heat transfer coefficient on the outside of the tube is 8000 W/m²K, calculate the
overall heat transfer coefficient
c) Calculate the number of transfer units (NTU) and the effectiveness ().
d) Calculate the outlet temperature of the cold fluid.
[Ans: a) 4504 W/ m²K; b) 2212 W/m²K; c) 0.576, 0.438; d) 55.7C]
Mike Patrick
Page 2
18/02/2016
ICAM2
– Heat Exchangers
8. A counter-flow, double-pipe heat exchanger is used to heat 1.25 kg/s of water from 35C to
80C by cooling an oil (heat capacity = 2.0 kJ/kg K) from 150C to 85C. The overall heat
transfer coefficient is 850 W/m² K. A similar installation is to be built at another location but it
is desired to compare the performance of the single exchanger with two identical smaller
counter-flow heat exchangers connected in series on the water side and in parallel on the oil
side. The oil flow is to be split equally between the two exchangers and it may be assumed
that the overall heat transfer coefficient for the smaller exchangers is the same as that for
the larger one. If the smaller exchangers cost 20 per cent more per unit surface area than
the bigger ones, comment on the economics of the choice between the two systems.
[Single exchanger area 4.65 m², area of each smaller exchanger 2.47 m²]
9. A run-around coil heat recovery system is used for a laboratory in which the presence of
bacteria rules out any possibility of air re-circulation or a direct recuperative heat exchanger.
Air enters the room via a supplementary heater at 24C and leaves at 20C. The outside air
temperature is 5C. If the mass flow rate of air is 2 kg/s, mean heat capacity of air (constant
pressure) is 1.005 kJ/kg K, (UA)H = (UA)C = 4 kW/K, and the heat capacity (constant
pressure) of the secondary fluid is 2.5 kJ/kg K, calculate:
(a) the required mass flow of the secondary fluid
(b) the temperature of the air leaving the system
(c) the percentage energy saving obtained by using the run-around coil.
[Ans: a) 0.804 kg/s; b) 12.5C; c) 39.5%]
10. As part of an energy-saving initiative, a corrosive gas at a flow rate of 30 kg/s, ejected from
a process at a temperature of 300C, is to be used to heat 20 kg/s of water, entering the
process at 10C, by means of a run-around coil. Using the data given below, calculate:
(a) the mass flow rate of secondary fluid required
(b) the effectiveness of the overall heat transfer process
(c) the exit temperature of the water from the run-around system
(d) the operating temperature range of the secondary fluid.
Data:
Mean heat capacities
(constant pressure)
(UA) values
gas
water
secondary fluid
gas to secondary fluid heat exchanger
secondary fluid to water heat exchanger
1.2 kJ/kg K
4.2 KJ/kg K
3.8 kJ/kg K
40 kW/K
200 kW/K
[Ans: a) 18.1 kg/s; b) 0.549; c) 78.3C; d) 31.8C to 115.2C]
Mike Patrick
Page 3
18/02/2016